Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
245 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 tonight/...
surface hipres remains off ack taftn. However...morning clouds brought onshore
around periphery of high have held tough along and east of blurdg...
trapped under 800 mb ridging. Latest vsbl satpix depict some clearing along
I-95. West of there...in the Lee of the rdgs...a broken to almost overcast
layer of clouds remain. Trends...and lav...suggest that clearing will come
during the afternoon. Obviously...this has held temperatures down. But would
expect a rapid warm up once insolation appears...as theres plenty of
sndg support for it.
Meanwhile...ample sun in the ptmc Highlands have led to diurnal cumulus.
Latest mesoscale guidance runs still suggest precipitation will develop there...middle afternoon
into Ely evening. However...support to allow for these cells to emerge
from the mountains will be lacking. Have opted to scale back probability of precipitation okv-mrb-
hgr. The rest of the County warning forecast area will be rain free through the evening.
Deep southwesterly flow tonight will bring the humidity. Think that/ll support
plenty of clouds overnight. The added cldcvr and dewpoints up near 70f
suggest that warmer min-T guidance the way to go. Also cant rule out
patchy fog as we/ll be quite satd.
Short term /Sunday 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 Friday/...
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 /18z Saturday through Thursday/...
MVFR ceilings still holding on...as moisture trapped below 800 mb invrsn. Am
still hopeful that restrictions will clear by middle afternoon.
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.
Monday...targeted IFR possible at times with scattered thunderstorms and rain. Covering should be
less than sun.
southerly flow across the marine area through the weekend. With diurnal mixing...
incrsd spds in the aftn-eve. Small Craft Advisory in effect lt taftn-tonight for Bay
Sandy pt-Smith pt due to potl of 20 knots 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.
tidal departures jumped up yesterday aftn-evng. Departures have been
fairly steady between a 1/2 feet and 1 feet above normal. Winds should
remain southerly...so anticipate addtl increases through the weekend. The
next high tide /ovngt into Sun morning/ will need to be watched for
possible minor flooding. Guidance right now making for a close call at
Annapolis...but suspect its overdone by a few inches. Nonetheless...
this will be an issue through the weekend.
District of Columbia...none.
Marine...Small Craft Advisory until 4 am EDT Sunday for anz532>534.