429
FXUS21 KWNC 091805
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 09 2026

SYNOPSIS: Compared to yesterday, today`s dynamical model guidance maintains
more amplified mid-level high pressure across the south-central and
southeastern contiguous U.S. later into week-2, suggestive of a long duration,
significant heat event, particularly across the Southern Plains and Lower
Mississippi Valley where a high risk for extreme heat has been designated.
Antecedent dryness, combined with little precipitation favored over the next
two weeks supports a Rapid Onset Drought (ROD) risk for portions of the
Southern and Central Plains and Lower Mississippi Valley. A weak but broad area
of mid-level low pressure across eastern Canada favors enhanced northwesterly
mid-level flow and a progression of enhanced moisture and frontal activity from
the Four Corners to the Midwest and Eastern Seaboard where periodic heavy
precipitation is possible early in week-2.

HAZARDS

High risk of extreme heat for portions of the Southern Plains, Lower
Mississippi Valley, and Southeast, Mon-Wed, Aug 17-19.

Moderate risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi Valley, and Southeast, Mon-Fri, Aug 17-21.

Slight risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi and Tennessee Valleys, Southeast, and southern
Mid-Atlantic, Mon-Sun, Aug 17-23.

Slight risk of heavy precipitation for portions of the Four Corners, Northern
and Central Plains, Midwest, Tennessee Valley, Appalachians, Northeast, and
Mid-Atlantic, Mon-Thu, Aug 17-20.

Rapid Onset Drought (ROD) risk for portions of the Southern and Central Plains
and the Lower Mississippi Valley.

DETAILED SUMMARY

FOR WEDNESDAY AUGUST 12 - SUNDAY AUGUST 16:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR MONDAY AUGUST 17 - SUNDAY AUGUST 23: Dynamical model guidance is in good
agreement regarding subtropical ridging rebuilding across the south-central and
southeastern U.S. late in week-1 and persisting into week-2. The 0z ECMWF,
GEFS, and Canadian ensembles depict a 595-596 dm 500-hPa height maximum across
the central Gulf Coast states. Unlike yesterday`s guidance which showed some
weakening later in the period, today`s ECMWF and Canadian ensembles maintain or
increase the amplitude of the ridge by the end of the period. As a result,
confidence is increasing for a long duration heat event centered across the
Southern Plains and Lower Mississippi Valley where a high risk for extreme heat
is posted, Aug 17-19. Both the GEFS and ECMWF Probabilistic Extremes Tools
(PETs) depict 30-50 percent probabilities for maximum temperatures to exceed
the 90th climatological percentile. While the ECMWF PET initially focuses on
higher probabilities across the Southeast, both PETs consolidate larger
probabilities across the Lower Mississippi Valley and Southern Plains by the
middle of the period. A large contribution from increased humidity is expected,
with a strong signal emerging in calibrated skill-weighted GEFS-ECMWF heat risk
guidance. Some areas have at least a 50 percent chance the maximum temperature
or heat index value exceeds the 95th climatological percentile, with heat index
values potentially exceeding 110 deg F. In addition, the National Blend of
Models (NBM) also depicts a number of locations with nighttime lows nearing
and/or breaking daily records, where little overnight relief to daytime heat is
expected. The moderate risk for extreme heat is now extended through Aug 21 and
now includes portions of the Central Plains and Middle Mississippi Valley to
account for a greater expansion of the ridge axis and increased potential for
heat index values to exceed 105 deg F. A slight risk surrounds the moderate
risk area and continues for all of week-2. The GEFS is more robust in depicting
increasing heat expanding into the Northern Plains later in week-2. Since the
ECMWF and Canadian ensembles keep the heat across the Central Plains and south,
no northward expansion is indicated in today`s outlook but trends will continue
to be monitored.



Troughing initially near the West Coast of the CONUS favors an influx of
moisture across much of the western U.S., and expanding through the Midwest and
Eastern Seaboard as evidenced by anomalous precipitable water values in the 0z
ECMWF and GEFS ensembles. This is also tied to broad troughing over eastern
Canada leading to enhanced westerly to northwesterly mid-level flow across the
Midwest into the East, and a general progression of frontal waves or Mesoscale
Convective Systems (MCSs) overtop the ridge axis situated across the
south-central and southeastern states. Therefore, a slight risk of heavy
precipitation is posted extending from the Four Corners, through portions of
the Northern and Central Plains, and into the Midwest, Tennessee Valley,
Appalachians, Northeast, and Mid-Atlantic, Aug 17-20. The GEFS and ECMWF PETs
continue to depict at least a 20 percent chance 3-day precipitation totals
exceed the 85th climatological percentile and a half-inch across the Four
Corners. This region is still prone to flash flooding even with relatively
small precipitation accumulations especially along dry creek beds (arroyos) and
in canyons. Caution should always be exercised in such areas should
thunderstorm activity initiate nearby. Pattern recognition supports the
extension of the slight risk into portions of the central and eastern U.S.



Toward the middle of week-2, troughing along the West Coast is favored to
retreat with rising heights across the West tied to expansion of the ridging
over the south-central CONUS. This evolution would decrease the moisture
advection into the West, and also promote an increase in above-normal
temperatures and possibly extreme heat across portions of the Interior West.
However, probabilities in the PETs for maximum temperatures exceeding the 85th
climatological percentile are for the most part under 20 percent, and no
related extreme heat hazards are posted across the West at this time.



A Rapid Onset Drought (ROD) risk remains designated and includes parts of
Oklahoma, northeastern Texas eastward through northern Mississippi. This ROD
area continues to register 30-day precipitation deficits greater than 2 inches,
with little rainfall forecast during the next two weeks. Much of this ROD area
is also included within the moderate risk of extreme heat as the potential for
increased evapotranspiration rates also looks to reinforce this risk later in
August.



Across the eastern Pacific, the National Hurricane Center (NHC) is monitoring
three areas located near 140 deg W, 125 deg W, and 115 deg W, with 90 percent,
60 percent, and 10 percent chances of tropical cyclone (TC) development,
respectively, during the next seven days (as of 11am PDT). For the western
watch area near 140 deg W, both dynamical model and Google AI guidance are
indicating a clustering of tracks near or just to the south of the Hawaiian
Islands, although potential impacts are shifting more into the late week-1
timeframe. However, the GEFS and ECMWF PETs continue to depict at least a 20
percent chance 3-day precipitation totals exceed the 85th climatological
percentile early in week-2 across most of the state, and interests across
Hawaii are advised to monitor the latest forecasts from NHC
(https://www.nhc.noaa.gov/).

FORECASTER: Thomas Collow

$$