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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0639Z Sep 27, 2023)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
239 AM EDT Wed Sep 27 2023

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

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 guidance shows very good agreement.  The 500 hPa heights,
pressures, fronts, and winds were based on an even blend of the
12z Canadian, 12z ECMWF, 12z UKMET, and 18z GFS early before
swapping out the UKMET for the 12z NAEFS and 12z ECMWF ensemble
means.  This blend kept good continuity,  The days 4-7 QPF was a
blend of the 18z GFS, 12z ECMWF, 12z Canadian, and 01z NBM.  The
remainder of the grids were primarily 01z NBM based, with some
automatic adjustments made to adjust to the QPF.

...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 for
the southern High Plains for day 4/Saturday into Sunday morning as
the 18z/00z GFS have not given up on the heavy rain potential for
convection to develop across westernmost TX and near a possible
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.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

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: