Severe Thunderstorm Warning 118, NWS Des Moines, IA

842 
WWUS53 KDMX 202243
SVSDMX

Severe Weather Statement
National Weather Service Des Moines IA
543 PM CDT Mon Sep 20 2021

IAC117-125-135-181-202253-
/O.EXP.KDMX.SV.W.0118.000000T0000Z-210920T2245Z/
Monroe IA-Lucas IA-Marion IA-Warren IA-
543 PM CDT Mon Sep 20 2021

...THE SEVERE THUNDERSTORM WARNING FOR NORTHWESTERN MONROE...
NORTHEASTERN LUCAS...SOUTHERN MARION AND SOUTHEASTERN WARREN COUNTIES
WILL EXPIRE AT 545 PM CDT...
                
The storm which prompted the warning has weakened below severe
limits, and no longer poses an immediate threat to life or property.
Therefore, the warning will be allowed to expire.  However gusty
winds are still possible with this thunderstorm.

A Severe Thunderstorm Watch remains in effect until 1000 PM CDT for
south central Iowa.

LAT...LON 4102 9345 4126 9341 4135 9287 4116 9287
      4116 9286 4098 9285
TIME...MOT...LOC 2243Z 263DEG 29KT 4119 9283 

$$

Donavon

Area Forecast Discussion
National Weather Service Des Moines IA
153 PM CDT Mon Sep 20 2021

.DISCUSSION.../Tonight through Monday/
Issued at 149 PM CDT Mon Sep 20 2021

*Storms will traverse area today/tonight
 -A few strong/severe storms possible
*Quiet, seasonably cool/seasonable remainder of the forecast

Discussion:

Today/Tonight Storms...

The anticipated strong/severe storm threat this afternoon and
evening remains largely unchanged from previous few AFDs, though
some details have changed.

Mainly, the threat for prefrontal convection largely diminished with
passage of morning showers and lingering cloud cover initiated by
elongated prefrontal surface troughing. Had the surface troughs been
stronger/more organized or the mid-level shortwave speedier,
prefrontal convection may have been in play still. Regardless, the
main show now focuses immediately along the front.

The details along the front have changed little.

The Obvious: Strong mechanical lift along the front will have no
issue initiating convection.

The Largely Unchanged: Effective and deep shear profiles remain
mainly unidirectional and driven by speed differences, ML/MUCAPE
values to around 1500 J/kg, poor mid-level lapse rates under 7 deg
C/km, and inverted-V soundings ahead.

Potentially Sneaky Details: Low level wind profile (about 0-1.5km)
suggest potential window for convection to tap into orthogonal
orientation to the line/cold front. Approaching mid-level wave over
eastern Nebraska may catch up to the front over central Iowa, namely
potentially improving mid-level lapse rates and updraft strengths.

Putting it together: The main storm threat remains for
strong/damaging winds as the cold front lights up and the linear
storm mode takes hold, likely boosted when the eastern Nebraska
shortwave catches the front (most likely along/east of I-35).
Inverted V profiles ahead become less important considering the
forcing mechanism, but while deep shear is not well oriented for a
widespread/high confidence wind or QLCS tornado threat, the low
level winds (0-1.5km) do suggest some potential for at least brief
organization/rotation/enhancement. QLCS tornado or two may be
possible, but am uncertain if the more orthogonal low level shear
will be strong enough (about 20kts shear eyeballing 0-1.5km).
Regardless, cannot be ruled out. Am not bullish on a damaging hail
threat given the lackluster mid-level lapse rates and inability
for much residence time due to questionable ability to organize
and the progressive forcing mechanism.

Beyond Today/Tonight...

A predominantly quiet forecast prevails after tonight with large
scale NW flow taking hold much of the time. While the currently
approaching/passing upper level wave is expected to cut off and hang
out in the Great Lakes Region into the end of the week, it is not
anticipated to adversely affect our sensible weather. A subsequent
upper low is then anticipated to move down from the Canadian Plains
and into the Upper Midwest/Great Lakes late week into the weekend. A
few recent runs have suggested an outside shot at some light
precipitation as a weak cold front slides through, but is likely to
struggle to produce anything with a quite dry air mass in place.
Overall, expect highs to be seasonably cool to seasonable from
Tuesday into the weekend with highs in the mid 60s to mid 70s
dominating. Some warming to end the weekend and start next week
appears likely with upper level ridging depicted amongst the
synoptic guidance, resulting in forecast highs warming into the
upper 70s to lower 80s Sunday into early next week.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1239 PM CDT Mon Sep 20 2021

Unsettled initial 6 to 9 hrs within the TAF period as a line of
thunderstorms traverses the area this afternoon and evening. The
main consequences will be reductions to MVFR and IFR conditions
driven by ceilings, visibilities (rain/driving rain), and
thunder. Improvements back to VFR will be seen overnight, but may
be delayed a few hours by lingering MVFR to IFR cloud cover across
northern sites KMCW/KALO. Elsewhere, expect VFR to return by
around 03z at KFOD/KDSM, 06z at KOTM, and 09z at KMCW/KFOD.

&&

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

DISCUSSION...Curtis
AVIATION...Curtis

Hazardous Weather Outlook
National Weather Service Des Moines IA
225 PM CDT Mon Sep 20 2021

IAZ006-007-016-017-024>028-035>039-046>050-058>062-071>075-
081>086-092>097-211930-
Winnebago-Worth-Hancock-Cerro Gordo-Humboldt-Wright-Franklin-
Butler-Bremer-Webster-Hamilton-Hardin-Grundy-Black Hawk-Greene-
Boone-Story-Marshall-Tama-Guthrie-Dallas-Polk-Jasper-Poweshiek-
Adair-Madison-Warren-Marion-Mahaska-Adams-Union-Clarke-Lucas-
Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-Appanoose-Davis-
225 PM CDT Mon Sep 20 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

A Severe Thunderstorm Watch is in effect for the area. Please
refer to weather.gov/desmoines for more details.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter reports of any storm damage would be appreciated.

IAZ004-005-015-023-033-034-044-045-057-070-211930-
Emmet-Kossuth-Palo Alto-Pocahontas-Sac-Calhoun-Crawford-Carroll-
Audubon-Cass-
225 PM CDT Mon Sep 20 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

CURTIS