Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 150145

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
945 PM EDT Sat Apr 14 2018

High pressure will remain across the western Atlantic overnight.
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.


A backdoor cold front is presently draped across central and
southeast Pennsylvania, stretching west to low pressure over
Missouri. Over the past couple of hours, since early afternoon,
this backdoor cold front has been showing signs of progressing
southwestward...similar to the NAM model indication. The
observations near Philadelphia clearly reveal a northeast wind
within the last hour or so. Clouds will be increasing,
especially after midnight, but the area should remain nearly
dry. Keeping lows in the 50s, except for northeast of Baltimore.

The challenge of the backdoor front will play a big role in
Sunday`s forecast. The differences are significant enough that
for now we will take the middle road with the backdoor front`s
progression, similar to the ECMWF. Highs 70-75 in Central VA vs
50-55 northeast MD. We knocked down our high temperatures in
northeast Maryland by a couple of degrees for Sunday, for now.
Should the progression of the backdoor cold front persist, we
may be knocking high temperatures down further southwest in our
zones. Elsewhere, we are staying on the high side of guidance.

There are other ramifications too. With the continued influx of
moisture, some instability will be able to develop. The
overrunning will lead to light rain/drizzle north of the front.
Showers and perhaps a couple of thunderstorms will develop to
the south. Regardless, PoPs will be increasing through the day.
have lowered them a little, but will still have a 70-80% chance
by day`s end.


A deep trough axis will become negatively tilted as it drives
cold front through Sunday night. It still is up for debate
whether the backdoor front will head north again or not. If it
doesn`t it will provide another lifting mechanism for an
increasingly moist atmosphere, enhancing the potential for heavy
rainfall. Would like to have more confidence in this aspect of
the forecast.

Surface low pressure will be traversing the stalled frontal zone
overnight. The atmosphere south of the low will hang onto
marginal instability. Regardless, an increased wind field will
provide plenty of shear to drive a convective threat. QLCS has
been a consideration for a couple of days, and the signals
continue to point in that direction, in spite of the late hour
of frontal passage (midnight-dawn). After coordination with SPC,
the Day 2 outlook has upgraded to Slight Risk CHO-EZF and
Marginal Risk for the rest except for northern Maryland.

No changes of consequence made to the Monday forecast. We should
be drying out through the day, although the presence of a
trough axis suggests it won`t be outright dry. Then colder air
returns Monday night, with maybe some upslope snow showers in
the mountains.


A closed upper level low will continue to spin near the
Ontario/Quebec border on Tuesday. A chilly west-northwest flow will
remain in place, and a few upslope snow showers will remain possible
until drier air arrives later in the day. As surface ridging builds
in from the south Tuesday night, some localized frost/freeze
concerns could arise (given the growing season has started now).

Shortwave ridging aloft 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 later on Wednesday and eventually drag a cold front
across the area by Thursday. There is still some model spread on the
timing and location of this system. However, overall consensus is
that moisture will be limited with this system. Rain chances, at
least east of the Appalachains, will remain low. There will be a
closed upper low associated with this system too, which will be slow
to dislodge from New England. So while dry weather is likely Friday
and Saturday (outside of another possible round of upslope snow
showers), temperatures will remain below normal.


VFR conditions will continue overnight. There is a chance that
low clouds will sneak down the coast with a backdoor cold front
toward dawn. Although some guidance is suggesting aob IFR, there
is low confidence in this outcome, and have capped ceilings at
1000 ft.

The backdoor will continue south invof MTN/BWI and maybe MRB. In
the most aggressive solution, if could make it to DCA/IAD, but
don`t believe that is likely. Regardless, south winds will turn
southeast or east, with 10-15 kt winds perhaps gusting to 20 kt
in the afternoon. Ceilings will slowly rise, but MVFR should
prevail. There is an increasing chance at either light
rain/drizzle or showers as the day progresses.

Greater uncertainty exists Sunday night. However, this much is
clear: a strong cold front will move west to east after
midnight, driving a swath of heavy rain and perhaps strong
gusty winds. This activity should clear the terminals prior to
the Monday morning push. Brief IFR plausible within this period.

VFR conditions will return Monday and are expected Tuesday, but
gusty west winds in excess of 20 kt will remain possible as low
pressure lingers to our north. High pressure to our south will
bring lighter winds with VFR conditions Tuesday night into
Wednesday. A low pressure system passing to the north may bring
some lower clouds and light rain late Wednesday into Thursday,
with a small chance of sub-VFR conditions.


Winds will continue to diminish the rest of the overnight,
dropping below SCA criteria.

Sunday`s forecast remains tricky, as a backdoor front will drop
south across the waters-- the extent of which is still up for
debate after the 12Z forecast cycle. Kept Small Craft Advisories
for Sunday afternoon, assuming that the waters will be behind
the backdoor, when east winds will provide a push of cooler air
thus improving the mixed profile. The SCA will be extended into
Sunday evening. We will likely be issuing a Gale Warning for the
waters soon for late Sunday evening and overnight due to
increased threat for showers and thunderstorms ahead of and
along a strong cold front.

Winds drop back to SCA on Monday behind the front.

Gusty WNW winds will continue into Tuesday as the low pressure
lingers to the north, and SCA conditions will remain possible.
High pressure will bring lighter winds Tuesday night into
Wednesday. A low pressure system will pass by to the north
sometime later Wednesday into Thursday, although winds remain
uncertain at this time due to the range of possibilities in the
low`s strength and location.


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. The presence of
a stalled backdoor front stretched east-west across the area
also suggests that there will be added forcing for heavy rain.
Believe the area will be within the path of a warm conveyor,
tapping both Gulf of Mexico and Atlantic moisture. These factors
all lead to flooding concerns.

On the other hand, fast storm motions and antecedent dry
conditions may be mitigating factors. WPC placed the western
portions of the CWA in a slight risk for excessive rainfall,
with marginal risk further east. The location of the backdoor
and transport paths will be key players. Have held off on any
Watches at this time, but they may become necessary in the next
forecast cycle or two.


Water levels near astronomical normals overnight through Sunday.
Onshore flow followed by a sharp southerly push Sunday night
may lead to an influx of water. Minor flooding will be possible
beginning Monday. Its unclear at this time how may cycles the
threat will last, but it could be multiple.


MARINE...Small Craft Advisory from noon Sunday to 2 AM EDT Monday for
     Small Craft Advisory until midnight EDT tonight for ANZ532>534-


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