837
FXUS61 KLWX 271434
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1034 AM EDT Fri Sep 27 2024

.SYNOPSIS...
Hurricane Helene has weakened to Tropical Storm Helene and is
rapidly moving northward into western North Carolina. The
tropical system will move into the southern Appalachians and
Tennessee River Valley later this afternoon through the weekend.
Widespread rain will overspread the region this afternoon,
especially across parts of central Virginia. Chances for rain
will linger into early next week as an upper low remains
overhead.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Helene has become a tropical storm as of this morning and will
continue to spread moderate to heavy rain northward into our
region the remainder of the day. Lots of clouds and rainfall
will lead to high temperatures only in the lower to middle 70s
this afternoon. Helene will bring several hazards to the area:

- Heavy rain threat... Given the tropical airmass with PW values
  well above climo norms for late September (~1.5-2.2"),
  repeated rounds of moderate to heavy rain are expected this
  afternoon. By this afternoon and into this evening, a broken
  line of showers/thunderstorms or several lines may develop,
  but coverage should be more scattered in nature. Given the
  SE`ly fetch off the Atlantic, abundant moisture is upsloping
  into the Blue Ridge and Alleghenies. The higher terrain of
  these locations have the greatest potential for Flash Flooding
  (especially where recent rains have exceeded 2-4"). Another
  2-4" look possible across the higher terrain of the watch area
  through this evening. Have maintained the Flood Watch for
  these locales. Further north and east, less forcing exists and
  lesser amounts of rain are expected.

- Strong Winds...The remnant wind field with Helene is
  extensive. As the remnants move north this afternoon, the
  winds will increase across the Blue Ridge and Alleghenies. A
  core of 40-60 kts of wind at H85 is forecast across central VA
  that pivots into the Ohio Valley. There is a good amount of
  disagreement between models on how far northeast these winds
  make it into the local forecast area, especially at lower
  elevations. There is a better signal for downsloping west of
  the Alleghenies, but northwestern Garrett County could
  downslope as well directly west of Keysers Ridge towards
  Friendsville. The saturated ground will likely allow for trees
  to fall as well. Across the lower elevations and metros,
  gusts to 20-30 kts are possible.

- Severe Potential (including tornadoes)... A broken line or lines
  of showers/thunderstorms are possible. The best overlap in
  instability/shear/SRH is across central VA. SPC has a Marginal
  Risk (Level 1 out of 5) that is tornado driven (2%). The
  further north into the forecast area, there is less
  instability. However, in tropical environments it does not
  take much instability. The one limiting factor is the forcing
  is definitely better closer to the center which is forecast to
  track well to the west.

The region will reside in a tropical airmass tonight with lows
only falling into the upper 60s in the metros with upper 50s
west of the Blue Ridge. Dew points will be near the air temp, so
fog formation appears rather likely. Rain chances look to be low
overnight with PWs actually decreasing heading into Saturday.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY NIGHT/...
Previous discussion...
Saturday should in large be dry for most of the area, aside for
areas along the Mason-Dixon from Hagerstown to Cecil County
where remnant showers are possible. Otherwise, cloudy with highs
in the 70s.

Come Sunday, the remnants of Helene and the ULL start to move
back into the region. Rain chances increase, especially heading
into the afternoon. Another day of highs in the 70s with
abundant cloudcover.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
A very slow moving upper low will continue its gradual trek toward
the Mid-Atlantic region on Monday. Some increase in forward speed is
likely thereafter once the system feels the influence of a longwave
trough crossing the Central/Northern Plains. Consequently, the upper
low evolves into more of an open wave while exiting into the
Delmarva Peninsula by Tuesday. In its wake, the upstream trough
across the Great Plains begins to track across the Great Lakes into
the northeastern U.S. by mid-week. This helps usher a fairly strong
cold front through the region on Wednesday with high pressure
settling in for the latter end of the work week.

Most of the action will focus on Monday given the presence of the
upper low and residual tropical moisture over the area. Some locally
heavy rainfall is not out of the question with the anomalous
moisture in place. Depending on how much rain will have fallen in
previous days, some residual flood risk may exist. Some of the
slower model solutions favor additional shower chances into Tuesday
as well. Given the tendency for upper lows to be slower than models
show, would expect Tuesday to offer a decent shot for rain. The true
drying trend does not unfold until mid-week as the mentioned cold
front sweeps through the local area. This helps lower humidity
levels and cools temperatures off from Wednesday onward.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
Widespread IFR conditions this afternoon. CIGs will be the main
driver, but given the tropical environment any periods of rain
can see VSBYs reduced given the higher quantity of small
raindrops. By this evening into tonight, continued IFR to
potentially LIFR CIGs are expected.

Conditions likely remain sub-VFR into Saturday due to a stalled
frontal boundary over the region.

Shower and possible thunderstorm chances continue Sunday as the remnants
of Helene make their way east. Additionally, persistent E/NE
flow is expected with surface high pressure to the north, so
sub-VFR conditions remain possible at times, especially during
any shower activity near the terminals.

With an upper low approaching from the west, there will be ample
opportunities for further shower activity on Monday into Tuesday. As
a result, expect restrictions, particularly in light of the
continued easterly onshore flow. Winds do shift toward north-
northeast by Tuesday, but with continued low ceilings at times.

&&

.MARINE...
SCAs remain in effect for all waters through this evening with
easterly winds up to 25 kts expected.

Winds decrease Saturday to only about 5-10 kts, but turn
southerly.

Come Sunday, the remnants of Helene and associated ULL will
swing through the area into Monday. This will result in elevated
winds over the waters out of the east, with SCAs possible.

Northeasterly winds could near Small Craft Advisory levels on Monday
across the northern half of the Chesapeake Bay. Otherwise, gradients
weaken with gusts dropping to around 10 to 15 knots through Tuesday.
Winds shift from easterly to mainly north-northeasterly by
Tuesday.

&&

.TIDES/COASTAL FLOODING...
Given the decrease in astronomical high tides, water levels have
lowered some relative to previous days. As for this afternoon,
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. Currently the D.C. Southwest Waterfront and Annapolis
are in Coastal Flood Watches due to the potential for moderate
tidal flooding early Saturday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Watch from late tonight through Saturday morning
     for DCZ001.
MD...Small Craft Advisory from 4 PM this afternoon to 3 AM EDT
     Saturday for MDZ008.
     Wind Advisory until 11 PM EDT this evening for MDZ001-501.
     Coastal Flood Advisory until 8 PM EDT this evening for MDZ014.
     Coastal Flood Watch from 8 PM EDT this evening through
     Saturday morning for MDZ014.
     Coastal Flood Advisory until 3 AM EDT Saturday for MDZ017.
VA...Wind Advisory until 11 PM EDT this evening for VAZ025-503-504-
     507-508.
     Flood Watch until 8 PM EDT this evening for VAZ025-026-036>040-
     503-504-507-508.
     Coastal Flood Advisory from 3 PM this afternoon to 7 PM EDT
     this evening for VAZ054.
WV...Wind Advisory until 11 PM EDT this evening for WVZ501-503-505-
     506.
     Flood Watch until 8 PM EDT this evening for WVZ505-506.
MARINE...Small Craft Advisory from 4 PM this afternoon to 3 AM EDT
     Saturday for ANZ530>532-538>540.
     Small Craft Advisory until 3 AM EDT Saturday for ANZ533>537-
     541>543.

&&

$$

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