Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Sat Sep 20 2025
Valid 12Z Tue Sep 23 2025 - 12Z Sat Sep 27 2025
...Heavy rain threat for portions of the central U.S. to
southern/central Appalachians next week...
...Overview...
The main system of note during the medium range period will be a
deepening shortwave into the Central U.S. on Tuesday with
increasing confidence the energy will close off into a cutoff low
and move rather slowly eastward across the Midwest and Ohio Valley
for the latter half of the week. This should focus rain that could
be heavy near a resultant surface low pressure/frontal system in
the lower/mid-Mississippi Valley, Ohio Valley/Mid-South, and
southern/central Appalachians. Meanwhile, another cutoff low off
the California coast could lead to a round of rain over southern
California into the Southwest as it lifts north and then east,
while an upper ridge ahead of this low over the Southwest could
lead to a period of triple digit heat early-mid week.
...Guidance/Predictability Assessment...
An omega block pattern looks to set up over the CONUS next week,
consisting of an upper low near/over California, a large cutoff low
over the Central U.S. moving east, and an amplified ridge in
between. Recent model agreement is better than a day or two ago
with the evolution of the upper low forming near the central
Rockies/High Plains by Tuesday and tracking east slowly through the
mid-Mississippi Valley and reaching the Ohio Valley by late week.
The associated surface low also shows reasonable consensus. A
multi-model blend with some ensemble inclusion worked well for
these features and maintained good continuity with the previous
forecast. But there is still some model spread with the timing of a
northern stream trough moving across the Northeast, affecting a
cold frontal position ahead of it and precipitation chances. The
00/06Z ensemble means, especially the CMC/ECens, and the 00Z CMC
tended to be farther west than the GFS and ECMWF runs that were
east. Now with the 12Z runs, the ECMWF has slowed down but the CMC
has sped up. So will continue to wait for better consensus in that
region.
There is general agreement for a smaller southern stream upper low
to drift northward from the eastern Pacific into California and
then move east through the period. Some timing and placement
differences are seen, but typical for the medium range period. In
the northern stream, model agreement has increased with showing low
amplitude flow as a weak shortwave moves through British Columbia
around midweek, and a northeastern Pacific trough perhaps edging
just into Washington late week. This leads to the southern stream
low potentially remaining a separate feature in the Intermountain
West even into late week. The West could also be handled with the
WPC forecast approach of a multi-model blend early in the period
with increasing proportions of the ensemble means to half by the
end of the period.
...Weather/Hazards Highlights...
Widespread rain will focus near the upper low and associated
frontal boundaries as it becomes more defined by Tuesday. Guidance
continues to show increasing agreement on an area of heavy rainfall
for eastern parts of the south-central Plains and the lower/mid-
Mississippi Valley. Show a Slight Risk for these areas in the Day
4/Tuesday ERO with a broader Marginal Risk for surrounding areas.
This is considered a higher-end Slight Risk atop parts of the
Ozarks, where heavy rain rates are forecast to train over this
typically flood-prone area. By Wednesday, the slow movement of the
upper/surface low eastward should carry the heaviest rain chances
east with it, into parts of the Tennessee Valley into the lower
Ohio Valley. Uncertainty increases with the details by then, but
this area seems to have enough consensus for a Slight Risk on Day
5/Wednesday, though its placement may move in future issuances.
There is increasing potential for localized heavy rain in unstable
air ahead of the low's cold front, so extended the Marginal Risk to
the Gulf Coast, despite some model spread in placement. There is
quite a bit of uncertainty in the northern side of the frontal
system that could allow for more rainfall in the Great Lakes and/or
Northeast Tuesday/Wednesday. The rain activity with the primary
low will move slowly eastward with time after midweek into parts of
the East, with a likely heavy rainfall focus in the south-central
Appalachians. Farther south, some diurnally driven showers and
storms are possible across the Florida Peninsula throughout the
period.
Meanwhile, an upper low lingering near California early-midweek
should result in some rainfall over the state and into the
Southwest as ample moisture advects ahead of it. Precipitable water
anomalies could be 3-4 standard deviations above normal into
California, though instability appears weaker in this event
compared to the recent rain event. Will show Marginal Risks for
parts of California and Nevada on Days 4 and 5 for isolated
flooding potential with the much above normal moisture. Burn scars
will be particularly vulnerable to flooding. There are some timing
differences with the upper low and thus precipitation timing, so
continue to monitor forecasts for refined details. Lighter
precipitation is forecast to spread into parts of the Intermountain
West as the energy moves north and east. The Pacific moisture
could allow for convection across the Southwest as well, so
delineated a Marginal Risk for portions of Arizona and New Mexico
into far west Texas on Day 5/Wednesday. The Northwest should
generally be dry until possibly late week ahead of a possible upper
trough.
A ridge axis will build from the Four Corners states into the West
with some above normal temperatures across northern California and
the northern tier states. The Desert Southwest may see
temperatures reaching or exceeding 100F, mainly Tuesday-Thursday.
Expect near to below normal temperatures for the Central U.S. under
the large upper low. As the low drifts eastward as the week
progresses, temperatures will cool a bit from above average across
the East to near average.
Tate/Santorelli
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