Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
304 PM EDT Tue Sep 26 2023

Valid 12Z Fri Sep 29 2023 - 12Z Tue Oct 03 2023

...Overview...

An amplified trough is forecast to develop over the western U.S.
while increasing ridging shifts into the East.  Convective rains
are possible near a lingering front along southern New England
coast Friday into Saturday but with a high degree of uncertainty. 
Meanwhile, portions of Minnesota could receive heavy rain ahead of
a front where it has been wetter than average this past week.  As
the amplified trough is forecast to close off into a large upper
low in the western U.S., higher elevation snows can be expected
over portions of the northern Rockies, while moderate to possibly
locally heavy rains are forecast for the lower-elevations from
areas of the Great Basin to portions of Montana Sunday and Monday.


...Guidance Evaluation/Predictability Assessment...

The ensemble means from the GEFS, ECMWF, and CMC show rather good
agreement with the synoptic pattern evolution over mainland U.S.
through Day 7.  Deterministic guidance from the GFS and ECMWF are
rather similar as well, except near the East Coast where
differences in the placement of the heavy rain near the southern
New England coast are quite noticeable.  The ECMWF, as well as the
EC mean, has been favoring a more northward intrusion of the heavy
rain into southern New England compared with the GFS/GEFS, which
may be attributed to a stronger upper trough that tends to amplify
a surface inverted trough/front together with formation of low
pressure waves.  Meanwhile, agreement remains very good with the
closing/cut off low over the West and ridging over the
east-central CONUS. The CMC shows a weaker surface ridge and upper
trough along the East Coast but similar to the EC and GFS
elsewhere.  A 40% blend of the 00Z ECMWF/00Z EC mean, 40% 06Z
GFS/06Z GEFS, and 20% from the 00Z CMC/CMC mean was used to
composed this morning's WPC medium-range forecasts, with higher
percentages from the ensemble means for Days 6/7.


...Weather/Hazards Highlights...

A stalled front over the Florida should progress southward as a
frontal wave forms to their east, progressing past Bermuda ahead
of a progressive and positively tilted upper level trough.  For
Day 4/Friday into Saturday morning, the excessive rainfall outlook
continues to have portions of Florida in the Marginal Risk as
instability lurks nearby across the very warm subtropical north
Atlantic and Gulf of Mexico, the 850 hPa slice of a front helps
focus activity, precipitable water values at or above 2.25", and
some enhanced cooler east-northeast flow aloft all conspire to
potentially leading to efficient showers and thunderstorms.  Since
the signal for heavy rainfall across the coastal Mid-Atlantic,
southern New England, and North Woods of MN has come up and there
appears to be instability available along nearby
fronts/convergence zones and precipitable water values are up in
the 1.25-1.5" range, went ahead and added a couple more Marginal
Risks to cover the excessive rainfall possibility.  The risk areas
generally cover regions where rainfall this past week has been a
bit above average, leaving them more sensitive than usual.

For Day 5/Saturday into Sunday 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 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 added a Marginal Risk
area.  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 area has been introduced over the southern High Plains
on Day 5 given better model agreement for convection to develop
ahead of a lee-side trough.

A digging western trough will close into a cut off low centered
over the Great Basin this weekend and 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 Northern Rockies 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 signal is not
strong enough for a Marginal Risk at this time. 

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
No significant hazards are expected over the CONUS during this
forecast period.

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