Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
916 PM EDT Sat Aug 30 2014
high pressure off Cape Cod will move southeast and become a
Bermuda high through Tuesday. A cold front will cross the area
Tuesday night. High pressure will follow for the second half of
Near term /through Sunday/...
at 01z klwx radar showing the line of thunderstorms that traversed
the Potomac Highlands earlier this evening dissipating. One area
of showers/thunderstorms remains over the Blue Ridge near Waynesboro.
This too appears to be dissipating as it moves northeast.
Skies will be partly to mostly cloudy tonight as middle and high
clouds from the convection over the Tennessee Valley continues to stream
to the northeast. With high dewpoints...will see areas of fog
developing...predominantly in sheltered locations where light
southerly winds will not be able to mix the boundary layer. At
01z...already seeing low surface temperature/dewpoint spreads at Stafford and
Warrenton and mist forming. Both met and mav guidance shows visibilities
Sunday will start out with lingering patchy fog through middle
morning...then remain mostly cloudy. The 18z GFS has a streak of
precipitation water great than 2.25" across our County Warning Area as a potent 500 mb vorticity
tracks from SW. This will lead to numerous rain showers/thunderstorms developing
by middle afternoon and lasting through midnight. Some of these
storms will have the potential to produce heavy rainfall...but
there is decent 20-30 kts in the cloud bearing layer which will
keep the storms progressive.
Short term /Sunday night through Tuesday/...
County warning forecast area will be west/in southwesterly flow for the frist parts of the weak. However the
flow will become a bit more westerly Monday...and especially Tuesday. The plume of
moisture evident on water vapor imgry will be running atop the ridge.
Latest guidance suggesting that the setup wont be quite as ideal as
prvs cycles. Thats not to say there wont be a moist/unstable air mass.
But thte ridge/advection as well as positive vorticity advection during the afternoon will be better in
the mountains and points west. Still will have a corridor of precipitable water at or above 2
inches...but there/S not as much to organize/force the precipitation. And
in the southeastern County warning forecast area...ridging will actually surpress it. Have therefore
backed off on probability of precipitation sun...still focusing on the afternoon-evening hours. Likely
probability of precipitation wont appear until evening...when lift arrives. Nocturnal stability
will allow positive vorticity advection to sneak through overnight.
Am forecasting a good deal of cldcvr sun...which should once again impede
strong warming. Have shaded down maxt a degf or two.
While there will be a sharp Lee trough Monday...all other features appear
to be either weak or subtle. Westerly flow will provide a little bit of
drying...which is evident in prognosticated precipitable water values. The impact is
evident in pop values...which are lower than sun. That seems quite
resonable. Am gnly in the 30-40 percent range...and am keeping a dry
forecast in centrl Virginia. Whatever thunderstorms and rain that do develop should mv to the Eastern
Shore by nightfall...and have a dry forecast for Monday night-- which largely
will continue into Tuesday due to downsloping.
Think that the best chance at a 90 degree day will come on Tuesday...as westerly
flow will keep probability of precipitation low and provide compressional warming. Have lowered
Monday maxt by a cpl...and raised Tuesday maxt also by a cpl. Min-T forecasts
on track...in step west/ prognosticated dewpoints.
Long term /Tuesday night through Saturday/...
500mb shortwave will track from the Great Lakes to southern Quebec
and drag a weak trough through the County Warning Area Tuesday night through
Wednesday. 500mb high will build over middle Atlantic region Wednesday
through Thursday. The high will weaken allowing a cold front to
enter northwest PA by Saturday night.
Band of moisture...pva...and passage of the cold front will support
a slight chance development of showers/thunderstorms Tuesday night
through Wednesday morning. Much drier airmass associated with
surface high will cross the County Warning Area later Wednesday through early Friday.
Increasing moisture due to the return south to southwest flow
associated with the high and positive vorticity advection out ahead of an approaching cold
front from the west will support an increased chance of more showers
and thunderstorms Saturday.
Aviation /01z Sunday through Thursday/...
Southerly flow will provide increasing low level humidity. Therefore...
overnight fog should be an issue. Fllwg climatology by keeping dca at VFR.
Otherwise...have mvrg visibilities across the board pre-dawn. IFR a possiblity...but
think there will be too much higher level cldcvr for thicker fog to develop.
Once morning restrictions clear...atmosphere will be unstable for scattered thunderstorms and rain to
develop. Could have local IFR in hvr rainers...but low confidence in a spot
forecast preclude a taf inclusion.
Outlook...Sunday night should see MVFR fog return after
rainfall...then Monday targeted IFR possible at times with scattered thunderstorms and rain.
Covering should be less than sun. Tue-Thu...primarily VFR.
southerly flow across the marine area through the weekend. Small Craft Advisory in effect
overnight for Bay Sandy pt-Smith pt as indicated by 01z marine observation
of upper teen gusts. It/ll be a simlr situation Sunday...though dont
think the mixing will be quite as good. In addition...potl thunderstorms and rain would
provide stronger wind gusts.
Offshore high pressure will continue to steer warm/humid air up across
the eastern Seaboard. Setup would support periods of southerly channeling...but
guidance not picking up on any just yet. Weak cold front lt Tuesday/Ely Wednesday...
marked by a slight shift in wind direction from SW to northwest.
departures at Annapolis are about 0.9 feet...and winds should remain
southerly...so anticipate addtl increases through the weekend. Barely made
it to 2.4 feet at 0030z...so missed a minor this evening. The next
high tide Sun morning will need to be watched for possible minor
flooding. Nonetheless...this will be an issue through the weekend with
new moon and southerlies.
District of Columbia...none.
Marine...Small Craft Advisory until 4 am EDT Sunday for anz532>534.