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!

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

3:30 PM UPDATE - SEE TONIGHT'S SNOW ACCUMULATION MAP BELOW

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 cell...one 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.

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.

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:

NOAA

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

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:

NWS

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

NWS

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.

January 16, 2013: Winter Storm Iago Coming

10 PM UPDATE - MODEL TREND IS FOR SLIGHTLY LESS SNOW ACCUMULATION

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.