Hail and Damaging Wind in Iowa, September 23, 2023

Area Forecast Discussion
National Weather Service Des Moines IA
1221 PM CDT Sat Sep 23 2023

...Updated for the 18z Aviation Discussion...

.DISCUSSION.../Today through Friday/
Issued at 421 AM CDT Sat Sep 23 2023

Key Messages:

- Patchy fog N of Highway 30 this morning
- A few storms possible across N Iowa this morning
- Severe thunderstorms possible this afternoon/evening
- Heavy rain...best chance along I-35 corridor

Early morning fog north of Highway 30 will persist through shortly
after daybreak before better mixing dissolves. Otherwise, a few
showers and non-severe thunderstorms possible across northern
Iowa this morning.

Thereafter, attention shifts to severe thunderstorm potential this
afternoon. On early morning satellite, easily discerned upper-low
evident spinning across eastern Wyoming will be the catalyst
for thunderstorm activity later today. The reflected attendant
surface front and strengthening LLJ blossoming severe-warned
convection across west-central Nebraska at the time of this
issuance. Models are similar in tracking this upper-low through
South Dakota today. Meanwhile, the surface low pressure will
accelerate northeast from W Kansas to near ND/SD/MN by tonight as
the system begins to occlude. As it does, a strengthening LLJ will
precede the approaching cold front, and will be positioned near
the MO River by midday. 06Z NAM and RAP points to the potential
for weak pre-frontal convection across central IA during the
morning within belt of strengthening LLJ, but thats in no way a
unanimous solution amongst the CAMs; and even so, it would be weak
should it occur. The one thing that it could do is delay
insolation/destabilization tho, thus effectively delaying cap
erosion & convection initiation. More organized convection will
await the surface cold front, which will move into western
portions of the CWA early this afternoon. This will provide the
lift necessary for convection to develop and organize, progressing
thru the rest of central Iowa during the late afternoon and
evening. 1-4P is the most likely timing of convection initiation
across western half of the CWA (earlier in the window with less
morning clouds; later w/ more).

Onto the severe threat. Ribbon of enhanced instability (SBCAPE of
1000-2000 j/kg) will exist prefrontal as the warm front races
from south to north. Meanwhile, strengthening low and mid-level
flow will push deep-layer shear above 40 knots. Both sufficient
for supercells and severe hail and winds. Isolated tornado threat
also a possibility, especially if areas of backed surface flow
can be realized (best chance across northern Iowa, but possible
anywhere with boundary interaction). LCLs are higher than optimal
though (4-5kft), which may help mitigate a greater tornado threat.
Discrete convection early will become more linear with time late
in the afternoon and especially evening, with severe potential
waning by late evening.

PWATs values of 1.5-2.0" this afternoon/evening suggest efficient
rainers are good bet. Dry antecedent conditions certainly help
mitigate any widespread flood threat. Will need to keep a close on
urban areas and any areas that may have recurring storms, however.

Thunderstorms push east of the CWA this evening, returning
conditions to a drier and more seasonal temperatures for Sun-Wed,
followed by a warming trend for Thu-Fri.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1221 PM CDT Sat Sep 23 2023

Primary concern for aviation interests will be periodic
thunderstorms moving across the area this afternoon and evening.
Have taken best estimate of timing worst impacts at each terminals
for the 18Z TAFs, but amendments are likely. Once the storms clear
out this evening/tonight, generally VFR conditions should prevail,
but there is some potential for MVFR ceilings and possibly LLWS at
times early Sunday morning.

&&

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

&&

DISCUSSION...Hahn
AVIATION...Lee

Hazardous Weather Outlook
National Weather Service Des Moines IA
435 AM CDT Sat Sep 23 2023

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-240945-
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-
435 AM CDT Sat Sep 23 2023

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Thunderstorms will be likely in western Iowa on Saturday afternoon
and spread into central Iowa by the evening, persisting into
Saturday night. There is a slight risk of severe weather, mainly
over about the western half of Iowa Saturday afternoon and
evening. Strong wind gusts and hail are the main hazards, with a
few tornadoes possible at times. A heavy rainfall threat will
also exist from late Saturday into Saturday night, however, the
overall threat of flooding is low.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

A few weaker storms are possible mainly across eastern Iowa
Sunday with the departing cold front. Otherwise low chances for
showers and a weak storm remain into early next week largely
across eastern Iowa.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed today.