Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 191439
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1039 AM EDT Fri May 19 2017

.SYNOPSIS...
A cold front will approach from the north today before passing
through tonight. The boundary will stall out to the south later
Saturday through Saturday night. The boundary will return north
as a warm front into our area later Sunday and Sunday night
before a stronger cold front passes through from the west
Monday. High pressure will briefly return for Tuesday before
another cold front impacts the area Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Mainly clear skies are allowing temperatures to warm quickly
into the 80s. Around 90 to lower 90s seems attainable for much
of the area, so have made an upward adjustment to the MaxT
forecast. It still looks like thunderstorm development mainly
holds until after 2 PM for much of the area. Modified 12Z IAD
sounding plus model forecasts indicate plenty of instability, so
convective forecast as described below seems on track. However,
feel like coverage may be pretty decent especially near and south
of DC, so have bumped up POPs. There may be a some storms which
drop south from PA as well which initiate along the actual cold
front. Clusters and storm mergers will pose the severe threat,
as deep layer shear is only forecast to be around 20 kt.
Convective evolution (how long the threat for showers/storms
linger) tonight remains highly uncertain, and is likely
dependent on what happens over the Ohio Valley today.

Previous discussion:

High pressure will remain near Bermuda today while a cold front
approaches from the north. An upper-level ridge will remain to
our south today and weak shortwave energy will approach the area
in the westerly flow aloft. More heat and humidity is expected
today due to the flow around the Bermuda high. Max temps will be
in the upper 80s to lower 90s for most areas. Did lean toward
the warmer guidance due to a downsloping westerly flow that is
anticipated.

The heat and humidity will lead to an unstable atmosphere once
again. A pressure trough is expected to develop
overhead...likely acting as the lifting mechanism to trigger
isolated to scattered thunderstorms this afternoon into this
evening. Despite weak shear profiles...moderate to high
instability is expected with over 2000 j/kg expected near and
south of the pressure trough. Steep low-level lapse rates are
expected and mid-level dry air in the soundings should lower
Min Theta E values...causing an elevated threat for locally
damaging wind gusts in stronger thunderstorms that do develop.
Latest forecast soundings show a bulk of CAPE in the mid and
upper-levels of the atmosphere which suggests large hail is
possible in stronger thunderstorms as well. Best chance for
strong to severe thunderstorms should be along and south of the
pressure trough which will setup near Interstate 66 in northern
Virginia into southern Maryland. Farther north...thunderstorms
are possible as well but a downsloping westerly flow may limit
coverage.

The cold front will pass through our area tonight from north to
south. A north to northeast flow behind the cold front will
usher in noticeably cooler conditions by daybreak. Min temps
will be in the 50s to lower 60s across most locations...perhaps
middle 60s in central Virginia. Isolated to scattered
thunderstorms should diminish in intensity this evening due to
the loss of daytime heating. However...isolated to scattered
coverage of showers may continue through tonight due to the cold
front passing through.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
The boundary will move to the south Saturday before stalling out
over North Carolina into the Appalachian Mountains Saturday
night. An onshore flow behind the boundary will usher in much
cooler conditions compared to recent days. Any leftover showers
early Saturday morning should dissipate and much of the day will
turn out dry. The one exception may be near the Allegheny
Highlands. Thunderstorms are likely to develop in the warm
sector west of the Appalachians. This activity may propagate
into the mountains before dissipating over the cooler air. Max
temps Saturday will be in the upper 60s to lower 70s for most
locations...with middle 70s possible across central Virginia
valleys of the Potomac Highlands. Current thinking is that there
will be breaks of sunshine Saturday...but should low clouds
from marine air develop quicker than anticipated then high temps
will be several degrees cooler.

More easterly winds and cool conditions are expected Saturday
night through Sunday. The boundary will return north as a warm
front Sunday. However...it should remain mainly to our south and
west across central Virginia into the Appalachian Mountains.
Overrunning ahead of the warm front along with more marine air
should cause more low clouds later Saturday night through
Sunday. Max temps Sunday will range from the upper 60s in
northern and eastern Maryland to the lower and middle 70s across
most other locations.

