521
FXUS21 KWNC 141812
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 14 2024

SYNOPSIS: Mid-level high pressure is forecast to expand across the Southern
Tier of the contiguous U.S. (CONUS) early in week-2. This favors increasing
chances for anomalously warm temperatures, and excessive heat conditions
possibly affecting parts of the Southern Plains, Lower Mississippi Valley, and
the Florida Peninsula. Mid-level low pressure predicted across the northwestern
and north-central CONUS early in the period could lead to a frontal passage and
brief moderation of temperatures at the outset of the period, with this and
additional shortwaves favoring increased chances for episodes of heavy
precipitation across portions of the central and east-central CONUS. By the end
of the period, models are in good agreement regarding weakening of the
mid-level low allowing heights to increase over the western CONUS.

HAZARDS

Moderate risk for excessive heat across southern Texas, Thu-Fri, May 23-24.

Slight risk for excessive heat across portions of the Southern Plains and Lower
Mississippi Valley, Wed-Tue, May 22-28.

Slight risk for excessive heat across the southern Florida Peninsula, Wed-Tue,
May 22-28.

Slight risk for heavy precipitation across portions of the Great Plains,
Mississippi, Tennessee, and Ohio Valleys, and Southeast, Wed-Sat, May 22-25.

Rapid onset drought risk across the southern Florida Peninsula.

DETAILED SUMMARY

FOR FRIDAY MAY 17 - TUESDAY MAY 21:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR WEDNESDAY MAY 22 - TUESDAY MAY 28: Subtropical ridging is forecast to
expand across portions of the south-central CONUS heading into the week-2
period. This favors increasing chances for anomalously warm temperatures
stretching from the Central and Southern Plains eastward to the southern
Mid-Atlantic and Southeast coasts. Given the time of year, many areas away from
the Gulf Coast are not expected to have temperatures reach excessive heat
criteria. The strongest signals for excessive heat are across portions of Texas
where the GEFS and ECMWF Probabilistic Extremes Tools (PETs) depict at least a
20 percent chance for maximum temperatures exceeding the 85th climatological
percentile and 90 deg F. The GEFS PET continues to show probabilities above 40
percent across portions of southern and southwestern Texas, although the ECMWF
PET generally depicts probabilities under 30 percent early in week-2. A passing
frontal system may result in more northerly flow and decrease temperatures
across the South compared to the late week-1 period, and given this possible
scenario, day-8 (May 22) is removed from the moderate risk of excessive heat.
The National Blend of Models (NBM) depicts a reduction in coverage of daily
temperature records on day-8 as well, but increases coverage on days 9 and 10
(May 23-24), with the ECMWF PET holding off on increasing probabilities for
maximum temperatures above the 85th percentile until later in the period. Given
the support from the NBM and the calibrated GEFS heat risk guidance, which
indicates at least a 30 percent chance heat indices exceed the 95th
climatological percentile across southern Texas, the moderate risk for
excessive heat is maintained for May 23-24. Along with the heat, dry conditions
could also elevate the potential for wildfires across parts of the Four Corners
into Texas, with troughing upstream possibly elevating wind speeds across the
region.



A broader slight risk continues across much of the remainder of Texas
(excluding the northern portion and the Panhandle), southeastern New Mexico,
and the Lower Mississippi Valley for all of week-2. For the western side of the
slight risk area, heat is likely to be temperature driven, but higher dewpoints
favored across eastern Texas and the Lower Mississippi Valley may push heat
indices above 105 deg F despite the relatively lower temperatures compared to
those further west. The slight risk for excessive heat also includes the
southern Florida Peninsula for all of week-2. The ECMWF PET is very robust
bringing in hot temperatures across this region by the middle of week-2. The
NBM also depict some parts of the Peninsula reaching the mid-90s, and combined
with dewpoints above 70 deg F, would result in heat indices possibly greater
than 105 deg F. While this would support a moderate risk for excessive heat
over parts of Florida, pause is given due to the weaker signal in the GEFS.
Decreasing soil moisture and a warm and dry pattern predicted in week-2 lead to
increased rapid onset drought risk across parts of south Florida.



Troughing forecast across the northwestern and north-central CONUS favors
below-normal temperatures across much of the West, extending into parts of the
Northern and Central Plains and Upper and Middle Mississippi Valley. The
resulting temperature contrast is likely to lead to enhanced frontal activity
across parts of the central and east-central CONUS during week-2, with a
passing front late in week-1 into the outset of week-2, and possibly another
shortwave ejecting out of the trough soon after. The ECMWF PET indicates at
least a 20 percent chance 3-day precipitation totals exceed the 85th
climatological percentile and 1-inch over portions of the Great Plains,
Mississippi, Tennessee, and Ohio Valleys, and Southeast through the early and
middle parts of the period, although the signal in the GEFS PET has decreased
compared to yesterday. A slight risk for heavy precipitation remains posted
across the aforementioned areas for 5/22-5/25.



No hazards are issued for Alaska, with near- to slightly above/below-normal
temperature probabilities favored for most of the state. Snowmelt season is
underway and frozen rivers are beginning to break up, leading to the potential
for river flooding related to ice-jams. However, there are no indications of
impending major river break-ups or serious threat of ice-jamming so no
flooding-related hazards are posted at this time. Caution will continue to be
exercised as river break-up can be unpredictable and local conditions can
change quickly. Anomalous ridging over the north-central Pacific favors an
enhanced storm track into western Mainland Alaska, although precipitation
amounts are expected to remain low and not reach hazards thresholds.

FORECASTER: Thomas Collow

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