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. 

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.

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.

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.

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!

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.

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.