652
FXUS61 KLWX 250748
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
348 AM EDT Wed Sep 25 2024

.SYNOPSIS...
Cloudy skies and shower chances continue through tonight. A weak
cold front approaches the area Thursday, while Tropical Storm Helene
moves north from the Gulf of Mexico into the southern Appalachains
and Tennessee River Valley. Widespread rainfall possible Friday
across parts of Central Virginia. High pressure builds in from the
north this weekend, but rain chances remain into next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Widespread clouds, passing showers, and on and off periods of light
rain continue today as a stalled frontal boundary and strong CAD
wedge remain draped across the region. Highs today reach the low to
mid 70s, with mild temps tonight in the mid to upper 60s.

The mid/upper-level pattern begins to amplify across the region
today. A strong mid-level trough over the Midwest cuts off, with the
low drifting southeast to the Middle Mississippi Valley, and the
trough continuing eastward across the Great Lakes to the Northeast.
A mid-level ridge builds over the western Atlantic. The interaction
of these features with Tropical Storm Helene will be important to
monitor toward the end of the week.

A pronounced surface boundary is in place from western PA south
across WV into southwest VA. Aloft, strong diffluence from the
approaching cutoff low and northward surging tropical moisture is
going to fuel widespread showers and some thunderstorms across the
Appalachians.

For our area, this could mean enhanced shower activity, and possibly
some heavy rainfall, along the I-64 corridor and Highland County
(possibly up along the Blue Ridge as well). Somewhere in this area
could see 1-2" of rain through tonight. Given the dry antecedent
conditions, not expecting significant flooding concerns. On/off
showers continue tonight, though some areas should see drier
conditions as we go into Thursday morning.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
Strengthening southerly flow Thursday is enough to break down the
CAD a bit, and bring warmer temperatures in the upper 70s to around
80F. Widespread cloud cover is still expected, and lingering showers
along the I-64 corridor are likely to keep temperatures lower in
that area.

The upper trough passing by to our north is going to drag a weak
cold front across PA. Depending on where the tail of that front is,
there could be additional showers that develop along/north of US-50
Thursday afternoon. This is not explicitly stated in the forecast as
rain chances are at 20pct, but something to monitor as there could
be a few hours of shower activity Thursday afternoon. Otherwise,
this looks to be a drier late morning to afternoon for most, though
a few stray showers cannot be ruled out.

Per the NHC forecast, Tropical Cyclone Helene makes landfall along
the northeast Gulf Thursday evening, then moves north toward the TN
Valley / Southern Appalachians Thursday night. Deep tropical
moisture moves into our area by early Friday morning, bringing
widespread rain/showers to Central/Northwest VA and surrounding
highlands. Models have trended weaker with a surface low moving
south from Canada, meaning most of the area is likely to see high
rain chances Friday into Friday night. The heaviest is going to be
along the eastern slopes of ridges south of I-66/US-48, and in
Central VA where 2-4" of rain is possible. Some ensembles indicate
the possibility of higher rain amounts, which could lead to flooding
issues if they materialize. The main circulation of Helene
retrogrades west toward the Mid-South as it interacts with the
cutoff low, keeping the better tornado environment to our west.
Still, need to continue monitoring the situation in the days ahead
as a slight change in where the remnants of Helene track could
change the forecast one way or another.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
A large upper low will have absorbed the remnants of Helene by the
start of the weekend as it drifts E/NE across the OH/TN Valleys.
Although the low will be filling and weakening, it will still
provide an expansive area of height falls/PVA, enhanced low- to mid-
level flow, and be infused with tropical moisture. This lends to the
potential of showers and a few thunderstorms much of the weekend.
Specifics beyond that are unclear and will depend on the exact track
and strength of the conglomerate gyre. The risk for rain and any
potentially more impactful weather (outside of coastal flooding, see
below) looks to be highest during the second half of the weekend at
this juncture given the latest suite of guidance and trends. But,
this is subject to change given the inherent volatility of cut off
lows at longer time ranges.

