Severe Thunderstorm Warning 124, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
634 AM CDT Fri Jun 20 2025

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Mild and quiet today, but scattered storms expected overnight
  into Friday morning, mainly north and northeast. A few storms
  could be strong to severe, with hail the primary threat but
  strong winds also possible.

- High confidence in hot, humid, and breezy conditions from
  Friday into Monday. The most intense heat is likely over the
  weekend. Extreme Heat Warnings, Extreme Heat Watches, and Heat
  Advisories are in effect for various areas of Iowa between
  Friday and Sunday.

- Next week will be active with multiple rounds of
  thunderstorms/heavy rain toward the northwest and persistent
  heat toward the southeast, but low confidence in where the
  zone between these two areas will fall across Iowa.

&&

.UPDATE...
Issued at 340 AM CDT Fri Jun 20 2025

Strong to severe storms continue to develop over northern Iowa,
as a stout low level jet noses up into central Iowa. At the
apex of this jet is a strong line of storms which has a history
of producing wind gusts of 50 to 60 mph. This storm is expected
to follow along the nose of the LLJ as it oscillates south and
east through the morning, impacting portions of north central
into northeastern and east central Iowa. Damaging winds will
continue to be the primary threat as storms follow along the
instability and DCAPE gradient, however, occasional hail remains
possible. Storms are expected to persist through the early
morning hours, eventually departing the area through mid-
morning.

Beyond the ongoing storms over the area, the main concern in the
near-term will be the heat developing today and Saturday as the
thermal ridge begins to spill into Iowa. 850mb temperatures over 20C
will drape over the entirety of Iowa Friday and Saturday, with 850
mb temperatures increasing to 30C in the west and northwest by
Saturday. This will translate to high temperatures at the surface in
the mid to upper 90s through the weekend, even pushing 100F in
western Iowa on Saturday. These warm temperatures will be
accompanied by a moist airmass boasting 70+ dewpoints both Friday
through Sunday. All of this will make for uncomfortably muggy
conditions through the weekend, with apparent temperature values
100F to 105F area-wide, and exceeding 110F at times in northwestern
Iowa on Saturday. In addition to daytime highs, there will be little
relief overnight with lows only falling into the mid to upper 70s
tonight and Saturday night. Therefore, while the warmest
temperatures will be during the day, not much relief will be in
place overnight either. Due to this long-lived nature of the heat
through basically Sunday afternoon, have decided to extend the
current Heat Advisory in time through Sunday. Likewise, have
upgraded the Excessive Heat Watch to a Heat Advisory for the
remaining counties in east central Iowa starting on Saturday and
lasting through Sunday. As of right now, the heat indices over 110F
seem to be limited in scope so have left the Excessive Heat Warning
as it was over the areas that are expected to be warmest.

&&

.DISCUSSION...
Issued at 212 PM CDT Thu Jun 19 2025

It is a sunny, mild, and quiet day across Iowa as a surface ridge
slides by to the south. Meanwhile a broad low pressure trough has
set up shop along the lee of the Rockies, while a large 500 MB ridge
dominates the southwestern U.S. and is pushing up into the Midwest.
A warm front lies well to our north, over North Dakota and northern
Minnesota. As a quick-moving shortwave impulse overtops the 500 MB
ridge later today, it will generate strong thunderstorms along the
front in North Dakota that will likely accelerate eastward and
southeastward overnight. It appears this complex will remain north
of our area over Minnesota and Wisconsin, however, a southwesterly
low-level jet developing overnight will push into Iowa and is likely
to generate thunderstorms during the early morning hours Friday.
While forecast soundings depict a deep, very warm layer above the
surface within a large-scale warm advection regime, they also
illustrate instability above that with MUCAPEs of around 1000-1500
J/kg overnight. The expectation is thus for the development of high-
based but occasionally strong thunderstorms capable of producing
hail. Coverage is expected to be scattered and 50-70% POPs are
carried in our northern and northeastern counties, closer to the
nose of the jet and the periphery of the 500 MB high. There is also
some potential that if storms can sufficiently organize a cold pool
some strong winds could punch down to the surface, particularly if
the storms from Minnesota curve southward enough to scrape the
northeast corner of our service area Friday morning. In any event,
the storms should push eastward and away from us by noon.

