201 
fxus61 klwx 040020 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
820 PM EDT Thursday Jul 3 2008 


Synopsis... 
a cold front will stall across the middle Atlantic through the Holiday 
weekend. Conditions will remain unsettled into the first part of 
next week. A second frontal boundary will move across the area on 
Wednesday. Showers and thunderstorms are expected each day. 


&& 


Near term /until 6 am Friday morning/... 


Current surface analysis shows a cold front that stretches from 
upstate New York through northern PA into Ohio. A pressure trough is 
located over the central portion of our County Warning Area and surface high remains 
over the western Atlantic. Water vapor imagery this evening shows a 
shortwave in the middle-level flow moving through Ohio towards our 
County Warning Area. 


The cold front to our northwest will slowly drop southeast into the 
northern and western parts of the County Warning Area overnight and the shortwave 
will also rotate through our region. Current radar imagery does show 
scattered showers and thunderstorms over PA and West Virginia ahead 
of the shortwave and cold front. Despite the middle-level cap still in 
place...there should be enough forcing with the shortwave and cold 
front for isolated to scattered showers/T-storms overnight. NAM/GFS 
BUFKIT along with sref guidance do show MUCAPES about 250-500 j/kg 
just ahead of the front. Will keep probability of precipitation confined generally from the 
metropolitan areas north and west overnight...closer to the cold front as 
well as the passage of the shortwave. Best chance for shower/T-storm 
activity will be closer to the Mason Dixon line and the Potomac 
Highlands. Have the highest probability of precipitation across these areas. Severe 
thunderstorms are not anticipated overnight due to the lack of 
significant instability and shear profiles being marginal at best. 


Min temperatures overnight will range from the upper 50s in portions 
of the Potomac Highlands to the middle 70s in downtown Washington and 
Baltimore. 


&& 


Short term /6 am Friday morning through Friday night/... 
cold front currently across New York/PA/OH will be driven southward tonight by ongoing 
cnvctn and exiting shortwave trough. Mean upper trough will establish over MS 
valley by Friday morning...with a second shortwave trough to move NE across forecast 
area by late Friday evening. Cold front to become stationary across County Warning Area by late 
Friday afternoon...with increasing precipitable water vals as moisture pools along boundary and higher 
level moisture is advected into region. 


Thunderstorms are expeceted to increase in covering as shortwave trough aprchs Friday afternoon and 
evening and interacts with boundary. Middle level flow of 25 to 30kt and weak 
low level Ely to northeasterly flow should result in deep layer shear of roughly 
30kt...which may lead to modest thunderstorm organization. While lapse rates 
should steepen again in response to aprchg shortwave trough...upper cloud 
cover is forecast to be extensive...which may limit instability and result in 
highly forced cnvctn. However...if cloud breaks can be realized... 
slightly higher covering of strong to severe thunderstorms will be possible. Either way...it 
appears that slow moving and training thunderstorms will be likely...with locally 
heavy rainfall. 


Maxima low to middle 80s f were retained from previous package...with eastern zones 
slightly warmer owing to lesser cloud cover during morning and early afternoon. 


&& 


Long term /Saturday through Thursday/... 
the long term portion of the forecast will remain unsettled well 
into next week. 


A frontal boundary will remain stalled across the County warning forecast area through 
Sunday...before dissipating on Monday. This boundary will be the 
primary focus for convection through the weekend. The front is 
expected to dissipate on Monday. Return flow will continue to bring 
moisture northward into the middle Atlantic though Tuesday. A cold 
front will drop south over the area on Wednesday. 


The atmosphere will remain unstable...especially along and south of 
the stalled boundary. The potential for strong storms to develop 
will exists especially for Saturday and Sunday. 


&& 


Aviation /00z Friday through Tuesday/... 
light winds expected overnight along with increasing middle-level 
clouds. An upper-level disturbance may trigger isolated/scattered 
convection overnight. This could briefly bring MVFR/visibilities or 
ceilings...but VFR conditions are expected for most of the time. Feel 
that activity will be isolated...so kept it out of the tafs for now. 
However...did include a cumulonimbus group for kmrb closer to the 
disturbance. 


Thunderstorms continue in forecast through weekend as moisture...a stationary boundary and 
several upper disturbances move through the area. 


&& 


Marine... 
local WRF continues to show southerly winds channeling up the 
Chesapeake Bay overnight. Will continue with a Small Craft Advisory 
from Pooles Island through Smith Point for wind gusts up to 20 
knots...but it will be marginal. Isolated convection possible 
overnight across the northern Chesapeake Bay and upper tidal 
Potomac. Cloud to ground lighting would be the primary threat. 


Cold front will stall over the middle Atlantic Friday through Monday...with periods of 
rain showers and thunderstorms and rain expeceted through the weekend. Some strong thunderstorms are forecast and 
will be capable of wind gusts in excess of 30 knots. Wind direction 
through the weekend is uncertain given presence of quasistationary 
boundary near Mason Dixon line...especially vcty northern Chesapeake Bay. 


Southerly winds to resume early next week...with moisture and instability expeceted. 
Therefore...appears thunderstorms will continue in forecast through extended periods. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory until 4 am EDT Friday for anz531>534. 


&& 


$$ 
Lasorsa/kramar/listemaa