Wednesday, January 30, 2013

January 30, 2013: Tornado Watch until 2 AM; Line of Storms Will Hit Within 1-2 Hours

The Storm Prediction Center has issued a Tornado Watch valid until 2 AM for the Washington-Baltimore metro area (yellow shaded region below).

A new squall line is rapidly developing and approaching our region from the southwest - depicted on radar below:

Note how the squall line (red colors) is bowing outward through central Virginia.  Bow echoes such as this can generate pockets of intense wind damage called downbursts.  Regarding the tornado watch, the concern is not for strong, supercell-generated tornadoes, but rather isolated, weak tornadoes that often develop with these bowing line segments.   Any small tornado that forms may be difficult to detect by weather radar, and they are often invisible during dark and/or when enshrouded by heavy rain.

Oddly, there is unlikely to be any lightning and thunder associated with this line of storms.

The likely time that this squall line will pass through our region is between 7:30-9 PM tonight.

A Flash Flood Warning is in place for counties to the north and west of DC-Baltimore.  Several inches of rain are possible from echo training - when more than one storm cell passes repeatedly over the same location.

January 30, 2013: Rainy, Windy Night On Tap

I'm continuing to monitor Winter Storm Magnus and its transit across the Mid Atlantic.   All day we have been grappling with the threat of severe thunderstorms and heavy rain.

Thus far, no severe weather in Central Maryland.   A band of heavy showers with gusty winds pushed through around 3 pm today.  There was no lightning activity.  Aside from the showers, winds have been gusting from the south at 25-30 mph.   Moderate rain will continue to fall as evening approaches.

Here is the 4 pm radar snapshot, showing an extensive band of showers (with some embedded heavy cells) pushing our way from the west, ahead of a strong cold front:

I've circled the region over southwest Virginia.  This is the nearest band of thunderstorms, producing local warnings for high wind.   While this is over 200 miles to our southwest, there is some concern that this line could develop further north and east, heading toward our metro region during the evening.   However, because of the extensive cloud cover all day, our region has not experienced intense solar heating, and thus there is very little instability in the atmosphere, to sustain a strong line of thunderstorms.  My thinking is that the Baltimore area may be a bit too far north to get in on this action;  the threat may be more for NOVA and Central Virginia.

The high-resolution computer forecast models continue to suggest that a band of moderate to strong rain showers will move through between 9-11 PM this evening, and these could create some locally strong wind gusts.  The band is not expected to generate any lightning or thunder.

Also this evening, the strongest part of the the storm's pressure gradient will move across our region.  This could bring higher sustained winds from the south, with gusts approaching 45 mph.  Because of this, NWS Sterling has placed our region under a Wind Advisory.  The winds covered under the advisory are widespread winds, generated by the larger region of low pressure, and NOT due to localized high gusts in showers or thunderstorms.  This distinction is probably confusing.  Here is the predicted range of wind gusts, at 10 PM tonite, the time widespread winds are expected to be their strongest:

The predictions are for gusts in the 40 mph range for D.C.-Baltimore, and in the 45-50 mph range over the higher terrain (foothills) to the west and north of the metro regions.

Also, the NWS continues its Flash Flood Watch for the entire region, as bands of moderate to heavy rain continue to work through our area overnight.

January 30, 2013: Squall Line Approaching Region

1:30 PM UPDATE:   Line of heavy showers with gusty winds will arrive by 4 PM today.  This initial line is well in advance of the main cold front;  an additional band of heavy showers, with wind gusts, will sweep through through between 9 PM - Midnight.  Thus far, the Storm Prediction Center has not forseen the need to issue severe thunderstorm or tornado watches for our region.

This is a very unusual weather situation for the Mid Atlantic in the dead of winter - an approaching squall line with temperatures soaring toward the 70 F mark.    If this were late April or May, it would seem much more like normal.