Following the departure of the upper low heading into early next
week, the region will likely remain in some sort of moist flow ahead
of troughing over the middle of the country. This trough has been
showing up as very deep across all model suites the past couple of
days, but its exact amplitude, timing, and whether or not it cuts
off remain in question. Regardless, there is the potential for a
strong frontal passage sometime mid to late next week.

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
Widespread MVFR to IFR CIGs are ongoing this morning at all
terminals, with LIFR CIGs at CHO. Passing rain at times is reducing
visibility. These conditions through late morning, with some
improved conditions likely late this afternoon. Additional periods
of showers could occur throughout the day. CIGs decrease back to
MVFR to IFR tonight.

Thursday looks to bring MVFR to possibly VFR CIGs during the
afternoon as most of the area remains dry. The exception will be at
CHO where showers and low ceilings remain.

The remnants of Tropical Storm Helene move into the Southern
Appalachians late Thursday night into Friday, likely bringing
widespread rainfall and sub-VFR conditions through Friday night.

Sub-VFR conditions are likely at times over the weekend in onshore
flow and potential rain showers/a few TS. Flow likely remains E.

&&

.MARINE...
High pressure move over New England pushes offshore, causing winds
to veer southeast this afternoon to tonight. There is likely to be
another few hours this evening of SCA conditions in the middle to
upper Chesapeake Bay due to local channeling of winds. Small Craft
Advisories are in effect from Pooles Island southward.

Winds decrease tonight and are forecast to remain below SCA
conditions through Thursday night. The remnants of Tropical Storm
Helene approach the area Friday into Friday night, with a surge of
east to southeast winds likely ahead of the tropical system. SCA
conditions are likely to develop Friday afternoon, and continue into
the weekend.

Depending on the evolution of low pressure approaching from the
OH/TN Valleys this weekend, onshore flow could be strong enough to
prompt the issuance of Small Craft Advisories at times. Uncertainty
is moderate to high at this time, and ranges from marginal SCA to
near gale-force.

&&

.TIDES/COASTAL FLOODING...
Southeast winds today will turn a bit more towards the south and
weaken slightly tonight through Thursday. This plus decreasing
astronomical tides will lead to slightly lower (though still
elevated) tides, with bouts of minor flooding likely around times
of high tide through Friday morning. The higher of the two daily
tides is during the overnight/early morning hours, and would be
susceptible to the most widespread minor to perhaps locally near
moderate tidal flooding.

By Friday, Helene will orbit around an upper low to the southwest as
high pressure holds firmly to the north. This pattern results in
enhanced onshore east to southeast flow at times through the
weekend, and has the potential to result in multiple rounds of
widespread minor to moderate tidal flooding through at least
Sunday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 6 AM EDT Thursday for DCZ001.
MD...Dense Fog Advisory until 8 AM EDT this morning for MDZ001-501.
     Coastal Flood Advisory until 5 AM EDT Thursday for MDZ011.
     Coastal Flood Warning until 6 AM EDT Thursday for MDZ014.
     Coastal Flood Advisory until 1 AM EDT Thursday for MDZ016-017.
     Coastal Flood Advisory until 2 AM EDT Thursday for MDZ018.
     Coastal Flood Advisory until 9 AM EDT Thursday for MDZ508.
VA...Coastal Flood Advisory until 6 AM EDT Thursday for VAZ054.
     Dense Fog Advisory until 8 AM EDT this morning for VAZ503.
     Coastal Flood Advisory until 1 AM EDT Thursday for VAZ057.
WV...Dense Fog Advisory until 8 AM EDT this morning for WVZ501-503-
     505.
MARINE...Small Craft Advisory from 11 AM this morning to 11 PM EDT
     this evening for ANZ531>534-537-538-542-543.

&&

$$

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