700
FXUS63 KDMX 161124
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
624 AM CDT Sat Aug 16 2025

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Lingering showers and storms this morning, then dry today.

- Hot and humid conditions continue today with heat indices of
  105 or higher expected. Heat Advisory now in effect for all of
  central Iowa this afternoon.

- Potential for showers and storms overnight into Sunday
  morning, mainly along and north of the Iowa/Minnesota border.
  Heavy rainfall is the main concern, although a few stronger
  storms are possible.

- Hot temperatures continue into Sunday with more shower and
  storm chances overnight Sunday into Monday.

&&

.UPDATE...
Issued at 340 AM CDT Sat Aug 16 2025

While conditions have generally quieted down overnight tonight,
isolated to scattered storms continue to linger over portions of the
forecast early this morning, primarily across the far eastern edge
of the forecast area and eastward. These storms are roughly
correlated with the edge of the 12 C isotherm at 700 mb, emphasizing
the presence of the elevated warm layer in place over much of
central and western Iowa. Areas beneath this warm layer have
generally remained dry this morning, although they haven`t been
completely immune to storm chances as weak updrafts have popped up
here and there. Fortunately, brief heavy rainfall and some thunder
have been the only impacts observed from any of the activity so far
this morning. Farther out to our west over South Dakota, a large MCS
being fueled by a 35 to 45 kt LLJ has been producing severe winds as
it barrels east northeastward through the plains. Current trajectory
of this system would suggest it will stay to the north of Iowa and
track through southern Minnesota, riding the nose of the oscillating
jet and propagating away from the warmer 700 mb temperatures. Short
range model guidance supports this progression as well, keeping the
storms well removed from the state. While it appears unlikely that
storms will develop southward into the more capped air, this
prominent of a cold pool will certainly warrant close monitoring
into the morning hours.

Assuming the MCS currently over the plains stays to our north, any
remaining scattered storm chances should diminish along with the LLJ
after sunrise, keeping conditions generally dry with the warm layer
overhead through the day today. While scattered cloud cover and
convective debris will likely be present overhead through the
morning, expecting temperatures to warm up quite a bit again today
as highs climb into the low to mid 90s. Dewpoints in the mid to
upper 70s will push heat indices into the 100 to 105F range through
the afternoon, with some likely seeing values over 105. Following
along with the northerly trend of this morning`s MCS/convection, the
warmer apparent temperatures have also begun to trend northward as
well. For this reason, in collaboration with neighboring offices,
have expanded the heat advisory up to the Iowa/Minnesota border.
There does remain some uncertainty on how warm temperatures will get
in far northern Iowa, especially those counties right along the
north border. Any convective outflow or even resultant cloud cover
into northern Iowa today could squash temperatures and keep those
areas well below 105F. However, with conditions trending drier
through the day and higher dewpoints pooling over northern Iowa, the
potential for hot and humid conditions is certainly still present,
hence the northward expansion.

As we get into this afternoon and evening, our next feature to
monitor will be the residual boundary left from overnight
convection. CAM guidance has struggled with the placement of said
boundary, but general consensus has it lighting up somewhere over
southern Minnesota Saturday evening before oozing southward into
northern Iowa. While some stronger storms could produce gusty winds
or hail along the boundary overnight, the main concern with
overnight convection will be storms backbuilding and training along
the boundary in what will be a 2"+ PWAT environment. This spells
some concern for hydrological impacts overnight, assuming that
boundary were to hang up over one area. However, the stronger LLJ
stays displaced to the south and west which should allow the
boundary and coincident storms to ooze southward, staying a bit
more progressive. The most recent 06z guidance also keeps
almost all of this rainfall north of the state through the
night, with only the farthest northern counties potentially
seeing rain briefly on Sunday morning before what`s left of the
LLJ ramps down. Therefore, while the risk certainly exists, the
likelihood of significant impacts in our forecast area seems low
at this time. The weather prediction center does have a slight
risk (level 2 of 4) clipping our far northwestern counties,
with a marginal (level 1 of 4) acting as a buffer down to
roughly the Highway 20 to Highway 30 corridors. This marginal
may be a bit generous on the southern extent, but covers the
uncertainty that is always evident this time of year. This is
especially true if this morning`s MCS behaves differently than
expected, which would inevitably affect almost everything
discussed up to this point!

