259
FXUS21 KWNC 241814
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 24 2024

SYNOPSIS: Heavy rainfall is forecast across much of the Southeast, extending
through the Middle Mississippi Valley to the southern Mid-Atlantic during
week-1 related to Tropical Storm Helene and its remnants. While the system is
forecast to dissipate by the start of week-2, flooding impacts are likely to
continue over these areas. Additional rainfall over parts of Florida and the
Southeast during week-2 could further exacerbate flooding concerns.
Above-average temperatures and a more tranquil pattern are forecast elsewhere
across the contiguous U.S. (CONUS). Surface low pressure may bring wind and
rain impacts to parts of southern Alaska around the middle of the period.

HAZARDS

Slight risk of heavy precipitation across portions of the Southeast, Wed-Sat,
Oct 2-5.

Flooding possible across portions of the Southeast, Appalachians, southern
Mid-Atlantic, and the Tennessee and Middle Mississippi Valleys.

Risk of Rapid Onset Drought across portions of eastern Oklahoma and
northeastern Texas.

DETAILED SUMMARY

FOR FRIDAY SEPTEMBER 27 - TUESDAY OCTOBER 01:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR WEDNESDAY OCTOBER 02 - TUESDAY OCTOBER 08: As of 2pm EDT today, the
National Hurricane Center (NHC) forecasts Tropical Storm Helene to intensify
into a major hurricane before striking the northeastern Gulf Coast later this
week. The remnants of this system are forecast to track inland and curve
northwestward resulting in heavy rainfall extending northward into the southern
Mid-Atlantic and Appalachians, and westward into the Tennessee and Middle
Mississippi Valleys. The heaviest rainfall is forecast to occur prior to the
start of week-2, although continued flooding remains possible over these areas.
Lingering cyclonic flow aloft may result in some areas of precipitation over
the eastern U.S., but coverage is favored to be more sporadic and lower totals
are forecast compared to week-1. There are some enhanced precipitation signals
in the ECENS Probabilistic Extremes Tool (PET) across the Northeast early in
week-2, but given the lack of antecedent heavy precipitation in week-1, and the
uncertainty regarding a more organized system versus lingering moisture from
the remnant circulation precludes a related precipitation hazard. Greater
concern exists across Florida where enhanced southerly flow may bring
additional rounds of heavy rain during week-2, which could worsen ongoing
flooding impacts in the wake of Helene. The ECENS PET depicts a 20-40 percent
chance 3-day rainfall totals exceed the 85th climatological percentile and
1-inch over portions of Florida and the coastal Southeast during the early and
middle parts of week-2. While the GEFS PET does not show a strong signal, the
uncalibrated 0z ECENS and 0z GEFS are in good agreement depicting enhanced
rainfall across the lower Southeast during week-2. Therefore, a slight risk of
heavy precipitation is highlighted across portions of Florida and the
Southeast, Oct 2-5.



Antecedent heavy rainfall across western and central portions of Oklahoma favor
decreased chances of drought development, with portions of eastern Oklahoma and
northeastern Texas remaining at elevated risk for rapid onset drought
development where precipitation deficits of around 3-inches remain and
below-normal precipitation is forecast during week-2. There is notably a tight
precipitation gradient related to Tropical Storm Helene and it is uncertain how
much moisture makes it into Oklahoma. The easternmost areas are favored to have
higher rainfall totals, but any westward expansion could help mitigate drought
concerns over the highlighted area.



Anomalously warm temperatures are forecast across much of the CONUS during
week-2, with the highest probabilities for above-normal temperatures over the
West tied to the enhanced mid-level ridge forecast over the region. High
temperatures over parts of the California Central Valley are forecast to be in
the mid- to upper-90s deg F, with the potential for triple digit heat (locally
greater than 105 deg F) over the Desert Southwest. However, these temperatures
are lower than those being forecast during week-1, and while some localized
areas could reach excessive heat criteria, a widespread event is unlikely given
the diminishing heat climatology in early October. Therefore, no related
temperature hazards are posted.



Surface low pressure is forecast across the Aleutians and Gulf of Alaska during
week-1 bringing the potential for high winds and heavy rain across the southern
coast of the state. This system is forecast to weaken by the outset of week-2,
with another area of surface low pressure taking a similar track across the
region toward the middle of the period. Given that Alaska is likely to be
in-between these two weather systems at the start of week-2, no related hazards
are indicated, but may be needed in future updates should models continue to
depict a more robust second system.

FORECASTER: Thomas Collow

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