Severe Thunderstorm Warning 254, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1208 AM CDT Tue Jul 30 2024

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

- Heat continues through Wednesday with heat index values over
  100F possible, especially in southern and southwest Iowa.

- A few strong to severe storms will be possible late tonight
  into Tuesday morning. Gusty winds and locally heavy rainfall
  will be the main concerns.

- More thunderstorms develop through the day on Tuesday. All
  severe hazards will be possible, but strong wind gusts will be
  the primary threat with any organized storms. Additional storm
  chances then expected again Wednesday into Thursday morning.

&&

.DISCUSSION...
Issued at 345 PM CDT Mon Jul 29 2024

A nebulous surface pattern this afternoon shows little in the
way of defined features, with generally light and variable flow
over the forecast area. However, looking further aloft we can
see warmer temperatures beginning to build and upper level
ridging fill in behind last nights shortwave. This is
translating down to the surface, where temperatures in southwest
Iowa are in the upper 80s to near 90. Cloud remnants from the
overnight/morning convection then cumulus developing through
mid-day has slowed temperatures somewhat, but much of the area
is under full sunshine and warming quickly. A few sites have
heat index values around 100F with only slow cooling expected
into this evening. No changes have been made to the Heat
Advisory from the previous shift, with the Advisory remaining in
effect for the same areas until 8 pm tonight.

Quick on the coat-tails of the trough from early this morning
comes a few additional waves with associated shower and
thunderstorm chances through mid-week. The first of these
chances arrives after midnight tonight, as storms firing in the
high plains grow upscale this evening. As is typical for this
time of the year, there is a large spread of solutions among
guidance on how these MCSs may evolve through the evening.
Models depict two areas of convection that could potentially
impact the state, one developing in North Dakota and riding
along the thermal ridge and another developing along the
Nebraska/South Dakota border and propagating east with the mean
flow. Uncertainty is pretty high with how these two areas of
convection evolve overnight and how they might impact the state
early tomorrow morning, if at all. That being said, should an
MCS maintain strength as it reaches Iowa overnight into
tomorrow, strong to severe wind gusts and locally heavy rainfall
would be possible.

The lobe of forcing responsible for the convection to our west
by this evening will make its way into the state tomorrow,
bringing more chances for storms throughout much of the day.
Again, there will be a lot of uncertainty on how these play out,
as location, timing, and severity will all depend on prior
convective trends. Therefore, rather than specifying timing and
locations, the main focus should be on the impacts of any storms
that do occur. The environment will be favorable for strong to
severe storms tomorrow, boasting 40-50+ kts of deep layer shear,
3000+ J/kg of ML CAPE, and plenty of moisture in place. Upscale
growth is expected, but there will also be enough low level
shear for supercells as well, meaning all severe hazards are
possible. That being said, winds look to be the primary concern
with DCAPE values of 1000+ J/kg, and would be especially
concerning if convection grows into an MCS. Locally heavy
rainfall is also expected, with PWATs nearing 2 inches and
strong moisture transport into the area. A warm front in place
may also promote training storms, which could lead to some
ponding water on roads in urban areas.

Finally, in addition to the storm chances tomorrow, heat
continues to build in with highs in the 90s and heat index
values pushing 105 to 110 in southern and southwest Iowa.
Another Heat Advisory has been issued to cover this, but the low
confidence in convective trends makes the temperature forecast
uncertain as well. Therefore, remained a bit conservative with
the northern extent of the advisory and held off on any
Excessive Heat Warning for the southwest CWA due to the
possibility for storms/cloud cover lingering over much of the
area through the day. Even so, if the sun makes it out for a few
hours tomorrow, it will be quite uncomfortable and potentially
dangerous, even if heat index values do not quite reach as high
as they could. The heat continues Wednesday as well before
another upper trough moves through and slows the heating through
the second half of the week. Showers and thunderstorms will
likely accompany this trough, with chances for more strong to
severe storms again Wednesday into Thursday morning.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1201 AM CDT Tue Jul 30 2024

Thunderstorm complex will sink into the area within the next 6
hours. Highest confidence in impacts at KFOD so have introduced
TEMPO group accounting for this. Expect gusts in the 35-40kt
range with thunderstorms with some sporadic 50kt gusts
possible. Have left VCTS at KDSM and VCSH at other sites due to
uncertainty in eastward extent of storms. Additional
thunderstorms will become possible near midday and beyond, but
remain uncertain until it becomes more clear how overnight and
morning thunderstorms progress. More thunderstorms anticipated
to arrive in the west at the end of the TAF period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from 1 PM this afternoon to 9 PM CDT this
evening for IAZ044-045-057>060-070>075-081>086-092>097.

&&

DISCUSSION...Hagenhoff
AVIATION...Jimenez

Hazardous Weather Outlook
National Weather Service Des Moines IA
Issued by National Weather Service Quad Cities IA IL
321 PM CDT Mon Jul 29 2024

IAZ004>007-015>017-023>028-033>039-046>050-061-062-302030-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-Hardin-Grundy-Black Hawk-Greene-Boone-Story-
Marshall-Tama-Jasper-Poweshiek-
321 PM CDT Mon Jul 29 2024

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Late this afternoon and tonight

Scattered to numerous storms are possible overnight, with any
organized storms that may develop being capable of producing
strong winds and heavy rain. A few may reach severe limits.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Heat and humidity will build further on Tuesday and Wednesday,
leading to peak heat index values in excess of 100 degrees each
afternoon.

Periodic storms are probable through Thursday with strong to
severe storms and heavy rainfall possible on both Tuesday and
Wednesday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed Tuesday or Wednesday.

IAZ044-045-057>060-070>075-081>086-092>097-302030-
Crawford-Carroll-Audubon-Guthrie-Dallas-Polk-Cass-Adair-Madison-
Warren-Marion-Mahaska-Adams-Union-Clarke-Lucas-Monroe-Wapello-
Taylor-Ringgold-Decatur-Wayne-Appanoose-Davis-
321 PM CDT Mon Jul 29 2024

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Late this afternoon and tonight

Heat index values in the upper 90s to 105 will continue across
southern Iowa through early evening.

Scattered to numerous storms are possible overnight, with any
organized storms likely to produce strong winds and heavy rain. A
few may reach severe limits.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Heat and humidity will build further on Tuesday and Wednesday,
leading to peak heat index values in southern Iowa of 105 to 110
degrees each afternoon.

Periodic storms are probable through Thursday with strong to
severe storms and heavy rainfall possible on both Tuesday and
Wednesday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed Tuesday or Wednesday.

MTF

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