The culprit is a large, extratropical cyclone (The Weather Channel has named it Winter Storm Magnus).   As I discussed in earlier blogs, it's another storm with a "split personality":  Heavy snow on its chilly, northwestern side, and severe thunderstorms on its warm and humid southeastern side.  You can see the wide variety of weather this system is generating on the morning analysis chart below:

The blue swath over Wisconsin and Iowa is heavy snow.  The dark green swath through the Ohio and Tennessee Valleys is a band of heavy showers and severe thunderstorms.  The squall line has developed along a cold front approaching from the west.  There is a warm front draped over Baltimore.  To the south of this warm front, over Virginia and the Carolinas, lies the storm system's warm sector.  The warm sector is where a warm and humid air mass streams northward from the subtropics.  The advancing squall line is feeding on this air mass, and as it sweeps through the warm sector, it will continue to generate very heavy rain.

Regionally, here is the morning radar, which shows a squall line - a fast-moving, narrow band of heavy storms (red colors) - crossing the Appalachians from the west:

That band of storms is expected to sweep through our region this evening, after the end of rush hour.   Thus far, there is no lightning, so this is technically not a line of thunderstorms - but the squall line is producing heavy downpours and gusty winds.  The Storm Prediction Center is monitoring our region for possible issuance of severe thunderstorm watches.  Additionally, a tornado watch is not out of the question - not so much from the squall line, but from any strong storms that develop along the warm front over Baltimore.  Low-level winds often shift direction along a warm front, and this veering can create the spin that leads to rotating thunderstorms (supercells) and tornadoes.

While the wind setup is very intense for severe thunderstorms, the limiting factor may be the air mass sitting over central MD - which is somewhat cool and stable compared to regions further south and west.  Thick, persistent cloud cover has kept the Sun from warming the surface and destabilizing the atmosphere. 

This will be a busy day and night for the NWS at Sterling, VA.  Below is the latest set of watches and advisories for our region:

These include:

(1)  Severe Thunderstorm Watch for the western slopes of the Appalachians;
(2)  High Wind Warning for the western slopes of the Appalachians;
(3)  Wind Advisory along the spine of the Blue Ridge;
(4)  Flash Flood Watch for Central Maryland and the Appalachians

Flash flooding is a concern because 1"-2" of rain may fall in a very short time as the intense squall line, feeding on a humid air mass, sweeps through.   High winds are a concern because the pressure gradient around this storm system is very compact - leading to widespread strong, sustained winds and high gusts.   The winds are blowing at 60-70 mph from the south just a few thousand feet above the surface.  Ridge tops that project up into this layer are therefore subject to experiencing many hours of high winds.

I will update this blog again in the afternoon, between 3-4 pm.

Sunday, January 27, 2013

January 27, 2013: Maybe A Slight Ice Glaze Tonight

Headlines are being issued by NWS Sterling about the potential for icy precipitation tonight and tomorrow morning.   However, this event seems to have more "have nots" than "haves" in terms of creating a mess.  The "storm" is a actually a warm front that will move north through the Mid Atlantic tomorrow morning.  But as warm air with moisture begins to stream in from the south, it will overrun what's left of the arctic air mass in place over the metro region.   This is the familiar cold air damming or "cold wedge" scenario - and that spells the possibility of sleet and freezing rain.

What this event lacks:  (1) moisture;  most models churn out about a tenth of an inch;  (2)  persistent cold air;  temperatures will be increasing through the day tomorrow.  

What this event could do:  Even the slightest ice accumulation i.e. a few 1/100ths of an inch can mean slick spots, for the morning commute.  Again, as has been the case in the past week, its not so much about the amount of precipitation, but bad timing of light amounts that coincide with rush hour.

At 3 pm today, NWS issued Winter Weather Advisories across a broad region:

The threat of significant icing (the kind that brings down limbs and power lines) is not expected. 

The warm front ushers in a dramatic change in our weather pattern for mid-week - the arrival of mild air and showery weather.  This will be caused by a vigorous low pressure system approaching from the Plains - and one that could very well produce severe thunderstorms in the Wed-Thurs time frame - for our region - perhaps even the threat of a derecho (!)  No's the Storm Prediction Center's forecast for Wednesday of this week, highlighting the potential for widespread severe thunderstorms, carrying a probability of wind damage:

Intense wintertime low pressure systems sometimes produce a type of derecho, called a serial derecho, in their warm sector - as part of a squall line of thunderstorms.  

To say the least, it's going to be a very interesting week.

