807
FXUS61 KLWX 091933
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
333 PM EDT Tue Jul 9 2024

.SYNOPSIS...
Excessive heat and humidity along with daily thunderstorm chances
look to continue through Wednesday as a warm front slowly drifts
northward away from the area. By late Wednesday into Thursday
morning, a cold front will approach from the Ohio River Valley.
The front will try to cross the area Friday before washing out
along the coast as broad high pressure builds for the upcoming
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
A quasi-stationary north/south oriented frontal zone has set up
along the Shenandoah Valley. Off to the east, a very hot and
humid air mass is in place with a slew of observations well into
the 90s. This is conjunction with dew points in the upper 60s to
mid 70s, locally into the upper 70s. Heat indices have soared
above the century mark across a vast portion of the Mid-Atlantic
region. Given recent observations with heat indices reaching 110
degrees, an upgrade to Excessive Heat Warnings was necessary.
This currently extends from southern Baltimore County southward
across the D.C. metro down to Prince William County and across
the Potomac into all of southern Maryland. Per the impacts
section of the heat product, extreme heat and humidity will
significantly increase the potential for heat related illnesses,
particularly for those working or participating in outdoor
activities. If outside, ensure time is limited in direct
sunshine, stay hydrated, and wear light/loose clothing.

Amidst all the heat and humidity, cumulus fields have fired up,
albeit with varying degrees of vertical development. The
healthiest convective towers currently are tied to the terrain
as well as right near the eastern border of Nelson County.
Elsewhere, towering cumulus are evident in GOES-16 visible
imagery from near Baltimore westward to the Blue Ridge
Mountains. The 17Z RAP objective analysis places a very unstable
and uncapped air mass characterized by 1,500-2,500 J/kg of
surface-based CAPE. Forcing is mainly tied to diurnal heating
with mid-level lapse rates being quiet weak, generally around 5
C/km or even less. Thus, any lightning activity has been
somewhat limited with -20C levels being around 26,000 to 27,000
feet. The risk for severe weather is very low, but torrential
downpours are possible in any storm, particularly given how slow
the cells are moving near the terrain. Elsewhere, the west-east
line of towering cumulus have begun to spark some isolated
showers and thunderstorms. While not very organized, these
could also produce locally heavy rainfall capable of isolated
flash flooding given very slow cell motions.

Tonight`s conditions will remain very warm and humid with low
temperatures remaining in the 70s to near 80 degrees inside D.C.
and Baltimore. The only true reprieve would be across the higher
elevations where lows are in the mid 60s to low 70s. While some
patchy fog may develop overnight over the Potomac Highlands,
much will depend on how much cloud cover gets scoured out into
the night.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
The remnants of Beryl are expected to pass well to the north
across the Great Lakes into New England. However, some of the
enhanced synoptic flow may extend southward into the local area.
This would be able to interact with the persistent hot, humid,
and unstable thermodynamic environment. With that said, the
atmosphere is certainly ripe for more widespread showers and
thunderstorms. Along or just ahead of a cold front, the latest
high-resolution guidance favors a later show than usual with a
north/south band of convection pushing eastward to around I-95
by the early/mid evening. This should continue a slow but steady
march eastward to the Chesapeake Bay before exiting after
midnight. The Storm Prediction Center has upgraded most
locations up to a Marginal Risk for severe thunderstorms. The
current exceptions would be the Allegheny Mountains and far
southern Maryland. Damaging wind gusts are the primary threat,
but a brief isolated tornado cannot be ruled, especially for the
more northern locations. Vertical shear is a bit stronger
coupled with decent cyclonically curved low-level hodographs.
Any threat for severe thunderstorms should halt by the overnight
hours.

On the other side of the equation, it will be another toasty day
accompanied by very humid conditions. Heat Advisories have been
extended into eastern Allegany County southward to eastern
portions of the Allegheny Front. This includes Hampshire and
Hardy counties as well with such advisories in effect over all
affected areas until 8 PM Wednesday evening. Heat index values
will be around 100 to 104 degrees west of the Blue Ridge, to
around 105 to 108 degrees off to the east. Given the duration of
excessive heat and humidity, continue to practice heat safety
techniques and find ways to stay cool and hydrated.

On Thursday morning, a slow moving cold front is slated to
drift offshore before stalling as a downstream upper ridge halts
its forward progression. This allows the frontal zone to stall
and remain in the vicinity during subsequent days. The period of
post-frontal air should allow for temperatures to drop some from
the past several days. Forecast highs will "only" be in the
upper 80s to low 90s, accompanied by dew points in the mid 60s
to low 70s. This should help keep heat index values below the
century mark and thus avoid another day of Heat Advisories.
Shower chances should decrease through the day before the front
retrogrades back toward the Chesapeake Bay late in the evening
and night. This brings a return of more unsettled conditions
along with more humidity. Thursday night`s lows will be in the
low/mid 70s east of the Blue Ridge, with 60s off to the west.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Stalled front along the eastern areas on Friday will
increase shower and thunderstorm chances. Afternoon highs will be
slightly below normal, with low to mid 80s expected as a result of
onshore flow. Shower and thunderstorm chances will continue into the
weekend with light winds turning more west to southwesterly. Sunday
looks to be the driest of the two days with less forcing to work
with along with limited low-level moisture.

