Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sat Sep 30 2023

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

...Overview...

An amplified pattern is in store next week, as an upper trough
tracks across the Interior West and then into the Plains and
east-central U.S., with amplified upper ridges on either side. The
upper trough will support cooler than normal temperatures in the
West and pushing into the central U.S. in moderated form as the
week progresses, while warmer than normal temperatures across the
northeastern quadrant of the CONUS will become more limited in
area by late week. Moist and unstable inflow just ahead of the
trough and an associated wavy cold front will likely produce an
emerging convective rainfall pattern for much of the Plains
Tuesday and focused in the southern Plains by Wednesday. Some
rainfall that will likely be lighter looks to push into the
eastern U.S. later in the week.


...Guidance Evaluation/Predictability Assessment...

Model guidance has at least been locked in with showing a fairly
deep trough to push through much of the CONUS through next week,
and show similar timing with its axis moving across the Rockies
Tuesday into the central U.S. Wednesday. Shortwaves within the
trough are somewhat more questionable but within reasonable
spread. The early part of the forecast period was based on a blend
of the 12/18Z GFS and the 12Z ECMWF, CMC, and UKMET that all
seemed within reason initially.

By the latter half of the week, models diverge with a few aspects
of the trough's evolution as it tracks east. For multiple model
runs, the GFS suite has been indicating more ridging across
western Canada than the ECMWF/CMC that have been stronger with
shortwave energy suppressing that northeastern side of the ridge,
which has implications for the axis and depth of the trough
downstream. The incoming 00Z GFS and GEFS are finally showing a
little more evidence for the shortwave, relatively more comparable
to other guidance. The shortwave energy serves to broaden the
trough by late week. Thus a narrow trough like the 12Z GFS was
showing has become more of an outlier especially in seeing the
newer 00Z guidance.

Meanwhile the base of the trough is another area of uncertainty,
as some model runs including the 12Z CMC and UKMET as well as the
18Z GFS (in a more phased way) hold some energy back in the
Southwest into late week. This has seemed like the minority
scenario for the past day or two, but the 00Z CMC and GFS have
come in indicating this as well, maintaining that possibility. The
ECMWF has been indicating ridging there.

Thus the WPC forecast transitions gradually to a more ensemble
mean-heavy blend by the latter part of the week due to the
increasing model spread as the forecast period progresses. This
serves to minimize the individual model differences but does
maintain the larger scale upper trough with ridging on either side.


...Weather/Hazards Highlights...

The trough and 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 Excessive Rainfall Outlook covers
most of the region, but guidance seems to have narrowed in on
heavier QPF along the surface low track in the northern
Plains/Midwest. 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 has been delineated for
eastern North Dakota/northeastern South Dakota/northwestern
Minnesota. 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. The cold front should gradually push through the
south-central U.S. Thursday and Friday, pressing rain chances
southward with it. Meanwhile showers are forecast to track across
the Ohio/Tennessee Valleys around Thursday and into the East by
Friday along and ahead of the front. 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 looks 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. 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. Another upper ridge coming in
behind the trough should raise temperatures along the West Coast
to above average.


Tate


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, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

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