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fxus61 klwx 201013 aaa 
afdlwx 


Area forecast discussion...updated 
National Weather Service Baltimore Maryland/Washington District of Columbia 
513 am EST Thursday Nov 20 2008 


Synopsis... 
clipper system to move across the area through Friday. High 
pressure builds in this weekend...before another strong upper 
storm system moves into the region Monday through Tuesday. 
Improving conditions are expected for the Thanksgiving Holiday. 


&& 


Near term /until 6 PM this evening/... 
surface analysis at 06z has a trough axis extending from a low over 
the eastern Great Lakes south across the eastern Panhandle of West 
Virginia and into southwestern Virginia. Temperatures across the 
region were in the middle 30s with dew points in the teens to near 20. 
Surface observations and satellite imagery have a solid middle deck 
of clouds across the region. Water vapor imagery has an upper low 
over Maine...with westerly flow aloft. 


The clipper system over the Great Lakes will gradually weaken after 
it drops southeast today. Expect upslope snow showers to develop 
later this morning across the west...with a few showers making it 
further east across the Potomac Highlands this afternoon. May see 
some minor accumulations out west today...less than an inch. 


&& 


Short term /6 PM this evening through 6 am Friday/... 
upslope snow showers really pick up overnight. Main challenge for 
tonight is the headline. There is currently a Winter Storm Watch in 
effect for the western five zones. While there will be some minor 
accumulations in the higher terrain during the late afternoon...this 
event will not really begin until tonight. Confidence is not high 
enough to upgrade to a second period warning. Therefore...will leave 
headlines intact. At this time...expect around 1 to 3 inches total 
accumulation overnight in the headlined area. Snow showers will 
spread east across the Potomac Highlands this evening...then 
towards the Baltimore metropolitan after midnight. Accumulation further 
east would be less than an inch and would not be included in the 
zone wording. 


&& 


Long term /Friday through Wednesday/... 
deep upper trough over aplcn mts will be in place Friday morning...with snow 
event expected to be ongoing high terrain of western zones. Total 
accumulations 4-6in possible for upslope areas over 24 hours. Will retain 
watch at this time...given low confidence in development of warning criteria 
conditions. 


Dry slot forecast to impact southern sections of County Warning Area...but precipitation may wrap 
around circulation into northern and eastern zones. With cold air in 
place...any precipitation should be mix of rain and snow...even into metropolitan 
areas by Friday afternoon. No significant accumulations expected. 


Northwesterly flow in wake of upper trough will continue moisture fetch from Great Lakes 
region...with possiblity of light snow showers continuing on higher terrain Friday 
evening. By early Sat morning...surface hipres builds...with light low level 
winds. Dry weather forecast through weekend. 


Strong shortwave trough deepens across north central Continental U.S. Sun evening...with southwesterly flow 
aloft over middle Atlantic region. As system aprchs...surface cyclogenesis over 
Great Lakes region will induce warm advection pattern over middle Atlantic. Although 
moisture return will be limited...southerly flow should be sufficient to erode 
low level cold air such that precipitation Monday will be liquid xcpt high 
terrain. 


Trough axis passes through Tuesday...driving cold front through forecast area. With 
cold air advection Tuesday afternoon...maxima may be reached early...and light 
snow should spread eastward through Tuesday. Accumulating snow will be possible in 
favored upslope areas to west...as sustained northwesterly low level flow expected in 
wake of cold front. 


Hipres builds after departure of system Wednesday afternoon...with cool temperatures 
forecast through remainder of next weak. 


&& 


Aviation /10z Thursday through Monday/... 
solid middle cloud deck to stay in place across much of the area 
through today and tonight. Snow showers will begin over the 
mountains later this morning...intensifying and spreading east 
overnight. Added vicinity showers at kmrb for tonight...but kept 
everywhere else dry. Winds will increase behind the clipper system 
today...so maintained a brief period of gusts of 20 to 25 knots this 
afternoon. 


Increasing chance for precipitation...low ceilings and visibilities as strong upper trough moves across 
region through Friday. Surface hipres and improving conditions this weekend...before 
another strong system aprchs Sun night and Monday. Conditions worsen Monday 
through Tuesday across terminals as rain and snow develop with arrival and 
passage of cold front Tuesday morning. 


&& 


Marine... 
winds already increasing over the southern marine zones. Have 
updated the headlines to start the Small Craft Advisory early this 
morning for the lower Maryland Chesapeake Bay and tidal Potomac 
River. Winds gradually pick up through the morning across the rest 
of the zones...with gusts up to 25 knots possible by the afternoon. 
Winds will be slow to diminish overnight with low pressure 
strengthening off the East Coast. 


Small craft adzy conditions to continue through Friday as cold air spreads 
atop warm waters and upper trough axis moves through. Surface hipres builds 
through weekend...with strong upper system to aprch Monday. Southerly low level flow 
Monday with increasing chance of precipitation Monday through Tuesday. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...Winter Storm Watch from this evening through Friday evening 
for mdz501. 
Virginia...Winter Storm Watch from this evening through Friday evening 
for vaz021. 
WV...Winter Storm Watch from this evening through Friday evening 
for wvz054-501-503. 
Marine...Small Craft Advisory from 9 am this morning to 7 PM EST Friday 
for anz530>532-535-536. 
Small Craft Advisory until 7 PM EST Friday for anz533-534-537. 


&& 


$$ 
Sar/kramar