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afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
1008 am EST Monday Nov 23 2009 


Synopsis... 
low pressure will track up the middle-Atlantic coast this afternoon and 
towards Long Island tonight. High pressure will build in behind this 
system Tuesday and Tuesday night. A storm system will approach from 
the northwest Wednesday. A stronger system will move across the area 
Thanksgiving and Friday. 


&& 


Near term /until 6 PM this evening/... 
current analysis shows low pressure off the North Carolina coast 
this morning. High pressure over eastern Canada extends into New 
England. A series of weak upper-level disturbances extend from New 
Jersey through the middle-Atlantic and into the Ohio Valley. 


The surface low off the Carolina coast will continue to slowly track 
up the middle-Atlantic coast this afternoon. With high pressure to the 
north...a low-level easterly flow will usher in plenty of moisture 
from the Atlantic Ocean. The warmer and more moist air at the 
low-levels will overrun the cool air at the surface...resulting in 
plenty of clouds along with periods of rain. Weak middle-level forcing 
will keep the steadiest rain across the Virginia Piedmont into lower 
southern Maryland...where rainfall amounts around one quarter to one 
half inch are expected. Further north and west...rainfall amounts 
will be closer to one tenth of an inch...with even lighter amounts 
near the Mason Dixon line. 


Maximum temperatures this afternoon will be several degrees cooler 
compared to recent days due to the cloud cover and rain. Maximum temperatures 
will 40s in the Shenandoah Valley to the lower 50s in Washington and 
Baltimore. 


&& 


Short term /6 PM this evening through 6 am Tuesday/... 
tonight the low will be tracking NE of the area. Will have Cat probability of precipitation in the 
extreme NE corner of the County Warning Area for the evening hours...dropping to chance 
everywhere aftr mdngt. Skies will remain cloudy. Again little moderation west/ 
the temperatures - expect the lows tonight to range from a40 in The Highlands to 
a50 along the Bay. 


&& 


Long term /Tuesday through Sunday/... 
weak hipres will build over the area behind the departing low off 
the New England coast. Despite hipres ovrhd...cloudy conditions will 
continue midweek as low level moisture remains trapped below subsidence 
inversion. Cannot rule out patchy drizzle with light southerly flow at llvls 
supporting weak isentropic lift in shallow moist layer. 


Leading upper shortwave energy embedded in c Continental U.S. Longwave trough 
will track over the Midwest Tuesday and the Great Lakes Wednesday. Although 
this weakening system should track well northwest of the area...enough lift 
aided by DPVA/warm air advection will develop to mention low chances for showers 
Wednesday/Wednesday night. 


The trailing shortwave trough drops southeastward while amplifying across the 
c Continental U.S. Thursday. With strengthening height falls ahead of the cutoff 
low...surface development is forecast to occur somewhere along the southeast coast. 
Best County warning forecast area-wide chance for rain or showers appears to be Thursday afternoon 
into Thursday night with the surface low tracking through the middle Atlantic region. 
With the thermal gradient tightening...strong cold air advection will ensue behind 
the surface low once the attendant cold front crosses the Appalachians 
late Thursday into Friday. 00z GFS BUFKIT soundings support a rain/snow mix for 
the higher elevations in the Potomac Highlands Thursday night and Friday. 


Cyclonic westerly flow around the upper low will lead to seasonably cold 
temperatures across the entire area while prolonging the chances for 
upslope snow showers across the western slopes of the Allegheny Front and 
Potomac Highlands through Sat. Hipres builds back into the area sun. 


&& 


Aviation /15z Monday through Friday/... 
low clouds will continue to affect the terminals throughout the 
afternoon. Rain will also overspread the terminals. MVFR ceilings across 
most of the airports this morning will drop to IFR levels once the 
rain begins. MVFR/IFR visibilities are also expected in areas of rain. More 
IFR ceilings are expected tonight with MVFR/IFR visibilities. 


Periods of sub-VFR ceilings and dz possible through midweek as marine 
moisture remains trapped at the llvls. A storm system will track northwest 
of the terminals Wednesday but close enough to bring chances for rain showers. A 
stronger disturbance is expected to move through Thursday into early Friday. 


&& 


Marine... 
will continue with a Small Craft Advisory this afternoon for the 
Chesapeake Bay and lower tidal Potomac. The pressure gradient will 
strengthen a bit resulting in wind gusts past Small Craft Advisory criteria. Across 
the upper tidal Potomac...wind gusts around 15 knots are expected 
this afternoon. Winds will turn more toward the north tonight as the 
low pressure tracks toward Long Island. More Small Craft Advisory wind gusts are 
expected across the Chesapeake Bay and lower tidal Potomac. 


Winds will be light during the middle part of the week with hipres 
ovrhd. The pressure gradient will tighten late in the week as 
developing lopres tracks up the middle Atlantic coast. Small Craft Advisory conditions 
appears possible Thursday afternoon and night as the low tracks over the region. 
Strong cold air advection behind the low and associated cold front will allow for 
widespread Small Craft Advisory conditions to continue into the first half of the 
weekend. 


&& 


Tides/coastal flooding... 
tidal anomalies currently are running about one half to one foot 
above normal this morning. As low pressure tracks up the 
middle-Atlantic coast this afternoon and towards Long Island 
tonight...a persistent onshore flow will continue. Tidal anomalies 
may increase as the low pressure passes by to the east late this 
afternoon into tonight. Latest cbofs as well as extra tropical 
models continue to keep water levels below thresholds for minor 
coastal flooding. Will continue to monitor the tidal anomalies this 
afternoon...with maximum high tide expected tonight any increase in 
tidal anomalies may warrant a coastal Flood Advisory. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory until 5 am EST Tuesday for anz531>534-537- 
539>541-543. 
Small Craft Advisory until 8 PM EST this evening for anz530- 
538-542. 


&& 


$$ 
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