Hail and Damaging Winds in Iowa, July 9 to 10, 2021

Area Forecast Discussion
National Weather Service Des Moines IA
342 AM CDT Fri Jul 9 2021

.DISCUSSION.../Today through Thursday/
Issued at 341 AM CDT Fri Jul 9 2021

...Severe Weather Possible Late Today Into Tonight...

The primary focus of the forecast is on convection and associated
severe weather/heavy rain potential today through tonight.
Currently a large swath of showers and thunderstorms has expanded
roughly northwest to southeast across much of Iowa. Some of the
storms were severe in western and northwestern Iowa earlier, but
have faded in intensity as they moved eastward into less unstable
air toward central Iowa. Additional storms are also forming in
north central iowa and upstream over southwestern Minnesota. Most
short-range/high-resolution model output continues fading the
large southern swath this morning, while the northern swath
persists a bit longer toward midday over northeastern Iowa. The
degree to which the atmosphere is worked over by this morning
convection, and the placement of any resulting outflow boundaries,
will be of great importance this afternoon.

As diurnal heating occurs today in areas southwest of the morning
convection it will lead to strong destabilization by the
afternoon, with an effective warm front extending out of a Great
Plains surface cyclone being enhanced by the differential
heating/outflow, sharpening the front somewhere over Iowa, likely
in the southwest and southern portions of the state. While extreme
instability may be limited to those areas, the boundary could also
rebound and push northward toward central Iowa, depending on the
ability of the heating/destabilization to overwhelm early-day
convective overturning. Even to the northeast of the front, there
will also be moderate to strong elevated instability by late
afternoon, though it remains to be seen whether daytime heating
will be sufficient to break the cap in that area. It is also
possible that the morning convection, especially the are currently
developing to the north, may just fester all day over the
northeastern half of the state as indicated by some of the high-
resolution model solutions. The bottom line is that the details of
convective evolution remain highly uncertain for later today.
However, the environment will clearly be supportive of severe
weather, especially over southern Iowa/south of the effective warm
front where surface-based initiation and supercell storms will be
possible in the face of unseasonably impressive shear/flow fields,
with all modes of severe weather possible. SPC convective outlooks
convey this threat well and all interests should monitor the
latest forecast updates and potential watches/warnings through the
day and into tonight.

As the nocturnal low-level jet redevelops tonight it may activate
additional rounds of showers and thunderstorms, mainly over the
southern half of Iowa, however confidence in this is even lower as
such a scenario would be highly dependent on the extent and
behavior of afternoon/evening storms, which could congeal into an
MCS and blast south into Missouri, subsequently eating up the
LLJ/inflow and limiting overnight convective reinitiation over
Iowa. If additional storms do develop, they would be elevated but
still able to tap into strong enough instability to support a
continuing hail threat tonight. In addition, high atmospheric
moisture and potential back-building/training of storms along
boundaries supports a heavy rain threat and has prompted issuance
of a Flash Flood Watch in our southeastern counties, as discussed
further in the Hydrology section below.

A broad 500 mb trough will approach from the Dakotas tonight,
subsequently stalling out and closing off, but also gradually
weakening, over Iowa/Missouri/Illinois through the weekend. This
will support continued rain/storm chances, especially over the
eastern/southeastern half of Iowa, through Sunday and perhaps into
Monday. While the severe weather risk will be very low during that
time, a heavy rain threat may linger and if flash flooding/heavy
rainfall does occur in southern/eastern Iowa later today or
tonight, those impacts could be exacerbated or prolonged by any
additional rainfall. The system should finally eject eastward
toward the Great Lakes around Monday, after which wester
northwesterly 500 mb steering flow resumes and we should see a
couple more opportunities for thunderstorms as impulses move
through that flow for the remainder of the coming work week.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1107 PM CDT Thu Jul 8 2021

VFR conditions at the moment, but storms have started to develop
over northwestern Iowa back into southeastern South Dakota and
northeastern Nebraska. These storms will move east-southeastward
and bring impacts to some portions of the central Iowa terminals.
These should wane or move out by mid-morning, but another round of
strong to severe storms is expected to develop in the afternoon
and push south-southeastward most likely impacting OTM and perhaps
ALO, FOD and DSM.

&&

.HYDROLOGY...
Issued at 322 AM CDT Fri Jul 9 2021

A round of light to moderate rainfall is heading into southern and
southeastern Iowa this morning. Additional thunderstorms are
forecast in that area late today through tonight, and perhaps
beyond into the weekend. While significant uncertainties remain
regarding the timing and placement of those thunderstorms,
atmospheric conditions are supportive of efficient rainfall rates
from this afternoon through tonight, and back-building/training
storms may also be possible. With the greatest probability of
these phenomena in the south/southeast, and that area also having
received heavy rainfall a week or two ago, have issued a flash
flood watch to highlight the potential threat. If the storms
develop/train over areas further north or west, which is possible,
then the watch may need to be expanded later today.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from this afternoon through Saturday morning
for IAZ085-086-095>097.

&&

DISCUSSION...Lee
AVIATION...Ansorge
HYDROLOGY...Lee

Hazardous Weather Outlook
National Weather Service Des Moines IA
350 AM CDT Fri Jul 9 2021

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>084-092>094-101100-
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-Taylor-Ringgold-Decatur-
350 AM CDT Fri Jul 9 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Thunderstorms are forecast at times today through tonight. There
is a potential for severe weather at times, especially over the
southern half or so of Iowa from mid-afternoon through this
evening when all modes of severe weather may be possible
including damaging winds, very large hail, and even tornadoes.
Locally heavy rainfall is also expected, especially south of
Interstate 80.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Periodic chances for thunderstorms remain in the forecast from
Saturday into next week, however the threat of severe weather
appears low at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening,
mainly across the southern half or so of Iowa.

IAZ085-086-095>097-101100-
Monroe-Wapello-Wayne-Appanoose-Davis-
350 AM CDT Fri Jul 9 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Thunderstorms are forecast at times today through tonight. There
is a potential for severe weather at times, especially from mid-
afternoon through this evening when all modes of severe weather
will be possible including damaging winds, very large hail, and
even tornadoes. Locally heavy rainfall is also expected and could
lead to flash flooding by tonight.

A Flash Flood Watch is in effect for the area late this afternoon
into Saturday morning. Please refer to the National Weather
Service website at weather.gov/desmoines for further details.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

A Flash Flood Watch is in effect for the area late this afternoon
into Saturday morning. Please refer to the National Weather
Service website at weather.gov/desmoines for further details.

Periodic chances for thunderstorms remain in the forecast from
Saturday into next week, however the threat of severe weather
appears low at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening.

Lee