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FXUS02 KWBC 101854
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 PM EDT Sun Aug 10 2025

Valid 12Z Wed Aug 13 2025 - 12Z Sun Aug 17 2025

19Z Update: The 12Z forecast models remain in good overall
agreement across the Continental U.S. for the middle to end of the
week, and a multi-deterministic model blend works as a starting
point in the forecast process. At the time of fronts/pressures
preparation, the 00Z CMC strayed from the consensus with the main
upper low/trough over southern Canada by Friday and into next
weekend, so more of the GFS/ECMWF was applied by Days 6 and 7, but
the 12Z CMC has trended closer to the consensus. The ensemble
means were gradually increased to about half by next Sunday amid
growing model differences across the Pacific Northwest and the
northern tier states. The previous forecast discussion is appended
below for reference. /Hamrick
-----------------

...Overview...

Increased troughing into the Pacific Northwest later this week
will linger through the upcoming weekend as above normal heights
dominate much of the rest of the Lower 48. Slow-moving frontal
boundaries in the East will spark convection and locally heavy
rainfall amid seasonably warm to hot temperatures, with at least
some signal for monsoon convection in the Southwest.

...Guidance/Predictability Assessment...

Through the 12Z/18Z guidance, most of the deterministic runs
offered reasonable clustering with the ensemble means except for
the 12Z Canadian over western Canada. The ECMWF AIFS EPS was a bit
slower/deeper than its dynamical counterpart which was more in line
with the ECMWF/GFS. Still, timing the shortwaves across Canada
exhibits lower confidence beyond day 3 as is typical in the summer.
Trended toward an even split between the ensembles and
deterministic GFS/ECMWF by the end of the period.

...Weather/Hazards Highlights...
Near and ahead (and south) of a frontal boundary astride the Great
Lakes, locally heavy rain is expected within a moist and unstable
air mass. The western end of this front over the High Plains will
push eastward as a warm front, igniting some locally heavier rain
across the Upper Midwest Wednesday into Thursday. Over the
Southwest, monsoonal moisture will make another attempt at pushing
northward midweek. These areas are denoted with a Marginal risk of
flash flooding (excessive rainfall) Wednesday-Thursday. For the
rest of the period (Fri-Sun), rainfall chances will be modestly
high over the Upper Midwest near a couple frontal boundaries, over
the Southeast, and across the Southwest.

Temperatures will start the period quite warm into the Northeast
to near record levels Wednesday with highs into the 90s. This will
push HeatRisk levels into the Moderate (level 2) to perhaps Major
(level 3 out of 4) category from DC northward into New England.
Temperatures will remain near to several degrees above normal over
much of the CONUS east of the Rockies, especially over the Midwest
into the Northeast/Mid-Atlantic (though likely below record
levels). Over the Pacific Northwest into California, temperatures
will trend cooler than normal as troughing digs into the West.


Fracasso


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






$$