Weather

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fxus61 klwx 280130 aaa 
afdlwx 


Area forecast discussion...updated 
National Weather Service Baltimore Maryland/Washington District of Columbia 
930 PM EDT Wednesday Aug 27 2008 


Synopsis... 
low pressure over eastern Tennessee associated with former 
Tropical Storm Fay will lift slowly to the northeast through the 
end of this week. Rainfall will be heaviest west of the Blue Ridge 
mountains. A cold front will move across the area Saturday...then 
high pressure builds in for Sunday. 


&& 


Near term /through Thursday/... 


Rain continues to fall mainly across the Shenandoah Valley where 
as much as an inch of rain was reported in the city of Staunton 
since last night. However...rain is tending to dissipate as it 
heads northeast with only sprinkles seen in The Wash/balt areas. 
Guidance along with radar trends indicate that the best lift will 
remain across the Potomac Highlands of western Allegany and Mineral 
counties. Over The Wash/balt areas is really only about a 50/50 
chance that they will measure due to large dewpoint depressions 
below 850 mb as seen on 00z iad sounding...very weak lift and 
continued strong influence from Dry Ridge to the north. 


&& 


Short term /Thursday night through 6 PM Thursday/... 


Not too optimistic about any sig rainfall tomorrow due to 
weakening lift. Most likely some showers with any precipitation probably 
less than one tenth of an inch. 


&& 


Long term /Thursday night through Wednesday/... 
500mb height/anomaly pattern during the past 5 days shows a 
trough/ridge/trough pattern in southern Canada has progressed to a 
deepening trough across western Canada...and a east central 
Canadian ridge. Water vapor loop shows the upper remnant circulation 
of Fay moving northeast into the Ohio Valley...with the ridge 
dominating the eastern Great Lakes. Surface map early this morning 
places a 1022mb high over New England...with a baroclinic zone from 
the Tennessee Valley east across the Carolinas. The remnant 
circulation of Fay was 1008mb and located in eastern Tennessee. 


First portion of medium range forecast revolves around the remnants 
of Fay. Per HPC am inclined to follow European/GFS blend as global 
models seem to be handling the evolution better than the NAM. 


Combination of warm advection of tropical airmass...isentropic 
upglide above frontal boundary to the south...and southeasterly 
upslope flow should provide an opportunity for replenishing rains 
continuing through Thursday...with a focus along the mountains. Its 
important to note that although probability of precipitation across the forecast area will 
be high...that the criteria is for 0.01" of rainfall. There will 
likely be a significant gradient in rainfall totals between the 
heavier rain across the western zones...and further east across the 
metropolitan areas where high pressure continues to feed a dry airmass 
southward. 


Beyond Fay...next cold frontal passage occurs Saturday...with high 
pressure building in early week. Also will have to keep an eye on 
the tropics as NHC continues to monitor Gustav...and two potential 
areas of development in the Atlantic. 


&& 


Aviation /01z Thursday through Monday/... 


Went more optimistic for ceilings overnight and tomorrow on the 00z 
issuance due to weak lift and continued influence of strong ridge 
to the north. Other than kcho and kmrb do not see any of the other 
taf sites dropping below 3kft. 


03z sref guidance indicates chance to likely probabilities of MVFR 
ceilings through the cold frontal passage Saturday. IFR ceilings 
are likely near the Appalachians mountains...perhaps as far east 
as the Blue Ridge through Friday morning. High pressure builds in 
for early next week. 


&& 


Marine... 


Cancelled Small Craft Advisory for northern two zones of the Bay due to weak gradient. 


East/southeast flow will continue through Thursday. Models 
indicate 20 knots or so that is available to come down...therefore 
will raise a Small Craft Advisory. 


Small Craft Advisory conditions may continue into Thursday evening 
as the pressure gradient strengthens between high pressure to the 
north...and low pressure approaching from the south. A cold front 
passes Saturday...with high pressure building in for early next week. 


&& 


Tides/coastal flooding... 
water levels within the tidal Potomac River and Maryland 
Chesapeake Bay are a half foot to a foot above astronomical 
predictions. With the easterly flow persisting until the cold 
frontal passage Saturday...water level anomalies will have to be 
monitored. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory until 6 PM EDT Thursday for anz532>537. 


&& 


$$ 
Update...Rosa 
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