Severe Thunderstorm Warning 111, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1142 PM CDT Thu Jun 19 2025

 ...Updated for the 06z 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.

&&

.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 /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1136 PM CDT Thu Jun 19 2025

Scattered thunderstorms are developing over western Iowa and
will continue to expand into parts of central and northern Iowa
overnight into early morning. The greatest chances are at
KMCW/KALO though all sites have potential through at least 12z.
MVFR cigs and vsbys are possible with the convection. Gusty
winds are also possible. VFR conditions will setup with VFR
conditions becoming widespread by late Friday morning. Gusty
south winds will develop. Low level wind shear is expected to
develop Friday evening.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Extreme Heat Warning from noon Friday 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 to 8 PM CDT Friday for IAZ006-016-025-
036-037-047>049-059>061-072>074-082>084-093-094.
Extreme Heat Watch from Saturday afternoon through Sunday
evening for IAZ006-007-016-017-025>028-036>039-047>050-059>062-
072>075-082>086-093>097.

&&

DISCUSSION...Lee
AVIATION...Donavon

Hazardous Weather Outlook
National Weather Service Des Moines IA
239 PM CDT Thu Jun 19 2025

IAZ006-007-016-017-025>028-036>039-047>050-059>062-072>075-
082>086-093>097-201945-
Winnebago-Worth-Hancock-Cerro Gordo-Wright-Franklin-Butler-Bremer-
Hamilton-Hardin-Grundy-Black Hawk-Boone-Story-Marshall-Tama-
Dallas-Polk-Jasper-Poweshiek-Madison-Warren-Marion-Mahaska-Union-
Clarke-Lucas-Monroe-Wapello-Ringgold-Decatur-Wayne-Appanoose-
Davis-
239 PM CDT Thu Jun 19 2025

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Thunderstorms will move across northern to central Iowa very late
tonight and into Friday morning. A few stronger storms may produce
hail.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Storms will continue Friday morning with damaging winds and hail
possible in especially the northeast and depart before noon.
Additional storms are expected late Sunday evening and into early
next week. Severe chances will be better determined in the coming
days.

Hot and humid conditions are expected from Friday through the
weekend, with heat indices exceeding 100 degrees. A Heat Advisory
is in effect for parts of the area Friday. An Extreme Heat Watch
is in effect Saturday through Sunday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed.

IAZ004-005-015-023-024-033>035-044>046-057-058-070-071-081-092-
201945-
Emmet-Kossuth-Palo Alto-Pocahontas-Humboldt-Sac-Calhoun-Webster-
Crawford-Carroll-Greene-Audubon-Guthrie-Cass-Adair-Adams-Taylor-
239 PM CDT Thu Jun 19 2025

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Thunderstorms will move across northern to central Iowa very late
tonight and into Friday morning. A few stronger storms may produce
hail.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Storms will taper off Friday morning. Additional storms are
expected late Sunday evening and into early next week. Severe
chances will be better determined in the coming days.

Hot and humid conditions are expected from Friday through the
weekend, with heat indices exceeding 100 degrees. An Extreme Heat
Warning is in effect for Friday. An Extreme Heat Watch is in
effect Saturday through Sunday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed.