Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 240751
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
351 AM EDT Tue Apr 24 2018

.SYNOPSIS...
Low pressure to our south will impact the weather today. The low
will slowly move up the Mid-Atlantic coast tonight before
passing by to our east Wednesday. A cold front will pass through
Wednesday evening and high pressure will return for Thursday.
Low pressure may impact the area overnight Thursday into Friday
before a cold front passes through Saturday. High pressure will
return by the end of the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Cutoff upper-level low pressure remains over the Tennessee
Valley this morning and the surface low associated with this
system extends from the Tennessee Valley into the southeastern
CONUS. Surface high pressure remains off the New England Coast
this morning and an easterly flow has developed in between the
high off the New England coast and surface low well to our
south.

A deep moisture fetch from the Gulf Stream is advecting into
this system. The warm and moist air from the Gulf Stream will
overrun surface cooler air resulting in a soaking rain for the
entire area. The tricky part will be the timing of the rainfall
arrival. Rain is expected to fill in early this morning across
the Potomac Highlands into central Virginia. A soaking rain is
expected for the morning rush across these areas. This band of
rain will gradually spread north and east...most likely making
it into the Washington Metropolitan area late this morning into
early this afternoon, then towards Baltimore by late this
afternoon. The heaviest rain is expected across central Virginia
into the Potomac Highlands where it will be raining all day.
Temp/dewpoint depressions early this morning remain pretty large
so evaporative cooling is expected once rain begins. Therefore,
ran a non-diurnal trend for the hourly temperatures and it will
turn out to be quite chilly for this time of year with temps
holding in the 40s across the mountains to the 50s across most
other locations. A few areas in northeastern Maryland will
approach 60 where it will take longer for the rain to move in.

The cutoff low will pass by to the south this evening before
getting picked up by the jetstream and eventually moving up the
Mid-Atlantic Coast overnight. Our area will remain on the cool
side of the boundary, and with more overrunning in place this
will allow for a soaking rain for much of the night. Rainfall
rates will subside a bit as the dry slot works its way in aloft.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
The surface low will continue to move up the Mid-Atlantic Coast
Wednesday while the upper-level low associated with this system
passes by to our south. A cold front will also pass through the
area Wednesday evening. Unsettled conditions are expected with a
few showers likely Wednesday due to the low passing by combined
with the approaching cold front. However, it does not appear
that Wednesday will be a washout since there will be breaks in
the precipitation. There may even be some breaks of sunshine,
but cloud should hang around for much of the day. Wednesday will
turn out a bit milder ahead of the cold front with max temps in
the 60s for most areas.

Drier air will move in behind the cold front Wednesday night due
to a northwest flow. High pressure will return for Thursday,
bringing some sunshine and dry conditions along with seasonable
temperatures.

Low pressure may impact the area overnight Thursday with rain
possible, but confidence remains low at this time and large
divergence among guidance as to how strong the low will be and
how far north it will track.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
An area of low pressure should move across our southeastern zones
Friday and bring a chance for rain showers to the region. This
chance of rain showers could extend back as far as I-81. There
remains some uncertainty on the track of the low and the
strength of the low between the models. For now, we will keep
the chance of rain showers. Temperatures should remain near to
slightly below average, and could vary greatly depending on the
track of the low.

A weak cold front Saturday will move across our region before
midday. A few rain showers cannot be ruled out along and ahead of
this front. Temperatures will be close to average for this time
in April.

High pressure builds in on Sunday and Monday. Dry conditions and
warmer temperatures will ensue each day.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
Rain will overspread the terminals from southwest to northeast
this morning into this afternoon. Along with the rain will be
lowering cigs/vsbys. Timing is a bit uncertain, but it looks
like MVFR conditions will overspread the terminals late this
morning into this afternoon and IFR conditions will overspread
the terminals tonight. However, conditions are expected to
deteriorate a little quicker for KCHO where IFR conditions are
possible this afternoon. Also, periods of IFR vsbys are possible
in bands of moderate rain, especially this afternoon.

Low clouds along with rain and drizzle will hang around for
tonight with IFR conditions expected. Conditions will gradually
improve Wednesday as low pressure passes by to our east, but
there will be a few showers around. Drier air and a northwest
flow are expected Wednesday night through Thursday with VFR
conditions. Low pressure may impact the terminals overnight
Thursday but confidence remains low.

Sub-VFR conditions possible Friday with a nearby low pressure
system. VFR conditions at all terminals Friday night through
Saturday night. Winds southwest around 5 knots Friday and Friday
night. Winds becoming northwest 5 to 10 knots Saturday and Saturday
night.

&&

.MARINE...
An easterly flow will gradually strengthen this morning as low
pressure approaches from the south. The low will move up the
Mid-Atlantic Coast late tonight and pass by to our east
Wednesday. The gradient will subside during this time. A
northwest flow is expected on the back side of the low Wednesday
night into Thursday. A Small Craft Advisory is in effect for the
waters today into this evening, and for the Bay and lower Tidal
Potomac River into late tonight. Will continue with a lull in
the winds for Wednesday as the low passes by to the east, then a
Small Craft Advisory will likely be needed Wednesday night and
Thursday for portions of the waters.

Low pressure may impact the waters later Thursday night but
confidence remains low at this time.

No marine hazards expected Friday through Saturday night. Winds
southwest 5 to 10 knots Friday and Friday night. Winds becoming
northwest 10 knots Saturday and Saturday night.

&&

.HYDROLOGY...
Widespread rainfall amounts from 1 to 2 inches are forecast
through Wednesday, with locally higher amounts likely along the
central Blue Ridge and adjacent foothills to the east. Due to
relatively dry antecedent conditions and extended time period
over which the rain falls, widespread hydrological concerns are
not anticipated. However, with increasing southeast flow,
localized amounts of up to 3" are possible, again mainly near
the higher elevations of the Blue Ridge and near-by highlands
which could lead to some minor hydrology issues.

&&

.TIDES/COASTAL FLOODING...
An easterly flow will strengthen today as low pressure
approaches from the south. The easterly flow will continue
tonight before becoming light Wednesday and turning northwest
Wednesday night.

Tidal anomalies will continue to increase today. Minor flooding
is possible with the high tide cycle this morning into this
afternoon, but confidence is too low for any headlines at this
time. In fact, most of the guidance keeps water levels below
minor flooding thresholds since the easterly flow will just be
increasing during this time. However, there is a better chance
for minor tidal flooding with the high tide cycle tonight into
Wednesday morning, despite it being the lower of the two
astronomical norms. Water levels should subside later Wednesday
night with the northwest flow.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 11 AM this morning to 4 AM EDT
     Wednesday for ANZ530-531-538>540-542.
     Small Craft Advisory from 7 AM this morning to 4 AM EDT
     Wednesday for ANZ532>534-537-541-543.
     Small Craft Advisory from 11 AM this morning to 10 PM EDT this
     evening for ANZ535-536.

&&

$$
SYNOPSIS...BJL
NEAR TERM...BJL
SHORT TERM...BJL
LONG TERM...KLW
AVIATION...BJL/KLW
MARINE...BJL/KLW
HYDROLOGY...MM/HTS
TIDES/COASTAL FLOODING...BJL



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