Aside from the Friday morning storms, the forecast is dry for the
next several days and increasingly hot and humid. Operational NBM
was only around the 10th percentile of the distribution on
temperature in our western counties, likely due to overly bias
correcting for recent cool weather, so increased MaxT into the 100-
102 range Saturday and Sunday which is still below the 50th
percentile. Operational NBM was only around the 10th percentile of
the distribution on temperature in our western counties, likely due
to overly bias-correcting for recent cool weather, so increased MaxT
Saturday and Sunday but still remain near or below the 50th
percentile. With the 500 MB ridge slowly moving overhead and
persistent, seasonally strong south southwesterly surface flow
helping to mix down the warm temperatures aloft, highs will range
well into the 90s by Saturday and Sunday, and perhaps reach triple-
digits in some western Iowa locations, with dewpoints in the upper
60s to lower 70s combining to produce Heat Index values easily in
excess of 100 degrees. Indeed, even on Friday our western counties
may reach these Heat Index levels, though our eastern counties will
remain somewhat less hot due to a slower arrival of the stronger
surface flow as well as potential lingering after-effects from
morning convection. Although Heat Index values in our western areas
Friday afternoon/evening are more in the 102-105 range, it will be
the beginning of a multi-day event with values Saturday and Sunday
more in the 105-110 range and overnight low temperatures only in the
upper 70s. Therefore, have opted to issue an Extreme Heat Warning
over about the western third of our forecast area beginning at noon
Friday and continuing through Sunday. Have also issued a Heat
Advisory for parts of central Iowa, roughly along the I-35 corridor,
for Friday afternoon and evening when Heat Index values will exceed
100 degrees at times. An Extreme Heat Watch remains in effect for
our entire area east of the warning for Saturday and Sunday, and
will likely be converted to a warning late Friday after the Heat
Advisory ends. All in all it will be a very hot and muggy few days,
with impacts enhanced due to it being the first really hot weather
of the year and falling over a weekend. The only factors mitigating
the heat impacts appear to be the persistent breezes, and the
potential for high-level smoke at times this weekend, but these
factors will only slightly blunt the excessive heat effects.

Over the weekend a broad 500 MB trough will build over the western
and northwestern U.S., with the big ridge/heat dome moving over the
southeastern U.S. and intensifying there. This synoptic pattern will
remain in place for about the first half of next week. As several
shortwave impulses eject out of the western trough, they will ride
over a quasi-stationary baroclinic zone beneath a strong belt of
southwesterly flow aloft between the 500 MB trough and ridge. This
will generate multiple rounds of thunderstorms within the baroclinic
zone over several days, with the zone wobbling around over the
Midwest due to the competing effects of the shortwaves/convective
outflow (pushing it to the southeast), and post-shortwave
subsidence/reassertion of the 500 MB high (pushing it back
northwest), with all of this occurring over or near Iowa. The result
is a forecast that is high in overall scenario confidence, but low
in confidence on details. Anywhere south of the frontal zone will
remain very hot and more or less rain-free, while along and
northwest of the boundary it will be relatively cooler, with severe
weather, heavy rain, and flooding all possible at times especially
as several rounds of storms could affect the same areas repeatedly.
However, with the location of the demarcation line between these two
areas still uncertain, for now the focus is on preparing for all of
these potential hazards, ranging from prolonged excessive heat to
severe thunderstorms and flooding. In any event next week looks to
be quite active across Iowa, which is climatologically typical of
the infamous "20s of June".

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 634 AM CDT Fri Jun 20 2025

Scattered thunderstorms will affect the terminals in the next
couple of hours, producing gusty winds and low
ceilings/visibilities. Thereafter, VFR conditions are forecast
through the remainder of the TAF period with gusty SSW winds and
clearing skies.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...

Extreme Heat Warning from noon today to 9 PM CDT Sunday for
IAZ004-005-015-023-024-033>035-044>046-057-058-070-071-081-092.

Heat Advisory from noon today to 9 PM CDT Sunday for IAZ006-
016-025-036-037-047>049-059>061-072>074-082>084-093-094.

Heat Advisory from noon Saturday to 9 PM CDT Sunday for IAZ007-
017-026>028-038-039-050-062-075-085-086-095>097.

