Severe Thunderstorm Warning 85, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1214 AM CDT Tue May 21 2024

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

.KEY MESSAGES...

- Storms will develop this evening and continue overnight into
  Tuesday morning. Severe storms are possible with wind as the
  main hazard. Flash flooding is also a concern with the
  overnight and Tuesday morning storms.

- Strongest storms are forecast Tuesday afternoon into evening.
  Tornadoes, damaging wind, and large hail are all concerns
  with a Moderate Risk (level 4 of 5). Localized heavy rain may
  enhance any flood concerns lingering from Tuesday morning.

&&

.UPDATE...
Issued at 825 PM CDT Mon May 20 2024

Some uncertainty regarding the southernmost location of possible
heavy rainfall overnight into early Tuesday. Subjective H850
analysis shows remnant upper level boundary sitting over central
Iowa with strong H850 front across northern Kansas and southern
Nebraska. Storms are congealing into an eventual MCS over
southeast Nebraska and are anticipated to drift east northeast
overnight. Any cold pool development may keep the area farther
south; as shown by a few HRRR runs today. In that event, higher
rainfall totals would arch from southwest Iowa northeast across
Central to northeast sections by 17z. A lot to digest and much
of tomorrows storm evolution will depend on how quickly the
synoptic forcing overcomes the overnight mesoscale forcing. With
all that, have expanded the FFA for portions of southwest to
south central given Sunday nights higher rainfall there already.

&&

.DISCUSSION...
Issued at 301 PM CDT Mon May 20 2024

All is quiet across Iowa early this afternoon as the thermal ridge
has sent temperatures into the upper 70s and mid 80s. Off to the
west the leading shortwave is sweeping across the central plains,
quickly noted in GOES imagery across South Dakota. By this evening,
after 7 pm, storms will develop across Iowa with this wave.
With the later onset, storms are expected to be elevated in nature
with wind as the primary severe threat. ML CAPE exceeds 2000 J.kg
through the late evening and overnight, along with 0-6 km shear
over 30 kts. The low level jet increases across the area
overnight which will help to fuel storms through the overnight
and lasting into Tuesday morning.

As we move into Tuesday morning the severe threat will diminish with
increase hydrologic concerns. Training storms overnight may
develop, which would result in multiple rounds of rain over the
same areas. Additionally, storms are expected to be efficient
rain producers with deep warm cloud layers and good deep
moisture transport into the area. PWATs exceed 1.5-2" through
the period. Antecedent conditions include near normal soil
moisture and river levels (rather than exception dry conditions
which have plagued the area previously). With heavy rain
overnight and into Tuesday morning flash flood problems are
possible for areas that see training storms, especially in urban
areas. A quick check for flash flood warnings before the
morning commute would be worthwhile. Additional details on the
possible hydrologic response is available in the Hydro
discussion below.

Expect to see a bit of a break in convection late morning to midday
before the next round of convection swings across the area from
west to east in early afternoon to early evening. The
environmental parameter space is primed for robust convection,
which led to the upgrade to a Moderate (level 4 of 5) Risk
issued by SPC for tomorrow. CAPE exceeds 3000 J/kg with deep
layer shear over 50 kts, supporting initial discrete supercell
development. With LCLs around 600m in the soundings and the sig
tornado parameter at 2-4 tornadoes are a risk, especially with
initial convection. Perhaps working against the tornado set up
is the relatively straight hodographs. If anything it would
squash longer duration tornadic storms and worth monitoring. As
the storms grow upscale into a line expect robust storms with
wind and hail worthy of elevated tags. Models also indicate a
strongly wrapped up system which may set up a wake low scenario.
What could mitigate convection tomorrow? The morning convection
may work over the atmosphere with less time for recovery. And
perhaps the morning storms will push the boundary south, and
shift the more robust axis out of the area. While both scenarios
are possible, the risk for severe storms is substantial
tomorrow. The mesoscale environment will be monitored closely
tomorrow for impact to storm development and mode. Worth noting,
additional thunderstorms on Tuesday afternoon could exacerbate
any flooding that does occur on Tuesday morning.