A cold front will move into the Ohio VAlley Sunday night.
Increasing southerly flow will allow for deeper moisture to move
into the area. Shortwave energy ahead of the boundary along with
a strengthening low-level jet will likely cause showers to move
into the area from west to east. Min temps will hold in the 60s
across most areas due to the southerly flow. Also included the
slight chance for thunderstorms due to some elevated instability
in the forecast soundings.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
Showers will linger into Monday as the cold front crosses the
area. A few thunderstorms will be possible too based on
CAPE/shear profiles, but significant instability likely won`t be
able to be generated due to saturation.

Anticipate there will be a quiet stretch from Monday night
through Tuesday night as high pressure crosses the northeast
and Mid Atlantic. That said, the return of warm advection on
Tuesday suggests that clouds will be making an early resurgence,
so the first part of the week won`t be all that sunny.

A deeply amplified northern stream trough axis will be pivoting
across the CONUS Wednesday into Thursday. The local impact will be
for moisture to be drawn off the Gulf of Mexico ahead of a cold
front Wednesday. While PoPs will reside in the database both
Wednesday and Thursday, the best chance (and higher PoPs) will
be on Wednesday.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
VFR conditions are expected through tonight for most of the
time. However...scattered to perhaps numerous thunderstorms are
expected this afternoon and evening. Some thunderstorms will be
capable of producing locally damaging wind gusts and large
hail. Confidence in severe thunderstorms is low but the best
chance appears to be near KIAD..KDCA and KCHO closer to a
pressure trough. A cold front will pass through the terminals
later tonight and northeast winds are expected behind the
boundary.

Onshore flow will continue Saturday through Sunday. The marine
air will increase the chances for low clouds...especially later
Saturday night and Sunday. More low clouds are likely Sunday
night along with increasing chances for showers ahead of a cold
front.

Flight restrictions will be likely Monday as a cold front
crosses the terminals. Its too soon to detail when/where/how
low, but an improving trend should take place during the
afternoon/evening hours. VFR likely will prevail Monday night
into Tuesday.

&&

.MARINE...
Bermuda high will continue to influence the weather pattern
through today. A southwest flow will continue through this
morning. A pressure trough will develop across northern
Virginia into southern Maryland later this morning through this
afternoon. Southwest winds are expected south of the
trough...but winds will shift to the west or even slightly north
of west across the Northern Chesapeake Bay and upper Tidal
Potomac River north of the trough. Across these areas...winds
will approach SCA criteria later this morning through this
afternoon but average winds through the mixed layer indicate
gusts should be limited.

A cold front will pass through the waters later tonight. The
pressure surge with the cold front will cause north to northeast
winds behind the boundary to gust around 20 knots. A Small Craft
Advisory is in effect for the waters later tonight into Saturday
morning. An onshore flow will continue through Sunday before
turning southerly Sunday night ahead of an approaching cold
front. A Small Craft Advisory may be needed for portions of the
waters Sunday night.

Behind a cold front Monday will be the best chance to see Small
Craft gusts in the extended part of the forecast. High pressure
builds Monday night and Tuesday.

&&

.CLIMATE...
BWI broke a record high minimum temperature for Thursday...May
18th. The high minimum temperature was 70 degrees which broke
the old record of 68 degrees in 1900.

More unseasonably hot conditions are expected today. Record
heat is not anticipated for BWI or DCA today...but it is
possible for KIAD. Here is a list of record high temperatures
and record high minimum temperatures for today...May 19th.

Friday (05/19) Record Warm Lows:
DCA: 72 in 2015
BWI: 75 in 1877
IAD: 66 in 2015

Friday (05/19) Record Highs:
DCA: 96 in 1997
BWI: 98 in 1962
IAD: 92 in 1997

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 11 PM this evening to 8 AM EDT
     Saturday for ANZ530-531-538-539.
     Small Craft Advisory from 1 AM to noon EDT Saturday for
     ANZ532>534-537-540>543.
     Small Craft Advisory from 1 AM to 8 AM EDT Saturday for ANZ535-
     536.

&&

$$
SYNOPSIS...BJL
NEAR TERM...BJL/ADS
SHORT TERM...BJL
LONG TERM...HTS
AVIATION...BJL/ADS/HTS
MARINE...BJL/ADS/HTS
CLIMATE...BJL



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