Monday, December 31, 2012

December 31, 2012: The Wild Winds of December

December, 2012 closed out with a series of three wind storms, from December 22 through the 30th.  Each time, the region endured several hours of cold winds gusting between 45-50 mph...during which the NWS issued Wind Advisories, and area utilities dealt with 100's and even 1000's of regional power outages.

Here are the maximum wind gusts (mph) reported at our region's three airports for each of these events:

Winter Storm           Date         BWI        IAD       DCA

Draco                       Dec 22     44            51          43
Euclid                      Dec 27     46            46          45
Freyr                        Dec 30     46            47          49

It is certainly unusual to experience three Advisory-level wind storms in the space of a single week.  But the series testifies to the ferocity of this season's early winter storms, and the overall pattern of a very energetic jet stream. 

In each case, the winds were generated on the back side of a retreating mid-latitude cyclone (low pressure region).   Draco was a powerful storm that generated a blizzard over the Great Lakes, then moved over interior New England.  Euclid was a Nor'easter (coastal low), as was Freyr.

As each storm moved out of our region (to the northeast), it intensified - meaning the central pressure dropped.  At the same time, a cell of strong high pressure approached from the west.  The difference in surface pressure between each low and its high - called the pressure gradient - determines wind strength.   The gradient became very strong for several hours over the Mid Atlantic during each of these weather systems.   An example of this is shown on the surface weather map for Winter Storm Draco:

Surface weather map December 22, 2012 showing Draco's pressure gradient. Adapted from Unisys Corp
Additionally, clearing skies behind each storm allowed the surface air layer to warm up a bit, while the upper atmosphere cooled (as cold air was swept in from Canada).  This destabilized the air layer, causing it to bubble and stir.   Pockets of fast wind were mixed down to the surface, adding momentum to the already rapidly streaming air, in the form of high wind gusts.  Meteorologists call this effect "mixing down of high momentum from aloft".  Consequently, tree limbs snapped onto power lines, leading to periods of brief power outages across the Baltimore-Washington region.






Saturday, December 29, 2012

December 29, 2012 Disorganized, Light Snow Event

Light, intermittent snow has erupted across the metro region this morning.  This event, well-predicted in terms of timing, may fall short in terms of expected intensity.   As the surface chart below shows, the storm is actually two separate low-pressure centers - one spreading rain over the Outer Banks and Tidewater, VA...the other spreading light snow across the northern Mid Atlantic.

Intellicast
The lows are embedded in relatively fast, west-to-east jet stream flow.  Accordingly, they will move quickly through the region today, and the jet stream troughs supporting their development are relatively low-amplitude.   While lacking upper-air support over the Mid Atlantic, the southern low is expected to intensify into a Nor'easter as it tracks away from our region...pummeling the New England coast with heavy snow...and will be named Winter Storm Freyr by The Weather Channel.

With the Atlantic Tidewater storm stealing moisture from the more northern storm, the rapid movement and weak intensity...snow totals will be light across our region.

Here is a more regional view of the two storm systems, as viewed by weather radar:

Adapted from NOAA

The fast, west-to-east jet flow will stick around through the coming week.   When the jet stream "superhighway" runs fast and lacks curvy twists (troughs), intense storms cannot develop at the surface.   The computer models are predicting a bit of storminess during the Wednesday timeframe - but any impact will likely be on the light side. 


Wednesday, December 26, 2012

December 26, 2012: Anatomy of Winter Storm Euclid

This storm is receiving lots of attention from me...because winter storm systems rarely get as complicated as this, around here.   Today we have experienced a mix of sleet, freezing rain, snow, moderate to heavy rain...and tomorrow, strong winds...all from the same mid-latitude cyclone.

My earlier posts have illustrated the complex thermal structure of this storm, tied to our region's geography, and the influence of that thermal structure on evolving precipitation types through the day.

In this post, we step back and look at the whole enchilada of this classic, textbook winter storm.

First, let's look at the synoptic surface map from this afternoon.  Winter Storm Euclid is a complex, sprawling and intense storm consisting of two low pressure centers.  The primary low is located over Kentucky.  Through the day, a secondary or coastal low has begun to develop over the Carolinas.  Eventually, Euclid will transfer its energy, and its moisture, to "Son of Euclid" as the new center tracks up the East Coast.

Euclid's primary and secondary low pressure centers, and associated precipitation pattern.  Intellicast
The precipitation shield surrounding Euclid's counterclockwise whirl is immense, wrapping from the southern tip of Florida, northward along the East Coast, then back west into the Great Lakes.   Green colors show rain and embedded thunderstorms (Euclid's "warm side") and blue colors indicate ice and snow (the storm's "cold side").

Cyclones such as Euclid have strong temperature contrasts; they bring air masses together from different source regions.  The juxtaposition of air masses creates weather fronts within the storm system.  The weather map below, showing the thermal pattern at 5,000 feet, illustrates the classic "ying yang" pattern whereby warm, tropical-source air (yellow) wraps into the system from the south and east, and cold, Canadian-source air enters (blue-purple) from the north and west:

Winter Storm Euclid's temperature contrasts.  Unisys Corp.
These air mass contrasts are what power large cyclonic storms in the mid-latitudes.

The next graphic shows Euclid from the vantage of weather satellite, using the moisture channel.  Dry areas in the atmosphere appear black, while moist areas show up in varying chromatic shades - from blue to red, dark red indicating the greatest degree of saturation.

Adapted from WeatherTap
I have added a few annotations that reveal the complex 3D anatomy of this storm.  First, note the primary low center over Kentucky, and the secondary center beginning to form near Tidewater, VA.  Next, the massive precipitation shield is this storm's distinguishing feature.  Euclid has an impressive feed of moisture from the Gulf and Atlantic.  The moisture influx assumes the form of a narrow, high-speed conduit or "river" of air entering the system from the south, and is called the Moist Conveyor (solid green arrow).  All along this corridor of high water vapor, clouds condense.  Along the East Coast, severe thunderstorms have erupted in the warm, unstable air.  Over the Mid Atlantic, moderate snow-sleet-rain has fallen from the Conveyor for over 12 hours.  And back through the Ohio Valley, heavy snow has fallen along the Moist Conveyor where air temperature has remained below freezing.  The Moist Conveyor is the one element that connects all the disparate forms of heavy and severe weather in this single storm system.

