Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Sat May 17 2025

Valid 12Z Tue May 20 2025 - 12Z Sat May 24 2025

...Severe weather and flash flooding forecast for parts of the
Mississippi and Ohio Valleys Tuesday...


...Overview...

A slow moving upper low over the central U.S. Tuesday shifts east
and strengthens over the Great Lakes Wednesday and the Northeast on
Friday before lingering over Atlantic Canada next weekend.
Meanwhile, subtropical ridging over the Southeast becomes more
suppressed from the eastern low/troughing by midweek. Finally,
general ridging over the west will lead to warmer than average
conditions across California and the Great Basin/Desert Southwest
midweek, expanding across the Four Corners and southern High
Plains late in the week.


...Guidance/Predictability Assessment...

12Z global guidance has improved agreement with the upper low
closing off Tuesday evening over the central Plains, a new and
deeper low development over the Great Lakes on Wednesday, a more
southerly track of the low over the eastern Great Lakes Thursday
and the departure of the low from the Northeast Friday into
Saturday. The GFS remains the most progressive solution as it opens
the low on Thursday while the EC/CMC keep it closed. One of the
more notable shifts with 12Z guidance is QPF on Wednesday focused
farther south over the Mid-Atlantic and central Appalachians
compared to prior runs.

Upstream, consensus on broad ridging over the Southwest midweek
that shifts east to the southern Plains by Friday. The 00Z EC was
the least with the next trough into the PacNW on Friday, but the
12Z consensus is now for a trough or low to reach Washington on
Friday.

The WPC forecast was based on a blend of mainly deterministic
models for Days 3-5 with ECENS/GEFS ensemble mean inclusion for
Days 6-7 as spread increased.


...Weather/Hazards Highlights...

Ample moisture and instability over the Miss/OH valleys Tuesday
ahead of a deep low over the east-central Plains along with strong
dynamical lift aloft will lead to widespread rain and
thunderstorms and mesoscale convective systems into Tuesday night.
This covers a broad area including most of the TN/OH Valleys and
back across the Midwest through southern MN where a broad Day 4
Slight Risk ERO remains with only minor edits to the shape. The
severe
weather threat area is focused over the southern portions of this
precip area in the warm sector over the TN Valley per the Storm
Prediction Center Day 4 Convective Outlook.

Rain and thunderstorm potential shifts east over the Appalachians
Wednesday as the low tracks east. Locally heavy rainfall is
possible particularly for the Mid- Atlantic region on Wednesday
where a Day 5 Marginal ERO remains. There is better agreement on
the Day 5 precip area today, but the overall magnitude seems to be
the greatest reason to not upgrade to a Slight Risk at time. Rain
chances will make their way into the Northeast Thursday-Friday as
the surface low pivots northward.

Elsewhere, some weak troughing aloft and frontal systems tracking
through the Northwest next week could produce rounds of modest
precipitation there. Most precipitation should be rain aside from
the highest peaks. Some rain may eject into the northern High
Plains by Thursday, with possibly another round of storms emerging
in the central U.S. by Friday, but with quite a bit of uncertainty.

Heat lingers over south Texas and the Florida Panhandle through
Tuesday, but the suppressed ridge should bring some temperature
relief by Wednesday. The trough/low aloft will promote below
normal temperatures across the northern tier with 10 to 15 degree
below normal temperatures working east mid to late week.
Upper ridging over southwestern U.S. will raise temperatures to
above normal there by Wednesday, expanding east across the Four
Corners states by Thursday and into the southern Plains late in
the
week. Highs will be well into the 100s in the Desert Southwest with
temperatures nearing 100 again in parts of Texas eventually.


Jackson


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