Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 200128 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
928 PM EDT Tue Jun 19 2018

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



Still a low chance for showers overnight mainly west of the Blue
Ridge as activity west of the Appalachians spills over the mtns
in WNW flow. Otherwise, dry with low clouds developing toward
morning. Most reliable guidance suggest stratus development
toward dawn rather than fog.

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.


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.


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.



Stratus development possible toward dawn but most guidance keep
cigs abv 3 kft.

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


Winds should stay largely below SCA levels outside of any
convection through Thursday.

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.



A 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.


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

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.




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