But along the back edge of the storm, very dry air has descended from high levels over the upper Midwest.   Along the track of this descending current, called the Dry Conveyor, clouds and precipitation have evaporated.   Dry air is wrapping deep inside the inner core of Euclid.  This is the hallmark of a cyclone entering its most mature and intense phase.  Eventually, the dry air will sequester the core completely, and the primary low will dissipate.   Meanwhile, the new coastal low will deepen as it moves toward New England, effectively robbing the old core of its Moist Conveyor.

Finally, and most importantly, it is important to understand where these intense cyclonic storms come from, and what sustains them.  The answer is the polar jet stream, a river of intense air blowing from west to east between 30,000-40,000 feet.  Below is the jet stream chart from this morning, which I have annotated:

Jet Stream level winds and their relationship to Euclid.  Adapted from Unisys Corp.
You can see the core of high winds zipping along from California to the Gulf Coast.  However, there are numerous undulations along the way.  Most prominent of these is a deep trough of low pressure over the Great Lakes (labeled).  The airflow around a trough has counterclockwise curvature, and the trough contains a core of very cold air.   As the air streams along the base of the trough, it speeds up as it exits and moves over the Mid Atlantic (think of a trough as a tight curve on the Beltway - you have to slow down through the curvy part, but once you are free, you accelerate).   I have shown a hypothetical red box in the exit region of the trough.  Imagine air exits the north edge of the box, faster than it arrives from the south.  This creates a "divergence" of air flow within the box.  Because the same amount of air must always remain in the box, air must be drawn up from below, from the surface, to help fill the void.   When we remove air from the surface, we create low pressure there.   Air rising up to fill the imaginary box creates widespread cloud and precipitation.  Thus, areas of low pressure and storminess - such as Euclid - develop near the "exit" region of large troughs in the jet stream. 

There is something else helping to sustain low pressure at the surface.  Note that in addition to the flow speeding up through box, it also spreads outward, away, in a fan-shaped pattern.   This is a consequence of very intense troughs such as the one shown here, which take on a certain tilt from NW to SE.  (In meteorological parlance, the spreading apart of airflow at high levels is called "diffluence").   Nevertheless, as the air fans away, more mass is trying to leave the box from the north side, than can enter from the south side.   Once again, extra air must rise up from below to fill the box.

Finally,  note a very intense pocket of high wind in the core of the jet stream, at the base of the Great Lakes trough.  This is called a jet streak.  When jet streaks zip around the base of a trough, they enhance both the divergence and diffluence of air in the trough's exit region.   This helps surface low pressure regions deepen even further. 





December 26, 2012: The Transition from Snow To Ice To Rain

Today's storm is a classic example of a winter storm starting off with a wedge of subfreezing air stuck up against the mountains - called Appalachian Cold Air Damming - creating several hours of snow and icefall across the region.

But as this storm moved closer to the Atlantic coast, it began drawing in relatively mild air off the Atlantic ocean.  At the critical 5,000 foot level of the atmosphere (the level in the cloud layers that determines precipitation type - rain or snow), temperatures warmed rapidly from several degrees below freezing, to several degrees above freezing.  By late morning, rain - not snow - formed in the clouds.  But air in the lowest few thousand feet still remained below freezing (courtesy the cold air wedge).  So rain drops froze into sleet grains - and sleet began to accumulate on all surfaces.

The 5,000 foot analysis map shows how the 0 C isotherm has pushed north and west of the metro centers (compare with the map from early this morning) - coincident with a changeover from snow to sleet:

Early Morning - Cold Air Damming;  Early Afternoon - Warm Air Invades from Atlantic.  Adapted from NOAA
You can see the change in precipitation types of D.C.-Baltimore on the radar between morning and early afternoon - noting how the snow-sleet line shifted north and west of the metro centers:

Morning:  Snow-Sleet Line south of D.C. - Baltimore.  Afternoon:  Snow-Sleet Line North of D.C.- Baltimore.  Adapted from WeatherTap
Around noon, winds at the surface transitioned from cold northeasterly to milder easterly flow.   The wind direction shifted as a new center of low pressure began developing along the Virginia coastline, setting up more of an oceanic fetch of wind into our region.  The arrival of slightly milder air at the surface began scouring out the cold air dam in place in the lowest few thousand feet over Washington- Baltimore.   As surface temperatures warmed, rain that forming at 5,000 feet, stayed rain all the way to the surface.  As of 2 pm, precipitation area-wide became plain rain.

This progressive transition from an air layer deep enough and cold enough for all snow, to an intermediate structure supporting sleet, finally to an atmosphere warm enough for all rain, took place over 12 hours.   The warming of this layer proceeded from top-down.  The final graphic below illustrates how meteorologists quantify the rate of temperature rise at 5,000 feet - a process called warm air advection.  The map shows a very strong pocket of warm air advection (solid red colors) over VA-MD early this afternoon.   Note the southeasterly winds, up to 75 mph, transporting mild air inland off the Atlantic.   D.C. and Baltimore lie within the northern edge of this intense, warming pocket.

Warm air advection early this afternoon changes the region over to all rain.  Adapted from NOAA



December 26, 2012: Cold Air Damming Bringing Frozen Precipitation This Morning

Overview of the Storm


This winter storm, called Euclid by The Weather Channel, is creating its heaviest snows and high winds across the Ohio Valley, where blizzard conditions prevail this morning along the back, cold side of this mid-latitude cyclone.  The center of the storm is over the Tennessee Valley, and the system is moving toward the northeast.  These enormous, intense, winter cyclones have "cold side impacts" and "warm side impacts" - a case of a storm with a split personality!  The band of heaviest snow usually develops 100-200 miles to the northwest of the storm center.  But in the southeast segment, called the "warm sector", the combination of warm, humid air and vigorous uplift often creates pockets of severe local storms, including thunderstorms and tornadoes.   Winter Storm Euclid has created both a blizzard on its northwest side, and tornadoes in its warm sector.   Here is the radar snapshot of this impressive storm from earlier this morning, clearly showing the cold and warm side precipitation features:

Surface weather in Winter Storm Euclid.  Adapted from WeatherTap
In this diagram, you can see the center of the cyclone, marked with a large, red "L".  To its north and west is the heavy snow band.  To the southeast, over the Carolinas, is a warm air mass containing bands of thunderstorms - some of which may produce another round of severe weather later today.
Note the orange-colored wedge of ice over the Appalachians - this is the region of overlap between warm and cold air masses - with the warm air sliding over dense, cold air trapped in mountain valleys.   The heavy magenta line roughly bisects the storm into its warm-side and cold-side.

