Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
235 AM EDT Tue Sep 26 2023

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

...MEDIUM RANGE GRIDS ARE CURRENTLY UNAVAILABLE...THE DISCUSSION
BELOW DESCRIBES THEIR BASIS...

...Overview...
An amplified trough is forecast for the West while increasing
ridging shifts into the East.  Convective rains are possible
across portions of the Mid-Atlantic coast, New England coast, and
portions of Minnesota which are areas that have been wetter than
average this past week, while higher elevation snows settle in to
the northern Rockies.


...Guidance Evaluation/Predictability Assessment...
Deterministic guidance shows very good agreement with the expected
flow pattern of a closing/cut off low over the West and ridging
over the east-central CONUS. Blending the 12Z ECMWF/CMC/UKMET and
the 18Z GFS worked well through Day 5 with inclusion of the 12Z
ECENS and 12Z NAEFS means for Days 6/7. QPF grids were generally a
blend of the 01Z NBM, 18Z GFS, 12Z ECMWF, and 12z Canadian.


...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 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.

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