Friday, January 25, 2013

January 25, 2013: Limited Snow Potential This Afternoon

Snow has now broken out across the D.C. - Baltimore metro region.  This is associated with a northern piece of energy (a weak wave in the upper atmosphere) sliding through from the Ohio Valley.   A second piece of energy - a wave moving out of the Tennessee Valley - is sliding well to our south.   You can see these two regions of precipitation in the afternoon radar view below:

Regional radar depicting two regions of snow.  WeatherTAP
The heavier snow region is over north central N.C.

In between these two regions lies a "snow hole", and in fact a wedge of dry air in the middle atmosphere - called a dry slot - is working across central VA;  you can see this nicely in the water vapor satellite image below:

Dry slot shutting down snowfall across central VA.  WeatherTAP
The back edge of the snow affecting our area this afternoon should clear the metro region between 6-7 pm this evening.  Snow accumulations are being limited by many factors:  (1) some of the snow was consumed by saturating a dry air layer near the surface;  (2) the northern wave producing our snow is moving very quickly;  (3) the southern wave is "stealing" some of the moisture from the northern one;  and (4)  the dry slot will eventually overspread NOVA and central MD.

The high resolution models I discussed in my earlier post today suggest that the snow will be intermittent from 3-6 pm, coming in two distinct, light batches out of the WV panhandle.  Either way, they also show a sharp back edge clearing our region after 6 pm.

January 25, 2013: Uncertainty In Today's Snow Totals

In a word, snow is coming.  It will be dry and powdery.  It will rapidly stick.  But just how much, that is the question.

NWS has painted a broad-stroke prediction of 1"-2" across the metro region.  

I took a look at two of the experimental, high-resolution forecast models for this storm.  These are called mesoscale models and they capture the small-scale processes that the more established, regional forecast models often miss. 

Here is a snapshot from one of them, showing a moderate snowband setting up across the DC-Baltimore metro region late this afternoon:

High resolution forecast of snow accumulation rate, Friday afternoon.  NWS

Blue is light snow, green is moderate snow.  This could easily spell more than 1"-2".

However, here is the output from another mesoscale model, for the same timeframe.  It basically puts the metro region in a "snow hole" with nada:

Another high resolution forecast of snow accumulation, Friday afternoon.  NWS

In this scenario, there is a heavy snow band to the south, and also along the windward side of the Appalachians (where snowfall is favored, along the "upslope" ridges).

One thing that struck me is the incredibly dry air layer in place in the lowest 10,000 feet (even though it is overcast above this).  This means that a significant portion of the snow falling overhead will sublimate (turn to vapor) as it passes through this dry layer.   It will take a few hours to moisten this layer, such that snowflakes can survive all the way to the ground.   Snow will show up on radar early this afternoon, overhead, yet what it detects is virga - snow disappearing before it reaches the surface. Depending on how long it takes to saturate the dry air layer, this may cut significantly cut down on our accumulation.

Stay closely tuned, because until this storm actually sets up across the region, the forecast of any heavier snow essentially becomes a "nowcast".

Thursday, January 24, 2013

Jan 24, 2013: Arctic Clipper #2 Bringing Friday Snow

We're still on track for another "snow event" (note my avoidance of the word "snow storm"),  tomorrow.   Even with a forecast 1"-2", timing here is bad, bad...right during peak Friday rush hour.

The culprit is a weak wave in the upper atmosphere, moving rapidly from west to east across the Ohio Valley.  The forecast map for tomorrow evening shows the weak area of low pressure moving off the Delmarva, with a pocket of light snow (green hatched area) exiting our region from the northwest:

Surface forecast chart for Friday evening
And yes, the 1032 mb high pressure over the northern Plains is part of the "Siberian Express", a sequence of sub-freezing air masses that have been invading the Mid Atlantic all week, from deep interior Canada.  Another Arctic front will sweep through Friday night, ushering in a new cold blast for the weekend.

Here are the snow totals envisioned by the NWS for tomorrow's event:

Predicted snow accumulation for Friday's snow event

Compared to last night's event, the snow is expected to be more widespread, without the strong north-south gradient.   The system has a bit more moisture to work with (but still not anything near what a coastal low or Nor'easter taps into).  And as this piece of energy passes through, it will merge with another wave to our south...phasing together and intensifying into a more powerful, single storm over the ocean.  But similar to last night, we anticipate a snow:liquid ratio in the 15:1 to 20:1 range, meaning very light, powdery snow that will accumulate on untreated roads instantly. 