Baltimore Impacts:  Why Geography Matters

 

Closer inspection of the morning radar shows a curious wedge of ice and snow over central and western MD, NOVA and the WV panhandle - extending along the spine of the Appalachians in Virginia:

Snow and ice wedge over the Mid Atlantic.  Adapted from WeatherTap
"The Wedge" as meteorologists refer to this setup, is classic Mid Atlantic, wintertime meteorology.   The Appalachians frequently trap cold, sub-freezing air flowing in from the northeast.  A shallow, frigid layer called Appalachian Cold Air Damming abuts the Appalachians over the Piedmont, and also gets trapped within the long, linear valleys of the mountains (such as the Shenandoah Valley).   This cold air is overrun above by warm, humid air flowing off the Atlantic.  The vertical layering of air masses provides a recipe for cloud layers containing various forms of frozen precipitation - freezing rain, sleet, snow.  Thus far, we have experienced all of these on this "multiple choice" winter day.

Cold air damming develops when a ridge of high pressure (caused by dense, chilly dammed against the mountains) sets up across the Mid Atlantic.   The pressure ridge east of the mountains is shown on this morning's surface map.  Solid lines are isobars (lines of constant pressure):

Surface map showing pressure pattern within Winter Storm Euclid.  Adapted from NOAA

The pressure ridge tightens the pressure gradient, accelerating a chilly, northeasterly breeze across the Piedmont.  This brings in more cold air at the surface, reinforcing the cold air wedge.

Let's see what this chilly air current does to the temperature field.   The morning surface temperature map (below) shows the cold air wedge in the pattern of isotherms (lines of constant temperature).   Red isotherms are temperatures above freezing;  blue are those below freezing;  the purple isotherm is the critical freezing line:

Surface temperature map showing critical freezing line (purple isotherm) and cold air wedge.  Adapted from NOAA
You can see the wedge-shaped pattern of sub-freezing air sitting over the Maryland Piedmont and mountains.   As is quite typical, the surface freezing line trends SW to NE, and this morning it bisects the major cities Washington and Baltimore.

Finally, the map below indicates that this morning's cold wedge extends upward through 5,000 feet.  This is a deep, sub-freezing air layer and it guarantees that snow will fall:

5,000 foot temperature map showing deep cold air wedge.  Adapted from NOAA
In fact, compare the location of this deep, cold wedge with the region of radar-observed snow, and you see excellent correspondance:

Location of snow wedge.  Adapted from WeatherTap.


You don't see correspondance with the ice storm extending south along the mountains.  This is because sleet is generated from a much shallower layer of cold air damming...temperatures at 5,000 feet are above freezing, meaning precipitation starts in the clouds as rain.  As it falls into a sub-freezing air layer perhaps only 2,000 feet thick (or less), it quickly freezes into sleet grains.



Monday, December 24, 2012

December 24, 2012: Surprise Christmas Eve Snow North and West of D.C.

Well, I do get it wrong from time time.

My call yesterday was for light precipitation this afternoon, mainly rain, mixing w/ pockets of sleet.  Instead...counties north and west of D.C. are being treated to an afternoon of light snowfall, and we will likely see 1"-2" of accumulation before the storm pushes out between 5-6 pm tonight.

I believe the NWS was also taken by surprise!  They were in "reaction" mode, rushing out winter weather advisories as the flakes began to fall.

First, the surface weather map (below) shows a weak region of low pressure approaching from the west.  A stationary front separates mild air and rain to the south, and cold air with snow to the north:

Intellicast
This is a fast-moving, weak system with limited moisture.  Early this morning, the cold (sub-freezing) air in the lowest 5,000 feet of atmosphere lay well to our north, across northern PA.   It certainly appeared that the precipitation, when it did break out over Baltimore-D.C., would be rain.

Alas, the early morning weather balloon launch at Dulles revealed very dry air in the lowest 20,000 feet of atmosphere, with temperatures hovering right around freezing at 5,000 feet.  When precipitation did begin to form in the cloud layer - perhaps as a mix of rain and snow at that level - it evaporated into the dry air.  Evaporation extracts heat from the air.  This chilled the air layer at the critical snow-making level (5,000 feet) several degrees below freezing.   The cooling continued, in spite of southerly winds at 5,000 feet pushing warmer air in from the south.  In the parlace of Meteorology 101:   Evaporative cooling won out over warm air advection.   Snow, not rain, was able to efficiently form in the cloud layer, and remain as snow all the way to the ground.

I've highlighted this pocket of evaporatively chilled air in the diagram below - showing isotherms of temperature at 5,000 feet.  Isotherms at or colder than freezing are shown in blue, those above freezing are shown in red:

Modified from NOAA
Here is a radar show that shows how the wedge of moderate snow (blue shades) line up with this cold pocket at 5,000 feet:

Weathertap
Now for the Wed-Thurs storm:   My thinking on this is basically unchanged from yesterday's blog post.   However, stronger cold air damming than forecast yesterday may lead to a more prolonged period of iciness (sleet, freezing rain) at the onset of this event.   I still see a strong push of warm air as the bulk of moisture moves through.   Upwards of 80%-90% of the precipitation in the metro will likely fall as liquid, but the percentage of ice will be higher farther west and north of D.C. - Baltimore.   Cold air damming, once established, can be slow to dislodge.  This means subfreezing air hangs tight in the valleys. 