This boundary hangs up over northern Iowa/southern Minnesota into
Sunday, but appears to stay dry through the day. Temperatures will
once again be warm and muggy, as little change comes to the air mass
overhead. The boundary to our north again becomes the focal point
for storms Sunday night into Monday as another wave propagates over
the upper ridge. This wave also looks to push shower and storm
chances farther south into the area towards Monday morning. However,
with multiple rounds of convection expected prior to this timeframe,
this forecast is bound to change. Therefore, main takeaways is
continuing shower and storm chances through the end of the weekend
and into early next week.

&&

.DISCUSSION...
Issued at 300 PM CDT Fri Aug 15 2025

A line of storms developed across northern Iowa midday today,
evolving from more isolated convection in Nebraska and South Dakota
earlier in the day. While early cores produced large hail, as the
complex has grown upscale cores have become messier and the elevated
mixed layer prevented gusts from elevated convection reaching the
ground. Most recent observations around 2 pm indicated gusts
increasing at the surface with increasing cold pool strength. Note
that recent CAM runs have struggled with convection, crush any
initialized convection. The 12z runs of the RRFS and HRRR had the
better handle on convective trend through this afternoon. As
convection pushes to the east and southeast along the instability
gradient through the afternoon additional advection is anticipated
to develop along the associated outflow boundaries. Note that 0-6 km
shear is best across northern Iowa with 35-40+ kts, dropping to only
25 kts towards the I-80 corridor and limiting severe potential
further south.

After convection along the outflow tapers off this evening a lull in
activity is anticipated ahead of the next round of convection,
arriving across northern Iowa as an MCS rides over the central US
ridge. Current trends place this activity along and just north of
the Iowa/Minnesota border, though these systems often push
south and will need to monitor trends closely for early morning
impacts across northern Iowa. Primary concern would be damaging
wind gusts. And while PWATS in the area are around 2 inches,
the progressive nature of the complex and lower soil moisture
should help to mitigate any hydro concerns.

The other side of this coin is the heat settling across the area as
the thermal ridge shifts across the area. So far today deeper mixing
has occurred and dewpoints a few degrees lower than forecast.
Similar conditions are anticipated for tomorrow, though temperatures
will be a few degrees higher as the ridge shifts directly over Iowa.
Even nudging Saturday dewpoints down a few degrees to account
for better mixing, still expect muggy feeling conditions and
heat indicies in the 100-105+ range. Northern Iowa should be
spared from the worst of the heat with morning convection
putting a damper on heating and pushing that boundary south.

Heat continues Sunday into early next week until the thermal ridge
is finally squashed south and east. Another overnight/early
morning MCS is likely to ride across northern Iowa on Sunday
with placement dependent on recovery of the northern Iowa warm
front. Early week the boundary gets hung up across Iowa and
expect to see several days with thunderstorm chances before
pushing out of the area midweek.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 621 AM CDT Sat Aug 16 2025

Minimal aviation concerns expected today. Upper level clouds
have begun to sink down into northern Iowa from storms ongoing
over Minnesota this morning. This activity is expected to stay
north of the area, but VFR cloud cover and breezy winds will
result due to proximity to the thunderstorm complex. VFR
conditions expected through the afternoon. Low end shower and
thunderstorm chances may return toward the end of the TAF period
on Sunday morning over northern Iowa. However, no mention in
TAFs given low confidence at this lead time.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon today to 8 PM CDT this evening for
IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097.

&&

$$

UPDATE...Dodson
DISCUSSION...Hagenhoff
AVIATION...Dodson