908
FXUS02 KWBC 221840
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
240 PM EDT Sun Sep 22 2024

Valid 12Z Wed Sep 25 2024 - 12Z Sun Sep 29 2024

...Tropical System Development threat for the Gulf of Mexico and
Central to Eastern Gulf Coast/Southeast by late week...


...Guidance/Predictability Assessment...

Guidance seems to be finally trending towards a more consistent
flow pattern across the CONUS during the medium range period, which
includes the eventual track of a possible tropical system through
the Gulf of Mexico and towards the central-eastern Gulf Coast
region late this week. Models have trended towards more splitting
of flow and a cutoff upper low lingering over the middle
Mississippi Valley much of the period. This should help to pull a
Gulf tropical system northward and faster than previous guidance.
Latest QPF/ERO forecasts from WPC and NHC thoughts reflected this.
Still, very little confidence in any of the specifics associated
with especially the tropical and until an actual system forms,
models will likely continue to flip-flop. Should also note that the
ensemble means are a little slower than their deterministic
counterparts, which highlights the uncertainties in individual
ensemble members. Elsewhere across the CONUS, there is some timing
and amplitude uncertainty with another shortwave into the Western
U.S. late weekend and reinforced troughing/a closed low near the
Northeast.

The WPC medium range suite used a blend of the deterministic
models Days 3-4, with more weighting towards the ECMWF which was
closest to NHC thoughts on potential Gulf tropical development.
After this, increased ensemble mean contributions to half of the
blend by day 6-7 given increasing uncertainty. Overall faster than
WPC continuity with the Gulf Coast system and faster to bring QPF
into the coast and the Southeast.


...Weather/Hazards Highlights...

Upper trough energy exiting the Great Lakes/Ohio Valley midweek
may be further enhanced by uncertain reinforcing energy into later
week over the Northeast. Wavy frontal systems will also focus
deepening moisture to fuel a threat of showers/thunderstorms with
runoff issues given slowed motion and possible closed low. The day
4 (Wednesday) WPC Excessive Rainfall Outlook (ERO) expanded the
marginal risk slightly across the Mid-Atlantic/central Appalachians
given newest guidance. On Day 5 (Thursday) ERO, and after
collaboration with the impacted WFOs, was able to remove the
marginal risk across New England as soils are dry and rainfall
should be mostly beneficial.

Meanwhile, NHC and WPC will continue to monitor possible tropical
development in the Caribbean over the next few days that looks to
lift into the Gulf of Mexico to threaten maritime interests and the
central to eastern Gulf Coast/Southeast mid-late week. Given
planned development, ultimate timing and track remains quite
uncertain in guidance. Even so, the vast bulk of recent guidance
offers a growing signal for impactful weather, including a threat
for heavy rainfall/winds/surf for the central to eastern Gulf
Coast, with impactful weather possible through the Southeast/East
late week/weekend. With the overall faster trend in the latest
guidance, the Day 4 ERO marginal risk was expanded northward to
include the FL Panhandle. On Day 5/Thursday, also expanded the
marginal and slight risks northward well into the Southeast states.
If the current forecast hold, it is likely at least a moderate
risk will be needed closer to the coast for this time period, but
with ongoing uncertainty, it seems prudent to hold off for now.
Regardless, latest models and ensembles continue to support a heavy
rainfall/storm surge/flooding threat across the central to eastern
Gulf Coast region.

Out West, most areas should stay dry. The Washington coastal
ranges/Cascades will be an exception with growing daily enhanced
rain potential with Pacific system energy and moisture approaches.

Santorelli/Schichtel


Additional 3-7 Day Hazard information can be found on the WPC medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw


















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