Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Thu Sep 28 2023

Valid 12Z Sun Oct 01 2023 - 12Z Thu Oct 05 2023

...Pattern Overview and Guidance Evaluation/Predictability
Assessment...

Model and ensemble solutions seem reasonably well clustered
overall Sunday into Tuesday, but diverge more significantly into
next midweek. A closed low/amplified upper trough over the
West/Southwest is expected to slowly eject northeastward to the
Rockies and gradually weaken with time, pending later forecast
period influx of uncertain northern steam energies over the Plains
midweek. Higher elevation snows over the Intermountain West and
Rockies into early next week should taper off as the system
weakens. Height falls that materialize over the Plains through
next midweek may combine with surface system instability/moisture
focus to support an emerging convetcive rainfall pattern, but
forecast spread and run to run guidance continuity has been less
than stellar at these longer time frames. Downstream upper ridge
building shifting from the central to eastern U.S. will bring
above normal temperatures, especially for the northern tier of the
regions. However under the base of the eastern ridge, an upper low
forms off the Southeast coast, which pushes a front through the
Florida Straits and ends the weekend heavy rain threat across the
Florida peninsula by early next week.

Accordingly, the WPC medium range product suite was primarily
derived from a composite of the 12 UTC GFS/ECMWF/UKMET/Canadian
models Sunday into Tuesday in a pattern with good predictability
before switching onward to a blend of best remaining compatible
guidance from the GFS/ECMWF and GEFS/ECMWF ensemble means as
uncertainty increases. WPC product continuity was well maintained
with this plan. 12 UTC guidance remains in line.


...Weather/Hazards Highlights...
A front drops through the Florida Straits as a frontal wave forms
to their east, progressing towards Bermuda before the low
stalls/tries to wrap back towards a mid-level closed low which
forms under the base of a strong mid-level height anomaly. For Day
4/Sunday into Monday morning, the signal for heavy rainfall across
portions of the FL peninsula continues due to instability from the
very warm subtropical north Atlantic and Gulf of Mexico,
precipitable water values over 2.25", the 850 hPa slice of the
front shifting slowly southward across the FL Peninsula while
helping to focus activity, and enhanced east-northeast flow just
behind it advecting some cooler air aloft all potentially leading
to efficient showers and thunderstorms, so a Marginal Risk areas
is in place. The risk area covers regions where rainfall this past
week has been a bit above average, leaving the areas more
sensitive than usual. In particular, enhanced low level onshore
wind/moisture influx in this pattern flow may favor heavy rains
for the West Palm Beach area as winds are directed westward from
the northern Bahamas.

Meanwhile, snow at higher elevations of the West begins to fade as
the upper trough weakens and edges eastwards with time.  While
portions of Montana appear wet early on, the QPF signal does not
appear to be significant enough for a Marginal Risk on days
4-5/Sunday into Tuesday morning.

Troughing across the West will initially lead to coolness,
initially 10-20F below average for high temperatures across the
Southwest (near 80F) and Great Basin (near 60F) before moderating.
Ridging to the east will allow portions of the Northern Plains,
Great Lakes, and Northeast to be 10-15F above average during the
afternoon (highs in the 70s and 80s). The drought-stricken western
and central Gulf will be hottest, with highs in the low to mid 90s.

Schichtel/Roth

Hazards:
- Heavy rain across portions of Florida, Sun, Oct 1.
- Heavy rain across portions of the Central Plains, Tue, Oct 3.

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