Sunday, December 23, 2012

December 23, 2012: Big Storm This Wednesday

Christmas Eve Storm

 

This will be a relatively small nuisance event, compared to what's coming this Wednesday.  The models appear warm enough for light rain tomorrow, ending tomorrow evening, as a weak wave in the polar jet stream passes through the region from west to east.   A few pockets of sleet may mix in, but any persistent frozen precipitation will fall in colder air to our north and west...where light accumulation may occur (< 1 inch). 

Wednesday's Winter Storm


This storm, Winter Storm Euclid, will intensify over the Tennessee Valley and is coupled to a stronger wave in the southern branch of the jet stream (the subtropical jet).   This will be a more intense and larger region of low pressure, with a strong feed of Atlantic and Gulf moisture.   There will be two phases to this storm - first cold, then warm. 

As the storm approaches from the southwest, a wedge of cold air will be trapped in place at the surface, piled up against the Appalachians.   This is called cold air damming and is a common winter occurrence in the Mid Atlantic.   The flow of sub-freezing air is fed by a region of high pressure over Ontario, which funnels the cold air southward, along the eastern slopes of the Appalachians (blue arrow in the surface weather map, below):

Modified from NWS image
Cold air damming means precipitation - starting as rain several thousand feet above the surface - will freeze before reaching the surface, or right at the surface.  Thus, Winter Storm Euclid may start as a period of light sleet/freezing rain late Tuesday night.

Phase Two occurs Wednesday morning, as the low intensifies and begins to draw relatively warm air off the Atlantic Ocean into its lower circulation.   A brisk southeast wind should scour out the cold wedge and warm the lowest layers of the atmosphere above freezing.   This occurs as the slug of heavy moisture arrives...meaning a rain event for Baltimore on Wednesday.    The rain could be moderate to heavy, and 1"-2" may fall before the storm moves away Wednesday night.  This surface weather map shows the arrival of warm air (red arrow) and heavy rain (solid light blue shaded region) on Wednesday:

Modified from NWS image
Thus, Euclid should begin as a "multiple choice" phase of sleet/freezing rain/rain, turning to all rain - possibly heavy.   This will NOT be a big snowmaker storm, since the source of cold air to the north cannot be maintained.  Nor is it a classic Nor'easter or coastal storm, because it is expected to track largely inland.  But the storm will wring out a fair amount of moisture.

Caveat:  Models such as these consistently underdo the intensity and persistence of cold air damming.   Worst case scenario - the iciness hangs on through Wednesday morning, and takes longer to transition to all rain. 






Saturday, December 22, 2012

December 22, 2012: Draco Winding Down, More Storms On The Way

High Winds Today


As advertised by NWS, strong "sunny day" winds have been buffeting the region, with greatest gusts in the 45-50 mph range early in the afternoon. This is validation of the Wind Advisory criteria.   For the past two days, NWS has posted a somewhat confusing string of watches and warnings to account for post-Draco winds:  First, on Thursday night, a  High Wind Watch, which was downgraded yesterday to a Wind Advisory, which was upgraded early this morning to a High Wind Warning, which was downgraded this afternoon to a Wind Advisory.   Especially Friday morning there was inconsistency in all this:  NWS was predicting maximum gusts of 55 mph for Saturday, which is below High Wind criteria, yet they had a High Wind Watch for Saturday.  It seems that more work needs to be done to clarify a confusing set of watches, warnings and definitions - and be more consistent in communicating the message to the public.

The cause of the winds:  A tight surface pressure gradient, and an unstable atmosphere that has allowed vertical air currents to mix down higher pockets of wind from a few thousand feet above the surface:

NOAA
The diagram shows isobars, lines of constant pressure.  Note how closely spaced they are along the Appalachians.   This means a large pressure gradient, hence strong horizontal winds.  As the large Draco low pressure region retreated slowly to the northeast, high pressure was rapidly approaching from the southwest - causing the isobars to "squeeze" together over us.

Power outages have occurred through the day, but have been isolated/spotty, numbering in the hundreds (not thousands) across area utilities.

Storminess Return This Week


My thinking on this, which I wrote about in detail in yesterday's blog, has not changed...so please reference that one.   Regarding the post-Christmas storm:  Today the models have trended toward a slightly warmer solution, meaning more rain than frozen precipitation for the metro region:



This map shows the center of the low taking more of an inland track, putting us on the warm side.  Note that when and where the heaviest precipitation occurs (solid, light-blue colored region over Baltimore), the freezing line (dark, solid blue line cutting across PA) is well north of Baltimore.  Zoom in on this figure to help see these features.  HOWEVER:  The event is still 4-5 days out.  It is normal to see the models waffle back and forth between "warm" and "cold" solutions, inland vs. coastal track locations...so I would not be surprised to see this forecast change.


Friday, December 21, 2012

December 21, 2012: Parade of Storms Now Through Christmas Week

Winter Storm Draco Exits The Stage

 

Cold air spiraling into the back side of intense winter storm Draco has filtered into our region.   Draco is the very intense, large mid-latitude cyclone that brought heavy snow and a blizzard to the Upper Midwest and Ohio Valleys.  Its tight pressure gradient is sustaining wind gusts in the 30-40 mph range across our region today.   Below is a snapshot of this impressive storm from this afternoon:

Surface Map Courtesy of WeatherBell
The storm has nearly attained its deepest minimum central pressure, and has occluded (the converging warm and cold air masses have become tightly wound-up).  When these storms occlude, they hit peak intensity, and move very slowly or even become stationary.  Thus, Draco will be slow to exit toward the northeast.  As it does, and as the pressure gradient relaxes gradually over the next 24 hours, winds will begin to abate across our region.  HOWEVER:  Winds are expected to surge once again, for several hours during the day tomorrow.   Reason:  Winds a few thousand feet above the surface, wrapping around the vortex from the west, are expected to intensify.  If there is sufficient sun and heating of the surface tomorrow, the atmosphere will become unstable and overturn.  This will mix down some of the faster-moving air aloft (70 mph at 3,000 feet) as intermittent, strong gusts.  The NWS has backed off the prospect of a High Wind Warning, and instead has issued a Wind Advisory, for winds that could gust tomorrow afternoon in the 40-50 mph range.   Below is the NWS wind gust prediction map, with maximum surface gusts (kts) shown at 1 pm tomorrow:


Max values on this map are around 50 mph.  I do not anticipate widespread power outages from this event - any outages will likely be spotty.  With sundown, the wind machine dies down and conditions turn calmer.

