Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 200224
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1024 PM EDT Wed Sep 19 2018

.SYNOPSIS...
High pressure will build over the region through Thursday
before migrating over the western Atlantic on Friday. A cold
front will approach from the northwest toward the end of the
week before stalling near or over the region through the
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
A secondary cold front is decaying over the Mid-Atlantic as of
early Wednesday evening. Surface high pressure is building over
the region in its place. Any convective activity has waned.

Patchy fog seems likely tonight given light winds, a mainly
clear sky and wet ground, especially west of I-95. Some fog may
be locally dense.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/...
1020PM UPDATE: made some tweaks to PoPs in the vicinity of the
central Shenandoah Valley for Thursday afternoon and evening as
it looks like some trapped moisture combined with weakly
convergent return flow should set of a few isolated to widely
scattered showers and thunderstorms.

Otherwise, high pressure will begin shifting to the east
Thursday, with light easterly or southeasterly return flow.

Low pressure will pass north of the Great Lakes on Friday,
continuing its path northeastward to close out the work week.
The associated frontal boundary will approach our region from
the northwest late Friday and into Friday night. Current
progression of the front by model guidance keeps precipitation
mostly northwest of the CWA through the day on Friday, with
coverage increasing Friday night. As such, have pushed back the
timing and have low end chance POPs Friday afternoon for only
our far northwestern zones (eastern WV/western MD). There is a
severe risk mainly north of the PA border where better
forcing/shear will be, and this risk should decrease with
southward extent towards our area as more favorable parameters
eject towards New England.

Rain chances increase further east Friday night as the front
sags southward. Temperatures Thursday through Friday will run
in the low to mid 80s for highs and lows generally in the 60s,
with near 70 degree readings Friday night ahead of the front.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
A cold front will be moving southward across the area on
Saturday. Showers and perhaps a few thunderstorms will be
possible near the boundary. It does appear the surface boundary
will move south of the area before stalling during the latter
part of the weekend, which will put the forecast area on the
stable side of the boundary. However, the frontal zone aloft
will likely remain near or perhaps even north of the area. At
the same time, surface high pressure will be building over New
England, resulting in a cold air damming pattern. Assuming the
frontal zone aloft remains nearby, moist ascent over the surface
wedge will result in plentiful clouds and the potential for
light to moderate rain at times. Temperatures (especially highs)
will likely fall below normal...and there`s potential highs
don`t get out of the 60s for some areas by Sunday and Monday. It
is worth noting that there is some potential the frontal zone
gets far enough south at some point to preclude appreciable rain
chances, but if the pattern over the past several months
indicates anything, that won`t be the case.

The frontal zone will be lifting north early next week as low
pressure moves through the upper Great Lakes. If/when we break out
of the surface wedge, it will result in a slight warming trend.
However, moist southwest flow ahead of a broad trough moving into
the Plains will continue to result in clouds and chances for showers
and some thunderstorms. The next cold front remains well to our
northwest through Wednesday, with its passage occurring after the
end of the forecast period. Given the multiple days of rain and
precipitable water rebounding to 1.5-2.0 inches by Tuesday and
Wednesday, some hydro concerns are not out of the question.

&&

.AVIATION /03Z THURSDAY THROUGH MONDAY/...
Patchy fog is likely west of I-95 tonight, could be locally
dense especially INVOF MRB/CHO. Low clouds also developing off
the New Jersey coast will advect southwest towards terminals
east of I-95 by tomorrow morning, and could bring CIGs down to
near IFR. Conditions should improve by mid morning.

Otherwise, mainly VFR conditions expected at all terminals
through Thursday night as high pressure moves overhead.

A cold front will approach the terminals late Friday and Friday
night, increasing rain chances. At this time, MRB would serve
to see the highest chance of restrictions with the remaining
terminals likely holding off until Saturday morning. Northerly
breezes today will subside tonight, veering southeasterly on
Thursday at less than 10 knots, then turning southerly and
increasing on Friday ahead of the front.

There will be a chance of showers and perhaps a thunderstorm as a
cold front drops into the area through the weekend. The frontal
zone will remain nearby (perhaps focused over central VA), and
showers may reduce visibility at times through the period.
Ceilings may eventually drop below VFR levels as well in
possible cool air wedge as high pressure builds to the north.

&&

.MARINE...
Winds have diminished. Sub SCA conditions should prevail through
Thursday night. SCA conditions may return on Friday as southerly
flow increases ahead of a cold front approaching from the Great
Lakes and Pennsylvania.

The cold front will drop across the waters on Saturday, then
stall nearby through early next week. This front will bring a
wind shift to the north, then eventually easterly. Winds may be
the strongest Monday as high pressure builds into New England.
Showers will be possible through the period, with the highest
chance of thunderstorms (though still low) on Saturday.

&&

.TIDES/COASTAL FLOODING...
Onshore flow will cause tidal anomalies to increase over the
next couple days, possibly persisting through the weekend. This
will likely lead to a renewed threat for coastal flooding,
especially given increasing astronomical tides.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...BKF
NEAR TERM...BKF/DHOF
SHORT TERM...BKF/DHOF
LONG TERM...ADS
AVIATION...ADS/BKF/DHOF
MARINE...ADS/BKF/DHOF
TIDES/COASTAL FLOODING...DHOF



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