Area Forecast Discussion
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FXUS61 KLWX 271838
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
238 PM EDT Tue Sep 27 2016
Weak high pressure will build overhead this evening bringing dry
and seasonable conditions before moisture increases again late
tonight. A strong upper-level low will then impact the area
Wednesday afternoon through Thursday night...bringing the
potential for heavy rain. The upper- level low will slowly move
away from the area Friday through the weekend.
.NEAR TERM /THROUGH TONIGHT/...
Ridging will build to the north of the region through tonight and
will lead to a dry and seasonable evening across much of the area.
With front just off to the south and east, clouds will struggle to
clear from southern Maryland and central Virginia, and can`t even
rule out a stray shower.
Overnight, will see northeasterly winds develop at the surface as
high pressure builds over New England. At the same time will see
south/southeasterly flow increase at 850 mb and southwesterly
above 800 mb. This will help to usher in additional moisture to
the region. Later tonight and towards morning will likely see the
development of a low stratus deck across central Virginia,
shifting northward towards the DC Metro. Could also be some patchy
fog and a few showers as well. Lows tonight will range from the
upper 40s in the far west where skies remain mostly clear to the
low 60s south/east.
.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
A strong cutoff upper-level low will drop south from the Great
Lakes into the Ohio Valley Wednesday and stall through the end of
the week. This will act to pump sub-tropical moisture into the
region on strong south/southeast flow from Wednesday through
Friday. At the same time, low level boundary that passed through
today will gradually lift northward through the region, and this
will provide for added forcing/focus for development of heavy
showers through the period.
Showers and possibly a few thunderstorms will develop Wednesday,
with coverage increasing through the day. May see enough
instability develop in southern Maryland and central Virginia that
we could see a strong storm or two. SPC has placed a Marginal
Risk for severe thunderstorms to the south. However, even with
relatively dry antecedent conditions, main concern will be
development of heavy rain.
The timeframe for the greatest risk of flash flooding will occur
Wednesday night through Thursday night. Will see low level
boundary shift northward and as it does so, precipitable values
surge to 1.75" across portions of the region, which is
significantly above the 90th percentile of around 1.5" for the
date. Warm cloud depths rise to well above 10k feet as well,
indicative of efficient warm rain processes. Divergence aloft from
the upper low sitting to the west will further enhance the
prospect for the development of heavy rain. Current indications
are for the greatest risks for flash flooding will occur across
northern/central Maryland and back across into northern
Virginia and the panhandle of West Virginia and western Maryland
as upslope component likely develops on the strong south/southeast
flow. Current forecast will show a relatively large area of 2-5",
with locally higher amounts possible in any training convection
that develops. WPC has placed portions of the region in a
Moderate Risk for excessive rainfall. Will continue mention of
possible flash flooding risk in HWO for now. The lowest threat of
heavy rain currently exists across our southwestern zones.
There may also be a secondary risk for a few stronger storms on
Thursday as well with strong shear profiles and marginal CAPE.
Showers then gradually taper off towards Friday. High temperatures
through the period generally 70-75F, with lows in the 50s/60s.
.LONG TERM /FRIDAY THROUGH MONDAY/...
Upper level low will be lingering to our west on Friday and start
moving north towards the Great Lakes Saturday. Unsettled weather
conditions are possible Friday into Saturday as moisture is
advected from the Atlantic towards our area. As the system moves
north, conditions should be clearing Saturday into Sunday from
south/southeast to north of our CWA. High pressure builds in on
Sunday into Tuesday. High temperatures will be in the upper 60s
.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
Mainly VFR conditions are expected through this evening and the
first half of tonight as high pressure builds in to the north.
Some lingering MVFR ceilings at CHO should scatter out shortly.
Overnight, low stratus deck expected to develop over central
Virginia and move northward. May also see some patchy fog and even
a shower. Greatest chance for MVFR/IFR conditions at CHO, with
MVFR possibly into IAD/DCA by morning.
An upper-level low will then impact the terminals Wednesday
through Thursday night. A mix of flight categories expected
Wednesday in areas of low clouds and developing showers. More
widespread showers and possible thunderstorms are expected to
impact the terminals Wednesday night through Thursday night with
widespread sub-VFR conditions likely.
Sub-VFR conditions possible Friday and maybe into Saturday as
upper level disturbance lingers to our west on Friday and then
moves away from us on Saturday. Rain showers possible Friday and
Saturday due to this disturbance. Conditions becoming VFR sometime
on Saturday and into Monday as high pressure builds into our area.
High pressure will briefly build overhead following the frontal
passage this morning. Sub-SCA winds expected to continue through
tonight and much of Wednesday. Low pressure will then impact the
waters later Wednesday through Thursday night. Widespread showers
and isolated thunderstorms are expected during this time which
will cause locally lower visibilities. A small craft advisory may
be needed for portions of the waters Wednesday night through
Small craft advisory possible Friday with winds near the
threshold. Winds will be diminishing later on Friday as they fall
below 15 kt.
No coastal flooding concerns through this evening. Annapolis may
approach minor flood stage early Wednesday morning, but is
currently forecast to remain below.
Onshore flow then develops on Wednesday and persists through the
end of the week, leading to increasing anomalies and tidal flooding
threat from Wednesday afternoon through at least Friday.
Baltimore, Annapolis, Solomons Island, and Straits Point have the
greatest risk of flooding. All other sites possible as well and
will continue to be monitored.