Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180841
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
341 AM EST Sat Nov 18 2017

.SYNOPSIS...
High pressure will move offshore today. A strong cold front
will cross the area late tonight into early Sunday. High
pressure will build to the south of the region late Sunday
through Monday before moving offshore Tuesday. Another cold
front will pass through Tuesday night into Wednesday before
stalling well to our south and east on Thanksgiving Day.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Surface high pressure located over eastern NC early this morning
will push out to sea during the day today. At the same time, a low
pressure system organizing over KS/MO will move east-
northeastward towards the eastern Great Lakes today and
through the Saint Lawrence Valley tonight, deepening to ~980 mb.

The main weather feature that will affect the region today is a
warm frontal zone associated with the aforementioned low
pressure system. Clouds and showers driven by the warm and moist
air advection are spreading eastward through OH/IN early this
morning, and while the greatest forcing will push north of the
region into PA/NY, enough will exist to bring a period of
rain/showers to the area today, mainly north of the immediate DC
metro. Temperatures have dropped below freezing overnight
across portions of WV and western MD, so can`t rule out a brief
isolated patch of light freezing rain early this morning at
onset, however coverage will be limited.

Strong southwest winds will overspread the area through the day,
and while mixing will be limited enough in the lowlands to
keep gusts below 30 mph, across the higher elevations, gusts are
expected to reach 45-50 mph by late in the day and a Wind
Advisory has been issued for these locations after 4 PM, mainly
above 2000 feet. Some isolated gusts to this magnitude may occur
earlier across the highest elevations.

Highs will occur late in the day and generally in the 50s,
although some low 60s are expected towards central VA and
southeast MD. Locations in western MD where cold air will remain
trapped, temperatures will likely hold in the 40s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/...
The surface low will be near far northern NY and the Ontario/Quebec
border by Sunday morning with the strong surface cold front
crossing our region tonight. This will bring a period of rain
showers as it moves across with gusty south/southwest winds
continuing out ahead of the front, especially across the higher
elevations where a the Wind Advisory is in effect. While
instability will very limited, with strong wind field aloft, a
shallow-topped convective fine line cannot be ruled out where
gusts may be locally higher.

Behind the front, low level temperature profiles will crash, with
precipitation changing to snow along and west of the Allegheny
Front and taking on an upslope characterization as northwest
winds increase. Wind gusts will be increasing area-wide behind
the front as well, and gusts to 50 mph are possible beginning
early Sunday morning. Therefore the Wind Advisory may need to be
expanded. Lows by morning drop into the 30s along the Allegheny
Front to the 40s and low 50s elsewhere.

The strongest winds will occur Sunday morning when the best
combination of mixing and strong wind field aloft overlap. The
winds will gradually lessen in the afternoon hours. Upslope
snow showers will continue along and west of the Allegheny Front
through the day with several inches of snow accumulation
likely. Temperatures Sunday won`t rise much following the front
so highs very similar to tonight`s lows.

A potent upper level shortwave rounding through the base of the
large scale trough will then cross the region Sunday evening. This
will not only act to re-invigorate the upslope snow showers, but
may also bring some scattered flurries and snow showers
eastward towards central and northern MD, the eastern WV
panhandle and northern Shenandoah Valley of VA. Snow showers and
gusty winds will then gradually dwindle Sunday night with lows
falling into the 20s and 30s.

By Monday high pressure will be building into the region with much
quieter weather expected. Highs in the 40s to around 50F, with
lows Monday night in the upper 20s to mid 30s.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
High pressure will move offshore Tuesday and a return southwest
flow will usher in somewhat milder conditions. A cold front will
pass through the region Tuesday night into Wednesday...possibly
triggering some showers along with more clouds.

