Severe Thunderstorm Warning 142, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
704 PM CDT Fri May 15 2026

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Severe storms develop between 6 and 7pm this evening. Large
  hail is the main concern the first few hours, then storms
  transition to more of a damaging wind gusts threat. Storm
  threat ends for our forecast area by around 2am Saturday.

- Additional severe storms and locally heavy rainfall chances
  exist at times from Saturday afternoon through Monday. With
  any weekend outdoor plans, monitor the forecast for the latest
  forecast updates and changes.

- Cooler and drier Tuesday through at least Wednesday.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 301 PM CDT Fri May 15 2026

It is a warm, mid-May day as temperatures have soared into the 80s
under a sunny sky with a wind from the south or southeast. GOES-East
visible satellite imagery shows stable clouds developing from around
Sioux Falls into our northwestern forecast area at present, which
are tied to the surface cold front that is sinking towards the
state. Additionally, there is a patch of clouds over southeastern
Nebraska, which have shown attempts at sustained convective
initiation. However, these have only sporadically produced lightning
with the forcing mechanism for these clouds likely tied to weak, low
level warm air advection. These showers should continue to struggle
and remain elevated per the 18z KOAX RAOB, which shows capping
around 150 J/kg. As the cold front continues southeastward, forecast
soundings show the capping weakening and it looks like isolated to
scattered storms will develop between 6 and 7pm in our forecast
area. There may also be a very narrow window for surface based
storms, but that varies depending on model sounding. Storms that
develop early will pose a risk of large and potentially
significant hail given the forecast hodographs. A tornado or two
cannot be ruled out initially, but high storm bases (LCLs
around 2km) and weak 0-3km CAPE and storm relative helicity
(SRH) point to this being a very low, but not zero, concern.
The hazard threat will transition quickly from hail to damaging
wind gusts concerns with gusts up to around 75 mph possible as
storms move quickly to the east. Mesovortex generation may be
possible, but the 0-3km bulk shear magnitude is just near the 30
knot threshold if storms can orient to the east-northeast. With
fast storm motions and dry soils per NASA SPoRT soil moisture
percentiles, the swaths of 1 to 2 inches of QPF shown in models
will keep flash flooding risk isolated to a low end threat to
an urban area if things line up just wrong. As these storms move
to the east, there is some indication that the storms become
out of balance with the outflow and wind pushing away from the
storms, especially into eastern Iowa. However, damaging wind
gust risk looks to persist through all of our forecast area.

All of the Friday night storm activity should be clear of our
forecast area by sunrise Saturday with clearing skies through
the morning as the surface boundary settles over far southern
Iowa. Forecast soundings show a capping inversion that will
inhibit storm development until at least mid if not late
afternoon. As the cap holds, the environment south of the
boundary will have dewpoints into the low to middle 60s, mixed
layer instability upwards of 2000 J/kg, favorable deep layer
shear of 30 knots for storm organization, and modest low and
steep mid-level lapse rates. The question becomes whether there
will be a forcing mechanism to initiate the storms with weak to
modest low level theta-e advection and no strong surface
convergence despite the surface front over southern Iowa.
Yesterday's and today's runs of CAMs have shown this struggle of
initiating storms. The less likely scenario at this time is
that if storms do develop in the afternoon or early evening,
they could be surface based as shown in the latest soundings
with the highest risks being large hail and damaging wind gusts.
As mentioned yesterday, the lack of stronger surface flow is
limiting storm relative helicity values (SRH). Thus, the risk
for tornadoes is conditional. Storms that do form in this
lesser scenario may have slower storm speeds and that may lead
to narrow paths of locally heavy rainfall that would pose only
an isolated flash flood risk to an urban area. However, the more
probable scenario is that little in the way of storms develop
Saturday afternoon with activity developing on the nose of the
lifting into Iowa low level jet Saturday evening. These
scattered storms would have a hail and gusty winds risk. In
addition, a mesoscale convective system (MCS) is forecast to
develop over the High Plains and move into some portion of Iowa
late Saturday evening into Saturday night/early Sunday morning.
The main concern would be gusty winds as this arrives and
locally heavy rainfall, though the intensity may be waning by
the time it reaches the state.

Any lingering MCS activity will be pushing out of Iowa by mid-
morning Sunday. Beyond this activity, much of Sunday during the
daytime even into perhaps some part of the evening is looking to
have limited storm chances as forecast soundings show stout capping
remaining in place. However, am cautious to say no storm chances due
to uncertainty in whatever outflow or differential heating
boundaries may exist post-MCS with an atmosphere that may otherwise
have plenty of instability. Sunday will otherwise be windy as
south to southeast winds prevail at 20 to 30 mph with gusts up
to 40 mph. Storm that develop west of Iowa on Sunday look to
move into the state later Sunday night.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 301 PM CDT Fri May 15 2026

Monday will dawn with storms either lingering over some portion of
eastern Iowa as a result of storms that push across Iowa Sunday
night. As these depart, the stronger synoptic scale forcing will be
moving closer to central Iowa through the day along with a surface
cold front. The forcing should be more than sufficient for storm
development and severe storms are likely given the strong
instability and shear parameter space. While four days away, the
most likely timing will be from mid-afternoon into the evening hours
before the front likely pushes the storms out of our forecast area
by Tuesday morning. In addition to the severe storm concerns, there
is a slight risk of excessive rainfall on Monday. Given successive
rounds of rainfall, will have to see if certain areas receive repeat
rainfall and become more susceptible to flash flooding. Of course,
urban areas will be in play in any event if rainfall of 2 to 3
inches occurs in just an hour or two. This risk was highlighted in
the 6z run of the experimental WPC Urban Rain Rate Dashboard for Des
Moines with a 5% chance of reaching the 5 year return interval.
Will need to keep an eye on rivers; however, many are near
baseflows so there is capacity along with soils expected to take
the first round to perhaps two rounds of rainfall. This is
shown in the 5 day National Water Model (NWM) forced by the GFS
that generally shows high annual exceedance probabilities (AEP),
though tributaries in both the Thompson and Chariton basins
shows 10% AEPs/higher flows.

With the cold front clearing the area by early Tuesday, cooler and
drier conditions are then forecasted through Wednesday and perhaps
Thursday as surface high pressure moves across the region.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 645 PM CDT Fri May 15 2026

Scattered thunderstorms have developed over portions of the
area early this evening with storms expected to persist through
much of the evening into the early overnight as they move to the
east-southeast. Have moved most thunderstorm impacts to tempo
groups but amendments may be needed for stronger storms to add
in wind impacts which could reach towards 50 knots. KDSM
remains with the most uncertainty in when storms will impact or
just miss this terminal. Storms will be clear of the area by 9Z
in the far southeast.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

SHORT TERM...Ansorge
LONG TERM...Ansorge
AVIATION...05

Hazardous Weather Outlook
National Weather Service Des Moines IA
453 AM CDT Fri May 15 2026

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-161000-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene-
Boone-Story-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper-
Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
453 AM CDT Fri May 15 2026

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

There is an Enhanced Risk (level 3 of 5) for severe storms today.
All modes of severe storms are possible. Storms are expected to
develop in northern and western Iowa early this evening, moving
across central to eastern Iowa late overnight.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

An active weather pattern will be in place through at least
Monday, with chances for strong to severe thunderstorms at times
each day. However the details of timing, location, and hazard
types will be better determined in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is possible at times Friday through Monday.

Hagenhoff

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