Tornadoes and Flash Flooding in Iowa, July 14 to 15, 2021

Area Forecast Discussion
National Weather Service Des Moines IA
643 AM CDT Wed Jul 14 2021

...Updated for 12z Aviation Discussion...

.DISCUSSION.../Today through Tuesday/
Issued at 353 AM CDT Wed Jul 14 2021

Forecast Highlights:
-- Gusty winds, brief heavy rains this morning over northern Iowa
-- Strong to severe storms develop this afternoon; hail and damaging
   wind gusts the main concerns; a few tornadoes cannot be ruled out
-- Heavy rains later in the day into the night over northern into
   central Iowa - localized urban water issues possible
-- Drier and seasonable into this weekend

Details: GOES-East upper level water vapor imagery shows a compact
mid-level shortwave trough over eastern Montana early this morning.
Ahead of this feature, deep moist convection is ongoing over central
South Dakota into central Nebraska aided by warm advection ahead of
the convection. Subjective analysis using current lightning shows
the 6z HRRR has a decent handle of current elevated convection over
South Dakota and Nebraska, which remains below severe limits with
Ainsworth Nebraska gusting to 54 mph before 2am. The convection
should reach our northwestern forecast area around or a little after
12z with expectation that gusty winds, brief period of heavy
rainfall, and lightning will be the primary concerns. The strongest
elevated storms this morning will generally be around and north of
Highway 20, though scattered storms could be as far south as Highway
30. As this first wave of storms depart around midday, it looks
likely that there will be a break in the precipitation over northern
Iowa. The amount of clearing, as is typical, remains uncertain over
this area and therefore the amount of atmospheric recovery for
redevelopment later today. South of this differential heating
boundary/outflow boundary over central and southern Iowa, there
should be enough clearing for strong instability to build with
MLCAPE values over 2500 J/kg. At the same time, forecast soundings
show warming at 850mb or just below, resulting in a cap. This should
hold off afternoon thunderstorms development til peak heating in the
21 to 22z timeframe. While the instability will be strong, deep
layer shear away from the boundary and a slow moving cold front
entering the state will be marginal around 30 knots with mid-levels
winds not robust around 25 to 35 knots.  With the likely scenarios
that storms will organize along these boundaries where convergence
will help to break the cap, the risk of tornadoes is lowering
compared to previous forecasts. Even the QLCS tornado risk is lower
with the current guidance showing more multicell convection outside
of the WRF-ARW, which shows more upscale growth into a bowing
segment. Therefore, the most likely threats for severe weather this
afternoon into this evening will be large hail and damaging wind
gusts. Thereafter, heavy rainfall will be the item to watch.
Throughout today into tonight, precipitable water values of 1.5 to
near 2 inches with favorable warm cloud depths of 3500 to 4000m will
lead to efficient rain rates. The first wave of storms this morning
will likely bring 1 to 2 inches with locally 3 inches to northern
Iowa. This should not cause any water issues outside of urban poor
drainage given the well draining, dry soils over this part of the
state. The afternoon into the overnight storms could bring similar
rain totals, though whether the highest totals from both the morning
and later storms line up over the same area is uncertain. Total
rainfall amounts from deterministic models through Thursday morning
have an average of 1 to 3 inches across northern Iowa with isolated
amounts up to around 6 inch, bullseyes. A review of ensemble members
from the GFS, ECMWF, and CMC are in agreement on this upper end with
the 00z HREF localized probability match mean also showing a 5-7
bullseye over north central Iowa. This rainfall, if it fell over an
urban area, could produce localized water issues. But with it hard
to pin down where these higher end totals will be, the soil over
northern Iowa being some of the drier soil in the state, and the
maturing crop being able to soak up much of this rainfall, have held
off on a flash flood watch.

The front will slowly move southeastward through the day Thursday
into Thursday night ending in central Missouri by Friday morning,
which will keep the focus for additional shower and storm chances
primarily over areas south of Iowa into this weekend. For Iowa,
we'll be on the eastern edge of an amplifying ridge over the western
US with troughing into the northeastern US. Weak flow aloft and
surface high pressure over the western Great Lakes should provide a
generally dry forecast even as a shortwave trough comes down the
eastern side of the ridge Sunday into Monday. As this shortwave
drops into Missouri, it should help the front south of the state be
pushed farther south. Temperatures during this period will be

&&

.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 644 AM CDT Wed Jul 14 2021

Storms are moving into central Iowa, mainly north of Highway 30
this morning. These may bring gusty winds and will bring
lightning and a brief period of heavy rain with restrictions for
a short time this morning over the northern Iowa terminals. There
should be a break with VFR conditions most places except over
northern Iowa later this morning or this afternoon. However,
storms should redevelop late this afternoon into this evening and
have handled with VCSH around the period of most likely SHRA/VCTS.

&&

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

DISCUSSION...Ansorge
AVIATION...Ansorge

Hazardous Weather Outlook
National Weather Service Des Moines IA
309 AM CDT Wed Jul 14 2021

IAZ004>007-015>017-023>028-033>039-044>050-151100-
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-
309 AM CDT Wed Jul 14 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Severe thunderstorms will be possible today into this evening
across the area with an enhanced risk of severe weather. The main
threats will be damaging thunderstorm winds and large hail,
although a tornado or two remains possible. Locally heavy
rainfall is also expected.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Additional isolated to scattered thunderstorms are possible on
Thursday, however no severe weather is expected.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed today into this evening.

IAZ057>062-070>075-081-151100-
Audubon-Guthrie-Dallas-Polk-Jasper-Poweshiek-Cass-Adair-Madison-
Warren-Marion-Mahaska-Adams-
309 AM CDT Wed Jul 14 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Severe thunderstorms will be possible later this afternoon into
this evening with a slight risk of severe weather. The main
threat will be damaging thunderstorm winds, with a secondary
threat from large hail and an isolated tornado. Locally heavy
rainfall is also possible.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Additional isolated to scattered thunderstorms are possible on
Thursday, however no severe weather is expected.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon into this evening.

IAZ082>086-092>097-151100-
Union-Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
309 AM CDT Wed Jul 14 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Scattered thunderstorms are expected tonight with some locally
heavy rainfall possible. Severe weather is not anticipated at this
time.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Additional thunderstorms will be possible on Thursday into
Thursday night although severe storms are not expected at this
time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

Special Weather Statement
National Weather Service Des Moines IA
717 AM CDT Wed Jul 14 2021

IAZ004>006-015-016-023-024-035-141315-
Palo Alto IA-Humboldt IA-Kossuth IA-Winnebago IA-Webster IA-
Pocahontas IA-Emmet IA-Hancock IA-
717 AM CDT Wed Jul 14 2021

...Strong Winds with Thunderstorms Across Northern Iowa This
Morning...

At 716 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Terril to 8 miles south of Pocahontas.
Movement was east at 40 mph.

Penny size hail and wind gusts up to 50 mph will be possible with
these storms.

Locations impacted include...
Estherville, Algona, Emmetsburg, Pocahontas, Dakota City, Humboldt,
Laurens, Swan Lake, Five Island Lake, Ingham Lake, Union Slogh, High
Lake, Lost Island Lake, Silver Lake, Rush Lake, Armstrong, Buffalo
Center, Graettinger, West Bend and Ruthven.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4350 9395 4268 9397 4256 9491 4347 9492
TIME...MOT...LOC 1216Z 255DEG 35KT 4334 9490 4262 9468

Cogil