064
FXUS02 KWBC 031840
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
240 PM EDT Sun Aug 3 2025

Valid 12Z Wed Aug 06 2025 - 12Z Sun Aug 10 2025

...Major to Extreme heat threat focus for the Southwest mid-late
week to expand through the south-central High Plains...


...Overview...

A cooling upper trough into the eastern third of the nation will
be periodically reinforced by disturbances digging from the
Plains/Midwest to the South/Southeast. This is to the lee of an
upper ridge anchoring over the Southwest and spread to the south-
central High Plains. A wavy stalled front will support heavy rain
threats over the Southeast past midweek, but as a Bermuda High
begins to build westward, wet weather is forecast to spread
gradually up the eastern U.S. later week/next weekend, albeit with
uncertainty of lift/expansion given variance with vorts/impulses
embedded in the upper trough and uncertain interactions with any
downstream coastal wave/potential tropical development and track.
Meanwhile, upper troughing will progress eastward over the north-
central U.S. to fire northern Plains to Midwest thunderstorms with
heavy downpours.

...Guidance/Predictability Assessment...

Updated 00/06Z deterministic guidance is in generally good
agreement through the first part of the medium range period mid- to
late next week. The synoptic pattern will feature a dominant area
of high pressure over the southwestern to south/central U.S. with
the main storm track shifted north along the U.S./Canadian border.
Periodic shortwaves help to reinforce mean upper-troughing over the
northwestern U.S. This will keep precipitation chances generally
limited to the northern tier of the U.S. along frontal waves ahead
of the upper-trough, as well as east of the upper-high over the
southeastern U.S. to Mid- Atlantic where a lingering frontal
boundary remains following the passage of a strong cold front this
past week and under the influence of an upper-level weakness. There
is one specific area of uncertainty with regards to the potential
for development of a coastal wave/low pressure off the coast of the
Southeast mid- to late next week with significant impacts on
precipitation chances. The National Hurricane Center has now
included low probabilities for tropical development as well. The
ECMWF/CMC and ECens mean show the potential for possibly more
significant heavy rainfall along the central Appalachians, with the
GFS along the Carolina Outer Banks. EC AI guidance/GFS Graphcast
show higher QPF further south along the southern Appalachians with
coverage varyingly more northward along the coastal Mid- Atlantic.
Updated QPF for this region used the NBM, with a local maxima along
the coastal Carolinas, and the ECens which helps bring up amounts
a bit in favorable upslope areas along the central Appalachians.
This also raises amounts broadly a bit compared to the NBM and
prior forecast given the signal for heavier rainfall with onshore
flow in the general vicinity, and the caveat that the ceiling could
be much higher.

Heading into next weekend, both the deterministic and ensemble
guidance also agree that additional more potent upper-energy over
the northeastern Pacific will help to strengthen the upper-trough
over the northwest and bring this feature eastward as the upper-
high over southwest to south/central U.S. weakens. However, there
is some disagreement in the progression, with the GFS in particular
more progressive with a stronger surface reflection in low
development over the north-central U.S. to south-central Canada.
All other deterministic guidance including from the EC AI suite and
GraphCast GFS remains slower and less aggressive. The updated WPF
forecast began with a composite blend of the deterministic guidance
given good agreement on the Synoptic pattern through mid- to late
next week. A contribution from the ensemble means was included for
next weekend as a replacement for the GFS, which notably diverged
from the other guidance, and for the time-limited UKMET.

...Weather/Hazards Highlights...

It generally still remains the case that a rather expansive cool
airmass will settle into the eastern U.S. for the first week of
August. High moisture content and instability near a front on the
leading edge of this airmass will focus some localized heavy rain
to raise flash flooding risks given wet antecedent conditions. WPC
Excessive Rainfall Outlook (ERO) Marginal Risk areas are depicted
for Day 4/Wednesday and Day 5/Thursday for the Southeast through
south-central portions of the Appalachians/Mid-Atlantic. The
potential for coastal wave development in addition to overall moist
onshore flow from the Carolinas northward into the
Appalachians/Mid- Atlantic would likely require at least one
embedded Slight Risk as confidence increases on location given
higher ceiling on rainfall amounts indicated in the model guidance.

Farther west, shortwaves rounding into the eastern edge of the
upper ridge from the Rockies/Plains are forecast to interact with a
downstream frontal zone. By early-midweek, there is a trend for
the northern Plains/Upper Midwest/Great Lakes to see increasing
rain and storms. WPC Day 4/5 Marginal Risk ERO areas are depicted
for Wednesday and Thursday given likely moisture/instability
pooling with shortwave passage. Activity will spread more across
the Great Lakes and Midwest later in the week. Meanwhile, the Four
Corners states could see some increasing monsoon showers underneath
the broad ridge, amid the relatively dry monsoon season especially
across Utah/Arizona. Some moderate rainfall chances may emanate
later period from over the Pacific Northwest under the influence of
a cooling upper-level trough given amplifying flow trends.

Cooler than average temperatures will continue on the cool side of
the main frontal system for the central to eastern U.S. and slowly
moderate through the week. Meanwhile across the Southwest,
dangerous heat will build throughout the week with some location
high temperatures as high as 110-115F, equating to major to extreme
HeatRisk. Some relative relief may come next weekend as the upper-
high begins to weaken. Temperatures will also be warming to above
normal into the southern and central High Plains as the upper ridge
expands. Some record high temperatures there could be challenged
and eclipsed through mid-later week over the Southwest. Florida is
forecast to see seasonable temperatures in the 90s, but with ample
humidity allowing for some areas of Major HeatRisk there as well.


Putnam/Schichtel


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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