High pressure is most likely to build into the area late
Wednesday through the end of the week...bringing dry and chilly
conditions. However...shortwave energy may interact with a
stalled boundary over the southeastern CONUS...causing low
pressure to develop. If this occurrs...it could have an impact
on our weather Thanksgiving Day into Friday with more clouds and
even possible rain. Confidence in this occurring is low at this
time since a majority of the guidance keeps the low far enough
to our south from impacting our area...but this will have to be
monitored over the next few days.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
Primarily VFR expected through today, although with thickening
and lowering clouds. A warm frontal zone will move into the
region during the day with some rain showers likely at MRB, and
possibly as far south and east as IAD/DCA/BWI/MTN. A period of
MVFR is possible at MRB with these showers.

An additional period of sub-VFR conditions is possible overnight
tonigt as a cold front moves across the region with another
period of rain showers.

However, the main aviation weather concern will be increasing
low level wind field today through Sunday. Low level wind
shear is expected this morning with 2000 foot AGL winds out of
the southwest around 40 knots. By this afternoon and tonight,
winds will begin to gust to 20-25 knots or so. After cold
frontal passage late tonight, gusty northwest winds will develop
with gusts 30-40 knots likely.

Winds will gradually lessen late Sunday into Monday as high
pressure builds in.

VFR conditions are expected Tuesday. A cold front
will pass through the terminals Tuesday night into Wednesday. A
few showers and subVFR conditions are possible during this time.


&&

.MARINE...
High pressure will move offshore Tuesday and a cold front will
pass through Tuesday night into Wednesday. A Small Craft
Advisory may be needed for portions of the waters during this
time. High pressure is most likely to return for late in the
week...but low pressure has the potential to develop over the
southeastern CONUS and it could impact our weather. Confidence
in this occurring is low at this time.

Small Craft Advisory is in effect for all waters through tonight
with increasing low level wind field. Gusts may approach gale
force later tonight associated with cold frontal passage, but
will keep SCA for now.

The strongest winds are expected to occur Sunday morning behind
the cold front, and a Gale Warning has been issued for Sunday.

Winds gradually lessen late Sunday and Sunday night, however
SCA level winds still likely, and may continue into Monday.

&&

.TIDES/COASTAL FLOODING...
Southerly winds have developed over the waters and anomalies are
increasing this morning. The southerly winds will continue to
strengthen ahead of a cold front today into tonight. Anomalies
will continue to increase during this time. Have already issued
a Coastal Flood Advisory for Anne Arundel county for the high
tide cycle late this afternoon and evening. Additional Coastal
Flood Advisories may be needed for the high tide cycle this
afternoon for Calvert and St. Marys Counties...but confidence is
too low at this time since the highest anomalies may not arrive
until after the high tide.

Coastal Flood Advisories for many locations will likely be
needed for the high tide cycle tonight as the southerly flow
continues. Moderate flooding cannot be ruled out for sensitive
areas ...but confidence was too low for a watch at this time
since it will be the lower of the two astronomical high tides.
A strong offshore flow will develop early Sunday and tidal
blowout conditions are possible later Sunday through Sunday
night.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Wind Advisory from 4 PM this afternoon to noon EST Sunday for
     MDZ501.
     Coastal Flood Advisory from 2 PM this afternoon to 9 PM EST
     this evening for MDZ014.
VA...Wind Advisory from 4 PM this afternoon to noon EST Sunday for
     VAZ025-026-503-504-507-508.
WV...Wind Advisory from 4 PM this afternoon to noon EST Sunday for
     WVZ501-503-505-506.
MARINE...Small Craft Advisory from 6 AM this morning to 6 AM EST Sunday
     for ANZ530-531-535>542.
     Gale Warning from 6 AM to 6 PM EST Sunday for ANZ530>543.
     Small Craft Advisory until 6 AM EST Sunday for ANZ532>534-543.

&&

$$
SYNOPSIS...BJL
NEAR TERM...MM
SHORT TERM...MM
LONG TERM...BJL
AVIATION...BJL/MM
MARINE...BJL/MM
TIDES/COASTAL FLOODING...BJL



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