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FXUS61 KLWX 191344
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
944 AM EDT Tue Jun 19 2018

.SYNOPSIS...
A cold front dropping south out of Pennsylvania will stall over the
Mid-Atlantic during the middle part of the week. A wave of low
pressure will move along this front Wednesday into Wednesday night.
The front will sag southward toward the North Carolina border Friday
as high pressure briefly builds in from the Ohio Valley, then the
front will return northward over the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Current analysis shows a weak cold front dropping south through
Pennsylvania this morning. A pressure trough is located over
southern Maryland into central Virginia. A northwest flow has
developed behind the pressure trough and a west to southwest
flow continues ahead of the trough over southern Maryland into
central Virginia. A downsloping westerly component to the wind
has allowed for sunshine this morning, and that has overcome any
weak cold advection. Therefore, max temps have been bumped up a
few degrees. Max temps are expected to range from the upper 80s
to lower 90s in northern Maryland, the lower to middle 90s in
the Washington and Baltimore Metropolitan areas, and the middle
90s across much of central Virginia. Dewpoints remain high this
morning with lower to middle 70s across most locations. The
dewpoints may mix down a few degrees behind the pressure trough
this afternoon, but it will still feel hot and humid across the
area. Heat indices are forecast to range from the upper 80s to
lower 90s near the Mason-Dixon Line, to the upper 90s and lower
100s near Washington and Baltimore, with lower to perhaps middle
100s across central Virginia and the Virginia Piedmont. Heat
indices will be close to advisory criteria, especially near and
south of DC, but confidence is not high enough for a headline at
this time due to the westerly component to the low-level flow
and some mid-level clouds moving through this afternoon.

Scattered strong storms are expected from around midday through
mid to late afternoon across the area as the cold front sags
south out of Pennsylvania intersects with the warm, humid and
unstable airmass. Modified KLWX sounding shows around 2000 J/KG
of MLCAPE developing, with the highest amounts near and south of
the pressure trough across southern Maryland toward central
Virginia. This coupled with marginal shear may result in
isolated severe wind gusts. Veering low-level flow and enhanced
low-level shear along the boundary would suggest an isolated
tornado is possible, particularly over central Virginia where
instability should be greatest (probability would be higher with
any boundary/river breeze intersections).

There is also a heavy rain/flood threat, but it appears too isolated
for a watch at this point (see Hydrology section for more
details).

Following the passage of a cold front and its scattered showers and
thunderstorms, temperatures and dew points will be a tad lower
tonight than previous nights.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As the front stalls over central Virginia Wednesday, a wave of low
pressure developing over the Ohio Valley will move eastward.
Widespread showers and a few thunderstorms are likely. Most of the
CWA should be on the "cool" side of the boundary, with a cooler
boundary layer meaning less of a chance of severe weather (though
still possible on an isolated basis near and south of the front).
Heavy rain will be a threat with high moisture content through the
atmosphere. Average rainfall amounts of a half to one inch are
possible.

The front will continue slowly sagging southward in the wake of low
pressure Wednesday night through Thursday night, with gradually
decreasing chances of showers and humidity levels.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
High pressure will be situated over New England on Friday with
the stalled front southwest of the area. Cooler and more stable
conditions can be expected, although moisture and overrunning
will be more prominent in southwestern portions of the CWA and
could result in showers at times. A closed mid/upper level low
in the central US will open and lift toward the Great Lakes on
Saturday. As the attendant surface cyclone takes a similar
track, the front will try to lift back to the north across the
area as a warm front. The northward progress of the front is
uncertain, and it's possible a secondary wave of low pressure
attempts to develop along it. At a minimum, rain will be
possible at times, but to the south of the front, there will be
a better chance of thunderstorms, and sufficient wind fields
could result in storm organization.

Sunday will likely be warmer as the low tracks up the St.
Lawrence Valley. However, a surface trough may linger across the
area and result in some showers and storms. The attendant cold
front will push southward late Sunday or early Monday with an
accompanying chance of showers and storms as well. The southward
progress of the front will determine lingering rain chances for
Monday, but it will likely be cooler.