Next Storm:  Getting Clipped On Christmas Eve

 

Next up in our parade of storms comes a weak area of low pressure, passing to the south of Baltimore, on Christmas Eve.  It does not have strong upper-atmospheric support and will be relatively starved for moisture.   Look for a brief period of light rain, possibly with a little wintry mix to the west and north of Baltimore:

WeatherBell

 

Post Christmas:  A More Potent Storm Sets Up

 

Storm #3 will originate in the Tennessee Valley and track up the Appalachians.  This track generally places our region on the warm (rainy) side.  This storm will have a much greater tap on Gulf and Atlantic moisture, so precipitation amounts will be heavier.  HOWEVER:  The computer models are suggesting that this system will develop a "secondary" low along the NC/VA coast - a classic breeding ground this time of year for Nor'easters.

WeatherBell
When the primary low transfers its energy and moisture to a secondary, coastal system - the forecast scenario can become quite complicated, with respect to (1) amount of precipitation;  (2) type of precipitation;  and (3) timing/duration of precipitation.   Whether this means a "multiple choice" type of day (wintry mix) for our region, remains to be worked out.   However, it does NOT appear to be a major "snowmaker" storm for Baltimore, for two reasons:  (1) the coastal low will pull quickly away from the coast;  and (2) a deep, sustained feed of very cold air to the north is lacking.





Thursday, December 20, 2012

December 20, 2012: Draco To Blow Through With Strong Winds

Winter Storm Draco is creating a mess of hazardous weather conditions across the eastern U.S. - including an outbreak of severe thunderstorms and tornadoes in its warm, southern side...and blizzard conditions on its cold, northern half.   Strong winds are blowing around the center of the entire system.  This is both an immense, and an intense, mid-latitude cyclone.   This radar mosaic snapshot from this afternoon illustrates the warm (green colors) and cold (blue colors) side of Draco:

Courtesy of WeatherTap
The center of Draco will pass north of our region tomorrow morning.  Tonite, the storm's cold front will push through.  Ahead of the front, the warm air and Gulf moisture will fuel bands of heavy rain showers, associated with gusty winds.    There are myriad weather hazards predicted across the Mid Atlantic.  Whenever you see lots of colors on the NWS watch and warning summary, you know we are in for a high-impact weather event:

National Weaterh Service
Heavy snows and intense winds will combine to create a blizzard scenario along the high mountain elevations to our west (pink and red colors).  The copper-shaded region across DC-Baltimore is a High Wind Watch.   The watch indicates a potential for sustained winds exceeding 40 mph, and/or gusts exceeding 58 mph.   The timeframe for these winds is Friday night and Saturday, peaking Saturday morning.   This is well after the precipitation has cleared the region, and in fact sunny to partly sunny (but chilly) skies may prevail during the period of strong wind.

The strong winds will be driven by two factors:  (1) an intense pressure gradient between Draco's low pressure center, retreating to the northeast, and high pressure building in across the southeast;  and (2) an unstable air mass (cold air aloft, heating of the surface by the sun) - which creates a tendency for the lowest air layers to "stir" or overturn.  With 60 mph winds at about 5,000 feet, some of this momentum may be occasionally mixed down to the surface as strong gusts.

I see a couple factors that might mitigate high winds across the region.  First, the models are weakening the low pressure center as it moves away from our region on Saturday morning, with the pressure gradient weakening throughout the day.  Second, any cloud cover that develops as a result of the unstable air layer may help limit the amount of instability, and therefore mix-down of higher winds aloft.   As we get closer to the event, NWS will either upgrade the High Wind Watch to a Warning, or downgrade it to a Wind Advisory.   Either way, it's fair to expect gusts into the 40-50 mph range for several hours Saturday morning and into the afternoon.   Highest wind gusts will be in elevated or mountainous area (remember that wind speed increases with height above the surface) and along the eastern shore of the Bay (winds pick up speed as they blow from west to east across the water surface).  With foliage off the trees and soils that likely won't become saturated, any power outages in the Baltimore region will likely be isolated.

Here is the predicted maximum gusts (kts) from the NWS, for 10 AM Saturday morning:

Note these values are in knots.  The 40 kts in vicinity of Baltimore translates into about 45 mph.






Wednesday, December 19, 2012

December 19, 2012: Winter Storm Draco Coming!

The Weather Channel has named a new storm, Draco, which is presently creating heavy snow and blizzard conditions across the Upper Midwest.   This is an especially powerful midlatitude cyclone, forecast to track across the Great Lakes and intensify tomorrow and tomorrow night.   The center of Draco will track well to our north, keeping us on the warm side of this winter system.   Draco's circulation will drag a powerful cold front through the region tomorrow night.  This will usher in periods of moderate to heavy rain overnight.  Following passage of the cold front, an intense pressure gradient with northwest winds will build across the Mid Atlantic on Friday.   While skies will become partly to mostly sunny, temperatures will plummet to near the 40 F mark on Friday and into Saturday, and winds will gust to 40-45 mph across the region.  The NWS may very well issue a Wind Advisory for this event.   High elevations to our west (summit of the Appalachians) may experience gusts higher than 60 mph, necessitating a High Wind Warning (this is because winds increase with altitude in mid-latitude cyclones).    A Winter Storm Watch is now posted for the windward side of the Appalachians, for possible heavy snows, during the period of intense W-NW winds (upslope flow) Friday night into Saturday.  Additionally, heavy Lake Effect Snows are likely along the lee shores of the Great Lakes this weekend.  So Draco will bring the first taste of December Winter across a broad region, from the Upper Midwest, Great Lakes, Ohio Valley, Mid Atlantic and into the Northeast.

The diagram below shows the forecast weather map for Saturday morning.  Note the intense low pressure system (the core of Draco) over upstate New York.  Black lines indicate isobars, showing a packed pressure field (tight pressure gradient) over Baltimore.   The colors show wind speeds at 5,000 feet.  Yes, the forecast is for 60 mph winds not far above the surface on Saturday morning!   Some of this momentum is expected to mix down to the surface on the day Saturday, causing very gusty conditions.  Note the 70 mph winds close to mountain top height along the Appalachians.

