815
FXUS02 KWBC 170700
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM 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...

...Hazardous and record heat could continue especially across Texas
and Florida into mid-next week...


...Overview...

As the medium range period begins Tuesday, a surface low pressure
system over the Mid-Mississippi Valley supported by strong upper-
level energy will provide ample lift and instability for rain and
thunderstorms in the Mississippi and Ohio Valleys. The low is
forecast to track east and spread rain and thunderstorm chances to
the Mid-Atlantic in particular on Wednesday, and shifting into the
Northeast later week as the low pivots northward. Aloft, the
subtropical upper ridge providing potentially hazardous and record
setting heat to parts of Texas and Florida in particular through
Tuesday-Wednesday should give way to combining energies that set up
troughing over the East for cooler temperatures. Meanwhile, ridging
behind this trough will lead to warmer than average conditions
across California and the Great Basin/Desert Southwest, expanding
across the Four Corners and southern High Plains into late week.


...Guidance/Predictability Assessment...

Model guidance is agreeable that consolidated mid/upper level
vorticity will be atop the central Plains early Tuesday, though
with some uncertainty regarding if the upper low will close off,
and also when the energy may transfer from a southeast extension of
a western Canada trough into a southwest expansion of a Great
Lakes/Ontario/Quebec upper low. The details of the upper trough/low
affect the exact timing and track of the surface low. The general
agreement of the surface low over the Middle Mississippi Valley on
Tuesday diverges especially with the timing by Wednesday. For the
12/18Z guidance, the 12Z CMC was on the slow/west side while the
12Z GFS was on the fast/east side. Some differences continue as the
low moves northeast, affecting precipitation timing. The 00Z GFS
ended up being faster with the low and lifting the precipitation
shield compared to other guidance.

Upstream, models indicate a Southwest upper ridge building and
sneaking gradually east into the south-central U.S. late week.
Farther north there is more uncertainty with weak shortwaves in the
main flow over the Northwest for some frontal system timing
differences. The main outlier there was with the 12Z and 18Z GFS
showing a notable upper/surface low spinning up over south-central
Canada by late week unlike other dynamical and AI/ML models. Did
not favor these GFS runs and the new 00Z GFS has backed off. There
is some signal for slightly deeper troughing to move from the
eastern Pacific into the Pacific Northwest by Friday or so, but
once again uncertainty with the details abounds.

The WPC forecast was based on a blend of mainly deterministic
models with minor ensemble mean inclusion early in the forecast
period, and gradually increased the proportion of ensemble means to
half Day 5 and more Days 6-7 as spread increased.


...Weather/Hazards Highlights...

The notable surface low pressure system over the Middle
Mississippi Valley Tuesday will draw in above average moisture
(with precipitable water values nearing the 90th percentile for
this time of year) and ample instability ahead of its associated
cold front. The upper low spinning aloft will provide good
dynamical lift, and this combination of ingredients will lead to
widespread rain and thunderstorms and mesoscale convective systems,
with best chances slowly progressing east from the central to
east-central to eastern U.S. as next week progresses. The greatest
chance for heavy rain causing flash flooding should be across the
Mid-Mississippi and Ohio Valleys into the central Appalachians on
Tuesday, as moisture and instability pool near a warm front
stretching west-east that could promote training storms. A
relatively expansive Slight Risk is in place in the Day 4/Tuesday
ERO. Severe weather is also possible per the Storm Prediction
Center, as they show a severe area centered over the Lower/Middle
Mississippi Valley on Tuesday. Rain and thunderstorm potential
should press toward the Eastern Seaboard by midweek as the low
tracks east. Locally heavy rainfall is possible particularly for
the Mid-Atlantic region on Wednesday. The Day 5/Wednesday ERO
indicates a Marginal Risk for flooding potential. A future Slight
Risk is not out of the question, but prefer to wait on better model
agreement for the low track and timing and thus the placement of
the highest QPF. Soils/streamflows are currently wet but should
have a few days of drying before midweek. 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.

Warm to hot temperatures are likely across the southeastern U.S.
as the subtropical upper ridge reaches the region. Southern Texas
in particular will remain hot into Tuesday, with temperatures well
into the 100s leading to Major HeatRisk through Tuesday. The
Florida Peninsula should see warm temperatures in the mid 90s. Both
areas could see record or near record warm lows and highs.
Meanwhile the trough aloft will promote below normal temperatures
across the northern tier, with highs only reaching the 50s in the
north-central Plains on Tuesday. As the trough tracks east and its
influence deepens, cooler than average temperatures are likely in
the eastern third of the U.S. under it, and this will also moderate
the temperatures in the South. But upper ridging poking into the
southwestern U.S. will raise temperatures to above normal there,
expanding east across the Four Corners states by Thursday and into
the southern Plains late next week. Highs will be well into the
100s in the Desert Southwest with temperatures nearing 100 in parts
of Texas eventually.


Tate


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



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