Flash Flooding and Damaging Wind in Iowa, June 23 to 25, 2021

Area Forecast Discussion
National Weather Service Des Moines IA
632 PM CDT Wed Jun 23 2021

...Updated Aviation Discussion...

.DISCUSSION.../Tonight through Wednesday/
Issued at 320 PM CDT Wed Jun 23 2021

In typical late June fashion, the script has flipped to a much
more active weather pattern across Iowa. Heavy rainfall
potential even returns to the state, at least for parts of
southern Iowa over the next few days along with token chances for
severe weather.

For tonight, an MCS is expected to develop over Nebraska
overnight. The strongest portion of this MCS should dive
southeast along the instability gradient and be more focused into
northern Missouri though the northern edge should move across far
southern Iowa with more scattered activity elsewhere. Much of the
area may storm free during the late morning and into the afternoon
tomorrow. Two areas of focus for storm development late tomorrow
afternoon and into the evening. The first along a cold front over
north central Iowa. Deep layer shear is lacking over northern Iowa
at 25 kts or less though some speed shear is noted above 700 mb.
This may yield multi-cell hailers or possibly a few damaging wind
gusts. The other area of focus will be on an advancing warm front
that should still be over northern Missouri after the morning
convection cold pool erodes. This area will be more favorable for
supercells. Numerous threats associated with these storms
including large hail, damaging winds and possibly a few tornadoes.
Maybe a bigger threat is very heavy rainfall with training of
supercells likely. Again, at this time, this threat appears just
south of the IA/MO border but a 100 mile shift north will place
southern Iowa under this threat. A constant low level jet pointing
into this boundary feeding 2+ inch pwats along with the training
factor could lead to an extreme rainfall event wherever this
boundary sets up.

A more vigorous upper short wave will arrive during the day Friday
and will have an attendant surface low lifting northeast with it.
This should lift a warm the surface warm front into central Iowa
in what is looking like a supercell environment over Iowa. Large
hail, damaging wind and heavy rain look all possible with these
storms. Isolated tornadoes also look possible with low LCLs and
back surface winds though hodographs and low level wind profiles
are not overly impressive. Will continue to monitor this period
but certainly the large scale forcing will be available and the
warm front should be lifting north into the state.

The remainder of the forecast period features on and off chances
for thunderstorms this weekend as the western ridge remains in
place and an upper trough remains through the mid Missouri Valley.
Temperatures will be near to below seasonal values this weekend
and into early next week with highs in the upper 70s to mid 80s.

&&

.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 629 PM CDT Wed Jun 23 2021

Expecting VFR conditions through Thursday morning before scattered
convection moves across the state. These storms are more likely to
impact DSM and OTM past 10z with MVFR ceilings and possibly
visibility with pockets of more intense rainfall. The majority of
CAMS are in decent agreement with timing b/t 09-14z to impact
central and southern Iowa.

&&

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

&&

DISCUSSION...Donavon
AVIATION...Podrazik

Hazardous Weather Outlook
National Weather Service Des Moines IA
343 PM CDT Wed Jun 23 2021

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-242045-
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-
343 PM CDT Wed Jun 23 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Thunderstorms are expected to move into Iowa after midnight
tonight. The greatest chance for storms is over the southwest. An
isolated storm is possible with damaging wind gusts possible.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Several rounds of thunderstorms are expected Thursday and Friday.
Strong to severe storms will be possible during this time.
Heavy rainfall is also possible over southern Iowa on Thursday
and across much of the area Friday. Given the current expected
rainfall, most rivers will see within bank rises with smaller
streams perhaps exceeding their banks.

Non-severe thunderstorms at times will linger into this weekend.

.SPOTTER INFORMATION STATEMENT...

Spotter activation might be needed at times later this week.