To summarize, Draco's main impacts for Baltimore will start with bands of rain, followed by onset of strong winds, with wind chills down into the low 30's on Friday and Saturday. 

Courtesy of Weatherbell.com
We will be watching the possible formation of another, potentially potent midlatitude cyclone, which could impact our region after Christmas Day.  The atmosphere appears to be switching gears into what could prove to be a colder than average, stormy period to close out December.  Winter is finally coming!


Monday, November 26, 2012

Nov 26, 2012: Weak Depression To Bring A Touch of Wintry Mix

10:30 AM

A weak low pressure system is expected to develop along the Delmarva Peninsula tonight.   Enough cold air will be in place at the surface and in the lowest 10,000 feet to bring wintry precipitation to parts of the Baltimore region.  However, there are several mitigating factors against significant accumulation:  (1)  the storm is weak (it is NOT a Nor'easter);  (2) the storm has limited moisture to work with;  (3) the storm will pass quickly to our south and east, exiting into the Atlantic on Tuesday.

The forecast map for tomorrow is shown below.  A stationary frontal boundary separates mild air to the south and east, and sub-freezing air to the north and west.   Thus, the further north and west from the front, the greater the probability of accumulating snow.   The highest amounts are expected across the Maryland mountains and along the Mason-Dixon Line (up to 3").   Within Baltimore, some surfaces may become slushy (< 1").  In Annapolis and along the Bay, just rain will fall.   So the temperature gradient is important in determining precipitation type, and this gradient straddles Washington-Baltimore, as is so often the case.

NWS

The NWS in Sterling, VA has issued their snow accumulation map.  It reflects the thinking that you must travel north and west of the D.C.-Baltimore metro to experience air cold enough for accumulating snow:

NWS
Timing of the event is critical.  The precipitation will develop overnight and continue through Tuesday morning.   In the cities, look mainly for a cold, light-moderate rain, with snowflakes mixing in at times.  However, the heaviest precip should be falling during the Tuesday AM rush.  This means slushy surfaces could create some delays.  The system clears out Tuesday afternoon.

Again, this is not a Nor'easter, and the storm is not intense enough to warrant a name given by The Weather Channel.





Wednesday, November 7, 2012

November 7, 2012: Winter Storm Athena Brushes Baltimore

What's In A Name?

Yes, the winter storm has received a name...courtesy of The Weather Channel (TWC).  While not officially sanctioned by the National Weather Service, TWC meteorologists took the unusual step of rolling out a naming convention for Nor'easters.   They have decided to name Nor'easters that generate impacts and cause disruption.   Personally I feel this is a good move and is long overdue.   We have been naming hurricanes since the 1950s and many Nor'easters are just as damaging and far-reaching as hurricanes.   I find it interesting that the NWS Buffalo has been officially naming Lake Effect snowstorms for many years now.  Time will tell if NWS, and other forecasting agencies, adopt TWC's somewhat daring naming convention.

A Brush With Athena

As of 2 pm today, Athena is a moderately intense Nor'easter, moving northward over the Atlantic about 150 miles east of Atlantic City, NJ.   The storm is rapidly deepening and expanding.   Moderate-heavy precipitation is occurring along the Delmarva, NJ and NY.   You can see the precipitation in relationship to the surface pressure field (isobars) in this image:

NWS

Note that the precipitation covers more area over the ocean than shown here, because weather radar along the coast has limited viewing range.

Not all the precipitation is rain.  A band of moderately heavy snowfall (1"/hr) has set up over NJ.  The snow band has developed in the "classic" location about 200 miles north and west of the storm's center, on the cold air side of the storm, in a region where a mid-level front is developing:

WeatherTap
The light green colors represent intermittent light rain over north-central Maryland;  however, because the air beneath cloud base is dry, much of it is evaporating before reaching the surface.

Baltimore Region Impacts

Yesterday I was not calling for much, if any, snowfall in our region, and this appears to be verifying.  The NWS has scaled back their snow accumulation totals for the Baltimore region...down to a dusting around the city, up to an inch in the far northern suburbs.   Only the back edge of the precipitation shield is scraping by Baltimore, because the storm is so far out to sea.   The precip shield has expanded a bit southwestward through the day as the storm intensifies.  But there is not a lot of moisture to work with on the back edge.  And air cold enough for sustained snow has not filtered in from the north.

The storm will continue to intensify this evening, but will also continue moving away to the north.  The precipitation shield should retreat east of Baltimore after midnight.   Winds will remain breezy at times, with peak gusts in the 20-30 mph range.  Tomorrow will become partly cloudy as the storm moves further away, but the breeze will continue.   Tomorrow's the breeze will be sustained by a developing pressure gradient between the storm, and high pressure advancing in from the west.  Wind gusts should not exceed about 25-30 mph.


Tuesday, November 6, 2012

November 6, 2012, 4 pm: Backside of Nor'Easter to Clip Baltimore Region

Now less than 24 hours from the impacts, the forecast models are in good agreement that the center of this Nor'easter storm will track 200-300 miles east of Baltimore, putting us on the back edge of the precipitation shield and wind field.   The storm is forming now off the Georgia coast.  It will track N-NE over the Atlantic tomorrow, intensify tomorrow night, then slowly start to weaken on Thursday evening:

Model Forecast Positions - Nor'Easter Track, NWS
Inland, an unseasonably chilly air mass will be in place, and the storm's circulation may pull in even colder air from the north on Wednesday.   Thus, the "S word" (snow!) is added to the list of our potential impacts.

Overall, given the trend in the models, this should be a low-impact event lasting about 24 hours for the Baltimore region.

Specific impacts:

1.  Precipitation Amount:  Precip will fall intermittently from Wednesday evening into Thursday morning.  Amounts will be light i.e. just a couple tenths of an inch:

NWS.
By far, the lion's share of rain is to our east, over the water.

2.  Precipitation Type:  A mix of rain and snow...ranging from a trace near D.C. to about an 1" north and east of Baltimore:

NWS.




