034 
fxus61 klwx 220157 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
957 PM EDT sun Mar 21 2010 


Synopsis... 
a potent upper low over the southern Mississippi Valley will 
slowly track toward the Tennessee Valley through Monday. The low 
will cross the region Monday night...and off to Cape Cod by 
Tuesday. High pressure will then build in for middle week...ahead of 
another complex low pressure system. 


&& 


Near term /until 6 am Monday morning/... 
latest WV Sat loop shows potent upper low pushing across the southern MS 
valley this evening. At 01z...surface lopres is located over central Tennessee with 
associated cold front situated from western Kentucky to southeastward into the West Coast of 
Florida. In advance of the upper low...an organized line of heavy 
precipitation and embedded convective elements continue to push northeastward just 
ahead of the cold front in a region of upper diffluence. Another 
area of precipitation continue to stream northward over the c 
Appalachians/mid-Atlantic region in associated with low level 
isentropic lift and a middle-level jet maximum. Regional radar shows light 
showers moving through our region so far this evening but has had a hard 
time reaching the ground thus far with dry air in place /as 
confirmed in the 00z iad raob/. 


The column should quickly saturate /as evident in the 00z rnk radiosonde observation/ 
with light showers later this evening across southwestern portions of the County Warning Area. 
Mainly light showers and lower ceilings should gradually overspread 
the rest of the County Warning Area from SW to NE overnight. With increasing cloud 
cover...temperatures will be warmer than the past few nights with lows in 
the mu40s west of the Blue Ridge and l50s to the east. 


&& 


Short term /6 am Monday morning through 6 PM Monday/... 
the strong upper low will move across the Tennessee Valley and towards the 
central Appalachians by the late afternoon. The aforementioned band of 
heavy precipitation ahead of the cold front will enter southwestern portions of 
the area during the morning and swing through the area during the late 
morning and afternoon. 18z NAM/GFS soundings show 50-55 knots of deep-layer 
shear along with 200-400 j/kg of cape ahead of the cold front/dry 
slot. Instability will be marginal enough to mention chance of 
thunderstorms. Qpfs will range from a half to one inch across the area 
with isolated higher amnts in convective bands. 


The dynamics of the system will be moving this activity steadily 
along...so flash flooding or high quantitative precipitation forecast amounts not anticipated. 


&& 


Long term /Monday night through Sunday/... 
GFS shows surface low over PA Monday night moving NE away from area. Enough 
leftover moisture to warrant continue of probability of precipitation into the night and into Tuesday. 
Drier airmass expected Wednesday and Thursday. Fnt drops down into the area 
late week with another chance for rain showers. Temperatures are fairly close to 
seasonal norms through the period. 


&& 


Aviation /02z Monday through Friday/... 
middle-level clouds will continue to move across the area this evening. 
Moisture will stream into the region tonight as a potent upper-low 
over the southern MS valley approaches. Pushed back onset of lowering clouds 
and light rain a few hours from previous tafs until late tonight-early 
morning hours with dry air initially in place. IFR ceilings-visibilities expected 
a few hours after start of rain around or shortly aftr daybreak. 


Did not have enough confidence to mention a cumulonimbus group in the 00z 
tafs. However may need to include in next set of tafs if models 
continue to support convective activity late Monday morning and afternoon. 
Southeasterly winds will increase during the late afternoon Monday with gusts 15-20 
knots possible. A frontal passage and windshift to the SW expected during the 
evening Monday. 


&& 


Marine... 
southeasterly winds will increase overnight sun into Monday. Small Craft Advisory conditions will 
become more likely into the daytime hours Monday morning...as a complex 
low pressure system approaches from the SW. The low will move over 
the region late Monday into Tuesday...W/ steady rain showers on Monday - 
mainly ending by early Tuesday. 


&& 


Tides/coastal flooding... 
current water levels over many of the tidal Potomac and Maryland 
Chesapeake Bay sites between 0.5-1.0 feet above normal. An approaching 
low pressure system from the southeast will create the possibility 
for at least minor coastal flooding at times of high tide about 
Monday night and Tuesday. Winds late tonight will back toward southeasterly...W/ fairly 
weak and intermittent gusts - but the effect will be a slow and 
steady channeling of water up the Bay/tp. Model forecasts extend 
the height differences about 1.5-2.5 feet above normal in some 
areas. Regardless of the exact amount...the steady southeasterly flow for 
an extended period of nearly a whole day should create a 
heightened awareness across coastal areas of southern/eastern Maryland for minor 
coastal flooding. However...phase of lunar cycle between new and 
full moon is not favorable for coastal flooding. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory from 9 am to 10 PM EDT Monday for 
anz530>543. 


&& 


$$ 
Update...Klein 
previous disc...gms/rdh