Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
147 PM CDT Tue Aug 10 2021

.DISCUSSION.../Tonight through Tuesday/
Issued at 145 PM CDT Tue Aug 10 2021

Hot and humid conditions remain in place over much of the state
again day. Heat concerns are most evident south and east of a
diffuse frontal boundary stretched southwest to northeast over the
forecast area. Dew points have been considerably high, ranging from
the mid 70s to low 80s. Cloud debris from various convection over
the region have hindered warming a bit today, thus keeping most heat
index values below advisory criteria to this point. 12z soundings
and model analysis through the day has suggested a rather strong cap
is place, which would help the extreme surface based instability in
check especially given the marginal convergence and upper support.
Up until around the lunch hour only some spurious elevated shower
and storms were noted over north central Iowa. However, surface
based CI already commenced in south central Iowa around 18z,
possibly boosted by a remnant outflow from convection to the
south. The northerly convection was also becoming more organized
and may further strengthen as it taps into deeper instability.
The evolution to this point casts further uncertainty into an
already murky convective forecast, which should be expected as
most hi- res model guidance will struggle in such a high
instability/low kinematics environment. The persistent elevated
convection may be a clue that deep layer support for ascent may be
stronger that currently progged, and thus the atmosphere may be
better suited to breach the cap later this afternoon and evening.
Current thinking still favors central and more likely southern and
eastern Iowa for further convective development later this
afternoon and evening as a mid-level wave and associated 700mb
speed max pushes northeast out of KS/MO. Should additional storms
develop, a highly unstable environment and 25-35 kts of deep layer
shear would be supportive of severe storms capable of large hail
and damaging winds. Convection may be sustained or redevelop as
the LLJ strengthens after dark, but models by this time period try
to push most convection into eastern Iowa.

Muggy weather will persist into Wednesday as little to no heat and
humidity is scoured out today. The heat advisory continues for areas
along and south of I-80 due to the potential for heat index values
pushing 105 degrees. Scattered storms will be possible as another
weak boundary traverses the state, but again convergence and support
aloft is weak. Any CI may hold off until late in the day or the LLJ
strengthens after dark. By then the boundary has advanced into
southern IA and/or northern MO.

Flow aloft becomes northwesterly into Thursday, which will push a
more robust frontal boundary through the region and finally bring
relief from the heat and humidity. Shower and storms are possible
along the front as it slowly sags south through Thursday night.
Still some uncertainty on timing and precip/cloud coverage during
peak heating, but a slower solution may necessitate another head
headline at least across the far south.

Much more comfortable and dry conditions are expected to end the
week as a Canadian surface high settles into the region. This will
bring high temps back into the low to mid 80s and dew points in the
50s to low 60s. Southerly return flow on the back side of the
departing high will gradually bring more humidity and possible some
precip chances back into the forecast early next week.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1231 PM CDT Tue Aug 10 2021

Area of MVFR cigs tracking across central Iowa as of early
afternoon. A few embedded showers were also noted over north
central Iowa, gradually tracking north and east. The stratus deck
should continue to track north east and eventually lead to a
return to VFR at most terminals by late afternoon.

Additional TSRA are possible by later this afternoon through the
overnight hours, mainly across central into southern Iowa.
Confidence is very low at the current time as to the location and
timing of any possible TSRA activity. Have opted to only include
mention of VCTS at KOTM for now, but this may need to be expanded
to other terminals or eliminated as trends become more clear.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon to 7 PM CDT Wednesday for IAZ057>062-
070>075-081>086-092>097.

Heat Advisory until 7 PM CDT this evening for IAZ027-028-037>039-
047>050-058>062-070>075-081>086-092>097.

&&

DISCUSSION...Martin
AVIATION...Martin

Hazardous Weather Outlook
National Weather Service Des Moines IA
337 PM CDT Tue Aug 10 2021

IAZ004>007-015>017-023>026-033>036-044>046-112045-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Sac-Calhoun-Webster-Hamilton-
Crawford-Carroll-Greene-
337 PM CDT Tue Aug 10 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

There is a slight chance of thunderstorms this evening, with a low
risk of severe weather, locally strong wind gusts the primary
concern.

