Severe Thunderstorm Warning 97, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
240 PM EDT Wed Jun 14 2017

.SYNOPSIS...
A cold front is crossing the area today. It will remain south
of the area through Friday before lifting back north of the area
as a warm front through the weekend. Another cold front is
expected to cross the area Monday.

&&

.NEAR TERM /THROUGH THURSDAY/...
Backdoor cold front continues to slowly progress across the
area. It's not the easiest to find, but seems to be stretching
somewhere near a Martinsburg to Quantico to Point Lookout line
at 1800 UTC. North and east of that, the chance for showers and
thunderstorms the rest of this afternoon is quite low (but not
necessarily zero). To the south, modified and model soundings
continue to indicate about 2000 J/kg CAPE, almost zero shear,
weak steering flow, and precipitable water values around 1.7
inches.

The more robust convection -- especially in storm mergers --
should be sufficient to produce gusty winds and perhaps some
small hail. The high moisture content and weak steering flow
also suggest heavy rain potential if areas get several of the
showers/storms in succession. Antecedent conditions are pretty
dry except in areas where it rained yesterday (Highland/
Augusta/surrounding areas). In those areas particularly,
localized flash flooding is possible.

Most of the shower/storm activity will diminish around sunset.
However, wherever the surface boundary ends up could continue to
be a focus for precipitation throughout the night, and therefore
at least a chance of showers was retained west of the Blue Ridge
all night long.

The boundary will be on the western periphery of the forecast
area on Thursday, and the onshore flow will promote some
potential for upslope precipitation on east-facing slopes.
However, that's the only source of lift so heavy rain isn't
too likely. Rain chances were retained mainly west of I-95, and
highest west of the Blue Ridge in the vicinity of the boundary.
Cannot rule out some isolated flood concerns where streams are
already elevated from today's rains, but this should not be
widespread.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
The Thursday setup stays in place at least until Friday, and
possibly through Friday night. Rain chances will spread further
east, although there is model disagreement about just how far
east and how quickly. Ultimately, I lowered PoPs in most areas
from the previous forecast due to uncertainty, but it will
likely be unsettled.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Unsettled conditions will accompany a wave of low pressure along a
front Saturday. A few thunderstorms could produce brief wind gusts.

Weak high pressure will push into the region Saturday night and
early Sunday to help give us a break from rainfall, but will be
short-lived as a cold front approaches from the west Sunday
afternoon into Sunday night.

A front could stall across the region Monday, bringing additional
showers and thunderstorms. Temperatures will be near normal.
Precipitation may actually be above normal.

High pressure will build in from the west Tuesday and Wednesday,
helping to push the stalled front to the east. Drier conditions and
seasonable temperatures expected during the period.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
Seems like even the potential of SHRA/TSRA is done at BWI/MTN
and pretty low at IAD/DCA. Most of the action will be near CHO,
where heavy rain and gusty winds could drop vsby to MVFR or even
below. Ceilings seem to be VFR even in the storms. Gusty winds
are possible in the storms but are not currently reflected in
the TAF due to uncertainty.

Cold front remains nearby, especially CHO, tonight, and there
could be a few hours of MVFR patchy fog at CHO/MRB/IAD. MVFR
ceilings also possible at CHO. Most areas should improve to VFR
on Thursday, though some sub-VFR conditions are possible mainly
at CHO.

MVFR conditions Saturday with a few showers and thunderstorms
around. Winds southeast 5 to 10 knots Saturday. VFR conditions
Saturday night and Sunday. Winds southerly becoming light and
variable Saturday night and Sunday. MVFR conditions with some
shower activity Sunday night. Winds becoming southerly 5 to 10
knots.

&&

.MARINE...
Some slight northerly channeling continues on the open waters of
the middle Chesapeake Bay, and the SCA has been extended until
5pm. Elsewhere, winds have gone light except near a couple
localized storms in the "Eastern Bay." After those winds
subside, the only remaining question will be any storms that
might produce gusty winds over the waters, mainly in the lower
bay and Tidal Potomac.

Onshore flow persists Thursday and model guidance shows wind
gusts to 15 knots are possible. That's close to Small Craft
Advisory criteria so it will need to be watched. Thunderstorm
chances return to the waters Friday as the frontal boundary
tries to lift back northward.

No marine hazards expected Saturday through Sunday night. Winds
southeast 10 knots Saturday, becoming light and variable Saturday
night and Sunday. Winds becoming southerly around 10 knots.

&&

.TIDES/COASTAL FLOODING...
Onshore flow has now begun. Tidal anomaly is currently almost
zero but will increase with the persistence of the onshore flow.
How much it increases is somewhat uncertain. But just given the
persistence, water levels are likely to approach minor flood
levels at sensitive sites (Annapolis, Straits Point, DC) with
the Friday morning/midday high tide cycle, and could remain high
for a few cycles beyond that time.

&&

.CLIMATE...
For today, the record highs were all 98 degrees set in 1994.
These are obviously safe. The BWI record high min -- 74 in 2001
-- is still in jeopardy of being tied today (current low for
today is 74). DCA and IAD have already been cooler than the
record high minimum values this morning.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for
  ANZ533-534.

&&

SYNOPSIS...JE
NEAR TERM...JE
SHORT TERM...JE
LONG TERM...KLW
AVIATION...JE/KLW
MARINE...JE/KLW
TIDES/COASTAL FLOODING...JE
CLIMATE...JE

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