Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
642 PM CDT Tue May 25 2021

.DISCUSSION.../Tonight through Tuesday/
Issued at 230 PM CDT Tue May 25 2021

Surface boundary has reached far northwest Iowa at 2 pm this
afternoon. Not much change in afternoon temperatures on either side
of the boundary with readings in the 70s to low 80s though a much
larger moisture discontinuity does exist with dewpoints in the 60s
pre-frontal to the 40s behind it. SPC mesoanalysis data is painting
a broad area of uncapped 1500 J/kg CAPE across much of central and
eastern Iowa currently. Proximity sounding analysis would suggest
some capping still exists and the fact that a broad area of
cloudiness still blankets much of that area suggests the SPC
mesoanalysis instability is a little over zealous attm. A few weak
scattered thunderstorms are currently moving across far southeast
Iowa while some agitated cumulus is beginning to develop in
vicinity of the surface boundary over parts of northwest Iowa but is
more prevalent into Minnesota though at this point it does not
appear much glaciation has occurred yet.

A few isolated to scattered thunderstorms may develop within the
warm sector yet this afternoon though the severe weather threat will
be limited. The better chance for convection and a few strong storms
will be further west along the surface boundary which will have some
kinematic forcing to work with as well with the upper short wave
arrival. Much of that convective development is on track for late
this afternoon into the early evening though the more widespread
activity will be to the northeast into Wisconsin where the currently
SPC slight risk exists. The scattered thunderstorm chances over the
area will persist along the boundary as it moves though overnight.
At this point, any severe weather looks isolated with large hail or
damaging winds the primary threats.

An overall outstanding day is on tap for Wednesday with plentiful
sunshine, less humid conditions and relatively lighter winds. High
temperatures will range from near 70 north to near 80 south.

The ingredients are starting to come together for a strong MCS
complex to develop Wednesday night over Nebraska and roll into Iowa
by early Thursday as a vigorous upper short wave moves in from the
west. This system will have a strong low level jet feeding a vast
amount of theta-e advection lifting north into Iowa which will have
maintain MCS maintenance and strength as it arrives. While the
convective updrafts may weaken a bit into Iowa, the potential for a
vast cold pool does exist to bring a damaging wind threat into the
state and very well could include an attendant QLCS tornadic threat
though medium range guidance is suggesting a less than impressive
<20 kt 0-3 km shear. The other question if the potential for
redevelopment during the afternoon hours and really depends on the
how strong the morning cold pool is and how far south it shoves the
surface boundary and if the surface low tracks far enough north to
pull the boundary back into southern Iowa where the chance for
destabilization occurs. Many times morning convection can run into a
mid morning destabilizing air mass which reinvigorates the
convection and it becomes a severe squall line through the day as it
moves east.  In those cases, the event is usually over.  If the MCS
decays and surface heating occurs, then it is possible for surface
based convection over far south central Iowa. While this possibility
is extremely conditional, should it occur, then the chance for a few
supercells hugging near the Missouri border are possible with the
chance for large hail and a tornado or two. This would likely be
short lived over Iowa with the Bunker's Right Mover supercell motion
begin east/southeast which would track supercells into Missouri.

Cooler weather will follow Friday and into the weekend. A few
showers and thunderstorms will be possible by late Sunday and into
early next week as the pattern again becomes more active.

&&

.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 642 PM CDT Tue May 25 2021

Main challenge is convection along the weak trough moving through
the area now. Both KFOD and KMCW will be clear of any impactful wx
for the period aft 00z with a few widely scat thunderstorms moving
across KALO/KDSM/again at KOTM between 01z and about 07z with the
progression of the boundary southeast. Upstream winds gusting to
24kts and will maintain some wind north this evening. Otherwise,
some mixing again north aft 13z north sites Wed. /rev

&&

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

DISCUSSION...Donavon
AVIATION...REV

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Special Weather Statement
National Weather Service Des Moines IA
1114 PM CDT Tue May 25 2021

IAZ059-071>073-260500-
Adair IA-Madison IA-Dallas IA-Warren IA-
1114 PM CDT Tue May 25 2021

...Line of Thunderstorms With Gusty Winds and Small Hail...

At 1113 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from Adel to near Greenfield. Movement was southeast
at 35 mph.

Pea size hail and winds in excess of 30 mph will be possible with
these storms.

Locations impacted include...
West Des Moines, Waukee, Norwalk, Winterset, Adel, Greenfield,
Stuart, Earlham, De Soto, Van Meter, Dexter, Cumming, Macksburg,
Greenfield Municipal Airport, Badger Creek State Park and
Winterset-Madison County Airport.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4134 9459 4151 9427 4150 9424 4152 9424
      4164 9401 4154 9379 4151 9379 4151 9373
      4148 9366 4117 9425
TIME...MOT...LOC 0413Z 295DEG 31KT 4162 9399 4135 9449

REV