Severe Thunderstorm Warning 291, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
329 PM CDT Wed Jul 31 2024

.KEY MESSAGES...

- Thunderstorms likely tonight with an Enhanced Risk of severe
  weather over much of the forecast area. Although large hail
  and a few tornadoes are in place, damaging winds are the
  primary concern.

- Lingering chances for showers & a few storms north and east
  Thursday

- Mainly dry this weekend

&&

.DISCUSSION...
Issued at 328 PM CDT Wed Jul 31 2024

The primary concern is obviously what is going to transpire
over the next 12-18 hours with another round of severe weather
and heavy rains anticipated. The airmass is becoming
increasingly unstable with MLCAPEs now approaching 4000 J/kg
west, however CINH is equally strong and still 100+ J/kg due to
a noted warm layer around 2km. This has suppressed even much
cumulus development. Forecast temps and heat indices have not
reached expected values due to morning convection lingering
longer than anticipated, but the sensible weather is still warm
and humid with dewpoints into the 70s so will leave the current
advisory in place until expiration.

This situation is expected to change later this evening however
as lift associated with the current Dakotas upper low is
expected to cross the MO River into IA. Vertical motion from
diffluence aloft and QG forcing is expected to cool the mid
levels and erode this warm layer allowing for convection to
rapidly develop to our northwest during the late afternoon or
evening hours, with a separate cluster likely occurring farther
southwest ahead of the current NE MCS. RAP forecast soundings
over western IA suggest MLCAPEs may reach 4000-5000 J/kg, mid
level lapse rates around ~9 C/km, and DCAPEs as high as 1500
J/kg. This alarming thermodynamic parameter space and very warm
airmass is very conducive for damaging winds, and both the 12Z
HREF and recent HRRR runs suggest that localized high end gusts
are possible (70+ mph). Recent HRRR runs have had good agreement
on this magnitude, but not placement, occurring anywhere from
the Siouxland area into MO. While the warm airmass and only
moderate effective shear ~40kts may temper the large hail
potential somewhat, and less than favorable 0-1km shear and SRH
aren't ideal for tornadoes, the magnitude of the instability
and stretching/parcel acceleration potential keeps both of those
in play. With organized wind/bowing segments favored, the heavy
rain potential may be minimized due to the progressive nature
of the MCS, and the 12Z HREF reflects that as well with only
spotty 1-4" amounts, lesser than noted for previous events.

The severe weather potential should be over by Thursday, but
the parent upper low is expected to mature over the Upper MS
Valley with lingering forcing in eastern IA aiding precip
chances in the morning, and the low modifying the environment
such that steep low level lapse rates may produce showers and a
few storms, and possibly even a funnel cloud or two near the
core of the upper low.

By Friday and the weekend, although there won't be a
significant change in airmass, surface high pressure should
dominate leading to mainly dry conditions and seasonal
temperatures and humidity. By early next week, the sprawling
Rockies upper high will become progressive enough that
increasing return flow and warm advection will return leading to
warmer temperatures, especially southwest. The models are in
good agreement that a northern stream short wave will bring
another frontal passage and chances for showers and storms
around Monday night, especially north, and this time leading to
a cooler temperatures and a more notable change of airmass.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1255 PM CDT Wed Jul 31 2024

Convection lingers south of KOTM, but VFR conditions can be
expected there and elsewhere with mostly clear skies over much
of the area this afternoon. However a period of strong to severe
thunderstorms is anticipated tonight, with the potential
starting at TAF sites around 01Z-04Z west to east. The
confidence in occurrence is sufficient to include thunder at
this lead time. Generic, VFR conditions have been mentioned, but
MVFR/IFR conditions are certainly possible but will not be
included until trends become clearer. Confidence in VFR
conditions increases again Thursday morning after the frontal
passage.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM CDT this evening for IAZ033>035-
044>048-057>061-070>075-081>086-092>097.

&&

DISCUSSION...Hagenhoff
AVIATION...Small

Hazardous Weather Outlook
National Weather Service Des Moines IA
323 PM CDT Wed Jul 31 2024

IAZ033>035-044>048-057>061-070>075-081>086-092>097-012030-
Sac-Calhoun-Webster-Crawford-Carroll-Greene-Boone-Story-Audubon-
Guthrie-Dallas-Polk-Jasper-Cass-Adair-Madison-Warren-Marion-
Mahaska-Adams-Union-Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-
Decatur-Wayne-Appanoose-Davis-
323 PM CDT Wed Jul 31 2024

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

A Heat Advisory is in effect for the area this afternoon. Please
refer to the National Weather Service website at
weather.gov/desmoines for further details.

Additional thunderstorms will arrive late this evening from the
west and move across the state overnight. Damaging winds and large
hail will be the primary threats. Heavy rain is also a concern
for areas that will have received the most rain so far this week.
A tornado is possible but the likelihood is low.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

A few non-severe thunderstorms may linger into Thursday.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed today through Thursday morning. Rainfall
amount reports are also appreciated.

IAZ004>007-015>017-023>028-036>039-049-050-062-012030-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Hamilton-Hardin-
Grundy-Black Hawk-Marshall-Tama-Poweshiek-
323 PM CDT Wed Jul 31 2024

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Additional thunderstorms will arrive late this evening from the
west and move across the state overnight. Damaging winds and large
hail will be the primary threats. Heavy rain is also a concern
for areas that will have received the most rain so far this week.
A tornado is possible but the likelihood is low.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

A few non-severe thunderstorms may linger into Thursday.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed today through Thursday morning. Rainfall
amount reports are also appreciated.

Dodson

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