Area Forecast Discussion
Issued by NWS Sterling, VA

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445
FXUS61 KLWX 170840
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
340 AM EST Mon Dec 17 2018

.SYNOPSIS...
A cold front approaching from the Great Lakes will cross the Mid-
Atlantic this afternoon. High pressure will follow Tuesday through
Wednesday, before another large area of low pressure approaches from
the Tennessee Valley later Thursday and Friday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Low pressure will lift further northeastward away from the region
today, while a secondary cold front and upper level shortwave
approach and cross the region this afternoon. Mostly clear skies
exist this morning across much of the region as widespread clouds
depart to the east, although some orographic clouds are still
present near and west of the Allegheny Front, and some lower
clouds/fog have developed across portions of central VA. It is
possible that some patchy drizzle/freezing drizzle occurs in the
higher terrain along the Allegheny Front early this morning, but the
probability is low. Otherwise, dry conditions are expected today
with partly to mostly sunny skies. As a cold front approaches from
the northwest today and crosses the region later this afternoon, an
increase in westerly and then northwesterly winds is expected. Winds
will peak near or just after frontal passage with some gusts up to
30 mph in the late afternoon/early evening hours, higher across the
higher elevations. Some lower clouds may follow the front and
approach portions of MD by late in the day. A light snow shower or
flurry is also possible along the Allegheny Front this
afternoon/evening. High temperatures from the upper 40s to upper
50s, mildest across central VA.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/...
Behind the frontal passage, gusty northwest flow will continue, with
gusts to 20 mph likely continuing through tonight. Some lower cloud
cover in the northwest flow will continue to be possible from the
DC/Baltimore metro areas northwestward overnight, in addition to
upslope clouds along and west of the Allegheny Front. Lows from the
upper 20s to low 30s.

High pressure will build eastward towards the region on Tuesday with
gusty northwest flow gradually waning through the day. Otherwise
a mostly sunny day is expected with highs in the low to mid 40s.
Clear and cold conditions will follow for Tuesday night with
high pressure directly overhead. Lows in the 20s.

The high will shift offshore during the day Wednesday with southerly
flow developing. As the flow turns southwest aloft, some high clouds
will likely make their return during the day. Highs will reach the
mid to upper 40s. Clouds will increase further Wednesday night with
increasing warm air advection ahead of the next low pressure system.
Lows will be milder, in the low to mid 30s.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Thursday into Friday, a strong upper level trough will approach the
region from the Midwest. The 00Z GFS and Euro are showing differing
solutions on how this trough propagates and transforms. The GFS has
the upper level trough becoming a cut off upper level low. As this
cut off low moves eastward it tracks a surface low northward along
the west side of the Appalachians. The track the GFS has the low
taking creates a easterly flow off the ocean. This would lead
to increased moisture and clouds. The European has the upper
level trough remaining elongated and doesn`t cut the trough off
into a closed off low. As this upper level trough tracks
eastward, the surface flow becomes more south to southeasterly.
This would increase precipitable water values to above average
ranges over an inch and half. Both the GFS and Euro agree that
the precipitation may start as early as Thursday afternoon but
the main bulk of precipitation will likely occur between
Thursday late evening and Friday evening. This event looks to be
a mostly rain event with the potential for some snow on the
back end over the higher elevations. Another threat for flooding
will be possible late Thursday through Saturday especially if
the Euro solution pans out.

Saturday into Sunday, the precipitation is expected to move
northeastward out of the region. Some upslope induced rain/snow
showers will be possible Saturday morning over the higher
elevations along our western parts of our CWA. High pressure
builds in from the south through Sunday afternoon. A generally
westerly flow should help clear out the clouds behind the
frontal passage Saturday morning. Temperatures during the
daytime periods over the weekend should hover near to above
average for December. Overnight lows will be mild in the mid to
upper 30s. Sunday evening into Monday, a front may affect the
region again bringing further chances for precipitation. The
Euro has this front bringing precipitation to the region while
the GFS remains mostly dry.

&&

.AVIATION /09Z MONDAY THROUGH FRIDAY/...
VFR expected through today with gusty west to northwest winds
developing by the afternoon and continuing through the evening.
Gusts up to 24-27 knots are likely, peaking in the late afternoon
and early afternoon hours. The winds will slacken somewhat tonight,
but gusts to 20 knots are still forecast. In addition, a period of
MVFR is possible in some lower stratus at MRB.