However...the devil could still lurk in the details.   There are some aspects of Clipper-type systems that we don't forecast very well.  Among these is the occasional appearance of an embedded snowband, containing moderate-to-heavy snow, within the larger region of light snow.  These bands are small-scale, meaning the computer models do not resolve the processes that create them very well.  I can recall several instances of Clippers Past in which a seemingly benign snow event mushroomed into a headache for a subset of our metro region.  In other words, Clippers are capable of generating small swaths of surprise, Warning Criteria snowfall.  Witness last night's Clipper, which dumped almost 6" to the southeast of D.C., and was therefore under-forecast for that region.   So I am always a bit wary of these beasts.  Sometimes the only way to anticipate these bands is to monitor the storm in real-time, i.e. radar surveillance, and rush out the necessary warnings.   You can bet I'll be keeping a close eye on the situation tomorrow afternoon.

Jan 24, 2013: Calvert County, MD: The Snow Jackpot

Well, it's not much of a winter when I get excited about blogging over a few inches of light powder.

The Arctic Clipper came through our region last night, and was well-forecast in terms of snow timing and amount.   And yes, the morning rush is a bit of a mess.

Here are the snow amounts tallied by NWS Sterling:

Thursday AM snow totals.  NWS
Note the north-south gradient, from a dusting around Baltimore, to over an inch in NOVA...and the Jackpot, over St. Marys and Calvert Counties (MD):  A small pocket of 5"!

The storm did not have much moisture (water vapor) to work with, but it did manage to freeze out the equivalent of 0.25" of liquid over these counties...if this were rain, it would be enough to wet the ground, but probably not form puddles.   Yesterday, the NWS predicted the region south of DC would pick up the highest snow amounts.  Keep in mind that it is very, very hard to forecast exact snow amounts, to the inch, when the snow:liquid ratio is 20:1. 

Nevertheless, I did some "forensic meteorology" this morning and determined that this pocket of snow was caused by a small area of enhanced uplift north of the low pressure center.  Anytime you observe a snowband, you look for something called "frontogenetic forcing" which means a region in the lower-middle atmosphere where a weather front is developing.  In this case, a mid-level front formed where chilly air to the north was sliding under slightly milder air to the south.  The cold air, acting like a narrow wedge, lifted up a narrow strip of moist air, forming the snowband that dumped on St. Marys and Calvert Counties.   You can see how this unfolded in the graphic below:

10,000 foot weather chart showing snowband (green) and pocket of frontogenesis (purple lines).  NWS.
This weather chart is at the 10,000 foot level, in the wee morning hours.  The black lines are isobars.  The blue dashed lines are isotherms (the air temperature over Baltimore is -24 C or about -11 F - plenty, plenty cold enough for snow).   The blue and green solid colors portray the radar's detection of snow.  Note the narrow, green band of moderate snowfall located south of D.C., over St. Marys and Calvert Counties.  Now, the solid purple lines indicate regions of frontogenesis.  Note the purple bulls eye, located over the snowband, marking the location of the mid-level front.   Subtle, yes.  But these are the features a meteorologist must look for, to determine where the heaviest pockets of snow will develop.

And all this while you and I were sound asleep!

Wednesday, January 23, 2013

Jan 23, 2103: Light Snow Tonight, Light Snow Friday


Mother Nature is trying very hard to make it snow this winter.   We finally have arctic-cold air in place, but we are lacking moisture and strong storms.   This trend will continue through the end of the week.

The first shot of light snow comes tonight.   As shown in the figure below, the computer models suggest an Arctic Clipper system will race across the Mid Atlantic tonight.  Arctic Clippers are weak upper air disturbances, moving along a fast jet stream oriented roughly west to east.   They are often associated with a blast of arctic air.   The figure shows the track of the Clipper system, across the Ohio Valley and Mid Atlantic.  The magenta colors portray an area of very light snow.  The surface low pressure associated with this Clipper is located over southwest Virginia at 1 AM.  Note the extremely intense high pressure region (1045 mb) over the Upper Midwest:  This is the reinforcing blast of arctic air on the heels of the Clipper.  The core of the really chilly stuff will stay just north of our region.  But 1045 mb is a very, very strong high pressure not often seen in any given year.

