Severe Thunderstorm Warning 261, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
250 PM EDT Fri Jun 28 2019

.SYNOPSIS...
High pressure will move offshore through this weekend. A cold
front will push south through the area Sunday before high
pressure returns for Monday. A weak front may approach the area
during the middle of the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A cluster of cumulus was capable of developing before noon in
the northern Shenandoah Valley. In spite of this, convection
overall this afternoon has been struggling locally. A band of
thunderstorms has been prevalent from ROA-AKQ, where instability
stronger and lapse rates steeper. That said, instability
parameters still respectable, especially near/south of DC-
Woodstock, and CAMs still suggesting the scattered thunderstorm
development anticipated mid-late afternoon. If that does
transpire, flow minimal so storms should be capable of
substantive vertical development. Thus, the threats remain the
same as analyzed this morning...for localized wet microbursts
and isolated flooding from slow moving storms. Did trim PoPs
slightly, placing the emphasis on Virginia. Since there is a
diurnal component to this activity, have a downward trend to
precip after dark, ending by midnight.

Persistence will be the temperature forecast for tonight with
patchy fog and lows in the 60s to lower 70s.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
Flow will become westerly Saturday as high pressure moves
offshore and a backdoor cold front drops across New
York/Pennsylvania. While the local area appears capped at
first, dropping heights, better shear, available instability,
and a focusing mechanism will provide the necessary ingredients
for a more organized convective threat to the north of
Maryland. The risk locally would be timing this activity as it
drops south of the Mason-Dixon line. The SPC Day 2 outlook has
upgraded the Baltimore- Washington area to Slight Risk, which
appears warranted based on the magnitude of the variables stated
above. Have forecast isolated PoPs during the afternoon (as
anything could develop given the inherent heat/instability), but
then increasing precip chances through the evening as the front
drops south.

Without widespread convection/cloud cover, believe we will have
a better chance at daytime heating, and have raised high
temperatures by a degree...which would place highs in the mid
90s. However, with dewpoints in the mid 60s, heat index values
would still remain under Advisory criteria.

As low pressure deepens over New England, the cold front will
sink south of the area Sunday. Features are subtle, but it
appears as though enough dry air (dewpoints lower 60s vs mid
60s) will become entrained to limit precip potential. Will back
forecast off to slight chance thunderstorms in the afternoon,
with dry conditions north of Martinsburg-Washington DC.
Temperatures will also be a category lower too...with highs
around 90F.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
High pressure will briefly build over our region Monday through
Tuesday afternoon. Winds will become light mainly out of the north.
Temperatures will be cooler on Monday in the mid to upper 80s and
will trend upwards on Tuesday into the upper 80s into the mid 90s.

A backdoor cold front will drop down into our region sometime
Tuesday into Wednesday and is forecast to stall near our region
through Friday. This boundary could combine with warm and moist
conditions to provide enough instability to kick off storms and
showers Tuesday through Thursday. The exact position of this
boundary will determine where the optimal areas for showers and
thunderstorms to form. There is a general warming trend through this
period which suggest we could have another period of 90 degree plus
weather.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
Scattered storm development will be possible this afternoon
again, with reduced visibility in heavy rain and gusty/erratic
outflow. TAFs still include VCTS mention, as low
coverage/confidence in specific preclude anything higher.
Patchy fog could occur again tonight, especially in climo
favored area. Have a few hours of MVFR fog at CHO/MRB. Will need
to see afternoon/evening convective trends to determine anything
more specific.

Thunderstorm development may be more limited Saturday daytime,
though one still can't be ruled out. Early evening may be the
favored time for a storm based on whatever drops south from
Pennsylvania. With the front pushing south on Sunday, the best
chance for an isolated storm will be near CHO. Otherwise VFR
conditions will prevail through the period.

Skies are expected to remain mostly clear Monday and Tuesday with
light winds. Some showers and thunderstorms will be potential late
Tuesday. VFR conditions expected at this time.

&&

.MARINE...
Light prevailing winds will continue with potential for onshore
breezes this afternoon. A few thunderstorms with gusty outflow
may also near the waters by late afternoon/early evening.

By Saturday, westerlies will increase flow still appears too
light to support Small Craft Advisory conditions. There will
also be thunderstorm potential...more likely in the evening vs.
afternoon, although cannot completely rule out an afternoon
storm given inherent heat/instability. A cold front will push
through Sunday and stronger northwest flow may require an
advisory for portions of the waters.

Winds will be light on Monday over the waters but a brief period of
18 kt plus gust will be possible early on Monday. Stronger
winds will be possible Tuesday afternoon as mixing taps into
stronger winds aloft. Small Craft Advisories may be needed
Monday morning and again on Tuesday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

SYNOPSIS...ADS
NEAR TERM...HTS
SHORT TERM...HTS
LONG TERM...JMG
AVIATION...HTS/JMG
MARINE...HTS/JMG

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
513 PM EDT FRI JUN 28 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0505 PM     TSTM WND DMG     5 SSE GREENFIELD        37.86N  78.81W
06/28/2019                   NELSON             VA   TRAINED SPOTTER

            TREES DOWN ON OLD ROBERT'S ROAD.

&&

EVENT NUMBER LWX1904872

DH

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