152
FXUS21 KWNC 191821
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 19 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 Middle and Lower Mississippi Valley.

HAZARDS

High risk of extreme heat for parts of the Desert Southwest, Southern Rockies,
Southern and Central Plains, and adjacent areas of the Mississippi Valley,
Thu-Sat, Aug 27-29.

Moderate risk of extreme heat for much of the Desert Southwest, Rockies,
Central and Southern Plains, parts of the Middle and Lower Mississippi Valley,
Thu-Mon, Aug 27-31.

Slight risk of extreme heat for portions of the California, Desert Southwest,
Great Basin, Rockies, Central and Southern Plains, Middle and Lower Mississippi
and Tennessee Valleys, and Southeastern CONUS, Thu-Wed, Aug 27-Sep 2.

Slight risk of heavy precipitation for the coastal Southeast, Mid-Atlantic and
Northeast, Thu-Fri, Aug 27-28.

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

DETAILED SUMMARY

FOR SATURDAY AUGUST 22 - WEDNESDAY AUGUST 26:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR THURSDAY AUGUST 27 - WEDNESDAY SEPTEMBER 02: Dynamical models remain in
good agreement and are bullish in regards to a highly amplified subtropical
ridge center and an accompanying extreme heat risk mainly focused from the
Southern Plains into the eastern Four Corners region next week. Over the past
few days, both the GEFS and ECMWF have trended stronger with the amplitude of
the ridge center, where the ECMWF, in particular, now features >50% chances of
500-hPa heights exceeding 597 dm later next week. A subtropical height feature
of this strength is unusual for late August, and translates to over 2-2.5
standard deviations above climatology consistent with a potentially major heat
event unfolding. By the middle of the week-2 period, ensembles show the ridge
center gradually becoming less amplified, but generally maintain the ridge
center over the eastern Four Corners. In addition, anomalous 500-hPa ridging is
also featured in the ensembles extending along an axis northward into Canada,
supportive of above-normal temperatures and possible extreme heat conditions
emerging over portions of the Interior West, Plains, and Mississippi Valley
next week. Bookended by a pair of mean troughs over the eastern Pacific and
eastern CONUS, the 500-hPa mean height forecasts continue to reflect an Omega
block presentation. Such a pattern would likely be quite stable, where the
elevated risks for extreme heat across lower Four Corners and Southern Plains,
as well as widespread, above-normal temperatures for much of the western and
central CONUS could be of long duration with little respite later in August and
possibly into early September.



Underneath the amplified ridge center favored, a high risk of extreme heat
remains posted, and is expanded to include additional portions of the Desert
Southwest, and now valid through Aug 29 based on increased support in a number
of temperature and heat tools. Within the high risk area, the NWS probabilistic
Heat Risk tool shows elevated chances for an extreme category heat event
extending from the Desert Southwest eastward to the Southern Plains and Lower
Mississippi Valley by late next week, and calibrated heat tools continue to
highlight >60% chances for daytime temperatures and/or heat index values
exceeding the 90th percentile. In the Southern Plains, daytime temperatures are
likely to exceed 100 deg F (except for the higher elevations) with heat index
values reaching above 110 deg F where higher dewpoints are expected. Over the
lower elevations of the Desert Southwest, maximum temperatures look to exceed
110 and 115 deg F in some locations based on the latest probabilistic and
deterministic guidance.



Surrounding the high risk area of extreme heat, a corresponding moderate risk
area remains posted for Aug 27-31 where raw and calibrated probabilistic tools
show lesser, but increased chances for temperatures exceeding the upper
climatological percentiles. The moderate risk area is inclusive of an increased
number of locations in the National Blend of Models (NBM) that are forecast to
approach and/or break daytime highs and nighttime lows, and where there are
elevated chances for positive daytime temperature anomalies in the
double-digits. 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
capped to the Central Rockies and Central Plains. While tools continue to
feature very warm signals tied to the anomalous ridge axis extending into
Canada, there is less confidence that actual temperatures and/or heat index
values will reach hazardous thresholds due to rapidly decreasing sun angles
over the higher latitudes in August. This is also supported in the heat index
tools depicting a limited potential for values exceeding 100 deg F over the
Northern Plains and Upper Mississippi Valley.



In the updated outlook, the one day extensions of both the high and moderate
risks of extreme heat ending later in August are not only supported by the
stable pattern favored, but also by feedback processes associated with rapidly
drying soils and drought being registered within the highlighted regions. The
hot and dry conditions favored are also expected to aggregate fuels and bring
increased chances for elevated fire weather conditions during the period. In
addition, a Rapid Onset Drought (ROD) risk also remains designated across
portions of the Central and Southern Plains and Middle and Lower Mississippi
Valley. This ROD area continues to register 30-day precipitation deficits
greater than 2 inches with little rainfall 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 to increase the risk of heavy precipitation later next week.
Although raw tools show the highest precipitation amounts remaining offshore,
uncalibrated GEFS and ECMWF, and the ECMWF Probabilistic Extremes Tool (PET)
continue to feature at least 20% chances for 3-day amounts exceeding a 1 inch
across portions of the Eastern Seaboard with an uptick in signals the tools
over the Northeast. Therefore, a slight risk of heavy precipitation remains
posted from coastal South Carolina and Mid-Atlantic, with coverage now
including parts of the Northeast for Aug 27-28.



Across the Desert Southwest, the aforementioned subtropical ridge center is
forecast to be in a less favorable position for the advection of monsoonal
moisture. However, 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 an
increased precipitation amounts 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.



In addition to the eastern Pacific, additional tropical cyclone formation is
also favored in the central Pacific during the next week which is highlighted
in the National Hurricane Center (NHC) Tropical Weather Outlook (TWO). Should
development occur with a track in the vicinity of the Hawaiian islands,
additional rainfall is likely to exacerbate saturated ground conditions
following the passage of Hurricane Lala to the south of the Big Island this
past week, as well as possibly bring other adverse impacts to the state later
next week.



For Alaska, there is some model support for the remnants of Tropical Cyclone
Lala (currently a Major Hurricane in the Central Pacific) bringing enhanced and
possibly heavy precipitation to portions of western and southwestern Alaska as
it recurves along a ridge favored in the northern Pacific. No hazards are
posted at this time due lack of support in the PETs, but this potential will
continue to be monitored in upcoming outlooks.

FORECASTER: Nick Novella

$$