244
FXUS61 KLWX 270759
AFDLWX

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

.SYNOPSIS...
Hurricane Helene has made landfall along the northeastern Gulf
Coast overnight, and is expected to quickly move into the
southern Appalachians and Tennessee River Valley today into the
weekend. Widespread rain is possible today, especially across
parts of Central Virginia. Chances for rain will linger into
early next week as an upper low remains overhead.

&&

.NEAR TERM /THROUGH TONIGHT/...
Hurricane Helene made landfall in the Big Bend of Florida this
evening. This storm has a forward propagation speed of nearly 25
kts.. which is near the upper percentile for tropical cyclones.
The upstream trough across the Lower Mississippi Valley is
pulling the cyclone quickly northward. The center is already
well into Georgia as of 03 AM with the outer rainbands making it
into Virginia thanks to jet support/WAA. Through the day today,
the remnants of Helene will continue to move northward, but get
pulled west slightly towards the trough. Other than another day
of cloudcover and highs in the 70s, 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 today.
  The greatest coverage will be across a line from roughly
  Highland County, VA to Albemarle County, VA this morning. 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 of 3 AM mesonets across the higher terrain of
  the Alleghenies and Blue Ridge are already gusting to 20-30
  mph. As the remnants move north through the morning and
  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. Have expanded the Wind
  Advisory in coordination with PBZ to our common border. Bufkit
  profiles suggest 40-45 kts should mix down around 2500 ft,
  with 45-50 kts closer to 3500/4000 ft. 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)... By this afternoon, a
  broken line or broken 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 /SATURDAY THROUGH SUNDAY NIGHT/...
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 /07Z FRIDAY THROUGH TUESDAY/...
Widespread MVFR decreasing to IFR conditions expected today.
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...
With the remnants of Helene moving west of the waters, the outer
windfield will result in elevated winds across the Tidal Potomac
and Chesapeake Bay. SCAs are 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. However, the usual more
sensitive locations are expected to reach minor tidal flooding this
morning.

For the rest of the day, 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 Advisory until 8 AM EDT this morning for DCZ001.
     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 from noon today to 11 PM EDT this evening for
     MDZ001-501.
     Coastal Flood Advisory until 6 AM EDT early this morning for
     MDZ011.
     Coastal Flood Advisory until 8 PM EDT this evening for MDZ014.
     Coastal Flood Watch from this evening through Saturday morning
     for MDZ014.
     Coastal Flood Advisory until 9 AM EDT this morning for MDZ508.
VA...Wind Advisory from noon today to 11 PM EDT this evening for
     VAZ025-503-504-507-508.
     Flood Watch through this evening for VAZ025-036>040-503-504-
     507-508.
WV...Wind Advisory from noon today to 11 PM EDT this evening for
     WVZ501-503-505-506.
MARINE...Small Craft Advisory from 4 PM this afternoon to 3 AM EDT
     Saturday for ANZ530>532-538>540.
     Small Craft Advisory from 1 PM this afternoon to 3 AM EDT
     Saturday for ANZ533>537-541>543.

&&

$$

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