It will be hot and muggy again today, with heat index values approaching
100 degrees this afternoon.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Additional thunderstorm chances are forecast late Wednesday and
late Thursday, especially toward central Iowa and further south
and east. Severe weather may be possible at times, with strong
winds being the primary threat.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated.

IAZ027-028-037>039-047>050-059>062-112045-
Butler-Bremer-Hardin-Grundy-Black Hawk-Boone-Story-Marshall-Tama-
Dallas-Polk-Jasper-Poweshiek-
337 PM CDT Tue Aug 10 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

A Heat Advisory is in effect for the area this afternoon and
evening. See weather.gov/desmoines for details.

There is a marginal to slight risk for severe weather this
afternoon and evening with strong wind gusts the primary concern.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Additional thunderstorm chances are forecast late Wednesday and
late Thursday, with a threat of severe weather at times primarily
in the form of strong wind gusts.

.SPOTTER INFORMATION STATEMENT...

Spotter activation could be needed this afternoon and evening,
and possibly again Wednesday and Thursday.

IAZ057-058-070>075-081>086-092>097-112045-
Audubon-Guthrie-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-
Union-Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
337 PM CDT Tue Aug 10 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

A Heat Advisory is in effect for the area through this evening.
See weather.gov/desmoines for details.

There is a marginal to slight risk for severe weather this
afternoon and evening with strong wind gusts the primary concern.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Additional thunderstorm chances are forecast late Wednesday and
late Thursday, with a threat of severe weather at times primarily
in the form of strong wind gusts.

The heat index may approach or briefly exceed 100 degrees again on
Wednesday afternoon.

.SPOTTER INFORMATION STATEMENT...

Spotter activation could be needed this afternoon and evening,
and possibly again Wednesday and Thursday.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
1257 PM CDT TUE AUG 10 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1245 PM     HAIL             2 W MURRAY              41.04N 93.98W
08/10/2021  M1.00 INCH       CLARKE             IA   TRAINED SPOTTER

            DIME TO QUARTER SIZED HAIL IN PROGRESS.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
211 PM CDT TUE AUG 10 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0208 PM     HAIL             2 SW FREDERIKA          42.86N 92.34W
08/10/2021  E1.00 INCH       BREMER             IA   EMERGENCY MNGR

            QUARTER INCH HAIL IN PROGRESS WITH HEAVY
            RAIN.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
310 PM CDT TUE AUG 10 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0300 PM     TSTM WND DMG     2 SW FERGUSON           41.92N 92.90W
08/10/2021                   MARSHALL           IA   PUBLIC

            SPOTTER REPORT OF STRONG WINDS WITH POWER
            OUT. TIME ESTIMATED VIA RADAR.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
318 PM CDT TUE AUG 10 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0317 PM     TSTM WND GST     2 E LE GRAND            42.01N 92.74W
08/10/2021  E60 MPH          TAMA               IA   PUBLIC

            PUBLIC REPORT OF 60 PLUS MPH WIND GUST EAST
            OF LE GRAND, SIGNIFICANTLY REDUCED
            VISIBILITY ON HWY 30.

&&

Special Weather Statement
National Weather Service Des Moines IA
359 PM CDT Tue Aug 10 2021

IAZ050-062-102145-
Tama IA-Poweshiek IA-
359 PM CDT Tue Aug 10 2021

...A strong thunderstorm will impact portions of northeastern
Poweshiek, southeastern Marshall and Tama Counties through 445 PM
CDT...

At 357 PM CDT, Doppler radar was tracking a strong thunderstorm near
Tama, or near Toledo, moving east at 30 mph.

HAZARD...Wind gusts of 50 to 55 mph and nickel size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

This storm will remain over mainly rural areas of Tama and
northeastern Poweshiek Counties, including the following locations...
Tf Clark State Park, Elberon, Montour, Toledo Municipal Airport,
Holiday Lake, Chelsea, Vining, Clutier and Le Grand.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Severe Thunderstorm Watch remains in effect until 800 PM CDT for
central and northeastern Iowa.

&&

LAT...LON 4181 9230 4181 9263 4195 9277 4203 9277
      4230 9230
TIME...MOT...LOC 2057Z 250DEG 26KT 4193 9249

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...55 MPH

Krull