458
FXUS02 KWNH 260658
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Sat Jul 26 2025

...Dangerous heat wave persists in the Southeast through midweek
with gradual relief later week...

...Overview...

A large and strong upper high meandering over the central/southern
Plains into the Mid-South will cause a persistent and major heat
wave for those areas and especially into the Southeast through
midweek. Dangerous heat is forecast to extend north through the
Ohio Valley and Mid-Atlantic/Northeast Tuesday-Wednesday but
gradually get pressed southward behind a cold front as the week
progresses. Complexes of showers and storms are likely along the
northern periphery of the ridge, across the northern/central High
Plains eastward into the Midwest. Scattered thunderstorms are
possible farther east at times, especially near the front as it
pushes south and along a retrograding trough/low near the Gulf
Coast. Additionally, southerly flow will bring monsoonal moisture
into the Southwest states for possibly heavy rain.


...Guidance/Predictability Assessment...

Model agreement is excellent for the overall pattern through much
of next week. Models are unified in showing an expansive upper high
of 2+ standard deviations above normal, with widespread 594+ dm
500mb heights but with low chances of reaching 600 dm. Guidance
shows the high largest into Tuesday as it encompasses the south-
central Rockies east across the Mississippi and Ohio Valleys and
Southeast, before shrinking in scope and being centered over the
southern Plains by later next week. Meanwhile the ridge axis will
shift west and build north across the northern Rockies into western
Canada starting Tuesday, coincident with the upper trough edging
into the north-central to northeastern U.S. anchored by a large
Hudson Bay to eastern Canada upper low. The main model difference
on the larger scale was by Friday-Saturday--the 12Z GFS indicated a
stronger shortwave in the northern Plains than the bulk of other
guidance, bringing it out of phase, so leaned against it.

Regardless of the large scale agreement, shortwaves riding around
the ridge will cause more differences in sensible weather like
frontal placement, and convection will have further uncertainties
due to small boundaries like outflows interacting. These tend not
to become clearer until the short range or near term, so the
details of QPF remain uncertain, but at least the overall pattern
seems set.

The WPC forecast was initially based on a blend of the 12Z ECMWF,
18Z GFS, 12Z CMC, and a bit of 12Z UKMET. Used some GEFS and EC
ensemble means by the latter part of the period, but was able to
maintain a majority of deterministic guidance with the good general
agreement.


...Weather/Hazards Highlights...

The persistent and strong upper ridge will lead to continued
dangerous heat that will be longest lasting across the Tennessee
Valley/Mid-South and Southeast into the Carolinas next week.
HeatRisk is in the Major (level 3/4) to Extreme (level 4/4)
categories for these areas through Wednesday or Thursday. High
temperatures will be in the upper 90s and 100s with heat indices
potentially exceeding 110 to 115 degrees, while morning lows well
into the 70s and nearing 80 will not provide much relief. This is
likely to set scattered daily records for maximum temperatures and
perhaps more widespread records for minimum temperatures. Farther
north, the Ohio Valley into the Mid-Atlantic and Northeast can
expect hotter than normal temperatures into the 90s with Major
HeatRisk into Tuesday, but these areas are likely to cool by
Wednesday or Thursday behind a cold front. This cold front
eventually making its way southward should finally lead to heat
relief in much of the Southeast later in the week, though Florida
could still be hot.

Rain and thunderstorms are forecast to focus on the periphery of
the ridge. Some upper jet and shortwave forcing combined with
sufficient moisture and high to extreme instability just ahead of a
front or two should lead to convection across the northern Rockies
into the north-central Plains and Mississippi Valley on Tuesday
and Wednesday. These ingredients will likely lead to heavy rain
rates, but model spread remains high with the axis of storms and
placement of heaviest rain. Another complicating factor for
potential flash flooding is the possibility of the maximum rain
amounts falling over the Nebraska Sandhills, not a sensitive area
for heavy rain. For those reasons, held off on any Slight Risk
areas in the Day 4/Tuesday and Day 5/Wednesday EROs at this point,
but broad Marginal Risks cover these areas and eventually Slight
Risks may need to be added.

Meanwhile to the west of the ridge, monsoonal moisture (with
precipitable water values generally in the 75th-90th percentile)
will be present over the Southwest with some instability. Marginal
Risks are in place in the Days 4 and 5 EROs for the Southwest and
reaching into the central Rockies/High Plains as the monsoonal
moisture meets the frontal system to the north. The central Rockies
and Plains look to be areas of focus for heavy rain into Thursday
and Friday as well.

Elsewhere, moisture and instability could lead to scattered
thunderstorms in parts of the eastern half of the U.S. from day to
day. There may be some focus for storms over northern
Florida/southern Georgia Tuesday and near the central Gulf Coast
Wednesday as a surface/low level trough/weak low retrogrades.
Marginal Risks of flash flooding are in place for those areas due
to high rain rates that may overcome the high Flash Flood Guidance.
Farther north, moisture pooling along the front pushing south
could enhance rainfall. Rain amounts do not look too high through
Wednesday, but by Thursday the rain is likely to increase in the
Mid-Atlantic and Carolinas on Friday.


Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw



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