Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
946 am EDT Sat Aug 30 2014
high pressure off Cape Cod will move southeast and become a
Bermuda high for the Holiday weekend and through Tuesday. A cold
front will cross the area Tuesday night. High pressure will follow
for the second half of the week.
Near term /until 7 PM this evening/...
surface hipres off ack this morning. Southerly flow across the middle Atlantic...which
will prevail through the day. Onshore flow around the periphery of the
high has brought low deck clouds to points along/east of the blurdg.
Forecast challenge will revolve around when these clouds scattered out. Lwx 12z
radiosonde observation depicts a stable/capped atmosphere. Meanwhile...pbz radiosonde observation contains some
instability above surface invsn.
Insolation across the mountains...aided by a weak surface warm front...will be the
sources for cnvctv initiation taftn. Not too impressive...but enough
to keep 20-30 probability of precipitation for terrain circulations. Think the influence of the
warm front will be more toward a ribbon of broken clouds. In the east...that/ll
be a period of clouds replacing the current clouds. Maxt will be kept
lower...and precipitation shudnt be a concern.
Short term /7 PM this evening through Monday night/...
it/ll be a warmer and more humid night tonight as southwesterly flow becomes
more entrenched across area. Fed by this warmth/instby...thunderstorms and rain may
attempt to emerge off mountains toward evening...but will be fighting the
loss of diurnal heating. Have leading edge /schc/ to catoctins-
centrl shen valley. It should remain rain-free east of there. Have
decreasing trends aftr midngt.
Sunday features the best chance of storms over the Holiday weekend
right now. Lee trough in place...weak short wave moving in...hi temperatures
in the middle 80s to around 90...some weak jet divergence at higher
levels...all of these are ingredients for some storms. The most
disturbing parameter is the GFS forecasting precipitable water to
be over 2 inches over a wide area. While there is enough wind in
the atmosphere to keep storms moving...could be isolated areas of
flash flooding with that much moisture. Will maintain mention in
hazardous weather outlook for Sunday into Sunday night.
Monday will still be warm and humid...but precipitable waters
fall...Lee trough slackens...and short wave moves out. For these
reasons expect lesser chances of storms Monday than Sunday for any
one spot. Still scattered storms expected though.
Long term /Tuesday through Friday/...
elevated heat indices will continue to be a concern heading into
middle of next week. Dewpoints will continue hovering around the 70f
degree mark...which is plenty humid in combination west/ daily highs
poking above 90f degree. From late Monday into Tuesday...a potent and large-
scale low pressure system will swinging through the Great Lakes/upper
Midwest. The parent low will be well to the north...pulling up into
the southern Hudson Bay region/Quebec. The dynamics of this system will
head due east...straddling the US/Canada border and taking the
heaviest precipitation along west/ it - well to our north Tuesday into Wednesday.
Some residual showers hanging around the coastal zones over to the
I-95 corridor from late Monday night will be dissipating early Tuesday...as
high pressure builds in from the southern Appalachians. This high...in concert west/
the offshore and larger Bermuda high will again press the
heat/humidity up into the middle Atlantic. The relative lack of organized
forcing across the region will keep skies generally clear Tuesday which
will be little relief in terms of the hot conditions. More dry and
hot weather to carry US through the second half of the week...though
Tuesday may be the warmest day - albeit only by couple of degree. Late Tuesday
evening...the tail end of the cold front from the Great Lakes low will
likely cross the region. The front will carry some scattered showers
and thunderstorms. Good low level lapse rates during peak heating and modest
instability will make for at least a handful of strong thunderstorms...more
Zonal upper flow over the northern half of the Continental U.S. For the second half
of week will essentially hold our warm/humid airmass in place. While
the cold front kicks off precipitation a couple of states to our
south...the middle-Atlantic will be mainly dry to close out the week until
an upper shortwave arrives from the west likely next weekend.
Aviation /14z Saturday through Wednesday/...
MVFR ceilings holding on at iad /borderline...ovc030/ and cho
/solidly...ovc010-015/. Ceilings should lift with diurnal heating.
So...flgt restrictions should ease at iad at or below 16z and at or below 18z at
With increasing low level humidity...overnight fog will be an
increasing issue. Expect some MVFR fog for Dulles and areas west
targeted IFR possible at times with any thunderstorms sun-
Monday. Only brief and short-lived periods of precipitation possible Tuesday-
Wednesday...but mainly dry and mostly sunny each day.
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. In addition...potl thunderstorms and rain would provide stronger
Offshore high pressure will continue to steer warm/humid air up across
the eastern Seaboard. Only minor periods of southerly channeling and/or Small Craft Advisory
conditions possible into the middle of next week. A weak cold front
will likely cross the waters by late Tuesday/early Wednesday. A few thunderstorms
possible west/ this feature. High pressure then builds in through the rest
of the week.
tidal departures have jumped up since yesterday...and now are hovering
between a 1/2 feet and 1 feet above normal. Winds should remain southerly...so
anticipate addtl increases through the weekend. Cannot discount
possible issues at sensitive locations before the weekend is over.
District of Columbia...none.
Marine...Small Craft Advisory from 4 PM this afternoon to 4 am EDT
Sunday for anz532>534.