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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1859Z Sep 30, 2023)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Sat Sep 30 2023

Valid 12Z Tue Oct 03 2023 - 12Z Sat Oct 07 2023

...Heavy rainfall potential over parts of the central U.S. next


Latest guidance continues to show an amplified upper pattern next
week. A mean trough over the Rockies on Tuesday should progress
into the east-central U.S. by next Saturday, while an eastern
Pacific ridge drifts toward the West Coast and initial eastern
U.S. ridge continues onward into the Atlantic. Moist and unstable
inflow just ahead of the upper trough and an associated wavy cold
front will likely produce an emerging convective pattern for much
of the Plains Tuesday and focused in the southern Plains by
Wednesday, with some of this rainfall likely to be heavy. Expect
mostly lighter rainfall to push into the eastern U.S. later in the
week. Cooler than normal early-mid week temperatures over the West
under the upper trough will likely moderate and push into the
central U.S. as the week progresses, though with potential for a
reinforcing cold front to bring cooler air east of the Rockies
toward the end of the week. Warmer than normal temperatures across
the northeastern quadrant of the CONUS will become more limited in
area by late week while the upper ridge nearing the West Coast
will bring warmer temperatures to that region.

...Guidance Evaluation/Predictability Assessment...

Operational models and ensemble means agree on the general idea of
gradual eastward progression of the amplified ridge-trough-ridge
configuration but there are notable detail uncertainties that
complicate the forecast of sensible weather over some regions.
Particular issues of note involve Tuesday-Wednesday frontal
wave(s) lifting from the northern Plains into Canada as shortwave
energy ejects northeastward from the Rockies, what happens with
energy in the southern Rockies part of the initial trough
Wednesday-Thursday, and specifics of eastern Pacific-West Coast
ridging/peripheral shortwave energy impacting the evolution of
eastern North American troughing late next week.

Fairly small-scale (thus having lower predictability) detail and
timing differences are affecting the precise evolution of northern
Plains into southern Canada low pressure during Tuesday-Wednesday.
In guidance through the 00Z-06Z cycles, the GFS/GEFS mean were
generally faster and more consolidated with the surface system
(fully over southern Canada by early Wednesday) while to varying
degrees the 00Z ECMWF/CMC/UKMET depicted a second trailing wave.
The ECens/CMCens means hinted at the ECMWF cluster. Overall
preference was to lean away from the slowest side of the spread,
and new 12Z model runs strongly favor a faster trailing wave if it

Regarding the southern part of the upper trough, 00Z and new 12Z
CMC/UKMET runs pull a notable amount of energy into the
southwestern U.S., in contrast to ECMWF runs that remain phased
and progressive. Interestingly, the GFS has trended from pulling
off some energy in the 00Z/06Z runs to being more like the ECMWF
in the 12Z run. The full ensemble spread includes all of these
possibilities, but the CMC/UKMET scenario is sufficiently in the
minority for the ensemble means to be more like the ECMWF and 12Z
GFS. Thus the preference for this aspect of the forecast was
closest to the ensemble means/ECMWF.

Differences continue with how energy over and upstream from the
Gulf of Alaska feeds into the mean trough based on the character
of the eastern Pacific ridge drifting toward the West Coast. The
06Z GFS and 12Z UKMET show some brief dampening of the northern
periphery of the ridge leading to somewhat broader downstream
troughing. Shortwave differences in the 06Z GFS get to be
significant enough by Day 7 Saturday to result in a central U.S.
surface pattern that becomes out of sync with most other models
and the ensemble means. 12Z guidance as a whole is starting to
look a little more similar though, hopefully providing a little
more stability for specifics of the leading cold front reaching
the East Coast and trailing front expected to drop into the
northern tier by late week.

An operational model blend was reasonable for Day 3 Tuesday but
then guidance preferences led to steady reduction of CMC/UKMET
input (replaced by 06Z GEFS/00Z ECens means), with the ensemble
means reaching 60 percent total weight by Days 6-7
Friday-Saturday. Lingering operational model input at that time
leaned more to the ECMWF than GFS, and among GFS runs more to the
00Z run versus the 06Z version.

...Weather/Hazards Highlights...

The upper trough and leading cold front emerging into the Plains
will support widespread rain and thunderstorms in the central U.S.
on Tuesday. A broad Marginal Risk in the Day 4 Excessive Rainfall
Outlook covers most of the region. Model and ensemble guidance has
recently narrowed in on an axis of relatively heavier QPF along
the surface low track in the northern Plains. The frontal low
along with a potent shortwave and some instability in the vicinity
should provide the ingredients for heavy rainfall, and a Slight
Risk area has been maintained--centered mostly over eastern North
Dakota and far northwestern Minnesota. Slightly faster 12Z model
trends are lowering confidence in some of the higher totals
previously forecast, but for now there is at least some
persistence of a relative maximum over this area. Then by
Wednesday, the northern side of the cold front may be moving more
progressively through the Midwest for some lighter rain, while
heavy rain and thunderstorms become more widespread in the
southern Plains where the right entrance region of the jet will
provide ample lift and plenty of instability is in place just
ahead of the front. Thus a Slight Risk is in place there in the
Day 5/Wednesday ERO, as despite the drought conditions, high rain
rates that could overcome the dryness are likely. Latest guidance
maintains support for this Slight Risk area. Uncertainty over what
happens to energy in the southern part of the upper trough leads
to some question marks over the longitude of heaviest rainfall,
with seemingly lower-probability UKMET/CMC solutions leading to a
farther west location. The western periphery of the Marginal Risk
area accounts for this possibility.

The cold front should gradually push through the south-central
U.S. Thursday and Friday, pressing rain chances southward with it.
Meanwhile showers will likely track across the Ohio/Tennessee
Valleys around Thursday and into the East by Friday along and
ahead of the front. Depending on the specifics of upper troughing
by Saturday, the cold front could begin to tap Atlantic moisture
within southerly flow to increase rainfall over parts of the
Northeast. Not too much precipitation looks to occur elsewhere,
though there could be some lingering snow over higher elevations
of the West in association with the upper trough crossing the
region through midweek. Some rain in the western Atlantic could
provide a few showers to eastern Florida, but otherwise expect the
rain to stay mainly offshore.

In terms of temperatures, the amplified trough will lead to cooler
temperatures than average while the ridge to the east will support
above normal temperatures. The Interior West should see cooler
temperatures than average especially through midweek, with with
lows 5-10F below normal but highs around 10-20F below average.
This puts high temperatures generally in the 80s for the Southwest
and the 50s and 60s for the Great Basin through around midweek,
warming to near normal for the latter part of the week. Meanwhile
warmer than average temperatures by around 15-20F above normal are
forecast for the Midwest into the Great Lakes and the Interior
Northeast, with highs in the upper 70s to low 80s. Some daily
records for highs/warm lows may be possible Tuesday-Wednesday. As
the cold front and upper trough march east into the latter half of
the week, high temperatures will cool to below average across the
central and then east-central U.S., limiting the above normal
temperatures to the Northeast. A trailing front may reinforce the
cooling trend by Friday-Saturday. Approach/arrival of the eastern
Pacific upper ridge should raise temperatures along the West Coast
to above average, by as much as 5-15F, and some record warm lows
may be possible.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at: