011
FXUS02 KWBC 051857
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Tue Aug 5 2025

Valid 12Z Fri Aug 08 2025 - 12Z Tue Aug 12 2025

...Major to Extreme Heat for the Southwest and South-Central
Plains; threat spreads to the Midwest/Great Lakes late week...

...Southeast U.S. coastal low being monitored by the NHC...

19Z Update: The 12Z model guidance suite features good synoptic
scale agreement across the nation for the end of the week into
Saturday, so a general deterministic model blend suffices as a
starting point in the forecast process. Looking ahead to Sunday,
the UKMET becomes faster with the low pressure system across the
northern Plains and southern Canada, so a non-UKMET blend was used
by this time to account for this. By early next week, the CMC
becomes more amplified with an upper level trough building over the
Pacific Northwest that the other guidance doesn`t support as much,
so it was dropped from the blend for next Tuesday. The GFS is
displaced to the west of the model consensus with the placement of
the southeast upper ridge by Tuesday as well, so the GFS was
trimmed back by early next week. The ensemble means were increased
to about half by Tuesday owing to increased differences across the
northern tier states and into southern Canada. The previous
overnight discussion is appended below for reference. /Hamrick
-------------------

...Overview...

An amplified upper trough will gradually transit east across the
Northern U.S., driving thunderstorm chances from the Northern
Rockies to the Midwest. A wavy frontal boundary will linger along
the Gulf and Southeast Coasts, resulting in unsettled weather for
portions of the Southeast and Mid-Atlantic, and NHC is monitoring a
coastal low for potential tropical development mid/late week.
Meanwhile, a strong ridge will be in place over the Southwest and
Southern/Central Plains, which will create a significant Major to
Extreme heat threat through the end of this week. High pressure
ridging should extend into the Midwest/Great Lakes late this week
into the weekend, resulting in increased temperatures and
potentially a Major heat threat in these regions. There may also
be an uptick in monsoonal flow into the Southwest/Southern Rockies
by the weekend given some eastern Pacific moisture connection.

...Guidance/Predictability Assessment...

Recent guidance continues to show really good agreement on the
upper level pattern through the period, resulting in above average
forecast confidence. All available guidance is capturing the
synoptic pattern well, with strong upper ridging anchored over the
Southwest and a deep trough slowly moving east across the northern
U.S. with several embedded shortwaves.

However, there is some uncertainty in the details of the sensible
weather at the surface, with some differences in precipitation
amounts and locations. Overall, the general consensus is for the
heaviest rain to focus over the north-central Plains/Upper Midwest
and from the Southeast Coast/Florida to the southern Mid-Atlantic.
In the Southeast, precipitation intensity and location is also pending
any frontal wave to tropical development later this week. Guidance
has varied with the system and coastal to Appalachians QPF focus.

The latest WPC medium-range product suite was mainly derived from
a composite of GFS/ECMWF/UKMET/Canadian features for Friday and the
weekend along with WPC continuity and the National Blend of Models
(NBM). Opted to switch to a composite of then best clustered
guidance of the GFS/ECMWF and GEFS/ECMWF ensemble means along with
the NBM and WPC forecast continuity valid for early next week.

...Weather/Hazards Highlights...

A rather expansive cool high pressure airmass will linger over the
Eastern U.S. this week. High moisture content and instability near
a front on the leading edge of this airmass will focus some
localized heavy rain over the Southeast, which may raise flash
flooding risks given wet antecedent conditions. WPC`s Excessive
Rainfall Outlook (ERO) depicts Marginal Risk areas for Day
4/Friday and Day 5/Saturday from southeast Georgia to the Hampton
Roads area of virginia, while the Marginal Risk area across Florida
was removed given a less impressive QPF signal there. This area is
quite uncertain with how the front and coastal wave/low will
evolve, and NHC is monitoring this area for potential tropical
development offshore later this week.

Locally strong convection and heavy rains may also be possible in
the north-central U.S. as shortwaves round to the eastern edge of
the upper ridge from the Rockies/Plains and interact with a
downstream frontal zone. Late week, expect the Northern to Central
Plains/Upper Midwest/Great Lakes to see periodic rain and
thunderstorms. WPC`s Day 4/5 EROs depict Marginal Risk areas for
these regions on Friday and Saturday given moisture/instability
pooling with shortwave passages. Activity will spread into the
Great Lakes and down to the central Plains/Midwest this weekend.
Meanwhile, the Southwest/Southern Rockies states could see some
increasing monsoon showers by Saturday underneath the broad ridge
given some moisture connection to an active east Pacific, with a
Marginal Risk area added for portions of eastern Arizona and
western New Mexico for Day 5.

Cooler than average temperatures will continue on the cool side of
the main frontal system for the East/Southeast U.S., while
dangerous heat builds across the Southwest and Southern/Central
Plains. Some locations may see high temperatures as high as 110-115
degrees, 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 across portions
of the Midwest and Great Lakes late this week into the weekend then
eventually the Northeast, which may create a Major HeatRisk. Some
record high temperatures could be challenged and/or eclipsed late
this 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.

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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