Tornado and Flash Flood in Iowa, June 4 to 5, 2024

Area Forecast Discussion
National Weather Service Des Moines IA
401 AM CDT Tue Jun 4 2024

.KEY MESSAGES...

- Few early day shower or storm chances mainly east (<20-30%).

- Scattered strong to severe storms and locally heavy rain
  remain possible this afternoon into evening as a front moves
  through the state. Main threats remain large hail and strong
  wind gusts, though localized flooding could develop.

- Mainly dry with seasonal temperatures and lower humidities
  Wednesday into the weekend. Breezy conditions expected
  Wednesday & Thursday.

&&

.DISCUSSION...
Issued at 345 AM CDT Tue Jun 4 2024

Today:

Upper-level trough that will help drive our convective potential
later today is moving into the Northern Plains early this morning
and is expected to amplify through the day today. An accompanying
cold front will extend to the south of this system and looks to be
nearing the Missouri River around 18Z (around midday to early
afternoon). Ahead of this front, we will be in a weak warm air
advection regime with a favorable stream of moisture that will help
send dew points further into the 60s and send our MLCAPE values
towards the 1000-2000 J/kg range this afternoon. As a result,
it will certainly feel warm and humid today as highs reach well
into the 80s.

Ahead of the front and upper trough to the northwest, a MCV rotating
over eastern Kansas is expected to lift through northern Missouri
into the southeast to eastern half of Iowa and has the potential to
bring some early day showers or storms. Despite having the moisture
to work with, this MCV is fighting some larger scale subsidence so
it remains unclear on the coverage of any early day activity, but
general expectation given current trends and CAMs will be spotty
showers or storms at best this morning southeast with a few models
suggesting more widespread development over eastern Iowa early this
afternoon which is generally not supported in forecast soundings in
our eastern forecast area given the dry sub cloud layer.

What is known about this MCV is it has brought cloud cover to
roughly the southeast half of the state which adds some significant
caveats to our severe threat later today with the approaching front.
Many models suggest this cloud cover will stick around through much
of central and eastern Iowa with any clearing generally over our far
western CWA which could severely limit our instability and heating
into this afternoon. With already marginal bulk shear values in
place of 20 to maybe 30 knots at best, any lesser instability
environment will continue to limit the extent and ceiling of our
severe threat.

Increasing convergence near the front should help kick off storms
with any storms that fire ahead of the front likely to be slow-
moving with mean wind speeds around 10 to 20 knots. These storms
could certainly pose a heavy rain threat given their slow movement
but could also produce marginally severe hail or gusty winds,
especially when they collapse due to evaporative cooling and the
generally expected pulse nature unless clearing occurs sooner
and a better environment can be realized.

Along to just behind the front is where the better shear parameter
space exists which should help make storms more progressive helping
to limit the hydro threat, but could still bring a threat for strong
wind gusts depending on storm organization which as mentioned
previously looks messy, and maintenance is certainly in
question.

All of this said, the overall environment looks only marginally
supportive of a severe threat this afternoon with hail and wind in
addition to heavy rain in any initial storms ahead of the front and
potentially strong wind gusts with the main line, especially early,
since storm intensity seems to wane with time as storms move
eastward. Given the messy environment, low shear and helicity values
the tornado threat looks very low today, but won't go as far to
completely rule a brief tornado in case a better environment
can materialize if cloud cover moves out faster and in turn
heating/instability values increase. See the hydro section for
some additional information on the hydro threat today.

Wednesday and Beyond:

Any lingering storms push out of the area by late tonight/early
Wednesday morning with a mostly dry forecast prevailing through the
end of the work week. A few sprinkles may be seen north on Wednesday
as a quick-moving shortwave moves to the north of the area in our
otherwise northwest flow behind the departing trough/low to the
east. This northwest flow looks to continue for the next several
days which may bring some low end additional opportunities for a few
sprinkles or showers at times but the overall forecast will feature
highs in the 70s to low 80s with less humidity through the end of
the week and weekend. An increasing pressure gradient behind the
departing system will make for some breezy days on Wednesday and
Thursday, especially north.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1139 PM CDT Mon Jun 3 2024

VFR conditions currently prevail over the area. Primary aviation
forecast challenge revolves around thunderstorm timing and
extent of impacts later today. Model guidance suggests a
disorganized area of iso to sct shra/tsra and pockets of low VFR
to MVFR stratus may move over portions of southern Iowa late
this morning into the early afternoon hours. No mention was
included at KOTM through that period due to low probabilities
and confidence. Much more widespread tsra is expected along a
frontal boundary moving across the area from late afternoon into
the evening hours. Tempo categorical restrictions and gusty
winds are possible as this activity moves east, however is only
covered with prevailing VFR conditions for now until confidence
increases in tsra timing and potential impacts at respective
terminals.

&&

.HYDROLOGY...
Issued at 345 AM CDT Tue Jun 4 2024

As mentioned in the previous hydrologic discussion, main concern in
terms of river flooding remains with the single flood warning at
Estherville with RFC contingency forecasts and HEFS output mainly
suggesting within bank rises on area rivers. Although heavy rain is
possible today with pwat values of 1.5 to 1.75", warm cloud depths
of 11-12kft, and 850-300 mb flow of 25-30 knots, the progressive
nature of storms near to behind the front should help preclude any
hydro issues unless storms move over the same exact location of any
pre-frontal storms which are expected to be slower moving, as
alluded to in the discussion. HREF 24-hr probability-matched mean
QPF values are generally only as high as 1-2", with a few isolated
areas in eastern Iowa with values of 3"+. Will certainly monitor the
situation today, especially if any early day showers can manifest
east, but overall weaker trends in system and less priming from the
events earlier in the week should help mitigate.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...KCM
AVIATION...Martin
HYDROLOGY...KCM

Hazardous Weather Outlook
National Weather Service Des Moines IA
446 AM CDT Tue Jun 4 2024

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-051000-
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-
446 AM CDT Tue Jun 4 2024

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Scattered strong to severe storms are possible later this
afternoon into this evening as a front moves through the state.
The main threats are damaging wind gusts and large hail. Locally
heavy rainfall is also possible and while a brief tornado is
possible, the environment is looking less favorable.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

No hazardous weather is expected at time.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed later this afternoon into this evening.