Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 170150
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
950 PM EDT Wed May 16 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 Thursday afternoon into Friday evening. As that
stationary front slowly dissolves overhead early next week, a cold
front will advance from the northwest before again stalling over our
region during the middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Showers and a few thunderstorms have continued to move into our
area this afternoon and evening as the frontal boundary
continues settled over us. Heaviest rainfall amounts have been
observed along the boundary, which is located near portions of
North Central Maryland, Eastern West Virginia and portions of
Northern Virginia, where flood warnings are currently in
effect. Plenty of moisture observed in the 00Z sounding with a
PWAT value of 1.77 inches. The instability and stationary
boundary will allow for additional rain showers in the overnight
hours. These showers and storms will be providing more moisture
to existing soils that are already saturated, making the
antecedent conditions more favorable for additional rainfall
Thursday- early Saturday. A Flood Watch is in effect through
Friday evening.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... 12Z GFS has
extended period of accumulating rainfall through the next 72
hours for most portions of the CWA, as an example, the BUFKIT
sounding from the 12Z GFS has nearly 3.5" falling through
Saturday morning. While it is difficult to pinpoint the exact
forcing mechanism, certainly daytime heating will cause an
increase in activity in the late afternoon through early evening
hours both Thursday and Friday. Additionally, deep moisture in
the entire vertical up to the tropopause becomes apparent
Thursday afternoon and continues through Friday evening, along
with a warm-cloud depth around 11kft. Based on this, WPC, 12Z
GFS & ECMWF guidance, 12-hour flash flood guidance, and
collaboration with our neighboring WFOs, we have issued a Flood
Watch for Thursday afternoon through Friday evening.

Even though rainfall rates generally will reach 0.25-0.50" per
hour at times through this period, the chances for flooding
will increase with time as the soils continue to saturate and
run-off increases. That is, the rainfall rates we are
experiencing today may not produce flooding, but the same rates
come Friday will likely cause flooding. Flooding is even
possible beyond Friday evening, but feel that their is not
enough certainty in forecasts beyond Friday night to continue
the watch past Friday evening at this time.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Conditions look to be not as wet on Sunday as the flow turns more
southwesterly in advance of a cold front pushing down from the
northwest. Precip should lower to only scattered showers and
thunderstorms for Sun and Mon, and temps should rise back to the 80s
during the afternoon. By mid next week that advancing cold front
will stall over the region as high pressure passes east across
Canada and New England. Most of our region is expected to be north
of that stalling front, turning winds off the cooler Atlantic for
mid week.

&&

.AVIATION /01Z THURSDAY THROUGH MONDAY/...
Most terminals have IFR to MVFR CIGS at this moment. There will
be a period of lower CIGS overnight from IFR to LIFR tonight
after midnight, continuing through daybreak Thursday. Conditions
will improve a bit in the late morning, but will once again
drop to IFR again Thursday afternoon.

VFR conditions expected for most of Sunday and Monday with
only scattered showers and thunderstorms to reduce visibility
and cigs at times. Winds should be light southwest. By mid next
week a cold front will advance and then stall over the region
setting up easterly winds over our region, and potentially lower
cigs and vsbys at times as cool moist air comes in from the
Atlantic.

&&

.MARINE...
Winds continue to remain below small craft advisory criteria overnight
and through Thursday night. Showers are expected to continue
across the waters this evening and overnight. Surface dewpoints
nearing 70F will cause formation of fog over the coastal waters,
reducing visibilities below 1SM at times tonight and early
Thursday morning. Winds may increase near or above the SCA
threshold for Friday, and expected to remain below criteria the
rest of the period.

&&

.HYDROLOGY...
Our ongoing hydrologic event is in the process of taking a turn
after the significant flash flooding from Tuesday night. Radar
estimated 5 to 6.7 inches of rain in a swath from extreme
southeastern Washington County up through the city of Frederick.
The highest ground truth total we`ve had in through the mesonet
was 6.56 inches just WSW of Frederick.

That kind of extreme rainfall in a very short period of time is
becoming less of a threat as we transition to just a continuous
moisture plume from the Gulf and Atlantic. But with this continuous
plume comes steady, occasionally heavy, rain that will add up more
over time and could still cause significant flooding.

Precipitable water is forecast to remain 1.6 inches or greater
through Friday and we generally remain on the cool side of the
boundary through Saturday, leading to an overrunning feed of that
moisture into the cooler Atlantic moisture-laden airmass. It`s still
too early to specify exactly where heavy rain axes will set up, but
it`s reasonable to assume one or more will be over this service area
sometime in the next 72 hours.

An additional three to five inches of rain, with locally higher
amounts, are expected through Saturday. Minor flooding is now
expected on many of the larger streams, and we are now issuing a
Flood Watch from early Thursday afternoon through Friday evening to
cover this potential as well as potential for flooding of smaller
streams and other areas. Depending on plumes/axes of heavier
rainfall, there is definitely potential for more significant
flooding, especially during the Flood Watch period.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flood Watch from Thursday afternoon through Friday evening for
     DCZ001.
MD...Flood Watch from Thursday afternoon through Friday evening for
     MDZ005-006-011-013-014-501>508.
     Flood Watch through Friday evening for MDZ003-004.
VA...Flood Watch from Thursday afternoon through Friday evening for
     VAZ053-054-506.
     Flood Watch through Friday evening for VAZ026>031-505-507.
WV...Flood Watch from Thursday afternoon through Friday evening for
     WVZ503-504.
     Flood Watch through Friday evening for WVZ050>053-055-501-502-
     505-506.
MARINE...None.

&&

$$
SYNOPSIS...CS
NEAR TERM...Lee/IMR
SHORT TERM...Lee
LONG TERM...CS
AVIATION...Lee/CS/IMR
MARINE...Lee/CS/IMR
HYDROLOGY...JE



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