Severe Thunderstorm Warning 179, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1005 AM EDT Mon Jul 18 2016

.SYNOPSIS...
A surface trough will remain over the region today and a cold
front will approach from the north tonight. The cold front will
slowly pass through the area Tuesday through Tuesday night. High
pressure will build overhead for Wednesday and Thursday. High
pressure over the Atlantic will control the weather pattern for
late in the week before a cold front approaches the area early
next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A line of showers and thunderstorms extends from Central NY
southwestward to the Ohio River Valley this morning. There has
been redevelopment along the front edge of this line as it moves
eastward. Across the Mid-Atlantic region...temps are climbing into
the 80s with dewpts in the low 70s leading to a muggy/humid
morning.

The line of showers and thunderstorms will continue to move
eastward this morning and move over the mountains around noon.
Marginal instability exists right downwind of the convection with
a lull across most of the Mid-Atlantic region this morning.
Thunderstorms are possible across the Potomac Highlands early this
afternoon. Uncertainity increases further east as dewpts are in
question. Model guidance has a large spread with dewpts ranging
from the 50s to the upper 60s. Most guidance has activity
diminishing as it moves into the Blue Ridge region and Northern VA
with scattered showers and embedded thunderstorms. However if
dewpts stay elevated thunderstorm coverage should be greater.
Behind intial line of showers and thunderstorms... redevelopment
is possible across WV and may move into the region late this
afternoon and into the evening. Best chance will be across the
SW...VA Highlands...Shenandoah Valley and central foothills.

Prev discussion...
Subtropical ridge will remain to our south today while low
pressure in the subtropical jet tracks through eastern Canada. The
cold front associated with the low will track through the Great
Lakes into Pennsylvania today. A southwest flow ahead of the cold
front will allow for hot and humid conditions. Max temps will top
off in the lower to middle 90s across most locations. Some areas
may even reach the upper 90s if convection holds off this
afternoon. Dewpoints across most areas should be in the mid to
upper 60s...which will cause heat indices to reach the upper 90s
to around 100...especially across central Virginia into the
Washington and Baltimore metropolitan areas.

A few showers and thunderstorms are possible...mainly this
afternoon and evening. Guidance diverges a bit on the exact
details...but an MCS over the Great Lakes and Ohio Valley early
this morning should dissipate as it moves toward our area since it
will be running in to more stable air.

However...a few showers and thunderstorms are likely to develop on
the outlow of this activity this afternoon before building to the
south and east into this evening. A surface trough may also
enhance convection a bit near the Washington and Baltimore
Metropolitan areas into central Virginia. Most guidance has MLCAPE
values around 1000 to 1500 J/KG with the higher amounts near and
east of the surface trough across the Washington and Baltimore
Metropolitan areas into southern Maryland and central Virginia.
However...shear profiles remain marginal at best and the stronger
shear profiles will be across northern Maryland and eastern West
Virginia. There is a threat for isolated severe thunderstorms
with locally damaging wind gusts being the primary threat...but
as of now the threat of widespread severe is low since the shear
and instability will be displaced. Will have to pay attention to
latest forecast because confidence is low. Timing issues of when
convection would develop on the outflow of the MCS will impact the
coverage of showers and thunderstorms this afternoon and evening.

The cold front will drop south approaching the Mason-Dixon Line by
early Tuesday morning. A few more showers and thunderstorms are
possible ahead of the boundary tonight...but coverage should
remain isolated to widely scattered due to the loss of daytime
heating. Shear profiles will strengthen overnight...and if
convection over Pennsylvania becomes organized there may be an
isolated threat for severe thunderstorms across northern Maryland
this evening. However...confidence is very low. Warm and quite humid
conditions will persist ahead of the front tonight. Min temps will
60s in the mountains to the middle 70s in Washington and
Baltimore.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
The cold front will slowly drop through the area Tuesday. In
fact...it may stall out over central Virginia and southern
Maryland for much of the afternoon. Tuesday will turn out to be
not as hot with temps closer to climo...maxing out in the upper
80s to around 90 for most locations. A few showers and
thunderstorms are possible into the evening hours...especially
across central Virginia into southern Maryland where dewpoints
will be higher closer to or just ahead of the cold front.

The cold front will continue to pass through Tuesday night. A
northerly flow behind the front will make Tuesday night quite
comfortable with dewpoints dropping into the 50s and lower 60s.
High pressure will build overhead for Wednesday through Wednesday
night...bringing dry and seasonably warm conditions along with
lower humidity.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
The return and amplification of heat and humidity will be the main
story for the end of the week and into the weekend.

Thursday will be the "coolest" and least humid of the period with
high pressure directly overhead and mostly sunny skies. Looking at
highs generally in the 80s to around 90F.

Temperatures will ramp up further Friday and Saturday under strong
upper ridge extending from the Central US. Could be a few isolated
showers or thunderstorms both Friday and Saturday that pop up in any
instability or any MCS's moving along the periphery of the ridge.
Likely looking at widespread 90s to locally near 100F possible. When
coupled with increasing humidity, heat headlines may be necessary.

Frontal system approaches from the northwest on Sunday, and this
will increase chances for some scattered showers/storms. However,
heat and humidity continue with widespread 90s for highs.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
VFR conditions are expected most of the time through through this
evening. A few showers and thunderstorms are possible this
afternoon and evening...especially between 18 and 22z. Locally
gusty winds are possible along with brief subvfr conditions.

Patchy fog is possible overnight into early Tuesday. An isolated
shower or thunderstorm cannot be ruled out ahead of the cold front
but most places will be dry. The cold front will pass through
Tuesday into Tuesday night and winds will shift to the north
behind the boundary. A few showers and thunderstorms are possible
near KCHO and perhaps toward KIAD and KDCA Tuesday into Tuesday
evening. High pressure will bring VFR conditions overnight Tuesday
through Wednesday night.

Generally VFR expected from Thursday and through the weekend. A few
isolated showers/thunderstorms are possible which may bring a brief
period of sub-IFR conditions. Patchy fog also possible during
overnights, but coverage not likely to be widespread.

&&

.MARINE...
A surface trough will remain near the waters through tonight. A
south to southwest flow is expected during this time. A small
craft advisory is in effect for this afternoon and evening and it
will be marginal. Left the SCA in effect due to a deep mixing
layer expected. A few showers and thunderstorms are possible this
afternoon and evening. Locally gusty winds are possible in any
thunderstorms that develop.

A cold front will pass through the waters Tuesday into Tuesday
night. A few showers and thunderstorms are possible ahead of the
boundary...especially across the middle portion of the Bay into
the Potomac River. Locally gusty winds are possible in any
thunderstorms that do develop. High pressure will build overhead
Wednesday into Wednesday night. A pressure surge behind the cold
front may cause winds to gust near SCA criteria overnight Tuesday
into Wednesday morning.

Mostly sub-SCA conditions expected for the end of the week and
weekend, however brief intervals of gusts up to around 20 knots
possible each night.

&&

.TIDES/COASTAL FLOODING...
Tidal anomalies are near normal early this morning. A southerly
flow will cause anomalies to increase a bit today...but the flow
should be just west of south. Therefore...minor flooding is not
anticipated at this time.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 9 PM EDT this evening for
     ANZ530>543.

&&

SYNOPSIS...BJL
NEAR TERM...BJL/HAS
SHORT TERM...BJL
LONG TERM...MM
AVIATION...BJL/MM/HAS
MARINE...BJL/MM/HAS
TIDES/COASTAL FLOODING...LWX

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