436
FXUS61 KLWX 201903
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
303 PM EDT Sat Jul 20 2024

.SYNOPSIS...
Showers and isolated thunderstorms are expected through this evening
as a weak piece of energy traverse a stalled front along the Virginia
and North Carolina border. High pressure briefly returns Sunday
before a warm front lifts into the region Monday and stalls out
heading into midweek. This will result in daily shower and
thunderstorm chances along with increased humidity and warming
temperatures throughout much of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
The early afternoon surface analysis places a stalled wavy
frontal boundary over southern Virginia back into eastern
Kentucky. Regional radar imagery shows plenty of activity across
the state of Virginia. A bulk of the heftier echoes remain south
of the local area, generally from the Northern Neck southward.
However, recent scans have shown enhancements along a north-
south line extending from near Warrenton southward into
Spotsylvania County. This appears to be a local area of
convergence owing to low-level easterly flow. Surface-based
CAPE values average around 1000 J/kg over this area which should
help sustain the activity until convergence weakens. Any
rainfall should prove to be beneficial given the increasing
drought over the region.

Elsewhere, a broad conveyor belt of weak echoes continue to
track from southwest to northeast. These are all in response to
a slow moving shortwave tracking through central Virginia.
However, such activity has not amounted to a whole lot given
their innocuous nature. The combination of isentropic lift over
the southern Virginia frontal boundary coupled with modest
instability should keep showers around into the early evening.
However, the expansive cloud deck over the area does cast some
doubt on the longevity of any convective elements. These
overcast skies have also kept temperatures down with current
readings locked in the mid 70s to mid 80s.

The slow moving trough begins to exit late this evening into the
night. While some showers linger over southern Maryland, most of
the area begins to dry out into the night. Forecast low
temperatures will mainly be in the 60s, locally hitting the low
70s in D.C., Baltimore, and near the milder waters (i.e., tidal
Potomac and Chesapeake Bay). Depending on where rain falls, a
night of calm winds and decreasing clouds may support some
patchy fog. The best chances would be along and west of U.S. 15.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
The stalled frontal boundary starts to make some poleward
progress on Sunday. This allows temperatures to crank back up
into the upper 80s to low 90s. Adding humidity to the mix yields
heat indices in the mid 90s. While seasonable in nature, these
certainly fall well short of readings over the past few weeks.
Subsidence behind the passing shortwave will also afford more
sunshine over the area. Some afternoon/evening shower and
thunderstorm chances loom across the Potomac Highlands eastward
to the Central Virginia Piedmont. The rest of the forecast area
is expected to stay dry underneath mostly sunny skies. Heading
into Sunday night, slightly milder temperatures are expected
with lows around 2 to 4 degrees above the previous night.

A longwave trough currently situated over eastern Iowa/western
Illinois begins to make its presence felt to start off the new
work week. Although the trough itself begins to open up and
somewhat shear out, the residual vorticity centers spread across
the Mid-Atlantic region on Monday. Ample lift from the forcing
aloft coupled with the stalled frontal zone will make for a
wetter day as showers and thunderstorms become likely. The
current forecast calls for a broad half inch to inch over the
area. Given the ongoing drought issues, these rains should be
welcome site. Any remote flooding concerns would be tied to the
metro areas given poor drainage. Monday`s temperatures should be
a bit lower than the previous day owing to more clouds. Most
stay in the mid/upper 80s, with 70s across mountain locales. For
the night, shower chances begin to decrease with seasonable
temperatures expected.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Key Messages:

1) Daily shower and thunderstorm chances continue throughout the
workweek.

2) Locally heavy rainfall possible Tuesday and Wednesday.

3) Temperatures near normal for the extended period.

Still looking at a stagnant, but active weather pattern as the
region remains sandwiched between the broad ridge over the southwest
Atlantic and ridge over the Rockies. In between the upper level
trough remains with it`s associated pseudo warm/stationary frontal
boundary draped just to the north of the region. With that said, a
steady supply of deep moisture will continue to advect in from the
Gulf of Mexico on south to southwesterly flow.

