732
FXUS61 KLWX 120120
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
920 PM EDT Thu Jul 11 2024

.SYNOPSIS...
Shower and thunderstorm chances return Friday and Saturday
especially east of the Blue Ridge with a stalled front nearby.
Weak high pressure briefly builds back into the area Sunday then
gradually slides offshore Monday into Tuesday allowing the high
heat, humidity, and daily thunderstorm chances to return.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Tropical moisture continues to increase across the area, which
made for a picturesque sunset. The IAD RAOB at 00Z showed an
increase in moisture compared to 12Z, with a PW of 1.16". Expect
this to increase through the overnight. PW values will approach
2-2.2" east of the Blue Ridge, which is near the climo max for
this time of year. There remains a good bit of spread in regard
to where the heavier QPF will fall. Regardless, the rain will be
beneficial for most given anteceding dry conditions. Previous
discussion follows...

The stalled front across the VA Tidewater slowly begins
advancing north into our area tonight into Friday morning. A few
showers push their way into southern MD by around midnight,
then coverage increases during the second half of the night. A
dramatic surge in PWATs accompanies the advancing front for
areas east of the Blue Ridge. Values go from under 1.2" this
evening to well over 2-2.2" come sunrise Friday. Widespread
showers, some producing locally heavy rainfall, move into the
I-95 corridor Friday morning. Multiple rounds of showers and
scattered thunderstorms are likely throughout the day on Friday.
A lull in activity is expected at some point Friday evening as
the front lifts north of the area, though given the very
saturated atmosphere some showers are likely to continue through
Friday night.

A tight QPF gradient is likely east of the Blue Ridge, and
especially east of I-95. The current forecast calls for half to an
inch of rain between the Blue Ridge and I-95, with 1-1.5" of rain
along the I-95 corridor, and 2-4" of rain east of I-05. The heaviest
rain is likely to occur late tonight through early Friday afternoon
in Southern MD, where localized higher amounts up to 5" are
possible. A Flood Watch may be needed for some areas east of I-95.

For those west of the Blue Ridge, the downslope nature of east to
southeast flow is going to reduce rain amounts significantly.
Overall only expecting around a tenth to a quarter of an inch of
rain reach the Shenandoah Valley, and stay mostly dry in the
Alleghenies.

Abundant cloud cover and precipitation keep highs well below normal
for most of the area Friday, only reaching the low 80s. The warmest
temps will be in the Potomac Highlands from Hagerstown to Cumberland
to Petersburg where highs reach the upper 80s.

Come Saturday the front itself washes out in vicinity of the area as
the mid-level forcing supporting the system weakens and moves east
over the Atlantic. Another wave of low pressure aloft approaches
from the west Saturday afternoon, that could fire off another round
of scattered showers/thunderstorms in the afternoon east of the Blue
Ridge. There remains a good amount of uncertainty regarding coverage
of storms Saturday afternoon as dry air filtering in from the west
could put a cap on much of the activity. Still, highs in the upper
80s to low 90s, dew points in the mid to upper 70s, and some broad
20-25 knots of shear could lead to some organized convection.
Conditions dry out Saturday night as another front that sweeps
through the area.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
The long term period will generally be unsettled, with our region
pitted between a strong upper ridge over the southern CONUS, and a
building positively-tilted trough to our northwest. This will
gradually swing through by mid-week, but the details on how strong
this trough will be and exactly where it tracks are still a bit in
question.

Overall, pretty much every day during this time period looks quite
hot, with highs well into the 90s, and even near 100 by the time we
get to Wednesday. Dew points each day will be somewhat questionable,
as model guidance suggests we get into the low to even mid 70s.
However, wind direction aloft could play a key role, and we could
mix down some lower dew points (mid-upper 60s) during the
afternoons. This would be the one thing keeping us from another
lengthy period of heat headlines, but will continue to monitor model
trends as we get closer. Either way, it looks very hot once again,
continuing an already very hot summer.