Otherwise, VFR is expected Tuesday through Wednesday night as high
pressure builds into the region.

Thursday, clouds will start to build into the region as a east to
southeasterly flow forms. Increasing cloud cover along with
increasing chances for precipitation Thursday afternoon into
evening will lead to the possibility for sub-VFR conditions.

On Friday, rain will be likely with mostly cloudy skies. Cloud bases
and visibilities will likely drop below VFR and MVFR as rain moves
through the region.

&&

.MARINE...
Small Craft Advisory remains in effect for portions of the central
Chesapeake and lower tidal Potomac early this morning, and this
expands to cover all waters by late morning with gusts 20-25 knots
out of the west/northwest. A cold front will cross the waters during
the mid to late afternoon hours, and this will bring a shift to a
more northwest direction, as well as an increase in speed,
especially along and just after frontal passage. Will show gusts to
30-32 knots across portions of the waters this evening, and it is
possible that a short-fused Gale Warning may become necessary,
but current thinking is most gusts will fall short of Gale
criteria (34 knots). Small Craft Advisory gusts will continue
through much of Tuesday as well before subsiding by Tuesday
evening. Sub-SCA winds will then prevail under high pressure
Tuesday night through Wednesday.

On Thursday, winds will become east to southeast with increasing
cloud cover. Rain will be possible starting Thursday afternoon.
Mixing heights look to be limited on Thursday which suggest Small
Craft Advisories won`t be needed at this time.

Friday, rain will be possible with winds out of the south.
Visibilities could drop in heavy rain and strong winds aloft
indicate that a small craft advisory will likely be needed.

&&

.HYDROLOGY...
Many larger streams and rivers will continue to have flood
issues through early this week. Detailed local information and
updates can be found in the various Flood Warnings.

&&

.TIDES/COASTAL FLOODING...
Coastal flood risk continues along the Potomac at Washington due to
freshwater input, especially at Georgetown, with a crest somewhere
in the 8.5-9.0 ft range expected late this afternoon.

Freshwater flooding typically spreads out downstream. While the
Southwest Waterfront likely won`t receive as extensive of a period
of flooding, there is a risk of a cycle of moderate inundation there
too this afternoon.

The impact at Alexandria likely to be less as freshwater input
typically fans out. However, minor flooding is also possible there
this afternoon. Once confidence grows, an Advisory may be
required.

&&

.CLIMATE...
Rainfall totals continue to creep upward, with Baltimore MD and
Washington DC setting the annual record already. Here are the
current rankings for wettest year on record (through December
16th):

Washington DC area (DCA)
1. 64.22 inches (2018)
2. 61.33 inches (1889)

Weather records for the Washington DC area have been kept at
what is now Ronald Reagan Washington National Airport (DCA)
since 1945. Precipitation records observed downtown extend the
period of record back to 1871.

Baltimore MD area (BWI)
1. 68.82 inches (2018)
2. 62.66 inches (2003)

Weather records for the Baltimore MD area have been kept at
what is now Baltimore-Washington International Thurgood Marshall
Airport (BWI) since 1950. Precipitation records observed
downtown extend the period of record back to 1871.

Dulles VA area (IAD)
1. 65.67 inches (2003)
2. 64.36 inches (2018)
3. 59.05 inches (1972)

Weather records have been kept at what is now Washington Dulles
International Airport (IAD) since 1960.

NOTE: All climate data are considered preliminary until
reviewed by the National Centers for Environmental Information
(NCEI).

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Warning until 1 PM EST Tuesday for DCZ001.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 10 AM this morning to 6 PM EST
     Tuesday for ANZ530>532-538>540.
     Small Craft Advisory until 6 PM EST Tuesday for ANZ533-534-537-
     541>543.
     Small Craft Advisory from 10 AM this morning to 1 PM EST
     Tuesday for ANZ535-536.

&&

$$
SYNOPSIS...MM
NEAR TERM...MM
SHORT TERM...MM
LONG TERM...JMG
AVIATION...MM/JMG
MARINE...MM/JMG
HYDROLOGY...MM/HTS
TIDES/COASTAL FLOODING...MM/HTS
CLIMATE...WFO LWX



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