782
FXUS02 KWBC 150741
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
341 AM EDT Mon Sep 15 2025

Valid 12Z Thu Sep 18 2025 - 12Z Mon Sep 22 2025


...Overview...

A blocked upper air pattern develops over North America by Thursday
with an anomalous ridge through central Canada with a low stalled
to the south over the northern Plains into this weekend. Meanwhile,
troughing lingers along the West Coast while ridging persists over
the Mountain West and Mountain West and Midwest through Southeast.
Wetter than normal conditions can be expected later this week into
the weekend over the Great Plains states, South Florida, and
California into the Desert Southwest. Cool air damming is looking
likely by Saturday down the Eastern Seaboard into the Carolina
Piedmont.


...Guidance/Predictability Assessment...

The 00Z deterministic model suite is in good alignment with the
rather anomalous ridge axis (blocking high) extending north from
Saskatchewan to the Canadian Archipelago by 12Z Thursday with a
trapped upper low centered over the Dakotas through Saturday.
Meanwhile troughing down the West Coast lingers into Sunday with
the EC still producing the most cutoff/retrograding low that
remains well off California while the GFS/CMC bring the cutoff low
back to the CA coast by Sunday. Ridging persists over the
Intermountain West and the Midwest with a deep low over far
northern Quebec and Labrador. Preference for the WPC forecast went
to the 12Z EC deterministic which was in good agreement with the
18Z ECAIFS for location of the Plains low. There is a noted slowing
to the low which means repeating rains for the north-central
Plains into the Upper Midwest Thursday to Saturday. More troughing
over the CA coast means more rain than previously forecast for
CA/the Great Basin as well.


...Weather/Hazards Highlights...

Enhanced Gulf moisture surges up the Plains to meet the low
pressure system over the north-central Plains Thursday/Friday with
only slow shifting east likely due to occlusion. A dry slot is
expected to form, but wrapping around it should be continued
convection. The Day 4 Marginal ERO continues a westward drift now
including eastern SD, southern MN and down along the KS/MO border
(including the KC metro). The Day 5 Marginal ERO is introduced just
east from southern MN, much of IA down along the MO/IL border
including the St. Louis metro.

A notable Pacific moisture surge up into California and the lower
Colorado River region into the Great Basin is on tap for Thursday
into Friday as the upper ridge axis developing over CA today
shifts east to the Intermountain West as troughing amplifies
along/off the coast. There is a bit of a west trend to QPF on
Thursday, so the Day 4 Marginal ERO is trimmed north of Mt
Charleston in southern NV with Death Valley removed. Uncertainty
with where to put the Day 5 ERO due to lingering anomalous
moisture, so for now the Sierra Nevada were drawn, but it may have
to be expanded over the Mojave and Sonoran Deserts depending on
model QPF trends.

A stalled front over the southern Florida Peninsula provides focus
for much above normal moisture and combined with wet antecedent
conditions Thursday with the Day 4 Marginal ERO maintained/trimmed
to just the Miami-Palm Beach metro corridor. The moisture focus
looks to shift a bit east on Friday, so no ERO is introduced there
for now.

An upper ridge will promote heat over the Midwest and Southeast
through Thursday to Saturday with max temp anomalies of 5-12
degrees above normal. A strong surface high looks to send a decent
cold front over much of the eastern states by Saturday with cool
air damming lingering east of the Appalachians and north from the
Carolina Piedmont at least into next Monday. Below normal
temperatures from onshore flow, clouds, and possible rain can be
expected.


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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