Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 200812
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
312 AM EST Mon Nov 20 2017

.SYNOPSIS...
High pressure will build to our south through tonight. The high
will move offshore Tuesday and a cold front will pass through
Tuesday night into Wednesday. High pressure will build back
overhead later Wednesday through Thanksgiving Day. The high will
build to our south for Friday and Saturday before a cold front
moves through Saturday night.

&&

.NEAR TERM /THROUGH TONIGHT/...
Upper-level trough overhead early this morning is beginning to
shift to our east while surface high pressure builds to the
south. Rising heights will continue to cause any stratocu along
the mountains and northern portions of the CWA to dissipate
throughout this morning. Plenty of sunshine is expected this
afternoon and it will turn out a bit milder this afternoon...but
temps will still be below average for this time of year. Max
temps will range from the upper 30s along the ridges of the
Allegheny Highlands...to the mid and upper 40s in northern
Maryland...to the middle 50s in central Virginia.

The high will remain to the south tonight. A weak shortwave in
a southwest flow aloft may bring a few high clouds...but dry
conditions will persist. Min temps will range from the mid and
upper 20s in the colder valleys to the mid and upper 30s in
downtown Washington and Baltimore.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
High pressure will shift offshore Tuesday while a cold front
moves through the Great Lakes. A southwest flow in between these
systems will allow for milder conditions. Max temps will be in
the upper 50s to lower 60s across most locations. There will be
some high clouds in a southwest flow aloft...but the cloud deck
should be thin enough for a partly to mostly sunny sky much of
the day.

The cold front will approach our area from the north and west
Tuesday evening before passing through overnight Tuesday into
early Wednesday morning. There will be separation between the
lift associated with the cold front and deeper moisture to our
south and east. Latest guidance shows that some of this moisture
can get drawn up into the front...bringing the chance for
showers. The best chance for showers will be across eastern
areas where moisture has a better chance to move in before the
frontal passage.

Any showers early Wednesday morning should dissipate as the cold
front moves off to the south and east. A northwest flow will
cause chillier conditions despite increasing sunshine. High
pressure will settle overhead and it will turn out dry and cold.

Did also want to note that a few snow showers are possible along
and west of the Allegheny Front Tuesday night into early
Wednesday. However...as discussed earlier moisture should be
limited so any accumulation would be light.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Longwave western US ridging and east coast troughing will
continue to dominate the weather pattern over the United
States through the holiday weekend. While no significant storm
systems are currently forecast, several upper level impulses
will move through the large-scale troughing.

Sprawling surface high pressure is expected to be in place on
Thanksgiving Day from the southern Plains extending northeastward
through the Appalachians. At the same time, weak shortwave
energy in both the northern and southern branches of the jet
stream will be moving towards the east coast and may lead to
surface cyclogensis off the southeastern US coastline. At this
time, the surface ridging and flow aloft is expected to keep
anything that can develop well to our south and east, leading to
a dry and uneventful weather forecast for Thanksgiving Day.
Surface ridging is expected to persist into Friday as well,
keeping the forecast dry.

By Friday night and Saturday, a more potent northern stream
system will move into the upper Midwest, with its associated
cold front barrelling towards the Mid-Atlantic Saturday night.
Once again, southern stream energy will be present as well near
the southeastern states, and the potential exists for surface
cyclogenesis to occur. At this time, fast flow aloft and
orientation of upper level trough suggests that a track well
offshore is favored. However, chances for rain showers increase,
either with the offshore low or with the cold front itself.

Following the front Sunday, blustery and colder conditions are
likely, along with the potential for snow showers over the
higher terrain.

Temperatures will start out below normal on Thanksgiving (40s),
moderate back towards normal or even a bit above for Friday and
Saturday (50s), before falling back below for Sunday (40s).

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
West to northwest winds are gusting around 20 to 25 knots early
this morning due to cold advection and a weak pressure surge. Do
think that there can be a break in the gusts for a few hours
between about 09 and 12z as the surge weakens a bit...but
confidence is low. Winds will gradually turn to the west this
morning and southwest this afternoon. Gusts around 15 to 20
knots this morning should gradually dissipate this afternoon.

VFR conditions will persist for tonight through Tuesday.
Southwest winds Tuesday will gust around 20 knots in the
afternoon. A cold front will pass through Tuesday night into
early Wednesday. A few showers are possible along with subVFR
cigs. The best chance for this will be across the eastern
terminals. VFR conditions will return behind the cold front for
later Wednesday through Wednesday night.

VFR and light winds expected Thursday and Friday with surface high
pressure

&&

.MARINE...
West to northwest winds early this morning will gradually turn
to the west later this morning and southwest this afternoon.
Wind gusts should gradually diminish throughout the day. Will
continue with a Small Craft Advisory for the waters through
mid-morning and for the Bay and lower Tidal Potomac River
through this afternoon. The SCA may be able to be cancelled
early across the middle portions of the Bay and lower Tidal
Potomac River this afternoon.

High pressure to our south tonight will move offshore Tuesday.
South to southwest winds will increase during this time ahead of
the next cold front. A Small Craft Advisory is in effect for the
middle portion of the Bay and lower Tidal Potomac River tonight
and for all the waters Tuesday. The SCA may need to be extended
into Tuesday evening for portions of the waters.

A cold front will pass through the waters late Tuesday night
into Wednesday morning. Northwest winds will pick up behind the
front and an SCA may be needed for portions of the waters later
Wednesday into Wednesday night.

Sub-SCA conditions expected Thursday and Friday with surface
high pressure.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 PM EST this evening for
     ANZ530>534-537>543.
     Small Craft Advisory from 9 PM this evening to 6 PM EST
     Tuesday for ANZ531>534-537-539>541-543.
     Small Craft Advisory from 10 AM to 6 PM EST Tuesday for ANZ530-
     535-536-538-542.
     Small Craft Advisory until 10 AM EST this morning for ANZ535-
     536.

&&

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


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