Of course, with the threat of heat comes the inevitable afternoon
thunderstorm threat. This will occur each afternoon, becoming
increasingly likely by Wednesday as the trough inches closer to the
region. Storms that develop Sunday will be very isolated and likely
driven by terrain circulations, thus would probably move quite slow.
Should a storm get locked over one location for an hour or two,
could see some isolated instances of flooding in the higher terrain.
However, the recent drought has flash flood guidance very high area-
wide. This would continue into Monday as well, though coverage could
be a bit higher.

By Tuesday and Wednesday, we start to tap a bit more wind shear,
thus there will be an increased threat for some strong to severe
storms. Looking into some model soundings, do think we will run into
a similar issue we have seen recently with a lack of substantial mid-
level lapse rates. The very warm air mass in place aloft is just
going to make it very difficult to get widespread severe convection
going in the absence of some cold advection aloft. Could see a
scenario where storms generally struggle to get developed, but the
ones that do will have plenty of DCAPE to tap into. Any storm that
manages to break through the mid-level warmth and grow a tall core
will have the potential for some damaging winds. Think the greatest
threat for this would be Wednesday into Wednesday evening, but we
are getting to the end of the forecast period at that point, so
details can certainly change this far out. Storms would also be
capable of some heavy rainfall rates, thus introducing at least
some risk for isolated instances of flooding. However, given the
recent rapid-onset drought, that would likely be relegated to the
metro areas.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
VFR conditions through most of tonight. A stalled front to our south
slowly lifts north of the area early Friday into Friday night.
Expect MVFR to IFR CIGs to slowly spread north by 12Z Friday, with
the worst conditions most likely at CHO, IAD, DCA, BWI, and MTN
Friday afternoon into Friday night. Persistent showers, some heavy
at times, are expected Friday late morning through Friday evening,
with MVFR or lower visibility likely. East winds around 5 knots
Friday morning become southeast in the afternoon.

Scattered to widespread showers continue into Saturday as the front
remains stalled over the area. Additional periods of sub-VFR
conditions are likely until another front sweeps through the area
Saturday night. VFR conditions return Sunday.

Sunday and Monday should both generally be characterized by VFR
conditions across all terminals. Winds are generally going to be out
of the SW at 5 to 10 knots throughout the period during the
afternoons, with light and variable winds overnight.

However, there is a slight chance for afternoon thunderstorms each
day, primarily at MRB and perhaps CHO. However, cannot completely
rule out an afternoon storm making it as far east as IAD Sunday, and
even all the way to I-95 on Monday. If these were to make it over
the terminals, could see some gusty winds, VSBY reductions, and of
course lightning. Probability at this point is 20 percent, so still
a low likelihood, but certainly not out of the question.

&&

.MARINE...
A stalled front over southeast VA lifts north over the local waters
tonight, stalling there through Saturday, before another front
sweeps through Saturday night. A period of SCA conditions is likely
in the open waters of the central Chesapeake Bay Friday morning into
Friday evening as southerly winds increase, gusting to around 20
knots. Elsewhere, variable wind pattern around 5-10 knots.
Widespread showers and scattered thunderstorms are expected all day
Friday into Friday night. A few Special Marine Warnings may be
needed for wind gusts of 35+ knots.

Rain/storm chances linger into Saturday, then a front sweeps through
bringing sub-SCA conditions Sunday.

Winds will generally be light out of the SW through Monday. However,
during the evening/overnight time period winds could turn more
southerly over the waters, resulting in channeled flow up the
Chesapeake Bay. SCAs may be possible both evenings, but are not
likely at this time.

Additionally, while the chance of thunderstorms is there on Monday,
they will be very isolated in nature.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Flood Watch from 6 AM EDT Friday through late Friday night for
     MDZ008.
     Flood Watch from 2 AM EDT Friday through Friday evening for
     MDZ016>018.
VA...Flood Watch from 2 AM EDT Friday through Friday evening for
     VAZ057.
WV...None.
MARINE...Flood Watch from 6 AM EDT Friday through late Friday night for
     ANZ530.
     Small Craft Advisory from 11 AM Friday to 6 AM EDT Saturday
     for ANZ534-543.

&&

$$

SYNOPSIS...KRR
SHORT TERM...CPB/KRR
LONG TERM...CJL
AVIATION...CJL/KRR
MARINE...CJL/KRR