748
FXUS61 KLWX 031946
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
346 PM EDT Wed Jul 3 2024

.SYNOPSIS...
High pressure will remain offshore through the first half of the
weekend. A cold front will move through the area Saturday night.
High pressure will re-establish itself offshore early next
week.

&&

.NEAR TERM /THROUGH TONIGHT/...
High pressure has progressed off the Mid-Atlantic/New England
coastline this afternoon. Meanwhile, much further north, low
pressure is tracking northeastward toward Hudson Bay.
Southwesterly flow aloft/southerly flow at the surface between
these two features is advecting a warmer airmass into the area.
Temperatures currently sit in the mid 80s for most, and should
reach highs in the upper 80s to near 90. Skies are partly cloudy
to mostly sunny in nature, with just some fair weather cumulus
around. No precipitation is expected through the remainder of
the daylight hours.

High clouds will start to increase across the area tonight, but
dry conditions are expected to persist for most. The exception
will along the Allegheny Front into the Potomac Highlands, where
a few showers may be possible during the second half of the
night. Temperatures will remain warm overnight, with lows in the
upper 60s and lower 70s.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
High pressure will remain in place offshore tomorrow,
maintaining southerly flow at the surface. Aloft, flow will be
west to southwesterly, which will draw a very warm and humid
airmass into the area from the center of the country. High
temperatures should reach into the low-mid 90s for most, which
when combined with dewpoints in the lower-middle 70s will result
in peak heat indices of 95-105. After a dry start to the day,
thunderstorms are expected to form during the afternoon hours
in response to daytime heating within a very moist, uncapped
environment with gradually falling heights aloft. The storms
will likely form in the vicinity of the higher terrain to the
west of the Blue Ridge during the early afternoon hours, and
then progress eastward through the remainder of the afternoon
and into the evening.

Flood Threat: Tomorrow`s environment will be characterized by
highly anomalous levels of moisture, with precipitable water
values of 2.0-2.5 inches across the area. Warm cloud layers will
be very deep, and MBE velocities are also very low. As a result,
any storms that form will be capable of producing very heavy
rainfall. There isn`t an obvious surface boundary present to
focus low-level convergence, which should serve as a limiting
factor (in terms of how long storms can last in a given
location). Recent CAM runs show storms being somewhat
progressive in nature, which makes sense given the lack of a
focusing boundary, and with around 20-30 knots of flow present
in the mid- levels. Antecedent conditions have also been very
dry. 1 hour flash flood guidance is around 3 inches in rural
areas, which will make flooding highly unlikely in those areas.
Urban areas have lower flash flood guidance, and may be a bit more
susceptible to flooding. WPC currently has the area outlooked
in a Marginal Risk, which seems appropriate given the potential
for very heavy rainfall producing storms, but also the limiting
factors listed above.

Severe Thunderstorm Threat: Model soundings show around
1000-2000 J/kg of MLCAPE and 20-30 knots of effective bulk shear,
with well- mixed boundary layers and steep low-level lapse
rates, but saturated profiles aloft. The saturated profiles
aloft limit DCAPE and resultant evaporatively driven downdraft
accelerations, but wet microbursts may still be possible with
any stronger storms. SPC currently has much of the area
outlooked in a Marginal Risk for severe thunderstorms, with
damaging winds being the primary threat.

Any thunderstorms should gradually wind down through the evening
hours, giving way to predominantly dry conditions during the
second half of the night. Overnight lows will be in the low to
mid 70s for most.

Friday looks like a similar day to Thursday, with continued hot
and humid conditions and additional chances for
afternoon/evening thunderstorms. Dewpoints may be a couple
degrees higher, which may make heat indices a few degrees higher
as well. An isolated downburst or instance of flash flooding
can`t be ruled out with any of the stronger storms, but the
overall coverage of storms may be slightly lower than Thursday.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
On Saturday, an upper shortwave and associated surface low pressure
system are expected over the Great Lakes. By early Saturday morning,
a weak cold front is expected to approach from our west and then
gradually move through the area through Sunday morning. Guidance
currently quickly fills in the upper trough and takes it northeast,
resulting in a continuing drier trend for Saturday as the best
forcing leaves with it. Scattered showers and thunderstorms remain
possible Saturday afternoon and evening with PoPs increasing west to
east, though with only slight PoPs along and west of the I-81
corridor. We`re continuing to monitor the potential for severe
thunderstorms and isolated flooding, but with the drier trend
confidence is low. Additionally, Temperatures on Saturday are
expected in the 80s to low 90s for most of the non-mountainous areas
and heat indices near 100-105 east of the Blue Ridge.

The front is expected to be mostly through the area by Sunday
morning, but since the upper low is expected to cut and run, the
weak boundary likely stalls and hangs around. Brief ridging looks to
keep us dry on Sunday, with highs a little cooler than Saturday, but
not as humid with dewpoints in the 60s.

Heading into the work week, continuing moisture advection will allow
afternoon showers and thunderstorms during the afternoon on Monday
and Tuesday, focused along the weak stalled boundary over the area.
This wet pattern likely sticks around as long as the boundary does,
which might not take its leave until another trough comes to pick it
up late in the week.

&&

.AVIATION /17Z WEDNESDAY THROUGH SUNDAY/...
Prevailing VFR conditions are expected through Friday. However,
afternoon and evening thunderstorms may lead to temporary
restrictions on Thursday and Friday. VCTS has been introduced to
the IAD, DCA, and BWI TAFs tomorrow afternoon, but some timing
uncertainty still remains. Winds will generally be light out of
the south over the next few days, although there could be a few
gusts to around 15-20 knots this afternoon.

Prevailing VFR conditions are expected Saturday, but sub-VFR
conditions are likely with any scattered showers and thunderstorms
passing over the terminals in the afternoon associated with a weak
frontal boundary. Sunday looks drier with high pressure building in
behind the boundary.

&&

.MARINE...
SCAs are in effect within channeled southerly flow through
tonight for all areas except the upper tidal Potomac. Gusts to
around 20 knots out of the south may be possible during that
time. Sub-SCA southerly winds are expected through much of the
day tomorrow, but the lower tidal Potomac and central portions
of the Bay may come close to SCA levels in channeled southerly
flow again tomorrow afternoon. Elsewhere, SMWs may be needed in
association with any thunderstorms that move over the waters
tomorrow afternoon into tomorrow evening. Sub-SCA southerly
winds are expected again on Friday, and SMWs may be needed once
again during the afternoon and evening hours.

Winds increase out of the south on Saturday and could approach SCA
criteria with any channeling through the Bay in the afternoon. Winds
then diminish overnight and Sunday as a weak boundary stalls over
the waters.

&&

.TIDES/COASTAL FLOODING...
Tidal anomalies are expected to continue their steady climb through
the rest of this week, into the weekend, and possibly next week as
south to southeast winds prevail for most of the period. Sensitive
locations are expected to reach Action Stage during the higher of
the two daily high tides. A Coastal Flood Advisory is in effect
in Annapolis for tomorrow morning`s tide cycle. Beyond that,
will need to continue monitoring trends to see if, and when, any
Coastal Flood Advisories are needed through the weekend.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Small Craft Advisory until 6 AM EDT Thursday for MDZ008.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ530>534-
     536>543.

&&

$$

SYNOPSIS...KJP
NEAR TERM...KJP
SHORT TERM...KJP
LONG TERM...CAS
AVIATION...KJP/CAS
MARINE...KJP/CAS
TIDES/COASTAL FLOODING...KRR