Forecast surface chart for 1 AM Thursday
Clippers are marginal snow-producers, because (1) they are moisture-starved (there is no inflow of air off the Atlantic or Gulf of Mexico) and (2) fast-moving (little time for snow to accumulate).  Furthermore, their snow:liquid ratio is on the order of 15:1 to 20:1, because the air is so dry and cold (our big coastal snow-maker storms have ratios on the order of 7:1 to 8:1, giving a heavy, wet snow).   Snow grains produced by Clippers are tiny and fluffy, leaving a fine powder on the surface.  It only takes b/t 0.05-0.1 inch of liquid equivalent to produce one inch of snow in a Clipper.  I find an effective way to clear our cars and driveway after one of these systems is to use a leaf blower, not a shovel.  With our roads now frozen, even an inch of this powder could create problems for the Thursday morning rush-hour.

Snow accumulation map issued this afternoon by NWS Sterling, VA:

Snow accumulation for early Thursday AM. NWS.

The second shot of light snow arrives Friday, in time for the Friday evening rush hour.  Again, amounts are expected to be light.  This is because there are two pieces of energy moving through the Mid Atlantic, and the models suggest that these pieces will not combine or phase in time to create an intense storm:

Forecast surface chart for 1 PM Friday

The phasing will take place out over the Atlantic.  The air mass in place will be cold enough for all snow, and once again we will be dealing with a light, powdery type of snow.  If anything falls, it will come from the northern ripple, which is another Alberta Clipper (heralding in yet another shot of arctic air for the weekend!).  It is even possible that our region will escape snow altogether - essentially a "snow hole" in between these two systems.  But should a light coating fall, again...the rush hour could be in peril.

Tuesday, January 22, 2013

January 22, 2013: More Snow Than Ice This Friday

The computer models are beginning to suggest a colder solution for Friday's winter storm.  Yesterday's runs showed some mild air invading aloft, meaning the precipitation would be a wintry mix, mostly in the form of ice.   However, this extremely cold air mass has been under-forecast, and a deep wedge of cold air is expected build in east of the mountains (cold air damming a.k.a. "the wedge") Thursday into Friday...just as the area of low pressure arrives from the Ohio Valley.   The model solutions today are showing a deeper, colder wedge of air in place during the precipitation.  This means more snow, and if some milder air were to invade from the southeast aloft, then any ice would take the form of sleet, not freezing rain.

The forecast map below shows the wedge of cold air at the surface, valid Friday evening, during the hours of heaviest precipitation.  

Surface forecast map for Friday evening.
Those deep blue colors over VA-MD-PA are sub-freezing air, and the 32F isotherm is the heavy maroon line dipping down into South Carolina.   This freezing layer extends all the way through 5,000 ft (the critical snow-formation altitude) across the metro region.

However, the models remain consistent with precipitation amounts, which are on the light side, and fast movement of the storm.   The storm will deepen once over the Atlantic, but the moisture will have departed our area by the time the storm begins to bottom out.

Bottom line:  Prepare for some snow and/or a snow-sleet mix on Friday, not a major storm, but enough to accumulate. 

The next major storm upstream (Wed timeframe of next week) at this time appears to be a rain-maker.

Monday, January 21, 2013

January 21, 2013: Potential Winter Storm Coming This Friday

Professor Storm will be back full-time in the classroom next week...but in the meantime, he may be offering up a "multiple-choice" winter day this Friday - that is, a wintry mix of snow, sleet and freezing rain. 

We are still several days away from the event, but the ingredients will come together, including (1) the remnants of an arctic air mass;  (2) relatively warm, moist air off the Atlantic sliding in overhead, and (3) an area of low pressure to pull it all together.   When this vertical, layer-cake type of temperature structure develops east of the Blue Ridge Mountains in Jan-Feb, it often spells a wintry mix.

