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000
FXUS61 KLWX 191826
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
226 PM 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 into
northern Maryland this afternoon. A pressure trough is located
over southern Maryland into central Virginia. A northwest flow
behind the pressure trough has caused enough downsloping and
sunshine for more hot and humid conditions. The heat and
humidity will continue to cause heat indices around or slightly
above 100 degrees across the Virginia Piedmont into central
Virginia, until convection arrives later this afternoon.

The heat and humidity has led to an unstable atmosphere, and
latest mesoanalysis shows around 1500-2000 j/kg of mlcape ahead
of the cold front across eastern West Virginia into the
Washington Metropolitan area and southern Maryland (ahead of the
approaching cold front). The instability has combined with
forcing from the boundary to trigger scattered to numerous
showers and thunderstorms. This convection will gradually
propagate south throughout the rest of this afternoon/early
evening. Shear profiles are marginal, but with the higher
amounts of instability, inverted V soundings, and mid-level dry
air there is a threat for locally damaging wind gusts in storms
along/ahead of the boundary into early this evening. The best
chance for stronger storms will be south and west of the Potomac
River in Virginia into southern Maryland. 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).

The boundary will stall out over the Potomac Highlands into
central Virginia overnight. A few showers are possible along the
boundary, but much of the time may end up dry due to the loss of
daytime heating. Patchy fog is expected along and ahead of the
boundary as well.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Low pressure will track along the boundary, passing through our
area later Wednesday into Wednesday night. Widespread showers
and thunderstorms are likely, especially later Wednesday
afternoon into Wednesday evening. There is an isolated severe
threat for locally damaging wind gusts and large hail. Also,
moisture convergence from the low and the instability will cause
thunderstorms to produce heavy rain. Localized flash flooding is
possible as well. More details are in the hydro section below.

The boundary will drop to the south overnight Wednesday through
Thursday night. A few showers are possible near the Potomac
Highlands into the central Shenandoah Valley, but most other
locations should be dry as cooler and more stable air moves in.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
On Friday attention will turn to an upper-level low located over the
central Mississippi Valley. At the surface, an area of low pressure
will develop ahead of the upper-level low and move into the lower
Ohio River Valley. Meanwhile, high pressure will depart off the New
England coast. We`ll be located in stable air to the north of a
frontal boundary, but rain chances can`t be ruled out by late
afternoon as warm advection commences aloft. Chances for rain will
increase Friday Night as deeper warm air advection and
associated overrunning precipitation overspreads the area.

By Saturday morning, the upper-level low will shear out into more of
an open wave as it tracks toward the Lower Great Lakes. The surface
low is also forecast to track toward the lower Great Lakes. As
this low moves off to the northeast, it will move a surface warm
front northward through the area Saturday morning. As a result,
Saturday will be noticeably warmer and more humid than previous
days. Model guidance suggests that areas to the south of the
warm front will destabilize by Saturday afternoon, with MLCAPE
values reaching around or slightly above 1000 J/kg. This
instability coupled with seasonably strong flow aloft (0-6 km
shear values around 40 kt), could lead to some stronger storms
Saturday afternoon.

The area of low pressure will continue to track off to the northeast
toward the Canadian Maritimes on Sunday. The cold front
associated with this system will progress through the region
sometime during the Sunday afternoon through Monday time period,
but differences between individual models and their respective
ensembles are substantial. Rain chances can`t be ruled out on
Sunday or Monday, but the general trend with time will be for
rain chances to lessen and drier, cooler air to work into the
region in the wake of the cold front.


&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
Scattered to numerous thunderstorms are expected along and ahead
of a cold front through early this evening. The best chance for
storms in DCA and IAD will be through 20z, and for KCHO 20-22z.
Farther north, the strongest storms have ended. Storms will be
capable of producing locally damaging wind gusts and IFR/subIFR
cigs/vsbys via torrential rain.

The boundary will stall to the south of the terminals overnight
into Wednesday, before low pressure rides along the boundary
later Wednesday and Wednesday evening. Some storms may be strong
to locally severe and IFR/subIFR conditions are possible in
heavy rain. The boundary will stall to the south of the
terminals overnight Wednesday through Thursday night.

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...
An SCA is in effect for the waters this afternoon. Otherwise,
winds should stay largely below SCA levels outside of any
convection through Thursday.

Scattered to numerous thunderstorms this afternoon will be
capable of producing gusty winds over the middle portion of the
Bay and the Tidal Potomac River. Another round of strong to
locally severe storms are possible later Wednesday afternoon and
evening.

The main concern in the long term will be chances for
thunderstorms on Saturday. Otherwise, there may be a chance for low-
end SCA winds on Friday/Friday Night in easterly flow to the north
of a warm front.

&&

.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 are 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 4 PM EDT this afternoon for ANZ530-
     531-538-539.
     Small Craft Advisory until 6 PM EDT this evening for
     ANZ532>537-540>543.

&&

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