Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1136 PM CDT Sun Aug 22 2021

.DISCUSSION.../Tonight through Sunday/
Issued at 228 PM CDT Sun Aug 22 2021

Key Points:
-Periodic storm chances overnight and through the upcoming week
-Heat builds with the hottest days of the week on Monday and Tuesday

Details:
The leading edge of a shortwave trough moving across the northern
and central plains has aided convective initiation across portions
of eastern South Dakota this afternoon. By 2 pm a small cluster of
storms had developed in far northwest Iowa at the South Dakota
border. CAMs have struggled with this system, as has been noted in
previous discussions. In fact, the ongoing convection is not
present in current runs with convection reaching Iowa after 06z in
most model solutions. Thus, confidence in convective trends this
evening and overnight is low. That said, the initial forcing with
the approaching shortwave may bring a few thunderstorms across
northern Iowa this afternoon, with additional activity through the
overnight as the low-level jet aid initiation. In favor of storm
development overnight is MLCAPE around 1500 J/kg and deep layer
shear around 30 knots. On the other hand, a strong cap will remain
in place and could limit development all together. The HRRR seems
to favor this solution with no convective development in Iowa
tonight while the NAMnest/RAP/NSSL-WRF favor a cluster of storms
moving across the state in the after midnight hours. Should
storms develop, elevated storms with hail would be the primary
threat.

Thermal ridging builds across Iowa Monday with the warm front
getting hung up across northern Iowa and southern Minnesota - a
scenario that remains in place through the upcoming week. This
results in warm air advection and moisture transport into the area
making for a hot and humid week. Monday and Tuesday will be the
warmest days with highs in the low to mid 90s and dewpoints
around 70 degrees. This will send heat indices into the upper 90s
north of I-80 to low 100s south of I-80. While just below heat
advisory criteria, those with outdoor plans should take
appropriate precautions to remain safe in the heat. The interplay
of convection and warm front placement will determine just how
hot and humid the area gets.

So what does the convective scenario look like? Convection on Monday
will depend on where the warm front stalls out overnight tonight
and will serve as the impetus for development. MLCAPE reaches
2000+ J/kg Monday with the advancement of a more unstable airmass.
Shear is weak however, around 20 kts, which will limit storm
organization and thus severe threat. SPC has added a marginal risk
to Monday which seems warranted based on the environment.

A shortwave will ride the ridge Tuesday, bringing better chances for
rain and a low end severe weather threat. With an unstable air mass
and a number of embedded disturbances in the flow, there are a
number of chances for convection throughout the week. While storms
are possible nearly everyday, it will not be a total washout and
most people will not see rain everyday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1136 PM CDT Sun Aug 22 2021

Current trends pointing toward convection across northeast areas
aft 09-10z with VCTS included at KFOD, KALO,and KMCW. Remainder of
the day likely VFR with little convection aft 12-13z east areas.
Another round of storms with daytime heating possible aft 23z
through 06z favoring north sites once again, though uncertainty of
position of any leftover outflow boundaries from morning convection
later in day may lead to farther south placement by evening hours.
/rev

&&

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

DISCUSSION...Hagenhoff
AVIATION...REV

Hazardous Weather Outlook
National Weather Service Des Moines IA
338 PM CDT Sun Aug 22 2021

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-232045-
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-
338 PM CDT Sun Aug 22 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Scattered thunderstorms are possible mainly over northern Iowa
tonight into tomorrow morning. A few storms may be strong to
severe with large hail the primary threat.

.DAYS TWO THROUGH SEVEN...Monday through Saturday

Periodic chances for storms continue through the work week. A few
strong storms are possible on Monday and again Tuesday. The
threat of severe weather beyond Tuesday remains uncertain.

.SPOTTER INFORMATION STATEMENT...

Spotter reports may be needed tonight and again tomorrow.

Special Weather Statement
National Weather Service Des Moines IA
335 AM CDT Mon Aug 23 2021

IAZ035>037-047-048-230915-
Hamilton IA-Boone IA-Story IA-Webster IA-Hardin IA-
335 AM CDT Mon Aug 23 2021

...A strong thunderstorm will impact portions of northeastern Greene,
northwestern Story, western Hardin, southeastern Webster,
southwestern Franklin, northern Boone and Hamilton Counties through
415 AM CDT...

At 333 AM CDT, Doppler radar was tracking strong thunderstorms over
northwest Boone county and into central Hamilton county, moving east
at 40 mph.

HAZARD...Penny size hail.

SOURCE...Radar indicated.

IMPACT...Minor damage to outdoor objects is possible.

This strong thunderstorm will be near...
  Boone around 345 AM CDT.

Other locations in the path of this storm include Story City, Jewell
Junction and Roland.

This includes Interstate 35 between mile markers 119 and 147.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 4211 9346 4202 9412 4214 9417 4221 9417
      4256 9348 4256 9346
TIME...MOT...LOC 0833Z 253DEG 34KT 4214 9405

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...<30 MPH

Donavon