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FXUS02 KWBC 231931
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
331 PM EDT Mon Sep 23 2024

Valid 12Z Thu Sep 26 2024 - 12Z Mon Sep 30 2024

...Potential Tropical Cyclone Nine brings heavy rain, high wind,
and coastal flooding threat to the Central to Eastern Gulf
Coast/Southeast late week...


...Guidance/Predictability Assessment...

All eyes will be on the pattern over the eastern U.S. with the
approach of newly initiated Potential Tropical Cyclone Nine during
the first part of the period later this week. Guidance has come
into better agreement in depicting a split flow pattern with an
upper-trough lifting northward over the Northeast as a cut-off
upper-low drops southward over the Middle Mississippi Valley/Ozarks
Region/ArkLaTex. This upper-low then lingers in the general
vicinity into the weekend as upper-ridging moves in to the north
over the Great Lakes/southeastern Canada. At the same time, the
official National Hurricane Center (NHC) forecast track for
Potential Cyclone Nine brings the system northward across the
Florida Panhandle/Big Bend Region and into the Southeast on
Thursday. The GFS/ECMWF have trended over the past few runs to
consistently depict Potential Tropical Cyclone Nine remaining a
separate entity for a longer period of time after moving inland,
moving northward into the Appalachians/Upper Ohio Valley through
the day Friday. The system is then eventually absorbed into the
upper-low by early Saturday over the Middle Mississippi Valley. The
CMC remains an outlier with the upper-low absorbing the system
faster. Specifics on the exact track of the system, the upper-low
to the west, and the timing in which the energy is absorbed will be
important to the overall QPF footprint, which will likely stretch
from the Southeast north into the southern/central Appalachians and
westward through the Ohio Valley into the Ozarks Region.
Uncertainty grows rather rapidly later Friday into Saturday
especially with westward extent, with updated 00Z guidance showing
a northward trend, especially with respect to the ensemble means.
The degree of upper-ridging to the north of the system becomes less
clear by early Saturday as well which will have an impact on
exactly how far the associated rainfall extends northward.

The position/strength of the upper-low not surprisingly becomes
less defined by the latter part of the period late this
weekend/early next week. Differences increase with the overall
pattern elsewhere by the end of the period as well with the
potential for more amplified northern-stream shortwave energy over
the northern Rockies/High Plains suggested by the deterministic
guidance, particularly the GFS. The 06Z GFS is also more aggressive
with a cutoff low dropping southward along the West Coast late in
the period. The updated WPC forecast includes a blend of the 00Z
GFS/ECMWF/UKMET early in the period given good overall agreement
with respect to the track of Potential Tropical Cyclone Nine and
the handling of the cutoff low to the west. Contributions from the
GEFS/ECens means are included and the GFS is removed by the latter
part of the period as uncertainty with the overall pattern grows
and the GFS diverges from the ECMWF/ensemble mean solutions in the
West.


...Weather/Hazards Highlights...

The NHC and the Weather Prediction Center among others continue to
monitor newly initiated Potential Tropical Cyclone Nine. The
official NHC track brings the system northward into the Gulf of
Mexico to threaten maritime interests and the central to eastern
Gulf Coast/Southeast mid-late week, with the system potentially
moving inland by later Thursday. Confidence is growing in a large
system that will bring significant rainfall/winds/surf/coastal
flooding for the central to eastern Gulf Coast/Florida, with
impactful weather possible through the Southeast and vicinity late
week into the weekend. There has been an overall faster trend in
recent guidance. For Day 4/Thursday, there are expansive WPC
Marginal and Slight Risk Excessive Rainfall Outlook (ERO) areas
spread northward across the Southeast both with advance moisture
feed inland and with the expected landfall of the system. A
targeted Moderate Risk has been introduced for the Florida
Panhandle/Big Bend region into southwestern Georgia where
confidence is highest for more significant flooding impacts near
the expected system track after landfall. Additional expansion of
the Moderate Risk further inland is possible as confidence in the
system track grows, particularly for upslope portions of the
southern Appalachians. The threat would continue for Day 5/Friday
with system proximity and moisture feed into the southern
Appalachians as well as over the Ozark Region under the influence
of the slow moving closed-low, with both regions covered with
Slight Risks. Uncertainty with rainfall coverage and placement
increases significantly into Friday so further refinement of the
outlook areas is likely.

Elsewhere, upper trough energy will dig through the Northeast mid-
late week. A wavy surface front will work offshore, leading to
coastal low development and exit. Expect a period of enhanced rain
over Maine Thursday with a closed upper low and wrapback moisture.
Out West, most areas should be dry. The Washington coastal ranges
and Cascades will be an exception with enhanced rain and highest
elevation snow potential late week with Pacific system passages.
Warmer than average temperatures are expected to translate from the
Northern Plains east into the Midwest later this week into the
weekend following upper-level ridging, with anomalies decreasing by
early next week. Cooler than average temperatures are expected
centered around the Ozarks Region under the influence of the upper-
low aloft, also returning closer to average by early next week as
the upper-low dissipates.

Putnam/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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