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fxus61 klwx 041933 
afdlwx 


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


Synopsis... 
a cold front will move into the Chesapeake Bay this evening. 
Canadian high pressure behind the front will bring colder 
conditions Friday and Saturday. A disturbance will move through 
Saturday night into Sunday Monday with a chance of snow showers. 
High pressure returns Monday. Low pressure will likely affect the 
middle Atlantic region during midweek. 
&& 


Near term /tonight/... 
cold front at 19z stretching east of mrb-west of shd. All precipitation in a narrow 
band bhd fnt...solid across PA/Maryland but breaks up across wva. Aloft...northern 
precipitation near rrq of upper jet but ahead of best positive vorticity advection. Shortwave evident in 
vapor imagery across deep S...which should have little impact lclly...aside 
from inhibiting Gulf moisture from advecting north. 


Fnt/precipitation continues to outrun upper support...and a weakening trend expeceted to 
continue through afternoon-into evening. MOS probability of precipitation catching onto that trend...as all 
in teens for 1st period of 12z cycle. Considering where forecast was... 
degree of surface forcing...and extent of thrml gradient...will lower probability of precipitation 
but would rather keep them mntnd. Have backed off to 30 probability of precipitation. 


Thermally...much of Virginia at or above 50f...but 2g4 at 32f. So...expect precipitation to 
mix west/ snow at higher elevs of ptmc Highlands lt taftn...and spread 
eastward. However...given scant width of precipitation...think it will depart prior to 
arrival of cold air. So...no mntn of mix east of mountains 


Surface hipres builds quite quickly...and shuts off feed from grtlks/upslp. 
Will carry chance probability of precipitation past midngt...but accums should be at or below 1 in. Thus no 
headlines this time. 


Owing to good cold air advection...have undercut guidance for min-T. 
&& 


Short term /Friday/... 
hipres builds through day. Thrml trough Parks atop County warning forecast area durng afternoon...so 
it/ll be chilly...but dry. Tight 500 mb flow and upper jetmax likely to 
keep hi clouds around...so a pt-mosun forecast will work. Once again...have 
undercut all MOS guidance. 
&& 


Long term /Friday night through Thursday/... 


Extend period begins west/ a chilly night leading into Sat morning west/ high pressure 
over the area. Majority of area will bottom out in the 20s...mu teens 
west of I-81. High pressure to continue over area during Sat b4 a fast moving 
cold fnt moves through the region Sat night. Temperatures do look cold enough to 
produce some snow across the area..qustns will be of areal 
covering...length of time...and probability of precipitation. For now do not feel confident to 
go higher than chance probability of precipitation east of the mountains..but west/ temperatures sufficiently 
cold some minor accums can't be ruled out for the metropolitan areas. 
This is still beyond allowable limits for putting accums in the 
zones...but this will need to be monitored by later shifts. 


The mountains could be a different story...as another round of snow 
looks to be on tap for the upslope regions of The Highlands. Have gone 
west/ Cat probability of precipitation for Sat night in the favored areas. 


High pressure returns for Sun afternoon and lasting through Monday. 


The middle part of the weak looks Lake A challenging forecast. Low pressure looks 
to come out of the Gulf states Tuesday. Precipitation may begin as a burst of 
snow...but predominant p-type would be rain. 


GFS and European model (ecmwf) both project further development of low pressure in the 
Gulf..W/ a more significant low moving into the region in one week's 
time. European has a more inland track for the storm...while GFS 
takes it off the CST. Obviously p-type would depend on tracking. 
But this is still a weak out and many model runs ahead to pin this down. 




&& 


Aviation /tonight through Tuesday/... 
cold front through mrb...marked by wind shift and sprnkls. Will carry frontal passage through 
rest of terminals by 22-23z. Mostly kept VFR conds through frontal passage... west/ 
risk of sprnkls. If ceilings do lower...it/ll be for several hours bhd fnt. Will 
carry tempo MVFR and monitor. 


Lower-Middle clouds scattered out aftr midngt...but hi clouds should linger through day 
Friday. 


Possible reductions in ceilings/visibility Sat night as a fast moving cold fnt pushes 
through the region. Next chance for weather will be Tuesday afternoon. 
&& 


Marine... 
marginal Small Craft Advisory at this time. Expect winds to pick up briefly bhd frontal passage. So...will 
carry Small Craft Advisory for now. Uncertain how long those conds will last though...as 
hipres builds quite quickly. Hvg doubts if favorable dwnwd transpt will be 
around by tmrw. At this time...will leaving headline up...but suspect it/ll be canx 
Ely. 


A cold fnt 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 through 
Thursday. 


&& 


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...Woody! 
Near term...heights 
short term...heights 
long term...Woody! 
Aviation...hts/Woody! 
Marine...hts/Woody!