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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 2032Z Sep 29, 2023)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
432 PM EDT Fri Sep 29 2023

Valid 12Z Mon Oct 02 2023 - 12Z Fri Oct 06 2023


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, while warmer than normal temperatures over most of the
central/eastern U.S. early in the week should moderate closer to
normal as the week progresses. 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 Monday-Tuesday and likely focused in the southern Plains by
Wednesday. Expect lighter rainfall to push into the eastern U.S.
later in the week. Meanwhile a reflection of the upper low/trough
now crossing the the East Coast may linger over the western
Atlantic for most of the week but most of the associated rainfall
should remain offshore.

...Guidance Evaluation/Predictability Assessment...

During the first two or three days of the period, the primary
forecast issues involve details for shortwave energy within the
overall western into central U.S. upper trough along with northern
Plains frontal wave development as the core of the initial western
trough ejects northeastward. Most of the guidance differences for
shortwave specifics and surface wave development appear fairly
typical for forecasts 3-5 days out in time, favoring a
blended/compromise approach among the latest operational runs.
However the 00Z UKMET offered one of the lower confidence
solutions in depicting more southwestward elongation of the upper
trough late in its run.

By late next week, individual models and ensemble members
increasingly diverge within a recent 2-3 day trend of the
GEFS/ECens means toward a stronger and eastward Pacific/West Coast
upper ridge and more amplified east-central North American trough.
There has been more consistency for western Atlantic into Canadian
Maritimes mean ridging. The most basic difference starts with
relative strength of the northern part of the Pacific ridge versus
Gulf of Alaska into western Canada shortwave energy, with some
influence just upstream as well. Latest GFS runs and the 12Z UKMET
favor a stronger/eastern upper ridge, maintaining more amplitude
of the central U.S. upper trough. The GEFS mean tilts in that
direction. The ECMWF/CMC are stronger with the shortwave energy,
yielding a farther west ridge axis that allows for this energy to
flow into the mean trough and eject leading energy more
quickly--ultimately resulting in a broader overall trough by Day 7
Friday. Not surprisingly the ECens mean hints at that idea.
Teleconnections relative to positive height anomaly centers near
the British Columbia coast and Canadian Maritimes in the D+8
multi-day mean charts favor fairly amplified east-central North
American troughing by the end of the period or just beyond, which
the ECMWF/GFS and their ensemble means show in principle.
CMC/CMCens runs appear to be lagging the trends and teleconnection
relations toward the majority scenario. However this does not
provide a lot of insight as to whether the GFS/GEFS or ECMWF/ECens
details would be more likely on a day-to-day basis, so preference
is to transition the early-mid week model blend toward greater
weight of the GEFS/ECens means relative to the GFS/ECMWF (given
decreasing confidence in exact operational model specifics) along
with equal consideration of the GFS/GEFS vs ECMWF/ECens clusters.

...Weather/Hazards Highlights...

The central U.S. will be the primary focus for heavy
rainfall/flash flood potential during the Monday-Tuesday night
period covered by the Days 4-5 Excessive Rainfall Outlooks. By Day
4, moisture and instability look to pool in the southern High
Plains just ahead of the cold front supported by the western U.S.
upper trough. The right entrance region of the upper jet will
provide ample lift which combined with the unstable conditions
could produce high rain rates, possibly overcoming the ongoing dry
conditions there to produce some localized flash flooding. Thus a
Marginal Risk is depicted for Monday across that region, with
guidance supporting minimal adjustment from prior issuance.
Rainfall chances are likely to increase in the north-central U.S.
as well, but uncertainty in the amounts and placement of enhanced
convection still precludes introducing any excessive rainfall risk
at this point. As the trough and cold front emerge into the Plains
on Tuesday, rain and thunderstorms should become widespread in the
central U.S., with model guidance showing embedded heavier amounts
through all the Plains states into the Upper Midwest, but with
little agreement in placement at this time. A large Marginal Risk
area remains in place for Tuesday as a starting point. The latest
cycle of guidance is starting to focus a little more on a
potential axis of heavier totals in the northern Plains as
ejecting energy aloft supports a northern tier frontal wave.
However there is still some spread for amounts/location, and how
this activity may overlap areas of wetter or drier soil
conditions. Thus prefer to wait at least another cycle before
introducing any embedded Slight Risk area.

Plains convection should focus more over southern parts of the
region by Wednesday, with some rain amounts of over an inch
already widespread in the deterministic forecast. Some rain and
storms should spread into the Ohio/Tennessee Valleys later in the
week, with greater uncertainty in timing and amounts. In general
the rainfall totals over the eastern U.S. should be lower than
over the Plains. Meanwhile, some lingering showers and storms are
possible over southern Florida especially early next week as a
front pushes through, but with lighter amounts than in the short
range. The Pacific Northwest may see a round of light to moderate
precipitation Monday-Tuesday before likely drying out by midweek.
Some snow may linger into early next week over higher elevations
of the West in association with the upper trough crossing the

In terms of temperatures, the early week trough atop the West will
lead to cooler temperatures than average, 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. Meanwhile, temperatures from the
Plains eastward will be near to above average on Monday, with the
largest anomalies of 15-20F above normal (for highs, even more
anomalous for lows) focused on the Upper Midwest to Great Lakes
and the Interior Northeast, with highs in the upper 70s to low
80s. The Plains should moderate to near normal or perhaps a few
degrees below normal temperatures as the week progresses, with the
area of above normal temperatures becoming more limited to the
Northeast by later in the week as the cold front reaches the
east-central CONUS.


- Heavy rain across portions of the southern High Plains, Mon, Oct
- Heavy rain across portions of the northern and central Plains,
and into the upper Midwest, Tue, Oct 3.
- Heavy rain across portions of the Southern Plains, Wed-Thu, Oct
4-Oct 5.
- Flooding possible across portions of the northern Mid-Atlantic
and New York City vicinity.

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: