Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181426
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1026 AM EDT Thu Apr 18 2019

.SYNOPSIS...
A warm front is moving north across the region today. 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 /UNTIL 6 PM THIS EVENING/...
Easterly flow and low clouds linger across northern and eastern
areas at this time, but they are eroding from south to north as
the warm front lifts northward. Warm air advection and a weak
disturbance aloft has led to the development of a few showers
and even an isolated thunderstorm or two across portions of
eastern WV and central/western VA. These will likely weaken as
the day wears on, but showers are possible through early
afternoon especially near and west of the I-81 corridor.

As an initial area of low pressure moves northeastward through
the Great Lakes and into Ontario/Quebec today and a more potent
low pressure system organizes in the south central US, southerly
flow will increase across the region, and the stalled boundary
will lift northward as a warm front this morning. This will
place the entire region in the warm sector by the afternoon
hours.

Temperatures are taking some time to rise with the clouds, but
once they break, highs are expected to reach the upper 70s to
lower 80s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
A strong upper level trough will then dig southward over the
Mississippi Valley tonight as a surface low ejects
northeastward into the Ohio Valley. Deep south to southwest flow
will be in place over the region with increasing warmth and
moisture. Some showers are possible towards morning, but most
locations likely remain on the drier side. Lows will only fall
into the 60s.

Very active weather expected for Friday and Friday night. The
upper trough is expected to close off and track eastward through
TN/KY during the day Friday and towards WV and the Appalachians
Friday night, with elongated surface low pressure moving
through OH/KY/TN and towards the eastern Great Lakes. Aloft,
the Mid-Atlantic and southeastern states will be under strong
diffluent flow, with forcing increasing through the day and
especially in the evening and overnight as the closed low
approaches. Deep southerly flow will be in place with a
connection to the tropics, and precipitable water values in
excess of 1.5 inches. This will promote the development of
showers/thunderstorms with the potential for heavy rain and
incidents of flooding Friday and Friday night. In addition, a
strong low level wind field will aid in the development of
potentially organized convection, possibly severe. The limiting
factor will be degree of stabilization (500-900 J/KG SB/ML
CAPE), but given strong wind field (850mb winds 50-60 knots) and
strong forcing, much of the region remains in a Slight Risk for
severe thunderstorms. Highs Friday will be in the low to mid
70s, with lows Friday night in the 50s to low 60s.

The main frontal push will exit eastward on Saturday, however
the filling surface low and closed upper low will remain near
or west of the region on Saturday. This will lead to continued
chances of rain showers, and even a few thunderstorms, with the
highest probabilities west of the Blue Ridge, closer to the
upper low. Highs Saturday in the 60s to near 70F. A few
snowflakes mixing in with lingering rain showers are even
possible in the mountains Saturday night with the upper low
still overhead. Lows Saturday night generally 40-50F.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Showers may linger Sunday into Monday as an upper low slowly
pivots overhead. High pressure is expected to build near and
south of the region from the Ohio Valley for early next week.
This should lead to warming temperatures and increasing
humidity. Mainly dry weather is expected during the first half
of next week, though a cold front may approach from the north on
Wednesday.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
MVFR stratus, with pockets of IFR, is currently in place at
IAD/BWI/MTN/DCA/MRB on the cool side of the backdoor frontal
boundary. VFR will return by the early afternoon at all TAF
sites under increasing southerly flow. Additional low ceilings
will likely overspread the area later tonight, with periodic
reductions to visibilities also likely Friday and Friday night
in showers/thunderstorms. Some thunderstorms may become strong
to locally severe. In addition periodic gusty southerly winds
and low level wind shear are likely.

Some improvement likely Saturday although isolated rain showers
will still be around.

Brief sub-VFR possible in any shower activity or lower CIGs Sun
into Mon in generally W flow as an upper low moves overhead.

&&

.MARINE...
Sub-SCA conditions are currently being observed this morning
across all waters. A warm front will move across the area during
the day, with southerly flow increasing behind it this
afternoon, this evening, and tonight. Will leave start time of
SCA at noon, however, there may be mixing issues initially, with
the higher probability of SCA gusts late in the day and
evening. SCA has been extended through Friday and likely will
need extension through Friday night with strong low level flow
and pressure gradient ahead of an approaching low and cold
front. The gradient is strong enough such that gales are
possible later Friday into Friday night. Stronger gusts may
occur with thunderstorms late Friday into Friday night, perhaps
50 knots or more and Special Marine Warnings may be necessary.

Low pressure will continue to reside near the waters Saturday
and a Small Craft Advisory may still be needed for the waters.

SCA may linger into Sunday before gradient relaxes.

&&

.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. Flood watches and/or flash
flood watches may become necessary.

&&

.TIDES/COASTAL FLOODING...
Southerly flow will cause tidal anomalies to increase through
the end of the week. This will likely result in at least minor
flooding at most tide sites, with near moderate flooding not out
of the question depending on exact evolution of approaching low
pressure later this week.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 PM EDT Friday for ANZ530>543.

&&

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


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