Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 181858
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
258 PM EDT Thu Apr 18 2019

.SYNOPSIS...
A warm front will be north of the region tonight. Strong low
pressure and its associated cold front will impact the region
Friday and Friday night, with an upper level low affecting the
area through Sunday. High pressure will return on Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Warm front has mostly cleared the region, and temperatures are
rising rapidly as the sun has broken through. Showers associated
with a weak disturbance have also now dissipated. Dry weather
should prevail for the balance of tonight, but after skies
become mostly clear for a time later this afternoon and evening,
they will start to cloud back up as the powerful system to the
west begins to encroach on the region. Southerly flow will also
increase as it approaches, and winds may gust 20-25 mph much of
the night. Between the wind and the clouds, temperatures will
not drop a whole lot, with much of the forecast area staying in
the 60s through the night.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Potent storm system will affect the region for week`s end and
the start of the weekend. Early Friday, a very sharp upper
trough will be over the Mississippi Valley, with a surface low
over the Ohio Valley. The warm front will be up in New York
while a cold front will extend south across the southern
Appalachians. Through the day Friday and night Friday night, the
sharp trough will cut-off into a closed low to our west, futher
increasing difluence aloft as it moves towards us. The surface
low will continue to strengthen as well as it moves towards
western Virginia and eastern West Virginia. Very strong low
level jet ahead of the cold front will bring very high moisture
(PW`s near 2 inches) into the region, and limited insolation
should provide some CAPE, perhaps as much as 1000-1500 J/kg.
Shear will be very high, especially low-level shear, thanks to
the powerful low level jet reaching 60-70 knots at 850 mb. Thus,
once we destabilize during late morning, showers and
thunderstorms are expected to blossom across the region ahead of
the main cold front. There may be a lull later in the day before
another round of storms associated more closely with the cold
front itself crosses the region during the night. Timing is
still a little uncertain, but confidence has increased overall.
With such a strong windfield aloft, the main threat is damaging
winds, which could easily be mixed down. However, the low level
shear and helicity is more than strong enough to produce at
least an isolated tornado threat, limited more by the CAPE than
anything else. Hail threat seems minimal given the warm
atmosphere and relatively weak CAPE in the hail growth zone, but
some cannot be ruled out. Even outside of storms, strong
southerly flow will bring gusts of 30, perhaps even 40 mph,
especially near the open waters of the bay on Friday and Friday
night.

Highs on Friday will be limited by the clouds and storms, but
70s look likely east of the mountains. Temperatures will fall
after the primary front crosses the region late Friday night,
with 50s east of the mountains and 40s in the mountains.

Precipitation wanes quickly early Saturday, and along the I-95
corridor, most of Saturday looks fairly dry and still on the
mild side, with limited cold advection. Further west, showers
will linger with the upper low nearby and late in the day and at
night, they will spread further east as the upper trough itself
also moves eastward. There is just enough cold air such that
snow levels may drop to the higher ridges, so have a little bit
of snow on the highest peaks, but not expecting anything in the
valleys. Lows will be in the 30s in the mountains, 40s further
east.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Closed off upper-level low will pass through Sunday before
moving off to the northeast Sunday night. Plenty of cold air
aloft associated with the upper-level low along with some wrap-
around moisture may trigger showers Sunday into Sunday evening.
However, precipitation amounts should be light due to limited
instability. Small hail cannot be ruled out either due to the
cold air aloft, but severe weather is not expected. Unseasonably
chilly conditions are expected as well with max temps holding
in the 50s and 60s for most areas.

High pressure is likely to briefly build overhead Sunday night
into Monday before settling to the south later Monday. A return
flow will allow for a milder afternoon Monday.

A backdoor cold front will likely move into the area Tuesday
and it is expected to stall out near the area for the middle
portion of next week. Unsettled conditions are possible during
this time.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
VFR at present, but its possible some low clouds may redevelop
late tonight. Main aviation hazard, however, will be reduced
cigs and vis with heavy showers and thunderstorms Friday and
Friday night, mainly in the afternoon and evening. IFR
conditions possible in storms, but strong winds are also a
threat, perhaps gusts of 50 knots in storms. Conditions improve
by early Saturday morning and should stay VFR most of Saturday
and Saturday night outside of any stray showers.

VFR conditions are expected most of the time Sunday through
Tuesday, but a few showers Sunday into Sunday evening may cause
periods of MVFR conditions.

&&

.MARINE...
SCA conditions starting to overspread the waters from south to
north and will likely cover all waters by this evening and
continue through Friday night at least. Gales are possible on
the waters at times Friday/Friday night, but confidence not high
given the stabilizing influence of the cool waters at this time
of year. Gusts of 50 knots are possible in thunderstorms
tomorrow afternoon and evening, so special marine warnings may
be needed regardless. Winds relax some behind the front on
Saturday, but SCA may still be needed.

Low pressure will gradually move off to the east Sunday and
high pressure will briefly build overhead Sunday night into
Monday before settling to the south Tuesday. A Small Craft
Advisory may be needed for portions of the waters during this
time.

&&

.HYDROLOGY...
Soils remain moist across the region with water levels remaining
at or above normal. With strong southerly fetch bringing very
high precipitable water values (in excess of 1.5 inches) and
strong diffluence expected with a warm atmosphere, high
intensity rainfall is expected late Friday/Friday night, with
potential prolonged moderate/heavy rain if training of rain
bands occurs. Therefore, flooding, both flash and river-type, is
possible Friday into the weekend. Latest model guidance has
become a bit more progressive, resulting in somewhat lower QPF.
That said, any training storms or locations which get multiple
rounds will still be at risk of flooding. Thus, have issued a
flash flood watch for Friday afternoon and night for eastern
portions of the CWA where multiple rounds appear most likely to
occur. This may need expansion further south and west by later
shifts, but confidence is not yet high enough to do so at this
time.

&&

.TIDES/COASTAL FLOODING...
Water level anomalies will be on the increase through Friday as
a southerly flow increases. The southerly flow will continue
through Saturday before slowly weakening and turning west of
south later Saturday night and Sunday. Anomalies will be well
above normal during this time, and minor flooding is expected
along with the possibility of moderate flooding. The highest
water levels are expected to be near high tide Friday afternoon
into Saturday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch from Friday afternoon through late Friday
     night for DCZ001.
MD...Flash Flood Watch from Friday afternoon through late Friday
     night for MDZ003>006-011-013-014-016-503>508.
VA...Flash Flood Watch from Friday afternoon through late Friday
     night for VAZ031-052>055-501-502-505-506.
WV...Flash Flood Watch from Friday afternoon through late Friday
     night for WVZ052-053.
MARINE...Small Craft Advisory until 6 PM EDT Friday for ANZ530>543.

&&

$$
SYNOPSIS...BJL/RCM
NEAR TERM...RCM
SHORT TERM...RCM
LONG TERM...BJL
AVIATION...BJL/RCM
MARINE...BJL/RCM
HYDROLOGY...RCM
TIDES/COASTAL FLOODING...BJL


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