Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 171907
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
307 PM EDT Thu May 17 2018

.SYNOPSIS...
A front will remain nearly stationary across the region through
Saturday. Ample moisture, from a non-tropical low pressure system
over the northeast Gulf of Mexico and Bermuda high pressure off of
the Southeast U.S. Coast, will bring the potential for heavy rain
and flooding through Saturday morning. A cold front will advance
from the northwest late Sunday, before becoming stationary over
the mid-Atlantic at the beginning of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Regional radar at 1830Z shows stratiform-like rainfall
continuing north of the boundary, which has been persistent in
bisecting the CWA, with convection south of this boundary. SPC
mesoanalysis has a tight gradient of SBCAPE with about 1000
J/kg extending along a line from Pt Lookout to KCHO. Lightning
networks shows activity on the stable side of this gradient, and
the 17Z HRRR layer comp reflectivity has this activity
continuing through 8 PM. Effective bulk shear is around 30KTS
over the mid-bay, with lesser shear over the land. Therefore,
not expecting any severe thunderstorms, but a stronger
thunderstorm may get going around the waters of Pt. Lookout
later this afternoon and evening. Late this afternoon and
before midnight tonight, the axis of heaviest rainfall will be
across the southern portion of the CWA in association with the
ongoing convection, which will produce 1-3 inches of rainfall
locally, especially the east slopes of the Blue Ridge and
Allegheny Mountains.

The 12Z GFS has strong omega throughout the vertical overnight,
associated with wind convergence throughout the vertical, and
warm air advection at the low levels as winds turn easterly
bringing in Atlantic moisture. The northern portion of the CWA
will see a brief respite from moderate or heavy rain until
midnight, then the areas of convection from later this evening
will move into the northern half of the CWA overnight, and be
heavy at times.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Deep moisture is still present from the surface to the
tropopause through Saturday morning, creating not much of a
change, except that the 12Z GFS showing slow tapering beginning
on Saturday into Saturday night.

During this time we will see an increase in both the local area
streams and mainstem rivers.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Lingering showers will continue on Sunday, as cold front drops
through the area from the northwest. High temperatures will be
in the low 80`s.

The dry pattern will not last too long, as a low pressure system
moving across the northeast will drag a cold front across the region
late Sunday into early Monday. This front will move through the
region by the end of Monday, but another low pressure system will
pass by closer to the region. The exact location is a it uncertain
at this point. However, it seems pretty clear that we will have
another cold front pass by in this general time frame, bringing
about another chance for rain. Based off of the latest model
guidance, these amounts do not look significant, and are generally
around a quarter inch of total precipitation. However, there will be
a chance for thunderstorms both Monday and Tuesday, so heavier
amounts may fall anywhere a thunderstorm sets up. Temperatures again
will remain in the low 80`s throughout this timeframe.

The details of Wednesday`s forecast are murky at this stage, as the
aforementioned cold front looks to stall somewhere along or south of
the area. Where this front stalls will determine if we things dry
out a little. Right now, maintaining a chance of showers and
isolated afternoon thunderstorms through Wednesday.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
All terminals will continue to see IFR conditions primarily due
to low CIGS through this evening, then becoming LIFR overnight.
KMRB and KCHO may briefly improve at times to MVFR through this
evening, but return to IFR overnight.  These conditions are
expected to continue through at least mid-morning on Friday.

Clearing skies Sunday afternoon, with another
approaching cold front late Sunday into Monday. Low ceilings
will be possible during this period. A chance for some scattered
thunderstorms on Monday as well.

&&

.MARINE...
Increasing easterly flow on Friday will bring small craft
conditions on Friday, predominantly in the northern portion of
the Chesapeake Bay, but extending to all waters during midday
Friday. Areas of fog will continue giving visibilities at or
below one mile at times, particularly overnight tonight.

Winds approaching SCA criteria will be possible on
Sunday afternoon as a cold front is exiting the region. Calmer
winds expected Monday, but the potential exists for some
thunderstorms.

&&

.HYDROLOGY...
Conditions continue to worsen as this slow-evolving hydro event
continues. A pretty widespread three inches or more of rain has
fallen across the northern half of the forecast area already, with
amounts that high more spotty to the south. (Parts of Culpeper and
Fauquier Counties haven`t even seen an inch so far.)

The evolution is so far going along similar to model projections,
with light to moderate rain across the north, and more showery and
heavy rain developing this afternoon to the south. This activity
will pivot northward, and it appears will be followed by multiple
additional rounds of precipitation.

Confidence has increased in the rainfall prolonging into Friday
night, and accordingly the Flood Watch was extended through that
time. Also, the area has been expanded to include more counties. We
still expect another 2 to 5 inches of rain between now and Saturday.

Streams (including the mainstem Potomac) are already elevated or in
flood, so flood potential is high. Forecasts now call for moderate
flooding on the mainstem rivers downstream of Shepherdstown and
minor flooding at several tributary locations. River Flood Warnings
will be in effect for these locations based on a high confidence of
at least minor flooding. It should be noted that depending on where
heavy rain axes set up during the course of the event, there could
be even more significant flooding than we are forecasting and
interests near any river should heed warnings and keep themselves
and their belongings away from the rivers.

&&

.TIDES/COASTAL FLOODING...
Fairly persistent onshore/up-the-bay winds increase the threat of
minor tidal flooding for the more sensitive locations starting
Saturday morning. Additionally, freshwater inundation will pose a
threat to the DC/Alexandria tidal zone, and a Coastal Flood Watch is
in effect from Friday through Sunday, with a focus really starting
Friday afternoon/evening, for moderate flooding or greater.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flood Watch through Saturday morning for DCZ001.
     Coastal Flood Watch from Friday morning through Sunday morning
     for DCZ001.
MD...Flood Watch through Saturday morning for MDZ003>006-011-013-
     014-016>018-501>508.
VA...Flood Watch through Saturday morning for VAZ025>031-036>040-
     050>057-501>508.
     Coastal Flood Watch from Friday morning through Sunday morning
     for VAZ054.
WV...Flood Watch through Saturday morning for WVZ050>053-055-
     501>506.
MARINE...Small Craft Advisory from 6 AM Friday to 3 AM EDT Saturday for
     ANZ530>533-538>541.
     Small Craft Advisory from 2 PM to 8 PM EDT Friday for
     ANZ534>537-542-543.

&&

$$
SYNOPSIS...Lee
NEAR TERM...Lee
SHORT TERM...Lee
LONG TERM...CJL
AVIATION...Lee/CJL
MARINE...Lee/CJL
HYDROLOGY...
TIDES/COASTAL FLOODING...



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