Weather

133 
fxus61 klwx 070738 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
338 am EDT sun Sep 7 2008 


Synopsis... 
relatively benign weather expected through Monday. Thunderstorms 
will be possible Tuesday and Tuesday night as a moist airmass 
moves into the middle Atlantic region. 


&& 


Near term /until 6 PM this evening/... 
Hanna off Cape Cod at this time...moving NE. Pressure rises in wake of low also NE 
of County warning forecast area. Subsequently...winds have dmnshd...and skies have clrd. Not much 
weather to talk about today...as hipres builds. Although flow northwesterly...not much cold air advection 
present. There is advection of lower dewpoints though...making day comfortable. 
Took maxt down a cpl degree from prvs forecast...in line west/ 800 mb temperatures near 14c. 


&& 


Short term /6 PM this evening through 6 PM Monday/... 
any diurnal clouds will dspt west/ sunset. Although upper jet across Great Lakes... 
perhaps a streak of cirrus or a few middle clouds possible per moisture seen at 700 mb. 
Based on dry air...little wind...and mostly clear sky...went on lower side of 
guidance. 


&& 


Long term /Monday night through Saturday/... 
shortwave trough will aprch Monday through Tuesday...with quasizonal middle and upper 
level flow increasing. Low level response will be increased southerly flow as Lee trough 
develops. Result will be strong wm air advection Monday night along with increased 
moisture as surface low deepens to northwest and low level jet forms. Will carry slight 
chance probability of precipitation late Monday night for possible rain showers/thunderstorms in such a regime. 


Thunderstorm chances increase further Tuesday through Tuesday evening as shortwave trough and cold front 
aprch. Thetae ridge will straddle forecast area Tuesday afternoon...as middle level flow 
increases. With moisture in place along surface trough...will increase probability of precipitation Tuesday afternoon 
and evening. Although upper level cloud cover may be extensive...sufficient 
breaks should exist in clouds to allow instability to be realized...and a few 
strong to severe thunderstorms may be possible given expected shear and instability. 


Cold front to push through forecast area Tuesday night...with chances for precipitation decrg. 
Hipres builds at surface Wednesday through Thursday. 


Medium range guidance and ensembles suggest mean trough developing across 
midwestern states by Friday...with increasing middle level flow across aplcn mountains 
this increasing flow should aid in Lee trough development Friday...as depicted in 
GFS and European model (ecmwf)...and limited moisture return on southerly low level flow. 


Differences in GFS and European model (ecmwf) for extended periods are in frequency and 
strength of shortwave troughs passing through. At this time...cannot carry chance 
probability of precipitation in extended periods given unclear dynamic support...so have lowered 
probability of precipitation Friday and Friday night to slight chance...with dry forecast thereafter. 


Maxima in upper 70s to lower 80s through forecast package...with cool mins especially 
Wednesday evening as hipres builds and low level drying ensues. 


&& 


Aviation /08z Sunday through Thursday/... 
VFR conds at this time...and will remain so through valid taf pkg. Winds have stayed 
up just enough to keep air mixed...and lower dewpt advection keeping fog from 
forming. Although suspect winds will go calm prior to dawn...especially in vicinity of 
cho/mrb...dont think it will be enough. Have imprvd forecast considerably... 
going west/ MVFR br for cho/mrb only. 


Thunderstorm chances increase Tuesday through Tuesday evening as moisture returns ahead of aprchg 
cold front. Hipres builds Wednesday and Thursday. 


&& 


Marine... 
winds on the Bay still gusting to 15-20 knots. All guidance Marks decreasing 
trend...which fits isallobaric pttn. Lwx-WRF-nmm12 suggests Small Craft Advisory will 
be done shortly...but arw4 hangs onto 20 knots gusts until about 10z. Although 
would like to drop headline entirely...observation dont support that. So...will 
hang onto Small Craft Advisory for lower ptmc and Bay a few more hours. 


Later today...winds will be safely below Small Craft Advisory...and decr as hipres builds. 


Thunderstorm chances increase Tuesday through Tuesday evening as moisture returns ahead of aprchg 
cold front. Aside from thunderstorms...no marine wind issues forecast in extended periods 
at this time. 


&& 


Tides... 
effects of northwest flow evident...as tidal anomolies decreasing. In 
fact...at District of Columbia water levels have lowered as high tide rolled in...and now a 
feet below predicted levels. Other sites west/in 6 inches of normal. Will canx coastal 
flood advisory. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory until 8 am EDT this morning for 
anz530>534-537. 


&& 


$$ 


Silverman/kramar