Hail and Lightning Strike in Iowa, September 17 to 18, 2022

Area Forecast Discussion
National Weather Service Des Moines IA
1151 PM CDT Fri Sep 16 2022

...Updated for the 06z Aviation Discussion...

.DISCUSSION.../Tonight through Friday/
Issued at 239 PM CDT Fri Sep 16 2022

Take Home Points:

*Unsettled weather this weekend
*Potential for a few strong to severe storms Sat/Sun
*Near record to record heat builds in early work week
*Much cooler latter half of next week

Discussion:

As the upper level ridge has pushed eastward over the Great Lakes,
larger SW flow has settled into the region, with a surface low and
associated boundaries remaining largely to the NW of the state.
General WAA/isentropic lift ahead of the surface front has allowed
for scattered showers and weak convection to fester across NW Iowa
much of the morning and into the afternoon. Additionally, a
shortwave trough exiting the Nebraska/Kansas area into Iowa and
Missouri, seen nicely on GOES WV imagery, has allowed shower
activity to extend down toward the Kansas City area, but too remains
weak with unsupportive temperature profiles/lapse rates. Hi-res
guidance has largely missed the weak S/SW Iowa into Missouri
showers, so have added some low end PoPs as the shortwave slowly
translates eastward, diminishing as the activity moves into areas
of drier low- mid level air. Nothing more than an isolated shot
at lightning is expected with the remaining aforementioned
activity.

As we move into the weekend, multiple opportunities for showers and
storms exist, and unfortunately the details remain quite messy.

The first round will begin over western Nebraska this evening as
storms initiate and grow upscale, aided by the developing LLJ.
Broadly speaking, HRRR/ARW/NMM cores are in fair agreement on
progression and development, including additional LLJ induced
activity over SC/SE Nebraska and SE Iowa into Saturday morning.
Expectation remains for generally non-severe and weakening storms
moving into Iowa around/after 10z with elevated nature, unsupportive
effective shear profiles, and weak to moderate lapse rates. SE Iowa
activity may be blossoming around/prior to 10z as well, again non-
severe. If nothing else, dependent on how widespread the activity is
and how long it holds on into the morning, it may play a factor in
storm magnitude potential for latter Saturday.

A shortwave riding the SW flow will move through the Dakotas
Saturday, aiding in development of a weak surface low. For the most
part, that would yield best potential for daytime activity into
Minnesota. Into Iowa, focusing mechanisms are distinctly missing,
but broad isentropic lift looks to yield scattered activity in the
afternoon/evening. Extent of activity may end up dependent on
lingering cloud debris, any lingering outflow boundaries from
morning convection, and just how much destabilization can be
achieved. Should solutions depicting MLCAPE 2000 to 3000+ J/kg and
surface based storms come realized, there will be the potential for
a few organized strong to severe storms. Strong directional shear in
the 0-6 km layer increases organization potential, even if the bulk
shear vector a bit more marginal around 30 to 35 kts. Storms could
be more elevated in nature though, especially if strong mixing and
destabilization is not achieved, with model depictions of moderately
warm mid-levels. Activity should slide eastward and likely expand
overnight with LLJ, but any severe potential expected to subside
through the night. SPC has upgraded the Day 2 to a Slight over much
of the CWA, and appears reasonable with the main threats for hail
and wind. The tor threat will be more conditional, likely needing to
latch onto an outflow boundary/augmented low levels. Was it
mentioned the forecast is messy?

Sunday will result in another round of showers/storms, this time
with more focus along a dropping cool front into northern and
central Iowa. Guidance remains torn on exactly where this will
reside by the afternoon/evening hours, with current best estimate
around I-80 corridor. Compared to Saturday, surface based storm
potential appears better with more chances for clearing and deep
mixing yielding MLCAPE 2000+ J/kg . Hodographs again depict moderate
bulk shear, this time driven more by speed versus directional, but
enough for organization potential near/along the boundary, moving
E/SE with the mean flow and boundary through the evening. Day 3 SPC
Marginal/Slight covers the S/SE CWA and continues to be reasonable
with main threats for hail and wind.

Phew. Activity quiets down overnight Sunday with the passing
surface high and building upper level ridge. This will yield dry
conditions through mid-week and the potential for near-record and
record highs by Tuesday. Synoptic guidance is in pretty good lock
step for mid 20s deg C 850mb temps overhead Tuesday. The GFS remains
way too hot in its depictions, as it has much of the summer, and is
a clear outlier over all other guidance. Utilizing model certainty
tool, the going forecasts of low to mid 90s across much of the CWA
are reasonable as they fall between the 50th/75th percentiles and
would yield a few broken records, including Des Moines which has a
record of 92 from 1931 on September 20th. The latter portion of the
week is on track to be significantly cooler as a strong cold front
drops through the region Wednesday/Wednesday night as a mean trough
moves across southern Canada.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1151 PM CDT Fri Sep 16 2022

VFR conditions will prevail likely until sunrise Saturday, but
scattered shower and storm chances will be increasing with low
chances throughout the day. Timing at a given location is hard to
pin down and thus have a window of VCSH at most terminals in the
morning hours. Enough confidence to include at least a short
period of VCTS at DSM, but additional VCTS may be needed as trends
are monitored. A more certain round of storms is expected to
develop around 0z and move southeastward in the evening. Given how
late in the TAF period this is, have chosen VCSH for now. No low
level wind shear is shown in the TAFs, but could be a sporadic
period at DSM this morning.

&&

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

DISCUSSION...Curtis
AVIATION...Ansorge

Hazardous Weather Outlook
National Weather Service Des Moines IA
335 PM CDT Fri Sep 16 2022

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-172045-
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-
335 PM CDT Fri Sep 16 2022

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Scattered thunderstorms are expected area wide by late tonight.
The severe weather potential is low.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

There will be additional chances for thunderstorms over most areas
this weekend. Some strong to severe storms are possible Saturday
and Sunday afternoon and evening. Large hail and damaging winds
are the primary threats. A tornado or two will be possible
Saturday.

It will be hot Tuesday when record or near record highs are
possible with peak heat indices in the lower to middle 90s.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed at times this weekend.

Jimenez