189
FXUS21 KWNC 201852
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 20 2026

SYNOPSIS: Dynamical models favor a potentially major and long duration extreme
heat event over the southern tier of the contiguous U.S. (CONUS), with the
greatest risk focused from the Desert Southwest to the Great Plains underneath
a potent mid-level high pressure center next week. Stalled frontal activity
over the portions of Southeast and Mid-Atlantic may bring locally heavy
precipitation across coastal areas of the Eastern Seaboard. Antecedent dryness,
combined with above-normal temperatures and low rainfall forecast during next
two weeks, supports a Rapid Onset Drought (ROD) risk for portions of the
Central and Southern Plains, and the Middle and Lower Mississippi Valley.

HAZARDS

High risk of extreme heat for parts of the Desert Southwest, Southern Rockies,
Southern and Central Plains, and Lower Mississippi Valley, Fri-Sat, Aug 28-29.

Moderate risk of extreme heat for much of the Desert Southwest, southern
sections of the Rockies, the Central and Southern Plains, the Lower Mississippi
Valley, and some adjacent areas, Fri-Mon, Aug 28-31.

Slight risk of extreme heat for portions of the Southwest, Great Basin, central
and southern sections of the Rockies and Plains, the Middle and Lower
Mississippi and the Tennessee Valleys, and parts of the South Atlantic Region,
Fri-Thu, Aug 28-Sep 3.

Slight risk of heavy precipitation for coastal sections of the Mid-Atlantic and
Northeast, Fri, Aug 28.

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

DETAILED SUMMARY

FOR SUNDAY AUGUST 23 - THURSDAY AUGUST 27:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY AUGUST 28 - THURSDAY SEPTEMBER 03: Dynamical models begin the period
similarly, with an amplified subtropical ridge, and an accompanying extreme
heat risk mainly focused from parts of the Lower Mississippi Valley through the
Southern Plains into the eastern Four Corners region next week. Today, the GEFS
is similar to yesterday, showing an amplified 500-hPa ridge that erodes
slightly by the end of week-2, but maintains a significant axis northward
through the central CONUS. After a quick jog to the west, the GEFS solution
drifts a bit east of yesterday`s solution. The Canadian ensemble (CMCE) and
European ensemble (ECENS) means also show a strong mid-level ridge at the start
of week-2, but more rapidly depict deamplification. They bring 500-hPa heights
down toward normal over the first few days, and allow the more suppressed
500-hPa height maximum to sink southward near the Mexican border later in the
period. Still, a subtropical mid-level ridge of this strength is unusual for
late August. Heights peak 2-2.5 standard deviations above climatology early
week-2, consistent with a potentially major heat event.



The mid-level ridge is bookended by a pair of mean troughs over the eastern
Pacific and near the eastern CONUS, but the 500-hPa configuration in the CMCE
and ECENS means is not as reflective of an omega block for very long today,
with the flow flattening substantially by the middle of week-2. The GEFS
remains more amplified for longer, though not quite to the extent depicted
yesterday. The forecast favors the GEFS today since it is considerably more
consistent with yesterday, but is shifted somewhat in the direction of the CMCE
and ECENS means, since their solutions are in the majority today even if less
consistent with their depictions from yesterday..



Underneath the amplified ridge, a high risk of extreme heat remains posted,
with the GEFS Probability of Extremes Tool (PET) actually a little more robust
with the potential for extreme heat relative to yesterday. Based on changes to
the internal heat index guidance, the high risk area is expanded throughout
Texas outside the Panhandle, and also somewhat farther east and south in the
Lower Mississippi Valley. Higher odds for temperatures to peak above the 90th
climatological percentile are greater than yesterday in these areas, especially
the ECENS-based tool. On the other hand, influenced by the aforementioned
500-hPa height forecast changes in the ECENS and CMCE, the high risk area has
been trimmed southward in the Central Plains.  The high risk is valid through
the same date as yesterday, Aug 29. Differences in the internal heat index tool
from yesterday reflect the changes in the ensemble mean solutions, with the
GEFS-derived version showing better chances for 2-day temperature and/or heat
index values to exceed the 90th percentile much farther north than the
ECENS-derived solution. Odds top 50 percent in the GEFS as far north as central
sections of the Rockies and Plains, the Middle Mississippi Valley, the
Tennessee and Lower Ohio Valleys, and the Southeast. Conversely, the ECENS
shows such chances only as far north as the southern Rockies, central Oklahoma,
Lower Mississippi Valley, and much of the Gulf Coast. The internal heat index
tools from both the ECENS and the GEFS show something of an eastward expansion
in the elevated odds for extreme heat, and this is reflected in the official
forecast. Heat index values reaching 105 to possibly 115 deg F are expected
over Southern Great Plains and parts of the Southeast, with higher dewpoints
playing a major role in driving extreme heat in the latter region. In lower
elevations of the Desert Southwest, maximum temperatures look to approach or
exceed 115 deg F in some locations, based on the latest probabilistic and
deterministic guidance.