3.  Winds:  Strongest over the Atlantic and coastal zone - generally from the N-NW gusting to 20-30 mph, peaking on Thursday morning, and highest along the Bay:

NWS.



Monday, November 5, 2012

November 5, 2012: Moderate Nor'easter Weather Impact for Baltimore

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November 6, 2012, 4 PM:  Baltimore Nor'easter Impacts
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Scenario and Timeframe
Our region is expected to pass through the back edge of a strong Nor'easter.
The various models portray the development of a Nor’easter off Cape Hatteras on Wednesday.   The storm will track N-NE and rapidly intensify off the NJ coast.   The storm may briefly stall off the NJ coast on Thursday, before moving NE away from the coast Thursday night.
The biggest impacts from this storm will be along the coastline from the Delmarva northward along NJ, NY and New England.
For Baltimore, the moderate weather impacts will occur during a 12-18 hr period from Wednesday night into Thursday morning.
Weather impacts will generally diminish moving inland from the coast (except at the higher elevations i.e. Catoctins, Appalachians – where winds will be higher and probability of snow is greater).
The following sequence of surface weather charts shows the general progression and intensification of the storm, at various times:
7 AM Wednesday. Storm develops, deepens, moves slowly north. NWS.

7 PM Wednesday.  Storm deepens, moving slowly north.  NWS.
7 AM Thursday. Storm reaches lowest central pressure.  NWS

7 PM Thursday.  Storm weakens, moves away toward northeast.  NWS

Another way to look at the evolution of the storm is to plot the likely storm center and intensity as a function of time:

Model Forecast Nor'easter Track and Intensity.  NWS

Precipitation
Up to an inch of precipitation will fall, with higher amounts along the coast, lesser amounts inland.  The rain may mix with snow Wednesday night.   Accumulation will be light, if any.   
This chart shows the liquid equivalent precipitation amounts:
Total liquid precipitation.  NOAA
This chart shows the probability of > 4” snow.  Note that there is only a slight probability of heavy snow well to our north, in the Poconos of PA.  
Probability of > 4" (Heavy) snowfall. NOAA
Wind
The most significant sustained and gusty winds will occur along the coast, from the Delmarva to Cape Cod.
Winds may gust 40-50+ mph along the Delmarva coast. 
Inland, including Baltimore:  Gusts 30-40 mph possible Wed night into Thursday morning.

Sunday, November 4, 2012

November 5, 2012: Strong Nor’easter Expected in Wednesday-Thursday Timeframe

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In the wake of Superstorm Sandy, the prospect of another major East Coast storm is most unwelcome.   Yet, these intense coastal low pressure systems, called Nor’easters, are a staple of our region’s cool season.  Before we get into specifics about the storm expected here mid-week, let’s review a bit about Nor’easters.
What Is A Nor’Easter?
Like hurricanes, Nor’easters are powerful cyclonic vortices, producing high wind, damaging ocean waves and heavy precipitation.   Aside from similar sensible weather and oceanic impacts, there are big differences between these two types of storm.  I’ll point out these differences below.  Also referred to as coastal lows, Nor'easters can produce hurricane-category wind and water damage along the immediate coastline.
Nor’easters get their name from the direction of strong winds, blowing from the northeast, inward toward the core of low pressure.   Since these storms track along the coastline, the northeasterly winds sweep inland, pushing large amounts of water against the shoreline.   It’s also true that these storms track towards the northeast.  This odd opposition between wind direction and storm movement was first explicated by Ben Franklin in the 1700’s.
How and When Do Nor’easters Form?
Nor’easters are a cool-season phenomenon, impacting the Mid Atlantic and New England coastal regions between October-April, with a peak in the December-March timeframe.   During the coldest months of winter, these are the big “snowmaker” weather systems that occasionally bury East Coast cities under one or more feet of snow.  
Our region experiences a few of these coastal storms each year, but some years are more active than others.   For instance, the winter of 2011-2012 experienced very few of these storms, while the previous winter (Snowmagaddon) was visited by numerous, crippling Nor'easters.   A typical year may average a half dozen or so in our region;  in any given year, we experience more Nor’easter than tropical-type systems in the Mid Atlantic.
Nor’easters develop during the cool season because they require strong contrasts in air mass temperature to form and intensify.  This occurs when cold, Canadian air masses sweep over the relatively warm waters of the western Atlantic. The East Coast, particularly along the warm oceanic Gulf Stream off Cape Hatteras, is a preferred breeding ground.
Nor’easters are also creatures of the jet stream.  Their formation requires an intense trough of low pressure in the upper atmosphere.  The most intense Nor’easters develop when two separate troughs or “waves” collide and merge (phase) along the coastline.  Jet stream troughs are most numerous and intense during the winter months.
What Are The Differences Between Hurricanes and Nor’easters?
Recent hybrid-type storms, such as Superstorm Sandy, have blurred the distinction somewhat – but Sandy was highly atypical.   Hurricanes develop and intensify in the deep tropics during late summer and fall, while Nor’easters develop in the middle and high latitudes during late fall, winter and early spring. 
Nor’easters draw their energy principally from jet streams and fronts, while hurricanes are fueled by rings of intense tropical thunderstorms.  These thunderstorms are sustained by ocean waters that exceed 80 F.   However, note that the Gulf Stream's warm waters boost the energy level of a Nor’easter by about 20%-30%.
How Is A Nor’easter’s Impact Measured?
Many are familiar with the Safir-Simpson hurricane intensity scale, which rates hurricanes according to five categories of wind speed.   Nor’easters also have a rating system, called the Dolan-Davis scale, developed at the University of Virginia.  The scale ranks Nor’easters according to five categories of coastal damage, including storm surge and beach erosion.   If a Nor’easter produces heavy snowfall, the impact is assessed separately by the five-category NESIS (Northeastern Snowfall Impact Scale).
What Are The Weather Impacts of Nor’easters?
In coastal areas, a combination of high seas, freshwater flooding and strong winds can be as devastating as a single hurricane.  What’s worse, if several Nor’easters visit a region in a given season, their effects are cumulative.
The magnitude of impacts, particularly storm surge and winds, depends on several factors, including (1) storm intensity (measured by minimum central pressure), (2) pressure gradient, (3) speed of movement, and (4) track.   All of these factors are highly variable from storm to storm.   In terms of storm surge and wave inundation, the phase of the lunar tidal cycle is also critical.  Since these storms tend to be larger in size than hurricanes, their effects often linger across multiple high-tide cycles.
Strong high pressure to the north of the Nor’easter – often situated over Newfoundland and Greenland – plays an important role in creating damage.  For one, high pressure blocks the northeastward progress of the storm, slowing it down or stalling it offshore.  This prolongs the action of wind, rain and high seas.  Secondly, high pressure intensifies the pressure gradient along the north along the north side, increasing the strength of onshore winds.  Some of the most infamous and devastating Nor’easters – including the 1962 Ash Wednesday storm, 1950 Great Appalachian Storm, 1991 Perfect Storm, and even the 2012 Superstorm, all featured a strong, blocking high to the north.
So What’s Expected With This Week’s Nor’Easter?
The various forecast models have not yet come into agreement regarding intensity and track, as is typical for the 3-5 day time range.
- For the Baltimore region, in short, the period from Wednesday to Thursday will be raw, but nothing nearly as intense as Superstorm Sandy.
- Those along the coast will experience a 24-36 hour period of strong winds (gusts to perhaps 40-50 mph), several inches of rain and battering waves.
- Those of us lying inland (including Baltimore and D.C.) can expect 12-18 hours of moderate to heavy rain and gusty winds (perhaps up to 40 mph, approaching Wind Advisory criteria).  And it will be chilly – temps will probably not break 40 F.
- For the mountains to our west and north, there will be 12-18 hours of precipitation, which may include a period of heavy, wet snowfall…and gusty winds (to 40 mph).