&&

UPDATE...Dodson
DISCUSSION...Lee
AVIATION...Lee

Hazardous Weather Outlook
National Weather Service Des Moines IA
502 AM CDT Fri Jun 20 2025

IAZ007-017-026>028-038-039-050-062-075-085-086-095>097-211600-
Worth-Cerro Gordo-Franklin-Butler-Bremer-Grundy-Black Hawk-Tama-
Poweshiek-Mahaska-Monroe-Wapello-Wayne-Appanoose-Davis-
502 AM CDT Fri Jun 20 2025

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Scattered thunderstorms will affect the area early this morning.
Severe weather is possible, with the primary threat being strong
winds and the secondary threat being large hail.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

A Heat Advisory is in effect for the area from noon Saturday
through Sunday evening. Please see our website at
weather.gov/desmoines for additional details.

Thunderstorm chances return to the forecast at intervals from late
Sunday into next week. Severe weather chances will be better
assessed in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed early this morning.

IAZ004-005-015-023-024-033>035-044>046-057-058-070-071-081-092-
211600-
Emmet-Kossuth-Palo Alto-Pocahontas-Humboldt-Sac-Calhoun-Webster-
Crawford-Carroll-Greene-Audubon-Guthrie-Cass-Adair-Adams-Taylor-
502 AM CDT Fri Jun 20 2025

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Scattered thunderstorms will affect the area early this morning.
Severe weather is possible, with the primary threats being strong
winds and large hail.

An Extreme Heat Warning is in effect beginning at noon today.
Please see our website at weather.gov/desmoines for additional
details.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

An Extreme Heat Warning is in effect for the area through Sunday
evening. Please see our website at weather.gov/desmoines for
additional details.

Thunderstorm chances return to the forecast at intervals from late
Sunday into next week. Severe weather chances will be better
assessed in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated.

IAZ006-016-025-036-037-047>049-059>061-072>074-082>084-093-094-
211600-
Winnebago-Hancock-Wright-Hamilton-Hardin-Boone-Story-Marshall-
Dallas-Polk-Jasper-Madison-Warren-Marion-Union-Clarke-Lucas-
Ringgold-Decatur-
502 AM CDT Fri Jun 20 2025

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Scattered thunderstorms will affect the area early this morning.
Severe weather is possible, with the primary threats being strong
winds and large hail.

An Heat Advisory is in effect beginning at noon today. Please see
our website at weather.gov/desmoines for additional details.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

A Heat Advisory is in effect for the area through Sunday evening.
Please see our website at weather.gov/desmoines for additional
details.

Thunderstorm chances return to the forecast at intervals from late
Sunday into next week. Severe weather chances will be better
assessed in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated.

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Preliminary Local Storm Report
National Weather Service Des Moines IA
632 AM CDT Fri Jun 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0627 AM     Hail             3 SE Hanlontown         43.24N 93.36W
06/20/2025  M1.00 Inch       Cerro Gordo        IA   Public

            Report from mPING: Quarter (1.00 in.).

&&

Fowle

Preliminary Local Storm Report
National Weather Service Des Moines IA
702 AM CDT Fri Jun 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0515 AM     Tstm Wnd Dmg     1 E Eldora              42.36N 93.09W
06/20/2025                   Hardin             IA   Dept of Highways

            Iowa DOT report of debris on Highway 175 in
            Eldora.

&&

Fowle

Preliminary Local Storm Report
National Weather Service Des Moines IA
720 AM CDT Fri Jun 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0355 AM     Tstm Wnd Gst     4 WSW Wesley            43.06N 94.07W
06/20/2025  M58 MPH          Kossuth            IA   Mesonet

            Delayed report - peak wind gust from
            personal weather station.

&&

Fowle

Preliminary Local Storm Report
National Weather Service Des Moines IA
739 AM CDT Fri Jun 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0708 AM     Hail             2 N Ames                42.05N 93.64W
06/20/2025  M0.75 Inch       Story              IA   Public

            Report from mPING: Dime (0.75 in.).

&&

Fowle

Preliminary Local Storm Report
National Weather Service Des Moines IA
757 AM CDT Fri Jun 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0530 AM     Tstm Wnd Dmg     3 SSW Conrad            42.19N 92.89W
06/20/2025                   Marshall           IA   Trained Spotter

            Spotter report and picture of large trees
            downed in this location. Time estimated from
            radar.

&&

Fowle