Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
932 am EDT Friday Aug 29 2014
high pressure builds over the northern middle-Atlantic today...then
moves offshore for the weekend. A warm front will lift north
through the area late Saturday. A cold front will approach during
the first part of next week.
Near term /until 7 PM this evening/...
surface hipres NY/PA...ridging into Maryland/Virginia this morning. In addition...ridging
building aloft /h8-5/. As a result...it/ll be awfully difficult to develop
clouds today. 12z radiosonde observation hints at perhaps a few flat cumulus in vicinity of 800 mb...and
cant rule out a streamer or two of cirrus.
Sndg based radiosonde observation maxt a pinch lower than forecast. West/ nearly full
insolation...am not going against erlr reasoning on siding warm.
Grids only a degf or two above lamp. Will adjust if ncsry.
Short term /7 PM this evening through 6 PM Saturday/...
as high pressure moves off the coast tonight into Saturday....winds
turn to the southeast. Return flow will bolster an increase in
dewpoints/moisture during this time...resulting in increasing clouds
overnight and partly to mostly cloudy skies for Saturday.
Followed srefs for probability of precipitation...keeping County Warning Area generally dry tonight then
bringing chance probability of precipitation into the northern Virginia Piedmont and areas
along/west of the Blue Ridge. This is in decent agreement with 00z
Long term /Saturday night through Thursday/...
the calendar will read September...but the conditions will resemble
what we would typically get back in July/Aug. Caught underneath a
strengthening upper ridge...the plume of moisture and warmer air
from the south will be on the conveyor belt into early next week.
After a couple of days gradually building to close out the current
week...the heating will reach its Climax over the eastern Seaboard on
sun...as the trough responsible for this build-up finally passes by
the area. The relatively minor and low amplitude trough axis will
actually be a series of offset kinks in the upper flow that connect
from Hudson Bay to the Gulf Coast. The combination of these features
centered a couple of states to our west will force the Atlantic coast
ridge to amplify and create a more pronounced fetch of warm/humid
air from the southern Atlantic states.
The southern half of the upper ridge will be strong enough to deflect the
incoming shortwave up/over the northern side - from about the Carolinas
and northward. A much more potent trough diving down into the northern plains
will also force the trough quickly over and north of our area sun into
Monday. Instability looks to be marginal but heating should be
sufficient for sct'd thunderstorms during the latter half of the day. Shear
profile not overly favorable west/ a largely westerly unidirectional flow in
the critical boundary layer...so the strongest activity would likely
be more localized where the best parameters/forcing come together -
possibly outflow driven as well.
Elevated heat indices will also be a concern heading into early next
week especially. Dewpoints will again be hovering in the l70s -
which can add a high level of discomfort to any ambient temperature. And west/
temperatures expected to crack the 90 degree mark...heat indices will easily
reach the middle...if not u90s - both sun and Monday afternoon. Skies will
waver between partly sunny and mostly cloudy which may help a little
in terms of maximum potential heating but it will still be plenty
hot/humid for this time of year.
The upper ridge being displaced only slightly to the south as the
trough slides by will rebound back over the middle-Atlantic through the rest
of the week. Long range models consistently showing US on the northern
fringe of the hotter conditions through much of the week...W/ a
cooler/more activity weather residing just north of the Mason-Dixon line west/
one trough after another sliding by just to our north. Brief periods
of convective activity may dot the area by middle week as a cold front
straddles the region but not much in the way of an organized or
airmass modifying system expected through the current forecast period.
Aviation /13z Friday through Tuesday/...
high pressure will be in control today with few-scattered cumulus/cirrus this
afternoon/evening. Potential exists for low clouds to develop late
tonight or Saturday morning as flow becomes southeast. Confidence is
low however. Per srefs...cho may have the greatest chance but even
that is uncertain. Latest tafs have MVFR at cho...otherwise VFR.
An upper ridge will continue to build-in heat/humidity sun into
Monday...W/ a possible wave of scattered showers/thunderstorms possible sun
aftn/eve. Another lesser potential exists on Monday as well...as the
hot/humid airmass stays in place for the next several days. Only
brief and short-lived periods of precipitation possible into middle next
week...but mainly dry and mostly sunny each day.
NE flow gnly at or below 10 knots /tplm2 10-15 knots/ at this time. As hipres builds...
winds shudnt go much higher. If anything...expect a little bit of a decr
through the afternoon.
High pressure moves off the coast tonight and Saturday...resulting
in winds veering to the southeast. Speeds expected to remain below Small Craft Advisory
at this time.
S-southwesterly winds will be in place for the first half of the coming
week...as a high sits just off the Atlantic coast and steers 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
front will likely cross the waters on sun...W/ another passage
possible from a different system to our north on Wednesday.
District of Columbia...none.