Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 121534 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
1034 AM EST Wed Dec 12 2018

High pressure will linger over the region through Thursday
night. A large area of low pressure will impact the area at the
end of the week bringing widespread rain and potential for



Thickening mid and high level clouds this afternoon as the next
surface and upper low nears the Great Lakes and additional
shortwave energy tracks overhead. High temperatures today will
be similar to yesterday, topping out in the upper 30s to middle

Clouds continue to increase tonight as warm air advection
improves aloft and the upper low tracks across northern PA and
southern NY. The best forcing and likelihood of precipitation
will reside to our north across PA, but the increase warm air
advection may result in light wintry precipitation over western
Maryland and the Allegheny Front. Moisture again will be very
limited, with just a few hundredths expected over the upslope
areas, but light freezing drizzle/snow will be possible. Cloud
cover will maintain temperatures overnight in the upper 20s to
lower 30s for much of the area.


The upper low will slide eastward into southern New England on
Thursday with mid to upper ridging again building overhead, with
dry conditions forecast after any light precipitation comes to
an end early Thursday morning. Temperatures a touch warm on
Thursday in the low to middle 40s, with a mix of sun and clouds

Skies will trend mostly cloudy by Thursday evening as the next
system approaches the Tennessee Valley. Models continue to trend
with precipitation holding off until later Friday morning and
into the afternoon. Any precipitation that is able to make it
into the Highlands and southern Shenandoah Valley early Friday
morning could be in the form of freezing rain with cold air
stubbornly locked in at the surface. If trends continue later,
this threat may not even materialize as temperatures will favor
above freezing. As temperatures do rise above freezing Friday
morning, rain will fills in over the area from southwest to
northeast during the afternoon and early evening hours. Moderate
to heavy rain is expected to fall Friday night as low pressure
nears the Carolinas. Rainfall amounts on the order of a half
inch to one inch will be possible by daybreak Saturday. This
combined with the snow melt may result in incidents of flooding.
Additionally, with this rainfall, the yearly rainfall record
for Washington DC will likely fall Friday night, as only 0.55
inches is needed to tie the current record (see climate section
below). Highs on Friday will range in the 40s to lower 50s, with
lows Friday night seasonably warm in the upper 30s to middle


A large and complex area of low pressure will linger over the
region through the weekend before exiting by early next week.
However, we do not expect a washout, and with temperatures
rising slightly above normal, it could feel a bit more
comfortable in some regards. Early Saturday, the heaviest rain
with the initial wave of low pressure should be exiting the
region, though some may linger in the morning. After that,
however, we will remain cloudy with a risk of showers as the
upper level system will still need to clear the region. There
are indications a secondary low pressure system will develop as
the upper low approaches, which could cause showers to persist
through the day Sunday. Temperatures will remain mild, with 40s
and even some 50s common, so do not expect significant wintry
weather, though the highest elevations could see a little wet

GFS and ECMWF both show a cold front crossing southeastward
through the region on Monday as the weekend system finally moves
out. It is fairly moisture starved, so the main precip threat
would be some upslope rain/snow showers. Of note is a
significant air mass difference between the guidance, with the
ECMWF showing little cold air behind the front, while the GFS
shows a brief shot of below normal temperatures. However, even
the GFS is brief with it, and temps likely return to normal or
even slightly above normal later next week as high pressure of
Pacific origin builds across the region.


VFR conditions forecast through Thursday night as high pressure
will reside to the north and south of the terminals and
predominately dry conditions forecast. Exception would be at MRB
where light wintry precipitation will be possible tonight into
early Thursday morning, but confidence in occurrence, and flight
restrictions, is low. Sub VFR conditions become increasingly
likely Friday and Friday night as low pressure nears the
terminals from the south with widespread rain expected.

Sub-VFR conditions likely linger Saturday and perhaps even
Sunday as complex low pressure system slowly lumbers through the
region. Showers are likely and reduced cigs/vis are possible,
perhaps even IFR at times. Conditions will likely start to
improve late Sunday.


Sub SCA conditions will likely persist through much of Friday
before the gradient increases late Friday afternoon and Friday
night as low pressure nears from the south. SCA conditions
likely Friday night into Saturday as a result.

Winds will diminish substantially Saturday and Sunday despite
low pressure lingering over the region. Showers and reduced
visiblity will continue to be a concern, however.


Rainfall totals continue to creep upward, with Baltimore
setting the annual record already. Here are the current rankings
for wettest year on record (through December 11th):

Washington DC area (DCA)
1. 61.33 inches (1889)
2. 60.83 inches (2003)
3. 60.78 inches (2018)
4. 60.09 inches (1878)

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. 65.67 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. 61.30 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




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