Weather

197 
fxus61 klwx 080729 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
329 am EDT Monday Sep 8 2008 


Synopsis... 
surface high pressure will move east into the Atlantic today... 
allowing a surface trough to develop across the middle Atlantic region 
this afternoon. An approaching upper wave and associated cold front 
will interact with a very moist and unstable airmass on Tuesday to 
produce thunderstorms throughout the area. High pressure returns 
Wednesday and Thursday before thunderstorm chances increase again 
toward the end of the week. 


&& 


Near term /until 6 PM this evening/... 
patchy fog this morning will give way to scattered cumulus by early afternoon. Surface 
hipres vacates eastward...with light southerly flow to ensue. Increasing middle level flow 
across aplcn mountains will aid in development of Lee trough this afternoon and evening... 
which will strengthen low level southerly flow this afternoon. 


&& 


Short term /6 PM this evening through 6 am Tuesday/... 
as surface low deepens to northwest in response to aprchg wave...southeasterly winds should 
increase this evening...with low level jet aiding in rapid transport northward of 
moisture. Expecting broken low to middle level clouds and advection fog to be common most 
areas late tonight. In addition...well mixed elevated layer in concert 
with strong moisture advection will yield elevated instability...especially left side of 
low level jet. Have reintroduced slight chance thunderstorms late tonight west of Blue 
Ridge. 


&& 


Long term /Tuesday through Sunday/... 
500mb height/anomaly pattern during the past 5 days shows East Coast 
ridging progressing into the Atlantic and weakening. Anomalously low 
heights have traversed the North Pole and built into northern 
Canada...with lowering heights as far south as the north central 
Continental U.S.. this has pumped up ridging over the eastern Pacific and 
western Atlantic. Water vapor loop shows a trough crossing over the 
northern Great Lakes...as an upper level jet digs over the northern 
rockies. Surface map last evening placed a cold front crossing the 
middle Atlantic. A second cold front was crossing the Midwest and 
plains...with 1026mb high pressure building across the Dakotas. 


Scattered to numerous showers and thunderstorms expected to develop 
ahead of and along the approaching cold front Tuesday and Tuesday 
night. Wild card will be amount of heating Tuesday after warm 
advection cloudiness during the previous night. On the synoptic 
scale...rapidly deepening low pressure will be accelerating across New 
England with strong height falls aloft. This leads to weakening low 
level convergence further southwest along the boundary. Perhaps 
convergence will tighten up during the evening across the southeast 
forecast area as drier air filters southward. With 21z sref 
indicating a likely probability of 30 knots effective shear...organized 
convection is expected. Best shear appears to be to the northeast 
from Pennsylvania into New England. Same guidance also indicates a 
good chance of 1000 j/kg MLCAPE...especially east of the Blue 
Ridge. With 50+ kts of flow within cumulus layer...locally 
damaging wind gusts will be the primary threat from when the 
diurnally forced convection develops midday...until frontal 
passage overnight across southern zones. 


In addition...will have to monitor rainfall rates. Precipitable 
water values are forecast to be near +1.5-2.0 Standard deviations 
for this time of year. Western Washington metropolitan area will be 
especially susceptible to flash flooding given the corridor of 
5-9" of rainfall from Hanna. In fact...current 1hr gridded flash 
flood guidance has values as low as a quarter inch from Quantico 
north to Fairfax City and Arlington. Hopefully continued drying 
will take place today ahead of the next system. Later shifts may 
need to consider a tactical Flash Flood Watch should confidence of 
storm coverage across the aforementioned area increase. 


The long range forecast hinges on the approach of the next cold 
front...and the interaction with Ike. By Friday evening...the 11 PM 
National Hurricane Center forecast track (advisory 28) places Ike in 
the northwest Gulf of Mexico or making landfall in Texas...Louisiana 
or Mississippi. Its still too early to say whether the steering 
flow of the ridge to its northeast will take it on a sharp right 
turn toward the central Gulf states...or if the ridge is stronger as 
advertised by the 12z European...taking the system further west into 
Texas during the weekend. If the later solution comes to fruition... 
approaching cold frontal boundary should take a bit longer to 
arrive given ridging over southeast Continental U.S.. 


&& 


Aviation /07z Monday through Friday/... 
patchy fog vcty kcho and kmrb will erode by middle morning. Otherwise... 
scattered cumulus to develop by early afternoon with light southerly flow as hipres exits eastward 
and surface trough forms. Strong winds above surface expected tonight as low level jet 
develops...with broken clouds at or above 5kft and advection fog possible with rapid return 
of moisture. Thunderstorms possible late tonight west of Blue Ridge. 


MVFR conditions possible within showers and thunderstorms Tuesday 
and Tuesday evening. Gusty southwesterly winds Tuesday...become 
northeast and diminish Wednesday behind the cold front. High 
pressure settles into the region midweek. 


&& 


Marine... 
hipres exits eastward today with light but increasing southerly winds expected. Southerly 
channeling expected tonight as surface low deepens to northwest...and may reach small 
craft adzy thresholds late tonight as low level jet should preclude development 
of strong inversion. 


Small Craft Advisory wind gusts continue Tuesday as higher winds 
mix to the surface. Scattered to numerous showers and thunderstorms 
expected Tuesday and Tuesday evening. Some of these storms may 
become strong to severe and may require special marine warnings 
for locally damaging winds. The most likely time period for 
inclement weather is between middle afternoon...lasting into the 
overnight hours across the Middle Bay. 


Pressure gradient relaxes late Tuesday ahead of the approaching 
cold front. A second period of Small Craft Advisory criteria winds 
is forecast to occur behind the cold front later Tuesday night into 
Wednesday. High pressure builds into the region midweek. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory from midnight tonight to 4 PM EDT Tuesday 
for anz530>537. 


&& 


$$ 


Kramar/rogowski 

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