Winter Storm Warning extended east, Maybe no school tomorrow?

Part one of today's storm was an overachiever, bringing more snow that expected to the area. Totals in the immediate DC metro ranged from 4 inches in the west to 1 inch to the south and east. Farther north and west, the totals were more impressive, with 7 inches in Frederick MD

Temperatures also stayed lower than expected today. this indicates that the ridging from the high pressure was stronger than expected and that the freezing precipitation will last longer. In response to the lower temperatures, The National Weather Service extended the Winter storm warning out to DC and PGCo.

The NWS also increased projected freezing rain accumulations to include DC and points north and west to receive more than .25 inches of ice. The 18z NAM model, which has higher resolution than the other operational models, predicts that the freezing line will stay south and east of DC and through 6AM Monday. 

As the day progresses, easterly winds of the relatively warm ocean will erode the cold air and the freezing rain will turn to just cold rain. Highs on Monday will be near 40. Tuesday features another chance at snow as another storm swings by to the south with highs in the mid 30's. 

Will we have school on Monday?

Loudoun County, VA has canceled school, which indicates that Montgomery County may follow suit. Loudon county lies just across the river from MoCo so whatever happens there is likely to happen here. MCPS needs to make it's decision by 5AM Monday. According to the models, temps will be below freezing at 6AM Monday for all of the county so MCPS will have to either call a 2 hour delay or no school. In addition, a quarter inch or more of ice is not something to mess with. It is much more hazardous to drive in than snow or sleet because it just freezes into something like an ice rink. the ice also may weigh down branches, resulting in scattered power outages. remember that just because the southern part of the county is above freezing does not mean there will be school. The northern portion may still be affected by winter weather.

WINTER STORM WARNING, Let the fun (or dread) begin...

Magenta denotes winter storm warnings and purple means winter weather advisory

The National Weather Service has updated the Winter Storm Watch into a Winter Storm Warning in effect from 8:00 AM Sunday to 10AM Monday. 1-3 inches of snow and sleet is expected before the changeover to freezing rain is complete.  The warning text is below with bold added for emphasis.

URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
834 PM EST SAT DEC 7 2013

MDZ004>006-009-010-VAZ039-040-042-050>053-501-502-080945-
/O.UPG.KLWX.WS.A.0008.131208T1400Z-131209T1200Z/
/O.NEW.KLWX.WS.W.0010.131208T1300Z-131209T1500Z/
FREDERICK MD-CARROLL-NORTHERN BALTIMORE-MONTGOMERY-HOWARD-MADISON-
RAPPAHANNOCK-LOUDOUN-ORANGE-CULPEPER-
PRINCE WILLIAM/MANASSAS/MANASSAS PARK-FAIRFAX-NORTHERN FAUQUIER-
SOUTHERN FAUQUIER-
INCLUDING THE CITIES OF...FREDERICK...WESTMINSTER...
GAITHERSBURG...COLUMBIA...WASHINGTON...LEESBURG...CULPEPER...
MANASSAS...MANASSAS PARK...FAIRFAX...WARRENTON
834 PM EST SAT DEC 7 2013

...WINTER STORM WARNING IN EFFECT FROM 8 AM SUNDAY TO 10 AM EST
MONDAY...

THE NATIONAL WEATHER SERVICE IN BALTIMORE MD/WASHINGTON HAS
ISSUED A WINTER STORM WARNING FOR SNOW...SLEET...AND FREEZING
RAIN...WHICH IS IN EFFECT FROM 8 AM SUNDAY TO 10 AM EST MONDAY.
THE WINTER STORM WATCH IS NO LONGER IN EFFECT.

* PRECIPITATION TYPE...SNOW AND SLEET...THEN CHANGING OVER TO
  FREEZING RAIN MID AFTERNOON AND CONTINUING THROUGH MONDAY MORNING.

* ACCUMULATIONS...1 TO 3 INCHES OF SNOW AND SLEET...FOLLOWED BY A
  QUARTER INCH OF ICE.

* TIMING...SNOW AND SLEET WILL BEGIN SUNDAY MORNING...THEN WILL
  CHANGE TO FREEZING RAIN SUNDAY AFTERNOON. FREEZING RAIN WILL
  CONTINUE SUNDAY NIGHT...EVENTUALLY CHANGING TO RAIN LATE MONDAY
  MORNING. THE GREATEST ACCUMULATION OF ICE WILL TAKE PLACE ON
  SUNDAY NIGHT.

* TEMPERATURES...UPPER 20S.

* WINDS...NORTH 5 TO 10 MPH.

* IMPACTS...HAZARDOUS TRAVEL CONDITIONS ARE PROBABLE DUE TO SLICK
  ROADS AND ICY CONDITIONS ESPECIALLY IN VALLEY LOCATIONS. DOWNED
  TREE BRANCHES AND LIMBS ALONG WITH POWER OUTAGES ARE LIKELY.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A WINTER STORM WARNING MEANS THAT A WINTER STORM SYSTEM WILL
IMPACT THE AREA WITH SIGNIFICANT AMOUNTS OF SNOW...SLEET...AND
FREEZING RAIN. TRAVEL IS LIKELY TO BE SEVERELY IMPACTED SUNDAY
AFTERNOON THROUGH MONDAY MORNING.

Why this wintry mess instead of some good old snow...

The storm setup does not favor snow, in fact, if not for our geography, we would probably just be getting rain. The mountains to the west allow for cool air to pool in a phenomenon called Cold air damming (CAD)
Why not snow?
Simply put,  the primary low will track to our west, drawing warm air on its east side north. we usually get snow when we are on the west side of the storm while the east side gets rain. We will be on the arm side of the storm. So why are we still getting wintry mix? Because of CAD
Cold Air Damming
High pressure to the north pumps cold air into the Mid Atlantic Region. This air jams up against the Appalachian mountains, creating an extension of the high pressure  along the Appalachian mountains .When a storm system arrives from the west, warm air overruns the cold, producing precip. Initially, the cold and dry air mass would cause any precipitation to evaporate, lowering the temperature of the air column through evaporative cooling, but eventually, precip will reach the ground. If the airmass is cold enough, the preip would be frozen. Below is a graphic drawn from the 00z CMC run, which demonstrates the cold air damming. The black arrows denote air flow from the arctic high to our north and the yellow lines show the isobars, which denotes a certain air pressure. Notice how a tongue of high pressure extends into the Mid Atlantic. Since the cold wedge is driven up against the mountains, precipitation over WV is in the form of rain as the warm air advects (travels) northwards with the low.

The precipitation should start out as a little snow. As the warm air  intrudes into the cold airmass at upper levels, the snow melts then refreezes forming sleet. As the warm air increasingly erodes the cold air, only a surface layer of cold air remains because warm air is less dense so it rises. Eventually, the cold air loses as a new low also forms to the east of the high pressure tongue. As a result, the cold northerly flow is cut off and replaced with warmer southerly and easterly flow.

The rate at which the cold air erodes is a major issue when forecasting. An difference of a degree or two could result in vastly different scenarios. Computer models tend not to do well with CAD events due to their low resolution. The NAM should handle the CAD better because of it's higher grid resolution.

NAM

GFS

Thanks for reading, 

Alex