Flash Flood Watch 2, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
941 AM EDT Fri Jun 16 2017

.SYNOPSIS...
A nearly stationary front will meander across the region through
tonight before lifting back northeast of the area as a warm
front over the weekend. A cold front is forecast to cross the
area Monday into early Tuesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
A nearly stationary remains draped across eastern West Virginia
then along the Blue Ridge Mountains. An onshore flow has caused
low clouds north and east of the boundary...but more breaks in
the clouds are occurring to the south and west of the boundary.

The boundary will slowly lift north and east as a warm front
this afternoon and evening. A southerly flow behind the boundary
will continue to pump in plenty of moisture overhead. There will
be enough breaks of sunshine for instability to develop. Latest
modified sounding from KIAD shows around 1000-1500 MLCAPE
developing this afternoon. Popup showers and thunderstorms are
likely to develop. The best chance for heavier showers and
thunderstorms will be across the Potomac Highlands and
Shenandoah Valley early this afternoon...then across the
Washington and Baltimore Metropolitan areas later this afternoon
and evening.

Shear profiles remain minimal so this does reduce the threat for
severe thunderstorms. However...taller updrafts could produce a
wet microburst so isolated severe storms cannot be ruled out.
That is represented by the Marginal Risk in the SPC outlook
today. In addition, these thunderstorms will be heavy rainers
with precipitable water values approaching 2 inches. Localized
flash flooding is possible...but due to relatively dry
antecedent conditions a Watch has not been issued at this time.

While there should be a decrease in thunderstorm coverage
by/after midnight tonight, there will be enough upglide and
vorticity advection to support a chance of overnight showers.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
Each day this weekend will be a little bit warmer as the front
retreats. Thus, it will reason that instability will increase as
well, which will support daily chances at diurnal showers and
thunderstorms, with scattered to numerous coverage. Shear
remains weak, but MLCAPE jumps up to perhaps 2000 j/kg per SREF
mean. Locally heavy rain will also be an issue, as precipitable
waters will remain around 2 inches. The cumulative effect may
become problematic, depending upon daily rainfall patterns.

Will also need to be mindful of early morning fog potential,
with ample low level moisture, dewpoints near 70 degrees, and
little flow.

By Sunday night flow turns more south/southwest as a cold front
approaches the forecast area. Forcing will become more organized
at that time.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
Large scale trough will be moving across the Great Lks early next
week supporting a cdfnt moving through the area Mon evening. Ahead
of cdfnt, a plume of very deep moisture (PWATs aoa 2.0) inches will
advect in with widespread showers and t-storms expected Mon some
capable of producing very heavy rainfall in a short period of
time. In addition to the deep moisture, there will be strong
upper level divergence on right entrance region of 85+kt jet
streak over southwest PA and good sfc convergence along front
that will encourage heavy precip. Flash flooding will be a
concern given anomalous deep moisture and strong lift, although
expected individual storm cell motion of 25-30 kt could offset
that somewhat from an otherwise favorable environment for flash
flooding.

Cdfnt will clear the fcst by 12Z Tue with significant drying
expected through Wed. Moisture begins to return again for the second
half of next week with sct afternoon/evening showers/t-storms
returning.

&&

.AVIATION /14Z FRIDAY THROUGH TUESDAY/...
Low clouds continue this morning across the terminals. MVFR to
IFR conditions will continue through early this afternoon. Cigs
should gradually improve later this afternoon. Confidence in
timing of improving CIGS remains low.

Do believe that there will be a better coverage of thunderstorms
this afternoon and evening. As is usually the case, point
forecasting more problematic, but am comfortable enough to
include VCTS for late day/early evening.

The pattern will be similar through the weekend, with
widespread flight restrictions due to low clouds and/or fog
early in the day, followed by scattered late day/early evening
thunderstorms.

Potential for IFR conditions Monday in heavy showers and
t-storms. Conditions improve late Mon night after frontal
passage.

&&

.MARINE...
Gradient flow should be 10 kt or less through Saturday, with
east winds gradually becoming south. The gradient will increase
Saturday night and Sunday, with Small Craft Advisory potential
increasing. That said, there will be daily chances at
thunderstorms, especially late day and evening...any of which
may contain locally gusty winds.

SCA conditions likely Monday and Mon night. Winds begin to
diminish Tuesday.

&&

.TIDES/COASTAL FLOODING...
Water level departures running around a foot above normal this
morning.

Believe departures should be gradually increasing though the
weekend, although guidance not exactly supporting that
assertion. That places the latest forecast at the upper end of
the guidance spread. Even with that, there should be no
additional issues-- aside from Straits Point-- until the
Saturday tide cycle.

Have extended the Advisory for St Marys County through Saturday
morning. Caution stages at high tide abound elsewhere.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Coastal Flood Advisory until noon EDT Saturday for MDZ017.
VA...None.
WV...None.
MARINE...None.

&&

SYNOPSIS...HTS
NEAR TERM...BJL/HTS
SHORT TERM...HTS
LONG TERM...LFR
AVIATION...BJL/LFR/HTS
MARINE...BJL/LFR/HTS
TIDES/COASTAL FLOODING...BJL/HTS

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