570
FXUS61 KLWX 260745
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
345 AM EDT Thu Sep 26 2024

.SYNOPSIS...
A weak front brings drier conditions to parts of the area, though
showers persist for some. Hurricane Helene makes landfall along the
northeastern Gulf Coast tonight, then moves into the southern
Appalachians and Tennessee River Valley Friday into the weekend.
Widespread rainfall possible Friday, especially across parts of
Central Virginia. High pressure builds in from the north this
weekend, but rain chances remain into next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Heavy rainfall has led to several instances of flash flooding along
the eastern slopes of the Blue Ridge from Nelson/Augusta counties
northward through at least Albemarle County. A lingering boundary
aided by strong 850mb moisture transport and deep moisture resulted
in training thunderstorms with high rain rates. This complex of
storms has pushed off the Blue Ridge, and will continue moving
north/east into central/northern VA, the DC Metro, and MD. Rain
rates have come down, though could still see some nuisance flooding
in low water crossings, or other low-lying areas.

Once this early morning precipitation ends we should be in for at
least mostly dry conditions, aside from the southwestern CWA. A weak
front moving south across PA could bring a few showers to north and
northeast MD this afternoon, though rain chances are low at 20-
30pct. Cloud cover finally lifts as southerly flow erodes the CAD
wedge that has been in place across the area since Sunday. Still,
expect plenty of clouds to remain, but could see some sunshine
peaking through for some. Highs today reach the upper 70s to low
80s. Passing showers continue along the I-64 corridor and parts of
the Alleghenies through this afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
Hurricane Helene is expected to make landfall this evening along the
northeast Gulf of Mexico, then track northward into the Southern
Appalachians tonight, and meander over the TN Valley region over the
weekend. The tropical low is expected to interact, and eventually be
absorbed by a large cutoff upper trough over the Lower Mississippi
Valley/Mid-South. Moisture surges north ahead of Hurricane Helene
tonight into Friday, along a pseudo warm frontal boundary, bringing
widespread showers to parts of Central VA and the Central Shenandoah
Valley. There is a very strong signal for a significant Predecessor
Rain Event (PRE) across the Southern Appalachians tonight into
Friday. While the better forcing (due to proximity to tropical low,
better mid-level moisture transport, and deeper moisture) is likely
to remain south of our area, there is going to be a period of heavy
rainfall along the eastern slopes of the Blue Ridge, and possibly
some of the higher terrain along the Shenandoah Valley. The current
forecast is for 1-3", though based on what occurred early this
(Thursday) morning, it would not take much to cause flooding in some
areas. A Flood Watch is currently being considered for Friday.

As the remnants of Helene move into TN/KY Friday, showers overspread
the entire area throughout the day. Could see some pockets of heavy
rain, otherwise not likely to see flooding east of US-15. The main
concern will be those higher elevations spots along the central Blue
Ridge and Shenandoah Valley.

Going into Saturday, the combined, very large stacked low pressure
over the TN Valley meanders in place. The previously mentioned
frontal boundary stalls near the area Saturday, keeping deep
moisture in place that results in scattered to widespread showers.
Near-normal temperatures in the 70s.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
The remnants of Helene will have been absorbed into a large cutoff
low by Sunday, with the large low gradually filling and weakening as
it tracks over the Mid-South toward the Mid-Atlantic through early
next week. With the low nearby, and pieces of embedded energy
pivoting around it, expect increased chances for showers and perhaps
a few thunderstorms during this time. Some uncertainty remains in
regards to the placement of the cutoff low and timing/placement of
embedded pieces of energy pivoting around it. The influence of
surface high pressure over New England wedging south along the
Appalachians will play into the overall coverage of precipitation as
well. Overall not expecting any washouts, although abundant cloud
cover and intermittent shower chances will continue as remnant
moisture from Helene overrides the wedge high overhead.

The upper-level low continues to fill and push to the east Monday
into Tuesday. Moist flow will likely remain with incoming troughing
and an associated cold front pushing out of the middle part of the
country. This deepening trough continues to show up on deterministic
and ensemble model suites. Its exact amplitude, timing, and whether
or not it cuts off remains in question. Regardless, there is the
potential for a strong frontal passage during the midweek timeframe
providing additional shower and thunderstorm chances. Drier
conditions look to return with high pressure by the latter half of
next week.

Due to the added cloud cover and cool air wedge, temperatures are
expected to remain at or below normal through Tuesday. Highs will be
in the 60s and 70s Sunday into early next week. Lows will fall into
the 50s and 60s.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
Widespread MVFR to IFR CIGs are in place across the region this
morning, as a complex of heavy rainfall and embedded thunderstorms
quickly moves northeast. These is going to produce some reduced VIS
at most terminals, but should be out of the area around 10-12Z.
After that, southerly flow erodes much of the low stratus, bringing
conditions to VFR this afternoon. Should be mostly dry today, aside
from VCSH at CHO.

This will be a short lived reprieve as low clouds build back in
tonight. IFR to possible LIFR CIGs expected at all terminals late
tonight into Friday. The remnants of Hurricane Helene are expected
to bring widespread showers Friday.

Conditions likely remain sub-VFR into Saturday due to a stalled
frontal boundary over the region. Sub-VFR conditions are likely at
times over the weekend into early next week as remnant moisture
from Helene continues to push over the region. Wedging high pressure
will keep abundant cloud cover and shower chances in place through
at least Monday. A few thunderstorms are possible during this time.

&&

.MARINE...
Winds turn southerly today across the waters, though they are
expected to remain below SCA conditions at 10-15 knots. As Hurricane
Helene moves northward to the Southern Appalachians tonight, its
wind field begins reaching the area. Increasing east to southeast
winds tonight will reach SCA criteria Friday afternoon into Friday
night. Widespread showers move across the local waters Friday into
Friday night.

Winds decrease Friday night, but a stalled frontal boundary over the
area into the weekend could produce additional periods of SCA
conditions.

&&

.TIDES/COASTAL FLOODING...
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 (particularly at Annapolis for moderate).

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 early next
week , 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 PM EDT this evening for DCZ001.
MD...Coastal Flood Advisory until 10 AM EDT this morning for MDZ008.
     Small Craft Advisory from 4 PM Friday to 3 AM EDT Saturday for
     MDZ008.
     Coastal Flood Advisory until 5 AM EDT early this morning for
     MDZ011.
     Coastal Flood Warning until 9 AM EDT this morning for MDZ014.
     Coastal Flood Advisory until 9 AM EDT this morning for MDZ508.
VA...Coastal Flood Advisory until 6 PM EDT this evening for VAZ054.
WV...None.
MARINE...Coastal Flood Advisory until 10 AM EDT this morning for ANZ530.
     Small Craft Advisory from 4 PM Friday to 3 AM EDT Saturday for
     ANZ530>532-538>540.
     Small Craft Advisory from 1 PM Friday to 3 AM EDT Saturday for
     ANZ533>537-541>543.

&&

$$

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