Below is the medium-range surface forecast map, for Friday morning, showing the storm approaching from the Ohio Valley, with cold air lingering over the Mid Atlantic.  Note that this is NOT a coastal low or Nor'easter type of system, one that is not expected to become very intense, and also a fast mover - so I don't expect prodigious amounts of wintry mix.  But there may be enough to create some local hazards, and prove to be an overall forecast headache:

 Stay tuned right here as I update this blog daily through the week.

Thursday, January 17, 2013

Jan 17, 2013: Nary A Flake Did Fall

This storm is one of the more spectacular examples of a "busted forecast",  made by just about every meteorologist...including Yours Truly.   Last night, we all bought into the following snow accumulation prediction:


Late this afternoon, about 24 hours later, and with the storm in progress - here is a major downgrade:

What a world of difference!  Yesterday's forecast painted up to half a foot of snow across D.C.;  today's forecast painted zilch.  We all have collective, meteorological mud on our faces.

Here's the storm tonight, which has become quite vigorous, dropping heavy snow across central Virginia:

Surface Map, Thursday Evening (Intellicast)
The storm center, which was over the Tennessee Valley earlier today, is transferring its energy to a coastal low. 

Why did it not snow here?  As explained in my earlier blog, a layer of extremely dry air slipped into the atmosphere's snow-forming layer this morning.   Even a solid deck of overcast cloud struggled to form.  And the really cold air did not arrive from the north.  You'll note a cold front in the surface map, above - still located north of the Mason Dixon line this evening.

Where did the air cold enough for snow come from, to our south, over South-Central Virginia?   (1) Much of the heavy snow that fell this afternoon, and continues to fall this evening, is occurring at high elevation - above several thousand feet in the Blue Ridge Mountains and Appalachians - where the air layer is naturally colder.  (2) Some of the chill was generated by vigorous cooling, as heavy precipitation fell through the same dry air mass that invaded our location early this morning.  (3) Rain has only turned to snow across VA this evening, as the sun set and the surface air cooled to near-freezing.  (4) Finally, vigorous upward motion - due to an energetic jet stream - created an effect called "dynamic cooling" (air cools as it is forced to rise and expand).

The really intense jet stream in fact is powering a band of heavy snow, with rates of 1"-2" per hour, and even thundersnow, across central VA.   The snow is heavy and wet and will undoubtedly cause widespread power outages.   The radar image below shows this rather isolated but heavy pocket of snow (blue shades):

Heavy snow over Central VA this evening.  WeatherTap

The snow band has set up in the classic location, within 200 miles to the northwest of the low's center.  I'm thankful that this heavy snowstorm was a miss...and it's unusual to see such an event to the south of D.C. - Baltimore!

Jan 17, 2013: Forecast Snow Totals For Metro Region Continue to Dwindle

This morning's radar tells much of the story with this storm:   At least for the next several hours, the moisture, uplift and energy are staying south of the D.C. - Baltimore region:

10:30 AM radar image of precipitation (green = rain, blue = snow);  Weathertap
And the light precipitation that is falling in the southern D.C. suburbs has been rain thus far.

The NWS has scaled back the accumulation threat to around 1"-2" of snow, and the entire metro region has been downgraded from a Winter Storm Watch to a Traveller's Advisory:

Snow accumulation forecast (NWS)
Winter Weather Advisory (NWS)

So what's happening with this storm in our region?    Super-dry air has invaded from the west, which is evaporating the precipitation before it reaches the surface.   Additionally, temperatures remain in the mid-40s across the metro regions.   We are essentially lacking the key ingredients for a heavy snow - cold air and moisture.

However, very high resolution forecast models that I have examined do suggest that light moisture will eventually overspread our region from the south later this afternoon.   Whether the temperatures can fall 10 F to be cold enough for all snow remains to be seen.  Certainly with the dry air layer, there is the potential that enough cooling could arise from evaporation - but the process of chilling the air may take too long - in other words, this storm is progressive and is expected to clear the region between 10 PM - midnight.   Now, this morning the surface winds have started to kick around from the north...meaning colder air may start filtering in...but the freezing line at the surface is way up in central PA, and I do not see it making any progress southward.

Bottom line:  The snowstorm may in fact be a nostorm.

Wednesday, January 16, 2013

January 16, 2013: Winter Storm Iago Coming


The Weather Channel will likely name a fast-developing, and fast-moving coastal low to impact the Mid Atlantic tomorrow, Winter Storm Iago. 

