Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 140135

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
935 PM EDT Fri Apr 13 2018

High pressure will remain across the western Atlantic through
Saturday. A large area of low pressure over the Central Plains
will move northeastward into the eastern Great Lakes by Sunday,
dragging a powerful cold front across the Mid-Atlantic Sunday
night and early Monday. High pressure will return for the middle
portion of next week.


Gusty southwest winds will diminish overnight, but persistent
gradient should keep these winds sustained around 10 knots. The
flow will be southerly around high pressure in the western
Atlantic. Subsidence will be sufficient to keep skies nearly
clear. Lows should only drop into the 50s in most areas with a
few 60s downtown DC and Baltimore.

There will be little change in the synoptic pattern through the day
Saturday. As south winds continue to advect warm air north, 850 mb
temps may jump up to 14 C. Going on the high side of temperatures
guidance seems prudent, which means that mid 80s are within
reach. Instability should be minimal, perhaps enough for
cumulus development.


Clouds will become more prevalent Saturday night as
isentropic/upglide lift continues and a mature low pressure
system marches across the CONUS. Do not believe that there will
be enough forcing for precipitation to develop locally. If there
were QPF, would think it would be confined to the mountains
(nearer the frontal system) or along the Maryland-Pennsylvania
border (close to a backdoor front). Have a slight chance for
these locations after midnight. Lows will be unseasonably
warm...near 60 degrees.

We should continue in the warm sector Sunday, but with increased
instability. The question mark though will be the progress of the
aforementioned backdoor cold front. The recent 18z NAM continues
to push the backdoor front south and west across most of our
CWA, perhaps pulling up short of reaching Harrisonburg, VA and
Charlottesville, VA. Nonetheless, it remains the colder
solution. The 18z GFS pushes the backdoor cold front south and
west to zones nearly parallel the I-70 corridor between
Hagerstown, MD and Baltimore, MD. The previous run of the ECMWF
offers a bit of a compromise, encompassing Baltimore and
touching DC. This will have a BIG impact on high temperatures,
as evidenced by MOS. Thus, there is a larger than typical
uncertainty. Have sought middle ground, siding a pinch toward
continuity, at this point. Thus, highs near 60 for northeast
Maryland vs mid 70s for central Virginia.

The instability and presence of a boundary should be sufficient to
encourage showers to develop in the afternoon...with perhaps a few
developing prior to that. South of the cold front, believe a rumble
of thunder will be possible as well. Granted, if the NAM solution
verifies, then the character of precipitation may be more toward
light rain, drizzle, and maybe even patchy fog. Hope to iron out
those details in the cycles to come.

There are hints that the backdoor could (should?) shift northward
again as a 50 kt low level jet heads up the coast ahead of the
approaching cold front. CAPE will be meager due to the nighttime
timing, but still present. Regardless of that, there will be plenty
of shear and moisture transport. With precipitable water reaching
1.5 inches, a period of heavy rainfall likely will result. In
addition, could not rule out a well-defined axis with gusty winds (a
la QLCS environment). Will be focusing on the enhanced wording for
heavy rainfall, and keep thunder chances at slight at this point.
Certainly an active period that should be monitored.


By Monday morning, low level jet induced rainfall will be exiting to
the east. A low pressure center tracking northeast along the
advancing surface front will likely be located nearby. There`s a
possibility some destabilization occurs in the narrow window between
the moderate rainfall and the surface front and vort max aloft.
Ample deep layer shear will be present, so a squall line may develop.
Temperatures will be highly dependent on the warm and cold frontal
progression. Drying will take place by later in the afternoon,
except along the Allegheny Front, where cold/moist WNW flow will
result in upslope snow showers which will persist into Tuesday.
Light accumulations will be possible. Temperatures will return below
normal areawide Tuesday into Tuesday night as the vertically stacked
low pivots toward eastern Canada and the thermal trough passes

Shortwave ridging will quickly move toward the east coast on
Wednesday, with surface flow becoming southerly and temperatures
rising closer to normal. A low pressure system will move into the
Great Lakes Wednesday night and drag a cold front across the area
early Thursday. Moisture will be limited with this system, with only
a small chance of showers east of the mountains. The upper level
trough will linger along the east coast on Friday as surface high
pressure builds eastward from the Mississippi Valley. Dry weather
will result with slightly below normal temperatures.


VFR should prevail through Sat, with gusty winds (up to 20-25 kt) in
the afternoon.

The forecast for Sunday more in doubt, as there is a potential for a
backdoor front to drop south. If that does occur, then IFR
conditions would result. There is high uncertainty in that solution,
Nonetheless, flight restrictions likely.

Rainfall and gusty winds will become the issue Sunday night in
advance of a cold front, along with an isolated thunder concern. A
40-50 kt low level jet may also create LLWS.

An area of low pressure and associated cold front will be moving
across the area during the first part of Monday. Sub-VFR cigs are
possible before the front crosses. There also may be a line of gusty
showers (possible thunderstorms) along the front. WNW winds gusting
in excess of 25 kt will be possible behind the front. Gusty west
winds likely return for Tuesday but with VFR conditions. High
pressure brings VFR and light winds for Wednesday.


Small craft advisory conditions continue for more than half of
the Chesapeake Bay and Potomac River due to the gusty southwest
winds. Winds will diminish some overnight, but should become
gusty once again tomorrow afternoon with diurnal heating.

Winds may be affected by a backdoor front Sunday. If this were to
occur, then the winds would back to the east. High uncertainty
in this solution as there is a significant spread in guidance.

A cold front will cross the waters Sunday night into Monday morning.
Strong wind gusts could reside along the front, but full mixing is

Small Craft Advisory conditions will likely continue into Monday as
a cold front passes and turns winds to the west. A line of gusty
showers and possibly thunderstorms may develop along the front as
well. Gusty winds continue into Tuesday as the low plods to the
northeast, and SCA conditions remain possible. High pressure will
bring lighter winds Tuesday night into Wednesday.


RH is expected to increase through the day Saturday, conditions
should relax. Widespread rainfall is expected to greatly reduce
fire danger Sunday night.


A strong upper trough and attendant surface front will barrel
through the area Sunday night. PWATs ahead of the front are
progged to be around 1.5 inches, near record highs for the month
of April. Unidirectional flow and the parent upper low cutting
off suggests training of heavy rain is possible, though the
flooding threat may be mitigated by fast rain/storm motions and
antecedent dry conditions. WPC is placing the western portions
of the CWA in a slight risk for excessive rainfall, with
marginal risk further east.


Anticipate water levels to increase slightly this weekend due
to southerly flow. The water levels will increase more markedly
Sunday night in strong flow in advance of a cold front. Minor
inundations would be possible for the Sunday night and Monday
tide cycles. However, uncertainty also increases in the


MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ530>534-
     Small Craft Advisory until midnight EDT tonight for ANZ535-536-
     Small Craft Advisory from noon to 6 PM EDT Saturday for ANZ535-


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