Similar probabilities linger into Monday with isolated showers and
thunderstorms possible. Afternoon highs for the weekend will
progressively increase from the low 90s on Saturday to nearing
triple digits by the start of the work week. Heat indices may
approach Heat Advisory criteria in some of the lower elevations on
any of those afternoons and early evenings.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
A west to east oriented axis of towering cumulus currently arc
from around KBWI/KMTN westward to just north of KIAD out to
I-81. Convection has now grown tall enough to produce lightning
around the Baltimore terminals. TEMPO groups have been added
until 21Z for this activity with accompanying restrictions to
visibility. Elsewhere, the KHCO and KMRB have VCTS groups until
00Z this evening. Otherwise, expect prevailing southerlies
through mid-week within the expanding warm/moist sector.

A cold front approaches the area from the west of Wednesday.
Southerly gusts increase during the late morning through the
daylight hours, generally capping out around 20 knots. A squall
line has been noted in a few of the high-resolution models. As
such, have introduced a VCTS group at the D.C. terminals around
22-23Z Wednesday, with possible expansion to other terminals in
subsequent TAF issuances. Restrictions remain possible into
portions of the night as the cold front drifts across the area.
This system eventually stalls off to the east before
retrograding back to the Chesapeake Bay late Thursday.
Prevailing winds on Thursday should be out of the west before
becoming more variable with return of the front.

Sub-VFR conditions are possible Friday into Saturday with any
passing showers or thunderstorms that cross the terminals with winds
out of the southeast on Friday/Saturday turning more westerly later
in the weekend.

&&

.MARINE...
Given the potential for southerly channeling, Small Craft
Advisories have been issued for portions of the waterways. Such
advisories continue into the night over the more southern
waters. Some of the ongoing convection near Baltimore could
impact the nearby waters. Will continue to monitor the
situation. Otherwise, Wednesday brings increasing southerly
channeling with gusts up to 20 to 25 knots, locally nearing 30
knots over the southern waters. Small Craft Advisories have been
issued through early Wednesday evening, likely needing to be
extended into Wednesday night as well. Additionally, convective
threats increase during the evening to early overnight hours.
Special Marine Warnings may be required as this frontal and pre-
frontal convection tracks toward the waters.

For Thursday, the region moves into a brief post-frontal
environment which yields a mainly dry day across the area. Winds
shift to westerly and could near Small Craft Advisory levels at
times. The frontal zone retrogrades back toward the Chesapeake
Bay late in the evening and night. This allows for a return of
more unsettled conditions.

A few showers and thunderstorms may impact the waters Friday and
Saturday. Cannot completely rule out an SMW if any strong
thunderstorms approaching the marine zones.

&&

.TIDES/COASTAL FLOODING...
Water levels gradually begin to increase in the next day or two
given the prolonged southerly flow. This carries the usual more
sensitive locations up to Action stage (most notably Annapolis).
Some drop off is expected by Thursday as the post-frontal
environment sees winds shift to westerly.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Heat Advisory from noon to 8 PM EDT Wednesday for DCZ001.
     Excessive Heat Warning until 8 PM EDT this evening for DCZ001.
MD...Heat Advisory until 8 PM EDT this evening for MDZ003>006-008-
     503-505-507-508.
     Heat Advisory from noon to 8 PM EDT Wednesday for MDZ003>006-
     008-011-013-014-016>018-502>508.
     Small Craft Advisory from 10 AM to 6 PM EDT Wednesday for
     MDZ008.
     Excessive Heat Warning until 8 PM EDT this evening for MDZ011-
     013-014-016>018-504-506.
VA...Heat Advisory until 8 PM EDT this evening for VAZ026>031-
     036>040-050-051-055>057-501-502-505-506-526.
     Heat Advisory from noon to 8 PM EDT Wednesday for VAZ026>031-
     037>040-050-051-053>057-501-502-505-506-526-527.
     Excessive Heat Warning until 8 PM EDT this evening for VAZ053-
     054-527.
WV...Heat Advisory until 8 PM EDT this evening for WVZ051>053.
     Heat Advisory from noon to 8 PM EDT Wednesday for WVZ050>053-
     055-502-504.
MARINE...Heat Advisory until 8 PM EDT this evening for ANZ530.
     Heat Advisory from noon to 8 PM EDT Wednesday for ANZ530.
     Small Craft Advisory from 10 AM to 6 PM EDT Wednesday for
     ANZ530>533-535>542.
     Small Craft Advisory until 2 AM EDT Wednesday for ANZ531>533-
     537-539>542.
     Small Craft Advisory until 6 PM EDT Wednesday for ANZ534-543.

&&

$$

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