Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Wed Sep 27 2023

Valid 12Z Sat Sep 30 2023 - 12Z Wed Oct 04 2023

...Overview...

An amplified trough is forecast to close off into a large upper
low over the western U.S. while an expanding ridge over the
mid-section of the country will tend to nudge the East Coast
trough farther offshore.  The threat of enhanced rainfall should
subside over southern New England on Saturday as the western U.S.
trough amplifies and spreads areas of moderate to possibly locally
heavy rain from the Great Basin on Saturday to northern Rockies
and northern High Plains Sunday to early Monday.  High-elevation
snows can be expected over portions of these same areas on the
backside of this system.  Meanwhile, convective rains are forecast
on Saturday over the southern High Plains, but they are expected
to linger into early next week and slide slowly southward across
the Florida Peninsula.


...Guidance Evaluation/Predictability Assessment...

The guidance continues to show very good overall agreement through
the medium-range.  The deterministic guidance shows higher
uncertainty regarding the placement of the moderate precipitation
across the western U.S. this weekend, with the CMC being the
fastest while the GFS and ECMWF solutions are comparable.  The
ensemble means show a closer agreement on the precipitation
distribution though.  Meanwhile, the ECMWF continues to offer the
most amplified solution regarding the low pressure system just off
the Mid-Atlantic coast on Saturday, which would keep the heavy
rain and gusty wind threat for southern New England as the
medium-range period begins on Saturday.  The 12Z CMC also
indicates a stronger low that edges closer to the southern New
England coast as well.

Ensemble means agree quite well that the large cutoff low over the
western U.S. will deamplify into a positively-tilted trough early
next week and slowly slide eastward into the Rockies by midweek
next week.  Much of the uncertainty is seen over the southwestern
U.S. regarding whether and where a closed low might form near the
base of the deamplifying trough.

The WPC medium-range forecast package was based on a 40% blend of
the 00Z ECMWF/00Z EC mean, with 40% from the 00Z GFS/06Z GFS/GEFS,
and 20% from the 00Z CMC/CMC mean.  This blend kept good
continuity,  The days 4-7 QPF was a blend of the GFS, ECMWF,
Canadian, bias-corrected QPF and 14z NBM.


...Weather/Hazards Highlights...

A front drops through the Florida Straits as a frontal wave forms
to their east, progressing towards Bermuda before
stalling/wrapping back towards a mid-level closed low which forms
under the base of a strong mid-level height anomaly. 

For Day 4-5/Saturday 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 Marginal Risk
areas are in place both days.  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 remains in
place for the southern High Plains for day 4/Saturday into Sunday
morning as model agreement remains good for convection-induced
heavy rain potential across westernmost TX and near an amplifying
lee trough in NM.

A digging western trough will close into a cut off low this period
which will focus moderate precip over the Intermountain West
through northern Rockies, and northern High Plains.  Snow at
higher elevation becomes increasingly favored with time across the
West as the upper level low develops and it cools off aloft. 
Portions of central Montana appear to have some instability
available to their east and enough moisture to wonder about
excessive rainfall, the QPF signal is not strong enough for a
Marginal Risk at this time for either days 4 or 5 (Saturday into
Monday morning). 

Increased ridging over the center of the country and troughing
across the West, will slowly shift temperatures of 10 to 15
degrees above normal east across the Central Plains through the
Great Lakes with temperatures below normal expanding from the
Northwest to across the West. Below normal temperatures over the
Mid-Atlantic States can be expected into Friday before moderating
on approach of the ridge axis.

Kong/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