Severe Thunderstorm Warning 174, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1151 PM CDT Mon Aug 19 2019

.UPDATE...
Issued at 942 PM CDT Mon Aug 19 2019

The active night still appears to be on target with little change
from previous thinking. 4-5K j/kg MUCAPEs are present along the
NE/IA border in the projected genesis area with 40-45kts of
effective shear. The only concern may be slightly delayed timing
and onset. 00z soundings and the latest model guidance do not
depict the strongest low level jet with RAP 305K inflow into the
base of the effective layer only 20kts or so. Nevertheless, once
it goes it may go quickly with fresh 00z CAMs all still depicting
a significant wind producing MCS NW-SE starting 06-09z and not
exiting southeast sections until 12z or so. If this caliber of MCS
develops as expected, it will be somewhat rare in magnitude
reflecting the infrequency of overnight Enhanced Risks in IA.
Possibly reminiscent of significant wind events in 2008 or 2010.

&&

.DISCUSSION.../Tonight through Monday/
Issued at 309 PM CDT Mon Aug 19 2019

High pressure remains ridged into central Iowa this afternoon with
return flow beginning to develop into far western Iowa and into
Nebraska and Kansas. The return flow will increase this evening as
the High departs to the east. The low level jet will form and
increase to 35-40kts between 03-06z and will bring a strong push of
theta-e advection into central Iowa. This combined with steepening
mid-level lapse rates will lead to moderate to highly unstable
environment with MUCAPEs exceeded 3500 J/kg. Expect thunderstorms to
develop across portions of northwest to west central Iowa around
midnight then roll southeast down the instability gradient. Severe
weather and heavy rain fall will both be concerns with this activity
overnight. Initial storms will be elevated with a few supercells
capable of large hail possible. The initial cells should congeal
into an MCS and propagate southeast with an attendant damaging wind
threat into central and southeast Iowa. In addition, an isolated
tornado or two may be possible given the amount of low level SRH if
the 0-3 km shear vector can orient correct in time. This would be
contingent on potential for increased mixing ahead of the line can
help erode the low level inversion and allow for low level vortex
stretching. A few areas of wind driven hail may occur given the
maintaining instability and potential for cold pool development.

The potential for heavy rainfall is increasing as the increasing
instability within a moisture rich atmosphere with good warm cloud
depths near 15 kft aiding in the efficient rain process. A rainfall
band of 2 to 4 inches is possible and cannot discount a heavier
localized band in excess of 5 inches. The intense rainfall amounts
will cause significant ponding in urban areas. If a 5 inch band does
develop, the threat may translate to a flash flooding threat given
the rates and amounts.

Warm advection following the the morning MCS will create a
significant elevated mixed layer that will lead to a significant cap
across the state. Convective debris looks like it will eventually
clear out over parts of the state. The biggest challenge will be how
much of the state will recover from the morning outflow. Have the
warmest temperatures over central and western Iowa with high
temperatures approaching 90 degrees and heat index values in the 100
to 105 degree range. Not anticipating any heat headlines at this
point especially considering uncertainties given the recovering.
Have lowered chances for thunderstorms Tuesday afternoon and
evening. A greater threat for storms will late Tuesday night into
Wednesday morning over as storms move east out of Nebraska.

Little change to the remainder of the forecast with cooler
temperatures Thursday through Saturday then a transition to more
westerly flow by Sunday into early next week. This will lead to
seasonal temperatures and a few more chances for precipitation.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1151 PM CDT Mon Aug 19 2019

The scenario is essentially unchanged since 00z. Isolated storms
should affect KMCW shortly and there is at least medium
confidence that a significant thunderstorm complex will develop
into portions of central IA during the early morning hours. Have
VFR either TSRA or VCTS mention for now depending on confidence,
but there will likely be a period of lower conditions for at least
one of the TAF sites overnight. No mention until timing and
location confidence increases however. Some lingering stratus is
possible after the precip, but confidence is medium to high that
VFR conditions will return by Tue afternoon.

&&

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

&&

UPDATE...Small
DISCUSSION...Donavon
AVIATION...Small

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Special Weather Statement
National Weather Service Des Moines IA
211 AM CDT Tue Aug 20 2019

IAZ004>006-015>017-023>027-033>037-044>047-057-200815-
Emmet-Kossuth-Winnebago-Palo Alto-Hancock-Cerro Gordo-Pocahontas-
Humboldt-Wright-Franklin-Butler-Sac-Calhoun-Webster-Hamilton-
Hardin-Crawford-Carroll-Greene-Boone-Audubon-
Including the cities of Estherville, Algona, Forest City,
Lake Mills, Emmetsburg, Garner, Britt, Kanawha, Mason City,
Clear Lake, Pocahontas, Laurens, Rolfe, Fonda, Gilmore City,
Humboldt, Eagle Grove, Clarion, Belmond, Hampton, Parkersburg,
Clarksville, Shell Rock, Greene, Aplington, Allison, Dumont,
Sac City, Lake View, Odebolt, Wall Lake, Schaller, Early,
Rockwell City, Manson, Lake City, Pomeroy, Fort Dodge,
Webster City, Iowa Falls, Eldora, Ackley, Denison, Carroll,
Jefferson, Boone, Audubon, and Exira
211 AM CDT Tue Aug 20 2019

...Thunderstorms Beginning to Intensify Early this Morning...

Thunderstorms are expanding in strength and coverage early this
morning across western and northern Iowa and this trend will
likely continue over the next few hours. Several storms will
likely produce dime to nickel sized hail with larger more severe
hail up to one to two inches or more possible later this morning.
Damaging winds may occur as well.

A Severe Thunderstorm Watch remains in effect until 7am.

Small