Showers and thunderstorms will be the continued theme of the
forecast especially during the afternoon and evening hours Tuesday
through Thursday next week. Several pieces of of mid-level shortwave
energy traversing the nearby front will be to blame for the
increased shower and thunderstorm activity along with the threat for
locally heavy rainfall. Any rainfall that we do see is likely to be
mostly beneficial, especially along and west of US-15 where
severe/extreme drought conditions are being observed. Locally heavy
rainfall in the I-95 urban corridor could result in some flooding
issues if storms persist over an area for an extended period of
time. PW values Wednesday and Thursday will range from roughly 1.50
inches across the northern portions of the area to around 2.00
inches across the south. The high end of this range touches the 90
to 97.5 percentile of the 30 year climatology. On Friday, values
decrease slightly to the 1.25 to 1.75 inch range. As for severe
weather, the threat remains low given the abundant cloud cover from
the stalled front nearby. This is highlighted by both the CSU and
NSSL learning machine probabilities which suggest a non zero chance
of severe weather during the middle and latter half of next week.
Later this week the upper level trough and associated cold front
approach the area from the Ohio River Valley and Great Lakes region.
Some uncertainty remains in the exact timing and strength of this
system. For now, will leave chance PoPs in the forecast through
Thursday and perhaps even Friday as models try to push the front
through during this timeframe.

As for temperatures, expect values closer to average for late July.
850mb temperatures will trend gradually milder through the period
running between +16 to +19 through Tuesday through Thursday
timeframe. Highs will range from the upper 70s and low 80s over the
mountains to mid and upper 80s and low 90s east of the Alleghenies
each afternoon.Overnight lows through the period will fall into the
mid to upper 60s and low 70s.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
Outside of KMRB, all terminals have been teetering between VFR
to MVFR ceilings given a scattered to broken 2,500-3,000 foot
deck. Have opted to TEMPO this MVFR group given the brief
lowering of ceilings will likely be tied to passing showers.
With that said, showers are expected to be in and around all
terminals except KMRB. The threat for thunder is fairly low so
have kept any mention out of the TAFs. Southerly winds weaken
tonight with a threat for some patchy fog, especially across
KCHO and KMRB where some restrictions are possible.

A stalled frontal zone to the south returns northward as a warm
front on Sunday. With the disturbance aloft passing off to the
east, it should be a dry day underneath mostly sunny skies. Some
afternoon/evening showers could push as far east as KCHO. It
reamins to be seen if any patchy fog is mustered overnight given
the dry soils over the region. A better chance for showers and
thunderstorms looms for Monday as a shortwave trough moves in
from Illinois. Restrictions are possible at times on Monday with
winds shifting to more southeasterly in nature.

VFR conditions continue into the middle of next week although the
opportunity for sub-VFR remains during the afternoon and early
evening hours as a front sits near region. This will lead to
increased chances for showers and thunderstorms during the afternoon
and evening hours at the terminals although confidence remains low
onto which terminals will be impacted. Shower/storm activity wanes
in the late evening, though a few showers could linger into the
overnight each day.

&&

.MARINE...
Some of the convective showers popping up within a broad swath
of light activity have begun to raise wind gusts to near 18
knots. With that said, Marine Weather Statements have been
hoisted across waters through 6 PM this evening. However, any
threat for such levels of gusts should wane with the decrease in
showers. Background winds remain on the light side through
Sunday as gradients remain weak. By Monday, an approaching
shortwave trough will interact with a stalled boundary over the
area. This promotes a much better chance for showers and
thunderstorms. While not obvious whether any of these will be
severe in nature, the threat for Special Marine Warnings is non-
zero. Any convective chances slowly diminish into Monday night.

South to southwesterly flow will continue to increase midweek ahead
of an approaching cold front and upper level trough over the Ohio
River Valley. This could result in SCA conditions mainly over the
central and lower open waters of the Chesapeake Bay as well as lower
tidal Potomac. The highest confidence for a Small Craft Advisory
appears be Tuesday night into Wednesday due to southerly channeling.
Outside of channeling expect daily shower and thunderstorm chances
each afternoon and evening next week as a stalled front sits nearby.
An SMW cannot be ruled out for any potential strong thunderstorms
(gusts up to 35 kts) that occur during the afternoon and evening
hours.

&&

.TIDES/COASTAL FLOODING...
Light southeast to south winds this afternoon and evening will
continue to result in a small increase in water levels. Although
minor flooding is unlikely, vulnerable shoreline (such as Annapolis)
could reach Action Stage on Sunday and Monday mornings. Water levels
are likely to remain elevated with south to southwest flow
persisting through much of next week.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

$$

SYNOPSIS...EST
NEAR TERM...BRO
SHORT TERM...BRO
LONG TERM...EST
AVIATION...BRO/EST
MARINE...BRO/EST
TIDES/COASTAL FLOODING...EST