Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Thu May 8 2025

Valid 12Z Sun May 11 2025 - 12Z Thu May 15 2025


...Multi-day Southeast U.S. Heavy Rainfall/Flooding Threat...


...Overview...

A closed low will meander over the Lower Mississippi
Valley/Southeast for several days before finally lifting out of the
region around next Wednesday. This will support deeply pooled
moisture to fuel daily heavy rainfall/convective threats into next
week over the Southeast including Florida, albeit with an eventual
gradual translation up through the Mid- Atlantic with slow system
ejection. Amplified upper troughing off the West Coast will move
inland by Sunday, with widespread precipitation extending across
the West into next week, including elevation snows. This looks to
eventually spin up a surface low pressure system over the north-
central U.S. by the middle of next week, with moderate to heavy
rain potential on the backside of the low across the Northern
Plains.


...Guidance/Predictability Assessment...

Model and ensemble guidance continues to be reasonably well
clustered through the first half of the period, despite some
differences still in the details and smaller scale sensible weather
particularly with the stalled upper low over the Southeast. A
general model compromise worked well as a starting point for this
period. By about Tuesday, there are some greater differences with
the evolution of elongated/amplified troughing over the West and
what may be ejection of energy and eventual closed low development
over the central U.S. next week. The GFS is quickest in this, while
the ECMWF was by far the weakest (showing a weaker shortwave vs
deep closed low) and farther north than the GFS. Regardless, there
is agreement that some sort of developed surface system will emerge
into the Central U.S. by mid- next week. To mitigate the detail
differences and take a more modest approach to start, the late
period WPC forecast trended more towards the ensemble means.


...Weather/Hazards Highlights...

A wavy frontal system and a closed upper low meandering into/over
the Southeast will continue to support a multi-day heavy
rainfall/runoff threat into next week as fueled by the pooling of
highly anomalous moisture. Given the increased signal for heavy
rains by Sunday (and some heavy rainfall potential starting on
Saturday), went ahead and introduced a slight risk for northern
Florida and southern Georgia on the Day 4 Excessive Rainfall
Outlook (ERO). By Monday, this potential expands southward into the
Florida peninsula and northward across much of the Southeast as
storms may train along a slow moving cold front lifting through the
region. As such, a slight risk is included for much of the
Southeast on the Day 5 ERO as well. Rainfall should gradually lift
northward with time into the Mid- Atlantic next Tuesday and
Wednesday.

Out West, rain and higher elevations snows will become more
widespread through the weekend and into next week with main
amplified system cooling and slow progression inland through early
and mid next week. Modest snows are possible in the higher terrain.
As the system progresses east, moderate to heavy rainfall may
develop on the north and west side of a surface low over the north-
central Plains.

General troughing across the South will keep temperatures near or
below normal at least through Monday. Upper ridging from the
Southwest to the Plains will support much above normal temperatures
across much of the intermountain West and northern U.S., with
daytime highs 20-30 degrees above normal possible in the Northern
Plains. Parts of the Southwest should approach or exceed 100
degrees with at least some localized heat threat given earlier in
the season timing Sunday and Monday. Above normal heat with
moderate some as it progresses across the rest of the Plains and
into the Midwest and East next week. Upper troughing over the West
will keep temperatures below normal for especially the latter half
of the period.


Santorelli


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 (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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