Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Wed May 21 2025

Valid 12Z Sat May 24 2025 - 12Z Wed May 28 2025


...Multi-day heavy rain threat in the Ozarks and vicinity this
weekend...


...Overview...

As the medium range period begins Saturday, a shortwave atop the
Rockies will undercut a broader omega-type pattern consisting of a
northeast Pacific upper low, a central Canada and U.S. ridge, and
troughing over the East. This shortwave and a frontal system will
provide support for heavy rain and thunderstorms in a moist and
unstable environment in parts of the central U.S. this weekend.
Flash flooding is possible in the Ozarks and vicinity. The
shortwave is forecast to merge with the western side of the trough
in the East and cause it to dig into the east-central U.S. early
next week, which should press the front south and east, shifting
precipitation across the southern tier into the Appalachians and
eventually the Eastern Seaboard. The rounds of troughing aloft will
cause cooler than normal temperatures in parts of the central and
eastern U.S., while upper ridging building in the Intermountain
West should allow for warmer than average temperatures there.


...Guidance/Predictability Assessment...

Model guidance starts out in good agreement with a wavy upper
pattern this weekend. The first model to diverge with the pattern
was the 12Z UKMET, which cut off energy in the Southwest underneath
the ridge by early next week and thus was out of phase with the
other guidance. There is some moderate spread with the way the
Rockies shortwave emerges and joins with the elongated eastern
trough early next week, causing the trough to dig southwest. Around
Monday, the newer 00Z models have the trough reaching farther
southwest than the 12/18Z guidance cycle that was available for
forecast generation, though with the 00Z CMC perhaps on the extreme
western side. The trough position will be meaningful for the
frontal and QPF position early next week, so forecast changes may
be needed.

Models are in good agreement that upper ridging will set up atop
the Interior West behind this trough, forming part of a
redeveloping omega block along with eastern Pacific troughing. The
eastern Pacific trough shows a fair amount of model spread though.
Guidance varies with how much energy may pinch off on the southern
side and even form a southern stream closed low moving into
California around Tuesday or so. The 18Z GFS and 12Z CMC indicated
this upper low forming, as do their new 00Z runs. The 12Z ECMWF did
not show it but the 00Z run shows hints of stronger energy even if
it may not quite close. 12Z AI models did not favor a closed low,
so leaned against it for this forecast by lessening the GFS/CMC
proportion in the forecast blend, but will continue to monitor this
potential.

The WPC forecast was based on a blend of the 12/18Z GFS, 12Z
ECMWF, and 12Z CMC early in the period. Included the ensemble means
in the blend later on, reaching half the blend Day 6 and a bit
more Day 7 as spread increased.


...Weather/Hazards Highlights...

A heavy rainfall pattern will be in place by Saturday as ample
Gulf moisture and instability stream north due to return flow from
surface high pressure over the Southeast and in response to a
shortwave/right entrance region of the upper jet coming into the
north- central Rockies and High Plains. Rain and storms are likely
to overlap with Friday's (Day 3 ERO period) convection where there
will be wetter antecedent conditions. A Slight Risk remains
centered over the Ozarks (which can be sensitive to flash flooding
anyway) in the Day 4/Saturday ERO. Convection ongoing from Saturday
night could continue east into Sunday across the Mid-Mississippi
Valley, with uncertainty in if/where it could persist into the
Tennessee or Ohio Valleys. Then additional convection is forecast
to form farther southwest in the southern Plains/Lower Mississippi
Valley as well on Sunday. Thus plan to show a fairly large Slight
Risk from northeastern Texas across parts of the Lower and Middle
Mississippi Valley for the Day 5/Sunday ERO to cover both areas.
Into the workweek, the front will press south farther into the
southern tier and east into the Appalachians and eventually the
Eastern Seaboard. Areas of heavy rainfall are possible and the
details will continue to get refined with time.

Precipitation, in the form of rain except for snow in the highest
peaks, is possible across the north-central Rockies and Plains into
Saturday underneath upper troughing and as frontal systems move
through. The details are uncertain based on the model spread with
the supporting energies. But the shortwave providing dynamical
support, above normal PWs, and the cusp of the instability gradient
could provide some heavy rain and a low end flash flood risk in
the north-central High Plains. A Marginal Risk is in place there
for the Day 4/Saturday ERO.

Elsewhere, light showers could continue across the Northeast but
gradually lessen in magnitude through the weekend as the upper and
surface lows are slow to pull away there. Summertime scattered
storms are likely across the Florida Peninsula through the period.
The Northwest may see renewed precipitation chances early next
week.

Warmer than normal temperatures will be in place across the
southern Plains on Saturday underneath a central U.S. ridge.
Temperatures well into the 90s to around 100 will support areas of
Major HeatRisk in parts of Texas and Louisiana this weekend. Then
as the mean upper ridge rebuilds farther west, the Great Basin and
northern/central Rockies can expect temperatures generally around
10 to 15 degrees above average. Meanwhile, cooler than normal
temperatures by 5 to 15 degrees will take hold over the north-
central to eastern U.S. into the weekend underneath upper
troughing. The trough will allow for temperatures to cool in the
southern tier during the workweek compared to the hot weekend, but
overall temperatures should gradually trend closer to normal as
next week progresses.


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