Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 171434
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1034 AM EDT Mon Jun 17 2019

.SYNOPSIS...
A front will be stalled near the area today and remain in the
mid-Atlantic region through much of the upcoming week. This will
lead to a prolonged period of unsettled weather. The front
should finally push southeast of the region Friday, with a
period of drier and less humid weather likely for week`s end and
the start of the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Made some minor tweaks to the POPs and weather with the morning
update. There will be an opportunity for showers and
thunderstorms areawide this afternoon, but focused in on higher
POPs along and east of the Blue Ridge. This is in line with
most of the CAM guidance, and conceptually makes sense given
westerly downslope flow, which could limit storm coverage a bit
immediately in the lee of the Appalachians. Think that storms
will initiate around 2 PM give or take an hour and press toward
the I-95 corridor through the mid-late afternoon. Multiple
rounds of storms are possible, and storms could linger into the
evening hours, especially south and east of I-95. This morning`s
IAD sounding shows a predominantly straight-line hodograph,
with around 40 kts of 0-6 km shear. However, there is a bit of a
weakness in the wind field above 6 km. MLCAPE is expected to
increase to 1000-2000 J/kg by later this afternoon. Given this
parameter space, the convective mode will favor multicell
clusters, along with some marginal supercell structures.
Splitting of the supercells will be possible given the straight
line hodograph. Damaging winds will be the primary threat with
these storms, but a few instances of larger hail can`t be ruled
out with the strongest storms. Flash flooding can`t be ruled
out either, with a slow moving boundary approaching the area,
and winds aloft paralleling the boundary.


Previous discussion...

Sfc front currently extends from east to west from near NYC
west across NJ and PA into OH and remains north of the PA
Turnpike. Sfc obs and MSLP fields indicate the sfc high over the
eastern Great Lks is very weak (~ 1016 mb) suggesting this
front will start slowing down significantly with little
southward progress over the next couple of days. The mid-level
flow is also becoming parallel to the sfc front which would
suggest potential for training convection. With shear and storm
motion weakening today, the risk of flash flooding will be
increasing over the next few days. Guidance indicate ample CAPE
higher than yesterday, but has trended lower in QPF likely due
to weaker forcing. Nonetheless, there is enough ingredients
conducive to flash flooding and spread in deterministic and
ensemble QPF to issue a flash flood watch. The greatest risk of
flash flooding is along the Mason Dixon line but could extend
far enough south into the highly urbanized areas.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
Nothing really is expected to change over the next 72 hrs.
Moisture and instability will be more than adequate and with
front nearby to support daily afternoon and evening risk of
convection. With the mean flow weakening, cells will be slower
to move and pose additional risk of flash flooding. Details with
summer time convection are typical uncertain especially in weak
forcing.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Active pattern will continue Thursday as a stronger wave of low
pressure moves eastward and northeastward along the stalled
front. This will initially allow the front to lift northward,
perhaps bringing some relatively quiet (albeit warmer and more
humid) weather to the region for a time Thursday before the
approaching cold front brings another round of showers and
t-storms with both a heavy rain and possibly a severe threat
late Thursday into Thursday night.

Guidance seems to be coalescing around the idea of this wave
passing faster and allowing Canadian high pressure to build in
on Friday. This would result in slight cooling and significant
humidity reduction on Friday, along with much less of a chance
of any rain.

The front doesn`t get too far south, however, and starts to make
its way back north as another disturbance looks to approach from
the west on Saturday. Right now, it still looks like most of the
forecast area will stay fairly dry, but humidity may increase
somewhat and there will likely be more risk of storms in our
southwestern zones (west-central VA and eastern WV), closer to
the front.

Front lifts back further north on Sunday as the aforementioned
wave moves through, so the risk of showers and t-storms looks to
become more widespread by then. Warm and humid weather will
otherwise prevail.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
Nmrs showers and sct t-storms this afternoon and this evening,
weakening late. Risk of storms continues through Tue. Brief IFR
conditions possible in any heavy showers/t-storms.

VFR overall Thursday and Friday, though patchy early morning fog
is possible especially at CHO/MRB, while scattered showers and
t-storms during the afternoon and evening hours (mainly
Thursday) may also result in a few instances of reduced
cigs/vis. The risk of storms looks significantly lower on
Friday.

&&

.MARINE...
Winds should remain below SCA outside of t-storms. Winds and
waves higher near thunderstorms. SMWs may be required especially
upper Potomac and north of the Bay Bridge.

Winds will be increasing ahead of an approaching cold front
Thursday, and small craft advisories may be needed. Special
Marine Warnings may also be necessary as the front brings
another round of showers and t-storms to the waters. By Friday,
the risk of showers and t-storms should decrease behind the
front, but winds may continue at small craft levels as
cooler/drier air moves in.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch from 2 PM this afternoon to 10 PM EDT this
     evening for DCZ001.
MD...Flash Flood Watch from 2 PM this afternoon to 10 PM EDT this
     evening for MDZ003>006-011-013-014-501>508.
VA...Flash Flood Watch from 2 PM this afternoon to 10 PM EDT this
     evening for VAZ028-031-053-054-505-506.
WV...Flash Flood Watch from 2 PM this afternoon to 10 PM EDT this
     evening for WVZ051>053-501-503.
MARINE...None.

&&

$$
SYNOPSIS...RCM
NEAR TERM...LFR/KJP
SHORT TERM...LFR
LONG TERM...RCM
AVIATION...RCM/LFR
MARINE...RCM/LFR



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