Severe Thunderstorm Warning 178, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
646 PM CDT Fri Aug 11 2023

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

.DISCUSSION.../Tonight through Friday/
Issued at 310 PM CDT Fri Aug 11 2023

Key Messages:

- Storms this afternoon and evening, some of which may be severe,
  across parts of northern and southern Iowa. Large hail and
  damaging winds are the main hazards, along with locally heavy
  rainfall.
- Additional storm chances Sunday with severe storms possible

Discussion:

There are two key features of note today, embedded within the same
trough. The first and most notable is a vorticity max moving across
Nebraska into Kansas, NW Missouri, and far SW Iowa this afternoon.
Convection has developed on the leading instability gradient, but
this diminished quickly moving north into Nebraska and Iowa. Here
soundings continue to show a decent cap in place that should
prevent the more robust development expected further south. Any
storms that do develop are anticipated to be quite elevated with
most CAPE above 700mb, as indicated both in previous discussions
and a recent SPC SWO. Storms that do develop will have plenty of
deep layer shear, exceeding 45kts at times.

At the same time, another embedded vorticity max in the shortwave
is moving across Minnesota and South Dakota. Convection from this
is expected to brush far northern counties this evening. With
area also have the potential for hail and damaging winds, though
with less instability, the severe threat is low than in the south.
The big question is whether these two areas will phase across
central Iowa this evening. Recent CAM guidance has suggested that
the cap in place will prevent this development, but it will be
worth monitoring through the evening. Given that CAM have
completely miss out on the early afternoon convection, would not
be surprised if they struggle to develop later convection across
parts of central Iowa into the evening due to poor initialization.
The final layer of this is the heavy rain threat. While
widespread hydro issues are not a concern, deep warm cloud layers
will support efficient rain rates in storms, especially across
southern Iowa.

After a quiet Saturday, another shortwave will move across the area,
starting after midnight through Sunday. With CAPE 1500+ J/kg severe
storms are certainly possible, though better shear lags the main
instability and could be a limiting factor in Iowa. Cooler
temperatures filter in behind this system, then warm into mid week.
next system looks arrive late week Thursday into Friday.

&&

.AVIATION.../For the 00Z TAFS through 00Z Saturday evening/
Issued at 626 PM CDT Fri Aug 11 2023

Two areas of storms are underway this evening, one to the north
over MN along the main cold front and one to the south, mainly
over MO. The area to the north is currently the stronger of the
two, although there is potential for additional development over
eastern Nebraska to move into southern Iowa later this evening.
One word of caution, the short term models have generally not been
handling the evolution of the ongoing convection well. Current
thinking is that storms to the north will continue to track to the
southeast, potentially impacting northern TAF sites this evening.
Southern storms could impact OTM later this evening, although
confidence is low at this time.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

DISCUSSION...Hagenhoff
AVIATION...DMD

Hazardous Weather Outlook
National Weather Service Des Moines IA
554 PM CDT Fri Aug 11 2023

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-122300-
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-
554 PM CDT Fri Aug 11 2023

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Thunderstorms are possible across the north and south this
evening. in the southwest through the evening. Some storms could
be strong to severe with large hail and damaging winds the primary
threats.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Storm chances return for Sunday with strong storms possible. Large
hail and strong wind gusts are the primary hazards.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed this evening and again on Sunday.

Zogg