Wednesday and Thursday should be quieter before another system moves
into the area by Friday.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1213 AM CDT Tue May 21 2024

Scattered storms overnight most areas except OTM. Toward 12-15z,
an MCS is expected to roll over the area from west central Iowa
toward ALO, impacting most sites with lower MVFR cigs/vsby and
VRB wind gusts over 45kts possible. Main storm will pull a
strong line of storms through the area from 20z to 00z affecting
most sites with higher end gust potential over 50kt along with
some severe hail/brief heavy rainfall. Timing details will be
refined as event unfolds with current timing best estimate at
this time. /rev

&&

.HYDROLOGY...
Issued at 301 PM CDT Mon May 20 2024

Just as our last high water and flood event has wound down recently
it looks as if we will enter another one in short order.

Before proceeding it should be stressed that the hydrologic response
will depend heavily on the amount, location and timing of the
rainfall forecast over the next few days. Having said that, the
hydrologic response described below is based on the best information
we have at the present time.

Heavy rainfall last night led to elevated soil moisture values
across southeastern 1/2 to 2/3rds of the CWA and especially from the
Osceola to Grinnell/Montezuma area. SAC-SMA soil moisture saturation
values in that area are as high as 40-60%. Flash flood potential
typically increases markedly once values approach or exceed 50% so
we will have to keep an eye on those areas tonight. That being said,
heavy rain across the CWA tonight may lead to flash flooding
concerns elsewhere due to the potential rainfall rates as well as
the sheer amounts of potential rainfall. Flash flooding will remain
a concern into Tuesday and Tuesday night due to the expected
continuing rainfall.

Concern then turns to rises on area rivers. Our latest official
river forecasts as of this morning incorporate QPF through 12Z
Tuesday. With additional rainfall expected beyond that time frame,
the rivers may indeed rise beyond what is presently shown in our
official river forecasts. Latest QPF ensemble hydrographs (which
incorporate up to 72 hrs of probabilistic QPF) are suggesting
significant rises will occur on area rivers especially in the Des
Moines (mainly above Saylorville Lake), Raccoon and Cedar/Iowa river
basins including their tributaries. The most likely scenario at this
time would be rises to within a few feet of flood stage with a
couple locations possibly exceeding flood stage with the crests
occurring within the two to seven days depending on location.
Additional rain is possible beyond the next few days and our longer-
range hydrologic tools are suggesting additional rises further out
are possible.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this evening for IAZ004>007-015>017-
023>028-033>039-044>050-057>062-070>075-081>083-092.

&&

UPDATE...REV
DISCUSSION...Hagenhoff
AVIATION...REV
HYDROLOGY...Zogg

Hazardous Weather Outlook
National Weather Service Des Moines IA
339 PM CDT Mon May 20 2024

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-212045-
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-
339 PM CDT Mon May 20 2024

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Scattered thunderstorms are expected to develop over portions of
western Iowa during the early evening, with additional storms
moving in from the west during the early morning hours Tuesday.
Although a tornado cannot be ruled out, large hail and damaging
winds are the most likely threats with the stronger storms. Flash
flooding is also possible with these storms.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Thunderstorms from overnight are expected to linger into Tuesday
morning with damaging winds and flash flooding the primary
threats. Additional storms are expected to develop during the
afternoon with tornadoes, damaging winds, flash flooding, and
large hail all possible.

Additional thunderstorm chances return at times Friday and
through the weekend. The severe weather potential for these days
will be better determined in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotters will likely be needed at times through Tuesday.

Castillo/Ansorge

Preliminary Local Storm Report
National Weather Service Des Moines IA
1044 PM CDT Mon May 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0921 PM     Hail             Mallard                 42.94N 94.68W
05/20/2024  E2.00 Inch       Palo Alto          IA   Public

            Corrects previous hail report from Mallard.
            Slightly larger than golfball size. Time
            estimated from radar.

&&

AB

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