In concert with changes in the high risk area of extreme heat, a corresponding
larger moderate risk area remains posted through Aug 31. Coverage was changed
in similar ways to the high risk area, with some removal from the Central
Plains but some eastward expansion from the Lower Mississippi Valley and
adjacent areas. A broad slight risk area of extreme heat remains posted for all
of week-2, covering the southern tier of the CONUS, with its northern extent
trimmed a bit over the central and interior western CONUS, but expanded a bit
on the east side to contain more of Georgia and most of South Carolina.



The episode of extreme heat is being exacerbated by feedback processes
associated with rapidly drying soils and drought being registered within many
of the highlighted regions. The hot and dry conditions favored are also
expected to aggregate drying fuels, increasing chances for elevated fire
weather conditions during the period. In addition, a Rapid Onset Drought (ROD)
risk is designated across portions of the Central and Southern Plains and
Middle and Lower Mississippi Valley slightly farther to the east and south than
yesterday. This ROD area continues to register 30-day precipitation deficits
greater than 2 inches and/or under half of normal, with several tenths of an
inch at most expected during the next two weeks. Much of this ROD area is
within the moderate risk of extreme heat as increased evapotranspiration rates
look to accelerate surface moisture depletion.



With troughing favored downstream over the eastern CONUS, stalled frontal
activity continues to be featured across the parts of the Southeast and
Mid-Atlantic. This enhances the risk of heavy precipitation later next week
despite raw tools showing the highest precipitation amounts remaining offshore.
The PETs derived from the GEFS and ECENS continue to feature at least 20%
chances for 3-day amounts exceeding the 85th climatological percentile along
the Atlantic Coast from North Carolina through Maine. Enhanced chances for over
1 inch of rain in the PETs, however, is restricted to the North Carolina lower
Delmarva Coast. A slight risk of heavy precipitation posted from coastal North
Carolina northward through Maine for Aug 28, with changes from yesterday
consistent with today`s PET guidance.



Across the Desert Southwest, the subtropical ridge center is forecast to be in
a fairly favorable position for the advection of monsoonal moisture into the
region. In addition, an uptick in tropical cyclone activity is expected over
the eastern Pacific in the coming weeks, where any transient tropical
disturbances nearing the Gulf of California could trigger a gulf surge event
and bring increased precipitation into the region. While there continues to be
lack of support for heavy precipitation in the tools, isolated areas of heavy
rain remain possible, and this region is prone to flash flooding even with
relatively low rainfall totals along dry creek beds (arroyos) and in canyons.
Caution should always be exercised in such areas if thunderstorm activity is
initiated nearby. Also, some convective activity may result in little or no
rainfall while still being capable of generating lightning, which poses a risk
of wildfire initiation.



Tropical Depression 2C  formed today in the Central North Pacific and is
favored to pass south of Hawaii as a tropical storm within the next week. The
National Hurricane Center (NHC) is anticipating heavy rainfall early next week
across portions of the state, likely exacerbating saturated ground conditions
associated with heavy precipitation from the passage of Hurricane Lala last
week. Above-normal precipitation is anticipated during week-2 as well,
potentially extending impacts related to the recent rainfall and anticipated
week-1 influence from the aforementioned potential tropical storm to the south.



For Alaska, there is some model support for the remnants of Tropical Cyclone
Lala bringing enhanced and possibly heavy precipitation to portions of western
and southwestern Alaska as it recurves along a forecast ridge in the northern
Pacific. No hazards are posted at this time since the preponderance of tools
demonstrate only lackluster support, if any, but this potential will continue
to be monitored in upcoming outlooks.

FORECASTER: Rich Tinker

$$