725
FXUS02 KWBC 170638
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 AM EDT Wed Sep 17 2025

Valid 12Z Sat Sep 20 2025 - 12Z Wed Sep 24 2025

...Desert Southwest Heat Wave First Half of Next Week...

...Overview...

The upper air pattern this weekend begins with a deep low lifting
north over the Labrador Sea with a descendant trough that also
lifts north from the Northeast, a filling/weakening low over the
Upper Midwest, and a potent trough reaching the British Columbia
coast by Saturday night which serves to further cutoff low off the
California Coast. That potent trough then likely develops into a
low over the Upper Midwest Tuesday and becomes the notable weather
feature through midweek. Meanwhile, strong ridging over the
Southwest through the first half of next week brings a heat wave.
Some locally heavy rain possibly lingers this weekend over the
Upper Midwest with some progression to development over the
southern Plains. Cool air damming will spread down the Eastern
Seaboard through the Carolina Piedmont Saturday and linger possibly
into Tuesday.


...Guidance/Predictability Assessment...

Global deterministic guidance is in fairly good agreement on timing
and strength of large low/troughs over Canada this weekend with the
progressive Labrador low and trough coming from the Gulf of
Alaska. The GFS remains more progressive with the western Canadian
trough that shifts over Montana Sunday night with EC and GFS in
decent agreement on deep low pressure developing for midweek over
the Great Lakes. However, troughing out west is notably later with
the EC, making the midweek forecast rather uncertain (though the
ensemble means are in better agreement with a slower than the GFS
solution out west. The WPC forecast was based on a general model
blend through Day 5 that then favored the GFS and EC and their
ensembles for days 6/7.


...Weather/Hazards Highlights...

Stalled low pressure over the Upper Midwest on Saturday continues
to provide a frontal focus for Gulf-sourced moisture streaming up
the Plains. A Day 4 Marginal ERO is retained/expanded north a bit
over MN/WI.

Farther south convergence of this Gulf-sourced moisture allows some
slowly progressive heavy activity over portions of the southern
Plains/Mid-South where Day 4 and 5 Marginals are raised for the
KS/OK/MO/AR area. Moisture is not that anomalous, but PW on the
order of 1.5" is sufficient for excessive rainfall concerns with
slow moving activity/flow parallel to any boundaries.

An upper ridge will allow heat to persist over the Midwest and
Southeast this weekend before shifting to the Interior Northeast
for early next week with max temp anomalies of 5-15 degrees above
normal. A strong surface high crossing the Northeast Saturday
will send a decent cold front down much of the Eastern Seaboard
with cool air damming lingering east of the Appalachians and likely
through the Carolina Piedmont at least through Monday and possible
tuesday. Below normal temperatures there from onshore flow,
scattered clouds, and possible showers can be expected.

The cutoff low off California and amplifying trough over the
northern Plains Monday spinning into a deeper low over the Upper
Midwest allows strong (though somewhat progressive) ridging over
the Desert Southwest for Monday, Tuesday, and possibly Wednesday
making for a heat wave. The high temperature record for Tuesday at
Phoenix of 108F may be challenged.



Jackson


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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