Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 160057
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
857 PM EDT Tue May 15 2018

.SYNOPSIS...
A cold front will approach the waters from the northwest this
afternoon, then stall over the waters tonight. The front will
remain nearly stationary across the area through Saturday. This
front, coupled with ample moisture from the combination of a
non-tropical low pressure over the northeast Gulf of Mexico and
Bermuda high pressure off the southeast U.S. coast, will
produce wetter than normal conditions through Saturday.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Showers and thunderstorms have developed ahead of a cold front
that is located over Pennsylvania. The heaviest activity has
been across north-central Maryland into the Washington and
Baltimore Metropolitan areas. Across these areas, latest
mesoanalysis shows MLCAPE still around 2000 to 2500 J/KG along
with marginal shear around 25 to 35 knots. The instability and
slow movement of the boundary will cause showers and
thunderstorms (with very heavy rainfall) to train over the same
areas across central Maryland. As of now, the heaviest rain is
expected from near Frederick to near Baltimore, but heavy rain
is also possible across the rest of the Baltimore and
Washington Metropolitan areas, into northern Maryland and
eastern West Virginia, close to the boundary.

Localized amounts around 2 to 4 inches within an hour are
expected, with even locally higher amounts.

A Severe Thunderstorm Watch remains in effect for the Washington
and Baltimore Metropolitan areas until 11 pm. Locally damaging
wind gusts and hail are possible in any storms through late this
evening.

Showers and perhaps a few thunderstorms are likely overnight as
the boundary over Pennsylvania drops south into our area, but
the intensity of the thunderstorms should diminish as
instability wanes a little. Having that been said, locally heavy
rain is possible due to the high amounts of moisture in place.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY NIGHT/...
A several day period marked with high moisture will move into
the mid-Atlantic region by the non-tropical low pressure system
in the northeast Gulf of Mexico coupled with the Bermuda high
off the southeast U.S. Coast. This will bring an anomalous
amount of moisture into the region, with precipitable waters
approaching 2 inches. Model ensembles bring on the average 2-4
inches of rainfall, with local amounts to 6 inches through
Friday.

While it is difficult to pinpoint at this time when the
heaviest rainfall will occur, certainly the 12Z GEFS indicates
Wednesday afternoon and Thursday late afternoon/evening with
the heaviest rainfall potential, with up to 3/4 inch rainfall
forecast in six-hours. This will likely lead to the region
having an extended and increasing flood threat through Saturday,
which is highlighted in detail in the flood section of this
discussion.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Zonal H5 flow with slight high rises through the long term, with
drying conditions expected on Sunday, before moisture returns
Monday and Tuesday.

Temperatures cooler than normal on Saturday in showers, then
rising above normal on Sunday, and continuing above normal
through Tuesday, with highs in the lower 80s.

&&

.AVIATION /01Z WEDNESDAY THROUGH SUNDAY/...
Showers and thunderstorms will impact the terminals this evening.
Locally heavy rain and gusty winds are the primary threats and
vsbys will briefly be significantly reduced in any thunderstorms
that develop.

Low clouds and IFR cigs are possible overnight along with more
showers. SubVFR cigs will continue through Wednesday night with
more showers and perhaps a few thunderstorms around.

Unusually moist airmass and a boundary will remain nearby
through the weekend. Low cigs along with showers and possible
thunderstorms are expected.

&&

.MARINE...
Marine hazards will continue to be strong storms this evening,
followed by low visibilities below one mile in mist and fog.
Small Craft winds will drop below thresholds overnight, and
remain that way likely through at least end of week with light
pressure gradient.

&&

.HYDROLOGY...
After seeing marginal to low-end minor flooding with the
precipitation yesterday, and overall a swath of two to four inches
of rain from near Cumberland to DC to Prince Frederick recently,
more rain is on the way.

A Flash Flood Watch remains in effect roughly along and north of the
US 48 / I-66 / US 50 corridor from until 1 am. More convective
development this afternoon and evening has led to more flooding
and flash flooding across Maryland. Showers and thunderstorms
with locally heavy rain are expected through this evening,
especially across central Maryland. Precipitable water remains
1.5 inch or greater along with a deepening warm cloud layer.

The surface front moves back into the area overnight and then
stalls somewhere in the neighborhood for almost as far as we can
feasibly forecast. With a deep and near-persistent moisture
feed from the Gulf and Atlantic pumping moisture in through at
least Saturday, and that boundary remaining somewhere nearby,
this acts as a recipe for heavy rain potential...which is
starting to look like more a matter of when and how much, rather
than if.

At this time, widespread rains of 2 to 4 inches, with locally higher
amounts, are expected through Friday, and although things get less
certain later on in the forecast period, there`s potential for even
more rain after Friday. These amounts of rain would definitely be
sufficient (given already saturated soil and elevated streams) to
cause flooding/flash flooding almost any day through the forecast
period, with some potential for significant river flooding late in
the week through the weekend.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch until 1 AM EDT Wednesday for DCZ001.
MD...Flash Flood Watch until 1 AM EDT Wednesday for MDZ003>006-011-
     013-014-501>508.
VA...Flash Flood Watch until 1 AM EDT Wednesday for VAZ027>031-
     038>040-051>054-501-502-505>507.
WV...Flash Flood Watch until 1 AM EDT Wednesday for WVZ050>053-055-
     501>504.
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>543.

&&

$$
SYNOPSIS...Lee
NEAR TERM...BJL/Lee
SHORT TERM...Lee
LONG TERM...Lee
AVIATION...BJL/Lee
MARINE...BJL/Lee
HYDROLOGY...JCE/BJL



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