Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 261433 AAA
AFDLWX

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
1033 AM EDT Thu Apr 26 2018

.SYNOPSIS...
High pressure will build overhead today. Low pressure will
approach from the south tonight before passing through the area
Friday. Another cold front will cross the region late Saturday.
High pressure will build across the region Saturday night and
hold through the middle of next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Weak high pressure will be over the area today, but mid-high
level clouds will be on the increase throughout the day. Rain is
expected to move in during the evening across the sw part of the
fcst area and then spready quickly northeast overnight.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
The low will track from the southeastern CONUS north and east
into the the Mid-Atlantic tonight. Clouds will continue to lower
and thicken this evening ahead of the approaching low. The low
will tap into some southern stream moisture, and warm/moist air
will overrun the relatively cooler air near the surface. The
overrunning will also be supported by decent frontogenetical
forcing both at the lower and mid-levels of the atmosphere due
to the closed off upper-level low and the surface low. The
decent forcing along with deeper moisture will cause rain to
overspread the area quickly from southwest to northeast
overnight. A soaking rain is expected late tonight into Friday
morning. In fact, with the forcing and deep moisture (pwats
around 1 to 1.25 east of the Blue Ridge is close to the 90th
percentile for this time of year), a period of moderate to
locally heavy rain is possible. Rainfall amounts for most areas
are most likely to be around one-half inch to one inch, with
lesser amounts west of Interstate 81, and perhaps a little more
(1-1.5 inches) across portions of central Virginia where forcing
is expected to be strongest. Most of the rain is expected to
fall within a three to six hour period, and latest 3hr Flood
Guidance is around 1.5 to 2 inches for most areas (1 to 1.5 in
urban areas), so current thinking is that the flood threat is
low at this time but not able to be completely ruled out.

The low will move away from the area later Friday morning
through Friday afternoon. An upper-level trough axis will remain
overhead, so a couple showers are possible Friday afternoon but
much of the time will likely be dry since most of the cold air
aloft associated with the upper-level trough will remain to our
north and west.

A cold front will approach from the northwest Friday night, but
our area will remain mostly dry ahead of the cold front. A light
gradient combined with a saturated ground may cause areas of fog
to develop overnight. Min temps will be in the 40s for most
areas, but near 50 in Washington and Baltimore.

A cold front along with along with a potent upper-level trough
axis will pass through the area Saturday. A few showers are
possible, especially late Saturday morning through early
Saturday evening ahead of the cold front. Shower coverage is
expected to be scattered so it will not be a washout, in fact
most of the time should be dry for any one particular location.
Having that been said, there is plenty of cold air aloft, so an
isolated thunderstorm cannot be ruled out across eastern areas
later Saturday afternoon. Latest nam bufkit shows around 100-200
j/kg of MLCAPE across these areas. Will continue with the
slight chance for thunder in the Washington and Baltimore
Metropolitan areas.

Canadian high pressure will build overhead Saturday night
bringing chilly conditions to the area, but it will be dry.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
High pressure will build into the region Sunday through Monday
night. Cooler temperatures and dry conditions expected Sunday
and Sunday night. Slightly milder temperatures anticipated for
Monday and Monday night.

As the high shifts to the East Coast then offshore Tuesday
through Wednesday, dry conditions will continue through the
period. Warm temperatures expected due to a developing and
persistent return flow Tuesday through Wednesday.

&&

.AVIATION /15Z THURSDAY THROUGH MONDAY/...
High pressure will build over the terminals today. Northwest
winds gusting around 15 to 20 knots this morning will gradually
diminish this afternoon. Low pressure will approach the
terminals tonight before passing through Friday. A period of
moderate rain is expected late tonight into Friday morning.
SubVFR cigs/vsbys are expected during this time and IFR
conditions are possible. Low clouds may hang around into Friday
afternoon before drier air slowly works its way in behind the
departing low.

A cold front will pass through the terminals Saturday. Scattered
showers and perhaps even a rumble of thunder are possible late
Saturday morning and afternoon. Northwest winds will increase
Saturday night and Sunday behind the cold front with gusts
around 20 knots likely. High pressure will build overhead Sunday
night through Monday night.

&&

.MARINE...
High pressure will build toward the waters today. A northwest
flow will continue through this morning and a Small Craft
Advisory is in effect. The winds will weaken this afternoon as
the high settles overhead. Low pressure will approach the waters
tonight before passing through Friday. The gradient should be
weak enough for winds to remain below SCA criteria, but it will
be close for middle portions of the Bay and the lower Tidal
Potomac River. A period of moderate rain is expected well after
midnight tonight through Friday morning as the low passes
through.

A cold front will pass through the waters late Saturday
afternoon into Saturday evening. A few showers and perhaps even
an isolated thunderstorm are possible ahead of the frontal
passage. Winds aloft will be light, but a deep mixing layer is
expected and if heavier showers or isolated thunder develop, an
inverted V in the forecast soundings suggests that locally gusty
winds cannot be ruled out in addition to the possible of small
hail (from cold air aloft). High pressure will build in behind
the cold front Saturday night through early next week. A Small
Craft Advisory may be needed for portions of the waters Saturday
night through Sunday night.

&&

.TIDES/COASTAL FLOODING...
Anomalies will continue to fall today due to a northwest flow.
Low pressure will approach from the south tonight before passing
through Friday. An onshore flow is expected ahead of the low.
This will cause anomalies to increase a bit but latest thinking
is that the flow will be light so water levels should remain
below minor flooding thresholds. However, it will be close for
sensitive areas.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 11 AM EDT this morning for
     ANZ530>543.

&&

$$
SYNOPSIS...BJL
NEAR TERM...LFR
SHORT TERM...BJL
LONG TERM...KLW
AVIATION...BJL/KLW
MARINE...BJL/KLW
TIDES/COASTAL FLOODING...BJL


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