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fxus61 klwx 201614 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
1114 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/... 
cold front/clipper system currently over the eastern Panhandle of 
West Virginia stretching through the Shenandoah Valley. No precipitation is 
associated with this feature at this time. However...upslope snow 
showers are expected along the western facing ridges this afternoon 
and overnight. 


Winds have shifted to the west behind the boundary. Several breaks 
in the overcast have occurred. Skies east of The Highlands should 
remain partly sunny/mostly cloudy for the remainder of the afternoon. 
Have also raised the maximum T numbers a few degrees. 


Have adjusted the sustained winds/wind gusts for the remainder of 
the morning and into the afternoon. Gusts to between 25 and 30 knots 
will be likely over the higher terrain out west. Gusts around 20 knots 
will be likely over the remaining land areas. 


Have gone ahead and issued a Winter Storm Warning for the far 
western three zones. An advisory has been issued for eastern 
Allegany...Mineral...Grant and Hampshire and Hardy counties. 


&& 


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 /16z Thursday through Monday/... 
ceilings will remain VFR into the afternoon hours. Ceiling heights between 
045 and 070 are anticipated. A wind shift to the west/northwest is expected 
later this morning and afternoon. Winds gusts to around 20 knots are 
possible. 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... 
have seen some wind gusts between 25 and 30 knots behind the frontal 
boundary. Have gone ahead and upped the gusts potential over the 
waters to 25 knots this 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 Warning from 7 PM this evening to 10 PM EST 
Friday for mdz501. 
Winter Weather Advisory from 7 PM this evening to 10 PM EST 
Friday for mdz502. 
Virginia...Winter Storm Warning from 7 PM this evening to 10 PM EST 
Friday for vaz021. 
WV...Winter Storm Warning from 7 PM this evening to 10 PM EST 
Friday for wvz054-501-503. 
Winter Weather Advisory from 7 PM this evening to 10 PM EST 
Friday for wvz050-055-502-504. 
Marine...Small Craft Advisory until 7 PM EST Friday for anz530>537. 


&& 


$$ 
Update...listemaa 

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