778
FXUS63 KDMX 190736
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
236 AM CDT Thu Jun 19 2025

.KEY MESSAGES...

- Scattered thunderstorms are again possible late Thursday night
  into Friday morning, with a Marginal Risk of severe weather
  most likely in the form of large hail.

- A potentially dangerous heat wave will settle across the
  region on Friday and intensify this weekend, with Heat Index
  values approaching or exceeding 105 degrees at times on
  Saturday and Sunday. An Extreme Heat Watch has been issued for
  the weekend.

- The first half of next week will be active with multiple
  rounds of thunderstorms expected and severe weather possible,
  however, confidence in timing and location is low at this
  range.

&&

.UPDATE...
Issued at 236 AM CDT Thu Jun 19 2025

Patchy light fog has developed in places this morning, remaining
fairly transient in nature thus far with visibility over 1
mile. With plenty of available moisture, cooling overnight
temperatures as skies clear out, and increasing high pressure
keeping winds light, expect this may expand through sunrise.
Our attention then turns to a quick moving shortwave that pass
across the upper midwest overnight tonight into Friday morning.
Better parameters exist to the north, though with MLCAPE around
1000 J/kg and 0-6 km shear around 40+ kts (the best we`ve seen
all week), a few stronger storms are possible. Storms should
remain elevated across the area based on model soundings but
hail is possible in the stronger updrafts and gusty wind might
be possible if it can make it to the surface.

After convection tapers off Friday morning warm air advection begins
in earnest. Breezy wind out of the south to southwest will gust 25-
30 mph at times as warm air surges north. Temperatures climb
into the 90s, the first in a series of hot and steamy days
across Iowa. Heat indicies through this weekend will top
100-105+, and with the potential for record warm overnight lows
in the mid to upper 70s there will be little relief overnight
(the heat index remains in the 80s overnight). The Excessive
Heat Watch remains in place for Saturday and Sunday, the hottest
days in this hot stretch. As the thermal ridge shifts east
early next week a boundary will shift across the area, bringing
several days of storms to the area for the first half of the
week. With PWATs exceeding 2" across the area into Monday, with
will be another window to watch for hydro concerns.


&&

.DISCUSSION...
Issued at 212 PM CDT Wed Jun 18 2025

A broad mid/upper level trough is moving across the region
today, generating scattered thunderstorms across Iowa. However,
cloud cover and unseasonably cool temperatures in the mid-70s
are mitigating instability, with MLCAPE only around 1000 J/kg at
best, and flow remains light with bulk shear values well below
30 KT, thus severe weather has not been observed and is not
anticipated. The showers and storms should end from west to east
this evening as the trough pushes through. A weak surface high
pressure ridge will then build in later tonight into Thursday,
allowing for gradually clearing skies and temperatures falling
to around 60 degrees by sunrise.

On Thursday a deep-layer thermal ridge will build across the
southern High Plains and Rockies, pushing up into Iowa and
bringing warm air advection aloft. Meanwhile, as the surface
ridge moves off to the east we will see a return of south to
southwest low-level flow by the afternoon, combining with sunny
skies to push temperatures into the mid to upper 80s late in the
day. From Thursday night into Friday morning the forecast
becomes a bit tricky, as a subtle mid-level impulse quickly
rounds over the top of the large-scale ridge, coincident with a
low-level jet developing over the Plains and turning into Iowa
shortly before sunrise Friday. Nearly all model solutions are
now generating widely scattered thunderstorms in our forecast
area in association with the conjunction of these two features,
despite a stout warm layer to overcome above the surface. Have
increased POPs into the 30-40% range in our northeast and
brought the 20% line down to around I-80, but a further increase
may be warranted on subsequent shifts. Any storms that do form
in this environment early Friday morning will be scattered and
elevated, but with sufficient instability and deep-layer shear
to pose a Marginal Risk of large hail as outlined by SPC.

By Friday, as any lingering morning storms push out of our area,
the large 500 MB ridge to our southwest will slide eastward
across the southern U.S., then subsequently intensify and build
into the southeastern U.S. This will result in steadily
increasing 500 MB heights over Iowa, with a slow but steady push
of warm air/moisture advection aloft. During this time frame a
broad surface low pressure trough will set up over the High
Plains and remain nearly stationary, with the gradient between
this trough and the large heat dome to the southeast resulting
in brisk south to southwest surface winds over Iowa from Friday
through at least Sunday. The resulting heat wave will set in
Friday and intensify over the weekend as the airmass continues
to warm, with no chance of convection or significant cloud cover
to hold it back. Forecast soundings depict good low-level mixing
which should hold dewpoints in the upper 60s to lower 70s for
the most part, but also allow temperatures to rise into the mid
to upper 90s on Saturday and Sunday. This combination will
produce Heat Index values in the upper 90s to a little over 100
on Friday, then well into the 100s and approaching 110 in some
areas Saturday and Sunday. There will be little relief overnight
too, as steady winds prevent temperatures from falling with
morning lows only in the upper 70s each day. The only factors
mitigating the heat will be that dewpoints are not higher,
partially thanks to the relatively early maturity stage of row
crops, and that there may be some possibility of smoke streaming
overhead that could put a small dent in daytime heating, but
that is a low confidence proposition. Regardless, given the
certainty of a pervasive, multi-day heat event, the likelihood
of Heat Index values approaching 105 or higher, the fact that
recent weeks have generally been cooler and more comfortable
than normal for the season and this will be the first intense
heat of the year, and the fact that it comes on a weekend when
many are planning outdoor activities, has prompted the issuance
of an Extreme Heat Watch for Saturday afternoon through Monday
evening. A Heat Advisory may be needed for some portion of the
area on Friday, but that determination will be made overnight
and tomorrow as it is more borderline. It remains to be seen
whether the Watch will be converted into an Advisory or a
Warning later, but given the above factors an Extreme Heat
Warning appears more likely.

From Sunday night into early next week the southeast U.S. heat
dome will remain entrenched, but a broad 500 MB trough over the
western and northwestern U.S. will eject multiple waves into the
High Plains/Upper Midwest, impinging on the ridge and allowing
for the aforementioned surface trough over the Plains to nudge
eastward and provide a focus mechanism for multiple rounds of
rain and thunderstorms. The swath of rain/storms may remain
nearly stationary for several days, resulting in heavy rainfall
totals, or may wobble southeast with convection/outflows, then
back northwest with the reassertion of the southeastern ridge
behind passing shortwaves. This leads to rapidly increasing
uncertainty in the forecast during the first half of next week,
as this zone will reside near or over our forecast area
resulting in low confidence temperatures and persistent 40-70%
POPs from Sunday night through the end of the forecast period.
As long as the rain remains northwest of our area the persistent
heat will hold, so any headlines from the weekend may need to be
extended into Monday or beyond, at least for parts of the area,
but this determination will be made later once confidence in the
convective scenario/location increases.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1040 PM CDT Wed Jun 18 2025

VFR conditions and mostly light west to southwest wind is
expected much of the period. There is a low chance for localized
fog by early morning. A few showers or storms may be possible
also late Thursday but greater chances are beyond this forecast
period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Extreme Heat Watch from Saturday afternoon through Sunday
evening for IAZ004>007-015>017-023>028-033>039-044>050-057>062-
070>075-081>086-092>097.

&&

$$

UPDATE...Hagenhoff
DISCUSSION...Lee
AVIATION...Donavon