- Factors that argue AGAINST measurable snowfall over Baltimore:  (1) the ocean waters are warm,  and the storm will draw significant amounts of warm air off the ocean, warming the lower layers of the atmosphere to temperatures above freezing;  (2) there is not a major source of deep, cold air inland associated with high pressure to our north.
In all, these are NOT hurricane-force impacts.   The Nor'easter's effects will be more of an inconvenience (likely including some local travel disruptions) but not a major, multi-day disruption.
Unfortunately, our concern and attention should shift further to the north and east, along the Jersey and New York coastlines, where much more significant impacts will be felt.

Monday, October 29, 2012

October 29, 2012: Ex-Sandy Undergoing Rapid Transformation

8 pm:  Vortex of Change

The central vortex is now inland over New Jersey, and rapid structural changes are taking place.   Most importantly, the central pressure is increasing...the storm is filling from below, which is starting to weaken the system.  Second, the system is now highly asymmetric, with most of the heavy rainfall on the west and north sides of the system.  Heavy rainfall is being maintained on the north-western side by jet stream energy aloft, and this is a very typical asymmetry that develops in tropical systems that interact with the jet stream.  Third, a punch of dry air aloft off the continent has entered the storm from the southwest, and has wrapped all the way around into the storm's interior.   The eastern semicircle is highly eroded and open.

Radar:  WeatherTAP
Water Vapor:  WeatherTAP
Additionally, warm air has invaded the storm to its north, cold air to its south - a highly wrapped-up, ying-yang type of pattern that spells early demise of the storm:
NOAA
The strong temperature differences that helped to maintain the storm for the past 24-36 hours - warm oceanic air juxtaposed with a cold air mass - are starting to mix out.

Is the worst over?  Not yet...but conditions should slowly start to improve after midnight.  These enormous wind engines take their time to wind down, even as they lose their principal sources of energy.


October 29, 2012: Sandy Coming Ashore


5 PM UPDATE:  SANDY MAKING LANDFALL, 90 MPH, EXTREME SOUTHERN N.J., MOVING RAPIDLY NW.

5 PM Track and Intensity Forecast:


National Hurricane Center

Forecast positions and max wind speeds:

5 pm Monday 90 mph
2 am Tuesday 75 mph York, PA
2 pm Tuesday 60 mph Altoona, PA

Check out the NWS warning coverage for the Mid Atlantic and Northeast - I am amazed to see both a hurricane warning and a blizzard waring, from the same storm, across a distance of only a few hundred miles!

NWS

4 pm, an amazing pressure gradient:


5 PM Heavy Rain and Snow:

5 pm radar composite, showing heavy rain bands and the western eye wall along the Delaware coastline.  Yes, that blue zone over the Appalachians is a blizzard!  What is this thing...a snowcane?  Blizzardcane?  White hurricane?

WeatherTAP

5 PM Wind Forecast

As of 5 pm, sustained winds are over 30 mph area-wide, with gusts into the mid-upper 60 mph range are being reported in the DC-Baltimore region.  The core of highest winds is still to come, tonight, at the point of closest approach of vortex near York, PA (due north of Baltimore) approximately 2 am Tuesday morning.

***From Previous Post Today***
High Wind Forecast:

We are still on track for a prolonged period of high, sustained winds, from the west, not shifting around to south winds until Wednesday.   As Sandy approaches the coast today, the winds will rapidly ramp up in intensity and gustiness.

I am introducing a new wind prediction product here, courtesy of Quantum Weather and my colleague Professor Robert Pasken (based in St. Louis, MO).   For several years Quantum has produced a highly sophisticated regional forecast model, designed to help area utilities plan for worst-case weather scenarios.   The max sustained wind plots, generated for the greater Baltimore region, show a significant ramp-up in high winds (> 45 mph sustained) after 11 AM this morning expanding westward across the region (4 pm), with widespread 50-55 mph sustained winds by 8 pm:

8 AM LOCAL
2 PM LOCAL
 
8 PM LOCAL
The wind model takes into account the detailed terrain and topography of the Bay.  Focus on the bottom panel, at 8 pm tonite, with an intense west wind sweeping across the entire region.  The areas of greatest wind damage potential (red colors) become concentrated (1) along the high terrain (Catoctin Mountains) west of Baltimore, and off the western shore of the Bay, southeast of Baltimore.   You can literally see streaks or corridors of high wind that accelerate over the Bay's waters and then impinge along southern Maryland.  The greater Baltimore region lies in a bit of a "wind depression" between mountain- and Bay-enhanced regions.