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fxus61 klwx 041523 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
1023 am EST Thursday Dec 4 2008 


Synopsis... 
a cold front will move across the middle Atlantic this afternoon and 
evening. Canadian high pressure behind the front will bring 
colder conditions Friday and Saturday. A disturbance will move 
through Saturday night into early Sunday with a chance of snow 
showers. High pressure returns Monday before a storm system 
approaches on Tuesday. 


&& 


Near term /until 6 PM this evening/... 


Cold front syr-jst-bkw-csv at this time. Swath of precipitation accmpny fnt. However...the 
organization falls apart taftn...as surface boundary outruns positive vorticity advection/jet 
support...and 700 mb-5 fgen forcing outruns surface convergence. So...expect 
areal covering/intensity to wane. Have retooled probability of precipitation...bumping up to 
likely/Cat probability of precipitation ptmc Highlands since precipitation nearby...and have taken 
probability of precipitation up to 50 percent between blurdg-i95 lt in the day...which should 
correlate west/ frontal passage. Dont think precipitation will be substantial near cold front passage... 
and will be none outside of this zn. Have also retooled skycvr based on 
satpix /ptcldy-mostly clear S/east of shd-dca-BWI/. However...skies will become cloudy 
where its not already. 


Cold air not that far rmvd from fnt...but limited moisture make accumulate snowfall 
doubtful today. Will indicate chgovr in ptmc Highlands. 


We still need a 10 degree boost southern/eastern County warning forecast area to make forecast maxt. Given 
skycvr and southerly flow...will give it a chance. But...am stating to have 
doubts if we/ll be able to give above normal today. If not now...it will 
be a while.... 


&& 


Short term /6 PM this evening through Friday night/... 
have made changes to tngts forecast...in line west/ changes to taftn. Based on limited 
moisture and lack of forcing...have lowered probability of precipitation from likely to chance...and 
trimmed time period as well. Hopefully...cold front will be east of County warning forecast area by 2z. 
12z lwx-WRF-nmm12g on board west/ 06 and 12z NAM and latest sref in a 
dry forecast by that time. Period of upslope snow limited as well...but will 
squeek one out in scattered covering...accums at or below 1 in. 


Canadian high pressure builds into the middle Atlantic Friday and 
Friday night. High pressure will infuse a colder air mass across the 
area. Save for a few upslope snow showers across The Highlands 
Friday morning...drier air will mean a precipitation free forecast 
through Friday night. However...a reinforcing shortwave /vort maximum/ 
is prognosticated by the models to shear across the area later Friday into 
Friday night so some cloud cover will remain. Grids reflect 40-60 
percent sky cover. Maximum T Friday and min T Friday night will be below 
normal by several degrees. Undercut GFS MOS especially Friday night 
with high pressure overhead promoting optimal radiational cooling 
conditions. /Peloquin 


&& 


Long term /Saturday through Wednesday/... 
high pressure scoots off the eastern Seaboard on Saturday...allowing 
a return flow to set up during the afternoon. Warm advection will 
develop ahead of a shortwave over the Great Lakes/Upper Middle west by 
afternoon. GFS indicates lift on isentropic surfaces most pronounced 
across the western County Warning Area during the afternoon...and this is where low 
probability of precipitation will reside. East of the mountains should remain dry during the 
daylight hours Saturday...but warm advection will denote an 
increase and thickening of clouds during the afternoon. 


Shortwave moves across the middle Atlantic Saturday night into early 
Sunday. Lift spreads further east across the County Warning Area. Moisture is not 
abundant but is sufficient for at least scattered precipitation 
Saturday night. Have the highest probability of precipitation /likely/ across The Highlands 
where an upslope component will enhance precipitation. We may need 
to consider increasing probability of precipitation to likely further east if the models 
continue to indicate a stronger shortwave. Temperature profiles 
suggest that precipitation will fall in the form of snow...except 
perhaps rain initially across lower southern Maryland. Best chance 
of accumulations will be across The Highlands...where Winter Weather 
Advisory criteria may be achieved. However...there is a chance for 
light accumulations of snow to extend further east across the rest 
of the County Warning Area all the way to the Chesapeake Bay. 


Precipitation exits early Sunday as the shortwave axis moves 
east...except for snow showers in favored upslope parts of The 
Highlands. Cyclonic flow will keep plenty of clouds around 
Sunday...then high pressure builds in Sunday night into Monday. This 
will mean an end to upslope snow showers and less clouds for Monday. 


Tuesday into Wednesday...operational models and medium range 
ensembles continue to indicate a storm system affecting the County Warning Area. 
However...there remains disagreement as to details /like timing and 
strength/ of the storm system. One thing which has been relatively 
consistent is that this appears to be primarily a rain event. 
However...if precipitation moves in early enough /Tuesday 
morning/...then some wintry precipitation is possible at the onset. 
Specifics will become more clear and we move closer in time to next 
week. 


&& 


Aviation /15z Thursday through Monday/... 
VFR conds across taf sites at this time...as all ceilings area at or above 8000 feet. 
Cold front passage taftn will lower ceilings and prompt wind shift from S/SW to northwest...W/ gusts to 
20 knots. However...am hvg doubts about appreciable precipitation at any pt 
location. Going tafs hint at that already...and will reevaluate at 
18z. If a period of MVFR exists...it will be brief and will come Post 
frontal passage. 


High pressure builds in Friday and Friday night and then moves off 
the coast on Saturday. A shortwave will move through Saturday night 
into early Sunday with a chance of snow showers. Another gusty 
northwest wind is expected on Sunday. High pressure returns 
Monday. 


&& 


Marine... 
marginal Small Craft Advisory conds exist at this time. It may briefly lull between now and 
frontal passage lt taftn-Ely evening...but will rtn aftr cold front passage. Will keep Small Craft Advisory for 
g20kt through Friday morning. 


A shortwave moving through Saturday night will bring Small Craft 
Advisory winds in its wake Sunday and Sunday night. High pressure 
builds in Monday...but a storm system may impact the waters Tuesday 
into Wednesday with at least Small Craft Advisory conditions. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory until 10 am EST Friday for anz530>537. 


&& 


$$ 
Synopsis...peloquin/hts 
near term...heights 
short term...hts/peloquin 
long term...bpp 
aviation...hts/peloquin 
marine...hts/peloquin 

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