The storm will spread a swath of snow across the Central Appalachians, West Virginia, Virginia, Maryland and Delaware, as it moves out of the Tennessee Valley and offshore late tomorrow night.

Based on the current model storm track, the center of the action is expected to stay south of the Washington-Baltimore region.  However, with sub-freezing air in place near the surface, and abundant moisture pulled into the storm, accumulating snow is likely to reach as far north as Baltimore.

The first figure, based on a computer model forecast for tomorrow evening, shows the surface low (1007 mb) over the North Carolina-South Carolina coastal border.   Colors represent precipitation amounts.  D.C. - Baltimore is on the far northern fringe, with 1/4" to 1/2" precipitation totals.  The precipitation bullseye - painted red, to the tune of 2" - is poised over the Central Appalachians, hundreds of miles to our southwest.

Surface Weather Forecast, Thursday evening (Unisys Corp)
The precipitation amounts are so high, partly because impressive jet stream dynamics are energizing this system - including a vigorous and intensifying wave (trough) in the jet stream, and a fast-moving pocket of air called a jet streak.

Below is another computer model forecast of snow amounts for tomorrow-tomorrow night:

Forecast Snow Amounts for Thursday-Friday (Weatherbell Analytics)
That's over a foot of heavy, wet snow in southwest Virginia.  D.C.-Baltimore, again, are on the northern fringe of the snow shield.   Snow that falls on the northern edge in drier, colder air will likely be dry and powdery.  

Finally, the NWS in Sterling has now issued a swath of Winter Storm Watches, including D.C., counties to the immediate south and north.  The city of Baltimore and Baltimore County are NOT included at this time:

Winter Storm Watch for Thursday (NWS)
Here is the latest snow accumulation guidance from NWS Sterling - note the large north-south gradient in expected snow totals, consistent with our reasoning on this storm:

10 PM - Forecast Snow Totals for Thursday(NWS)
It's likely that Baltimore will get placed in a Winter Weather Advisory, once the storm track becomes better refined.  Regardless, timing of this event is expected to be very bad - with a period of moderate snow coinciding with the hours of tomorrow's evening rush.

Of great interest, tomorrow we will be watching the radar evolution of this system very closely.  A narrow band of heavy snow, with snow rates up to 2" per hour, is expected to set up somewhere across Central VA.   Depending on the exact track of the storm...that snowband could shift north, as there has been a tendency in the models for the storm to track closer to D.C.-Baltimore. 

Saturday, January 12, 2013

January 12, 2013: Winter's Lies, Damn Lies and Statistics

The title of this blog is made in the context of Mark Twain, who stated simply "Lies, Damn Lies, and Statistics".   This is relevant to understanding the capricious nature of Washington and Baltimore's winters.

You often hear the statistic that our region receives an average of 18"-20" of snow each winter season.   This is indeed true.  But does this average truly convey the actual behavior of a typical snow season?   Far from it.

This past week, the NWS released a simple graph, showing annual snowfall recorded at D.C.'s Reagan National Airport, from 1992 through 2011.  A graph for BWI looks very similar, with slightly higher amounts across the board.  Here is the graph, which is called a time series chart:

Washington's annual snow has varied anywhere from nearly zilch (1997-1998, 2011-2012) to 50"-60" (1995-1996;  2009-2010).   There is tremendous annual variability in our seasonal snow - so much, in fact, there there is very little predictive power in trying to glean one year's expected snowfall from the previous.   A "Snowmageddon" season is very often preceded or followed by a "Snow Drought" year.   Basically, it's all or nothing here.  Meteorologists term this type of behavior "episodic".  In fact, most of our years appear to hover around the 10" annual snowfall mark.

The 18"-20" snow average is really an artifact of averaging lots of very lean years, with a very few episodes of heavy seasonal snowfall.   The average creates a false expectation that our "normal" snow should be in the 18"-20" range.  In fact, 17 of the 22 years above fall closer to 10" than 20".  That's a big difference.   So the intermittent nature of our blockbuster snow seasons has inflated an average value, to the point of conveying little meaning.

Beware, you must, of those wily statistics!