Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
205 AM EDT Thu Sep 28 2023

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

...Overview...
A closed low/amplified trough in the West will slowly move
eastward and weaken with time.  Higher elevation snows early
should taper off as the system weakens.  Ridging expands across
the Plains/East bringing above normal temperatures, especially for
the northern tier of the regions.  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 heavy rain threat
across the peninsula early next week.


...Guidance Evaluation/Predictability Assessment...
The guidance continues to show very good overall agreement through
the medium-range, though detail issues remain.  To deal with those
issues, the WPC medium-range 500 hPa heights, fronts, winds, and
pressures were derived from an even split of the 27/12z ECMWF,
27/12z Canadian, 27/12z UKMET, and 27/18z GFS from Sunday into
Sunday into Tuesday before swapping out the UKMET for increasing
amounts of the 12z NAEFS/ECMWF ensemble means mid next week.  This
blend kept reasonable continuity.  The days 4-7 QPF was a blend of
the 27/18z GFS, 27/12z ECMWF, 27/12z Canadian, and 01z NBM.  The
remainder of the grids were more heavily based on the 01z NBM.


...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.  A Marginal Risk area is now in place
for portions of the southern High Plains for day 5/Monday into
Tuesday morning as there is the signal for convection-induced
heavy rain potential across westernmost TX and near a front/lee
trough/dry line in southeast NM.

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
-- while 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.

Roth


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