Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 160807
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
407 AM 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 produce wetter than
normal conditions through Saturday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Thunderstorms have greatly diminished in coverage and intensity
during the past few hours. Some moderate rain continues to move
northeast from the northern Potomac Highlands into south-central
Pennsylvania. Other areas of rain showers are across northeast
Maryland and eastern Virginia. Expect additional light showers to
develop during the next few hours of the overnight.

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.

While it is difficult to pinpoint at this time when the heaviest
rainfall will occur, certainly the 12Z GEFS indicates this afternoon
through tonight, with up to 3/4 inch rainfall forecast in six-
hours.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
The heaviest rainfall during the middle to latter part of the week
looks like it will be Thursday afternoon and Thursday night. 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/...
Surface and upper level low will reside near the Ohio Valley region
on Saturday with likely POPs area wide thanks to deep southerly flow
and supportive PVA. Conditions look to be drier on Sunday as the
main trough axis migrates to our north and into the New England
states, resulting in a more westerly flow aloft as high pressure
builds in to the region. The drier conditions look to be short lived
as a cold front approaches from the northwest Monday and Tuesday,
bringing an increase in rain chances once again. Expect temperatures
to run near normal in the middle to upper 70s on Saturday with the
increased rain chances and cloud cover. Temperatures will return to
above normal, especially for nighttime lows, for the remainder of
the period with highs in the low to mid 80s and overnight lows well
in to the 60s.

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
Low clouds and IFR cigs are possible overnight along with more
showers. SubVFR cigs will continue through tonight 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.

Sub VFR conditions likely on Saturday with rain showers a plenty and
possible thunderstorms impact the terminals. Drier conditions return
on Sunday with brief high pressure building over the region,
bringing a return to predominate VFR VIS/CIGs. Winds through the
period will remain light and southerly at 10 knots or less.

&&

.MARINE...
Small Craft winds will drop below thresholds overnight, and remain
that way likely through at least end of week with light pressure
gradient.

Mainly sub SCA conditions forecast this weekend, with the greatest
threat to promoting adverse boating conditions would be passing
showers/thunderstorms over the waters on Saturday and Saturday
night. Drier conditions forecast on Sunday with winds remaining
below headline criteria.

&&

.HYDROLOGY...
The Flash Flood Watch was allowed to expire earlier this morning,
and the general trend has been downward in terms of rainfall and
flooding threat for most of the region. Continue to maintain Flood
Warnings across portions of central Maryland and northern Virginia
with the copious rains received late yesterday and early this
morning.

The surface boundary currently resides across the northern tier of
our CWA this morning, and shows no signs of deviating from this
position very much for the remainder of the week. 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 will act 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...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...KLW
NEAR TERM...KLW
SHORT TERM...KLW
LONG TERM...BKF
AVIATION...BKF/KLW
MARINE...BKF/KLW
HYDROLOGY...BKF/JE



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