&&

.AVIATION /14Z TUESDAY THROUGH SATURDAY/...
A cold front dropping out of Pennsylvania will induce scattered
thunderstorms mainly between 16-23Z, moving north to south. Any
thunderstorm may briefly reduce visibility to IFR. Isolated
severe wind gusts (50 kts) are possible mainly SW of a line from
MRB to IAD to DCA. Outside of any convection, winds are
expected to be NW around 10 kts.

Cloud cover should preclude widespread fog tonight, but patchy MVFR
is possible. Low-level dry advection will also tilt the scales
against fog (as well as low ceilings). Light and variable winds as
the front stalls overhead.

More widespread lower ceilings and possible visibility reductions
are likely Wednesday into Wednesday night as a wave of low pressure
moves along the stalled front, bringing numerous showers and a few
thunderstorms. VFR should return to most sites Thursday as the front
pushes farther south. S winds AOB 10 kts Wed becoming variable Wed
night as the front crosses, then N AOB 10 kts Thu.

Showers and thunderstorms will be the main concern Friday into
Saturday as a warm front slowly lifts northward. Sub-VFR clouds
may also occur before the front lifts north, particularly Friday
night.

&&

.MARINE...
SCA for northern Chesapeake Bay and upper tidal Potomac this
morning, mainly from increased gradient due to residual outflow
of overnight convection over Pennsylvania propagating south.
Otherwise, winds should stay largely below SCA levels outside of
any convection through Thursday.

Northeast to east flow Friday becoming southerly on Saturday as
a warm front lifts north. Low probability of SCA conditions at
this time. However, thunderstorms are possible mainly Saturday.

&&

.HYDROLOGY...
High PWATs (around 2 inches) are expected this afternoon ahead of a
cold front dropping southward out of Pennsylvania. Despite warm
cloud layers in excess of 12 kft and steering flow somewhat parallel
to the forcing boundary, low-level NW flow and forward storm motion
should prevent a more widespread threat of flooding. The forward
motion of the front forcing the convection may accelerate some as it
is modulated by residual cold pools from overnight convection, which
would help move storms along. We also have not had much appreciable
rain in about a week. Therefore, there is no watch at this time.
However, some training is still possible and given the conditions
listed above.

A second, more widespread and possibly persistent round of showers
and a few heavy thunderstorms is likely Wednesday into Wednesday
night as low pressure developing over the Ohio Valley moves eastward
over the Mid-Atlantic along the stalled front. High moisture
content, wetter antecedent conditions and additional training
potential would result in a higher flood potential, and may
ultimately require a watch.

&&

.CLIMATE...
Above normal temperatures are forecast today. While record highs
are unlikely, record warm lows are still possible. Below is a
list of daily record warm temperatures.

Washington DC area (DCA)
Date Record High Record Warm Low
June 19 99 (1994) 75 (2011, 1994, 1981, 1978)
Temperature records for the Washington DC area have been kept
at Ronald Reagan Washington National Airport since 1945.
Additional temperature records observed downtown date back to
1872.

Baltimore MD area (BWI)
Date Record High Record Warm Low
June 19 99 (1994) 74 (1905)
Temperature records for the Baltimore MD area have been kept at
Baltimore-Washington International Thurgood Marshall Airport
since 1950. Additional temperature records observed downtown
date back to 1872.

Dulles VA area (IAD)
Date Record High Record Warm Low
June 19 98 (1994) 71 (2014)
Temperature records for the Dulles VA area have been kept at
Washington Dulles International Airport since 1960.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until noon EDT today for ANZ530>532-535-
538>540.

&&

$$
SYNOPSIS...DHOF
NEAR TERM...BJL/DHOF
SHORT TERM...DHOF
LONG TERM...ADS
AVIATION...BJL/ADS/DHOF
MARINE...BJL/ADS/DHOF
HYDROLOGY...DFH
CLIMATE...LWX
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