868
FXUS63 KDMX 080933
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
433 AM CDT Tue Jul 8 2025

.KEY MESSAGES...

- A few isolated storms today and Wednesday but most areas will
  remain dry after 7 am this morning.

- Several rounds of storms to end the week, possibly beginning
  as early as Wednesday night.

- Monitoring Thursday into Friday timeframe for very heavy
  rainfall potential. Stay tuned on this.

&&

.UPDATE...
Issued at 428 AM CDT Tue Jul 8 2025

Thunderstorm complex is beginning to move out of the area with some
remnant light to moderate rain lingering over far southern Iowa.
This system did produce some wind gusts in excess of 60 mph. In
addition, a band of heavy rainfall did occur just southeast of the
Radar over southwest Ankeny and into the Saylor Township where 5+
inches of rain occurred. This did cause significant ponding in that
area but much of this runoff will have a short trip through the
Saylor Creek and into the Des Moines river. The Walnut Creek and
Four Mile Creek basins did have 1-3 inches across their basins that
will cause within bank rises.

The precipitation will continue to decrease in areal coverage over
the next several hours. Ridge of high pressure will settle back over
the state today. Similar to the past several afternoons, model
soundings are suggesting the potential for convective temperatures
to be reached though there remains some uncertainty how strong if
any of an elevated mixed layer settles over Iowa. The uncertainty
there along with the lack of another trigger would suggest
thunderstorm potential is spotty at best. Per trends, CAMs are
likely overzealous on weak convective potential during the afternoon
hours today and on Wednesday.

Looking ahead, thunderstorm chances may return to the area as early
as Wednesday night as the low level jet begins to lean back into
Iowa sending waves of theta-e advection across the state. This could
lead to another complex of storms moving over portions of central
and western Iowa. These storms have been following the more
southerly Corfidi vectors and would expect the same here if an MCS
does develop. More active weather is on the way for Thursday and
into Friday as a stronger upper level system arrives. While some
severe weather is possible, monitoring this timeframe for the
potential of a Maddox Frontal type of heavy rainfall event. The
current setup ahead of the main short wave has an east-west boundary
over Iowa with an abundance of Gulf moisture lifting north with 2
plus inch pwats continuously fueled by the low level jet. The
global models QPF fields are beginning to pick on this but
potentially not the extreme ends that could be possible. Continue to
monitor this period!

&&

.DISCUSSION...
Issued at 244 PM CDT Mon Jul 7 2025

Early afternoon upper level water vapor imagery from GOES-East
shows a cyclonic spin over the southern Manitoba province with
the shortwave trough axis into the Dakotas. Convection is ahead
of this area while over Iowa there are low level cumulus clouds,
particularly over eastern and southern Iowa, with a ribbon of
high clouds from southwest into north central Iowa. Temperatures
have warmed well into the low and middle 80s and this is
nearing the convective temperature in forecast soundings. While
there is a slight warm layer aloft, we may be able to overcome
this for a few storms this afternoon over southern Iowa with a
few storms already over eastern Iowa. With these storms favoring
more airmass variety with little available shear, main concern
would be sub-severe gusty winds as thunderstorms collapse with
this activity diminishing toward sunset.

As we head into this evening, will be watching clusters of
thunderstorms over South Dakota and northeastern Nebraska start to
head southeastward along the MUCAPE axis over western Iowa. These
storms could be severe with damaging wind gusts up to 70 mph
possible into a few of our western counties such as Sac and Crawford
mid to late evening. The outflow looks to push out ahead of these
storms making them becoming out of balance. Thus, current
expectation is that the severity wanes quickly east of roughly
Highway 71 and south I-80. Still, subsevere gusty winds of 40 to 55
mph may still occur with this most likely for areas south of I-80
and also likely west of I-35. Rainfall amounts will generally be
around or under an inch, though a few places in western or southern
Iowa could receive between 1.5 to 2 inches per latest HREF localized
probability matched mean. With the expected progressive nature of
these storms, not anticipating any short-term hydrology concerns.
All of these storms should be largely out of our service area by or
shortly after sunrise Tuesday. However, a few showers or storms may
continue Tuesday along a surface boundary over southern or eastern
Iowa. Additionally, a few storms may try to develop in the afternoon
over north central Iowa along the 850mb boundary. Subcloud dry air
may contribute to a few gusty winds, but this should all be below
severe limits with deep layer shear being less than 25 if not 20
knots. While the previous sentences make it sound wet,
expectation is that many places will stay dry during the day
Tuesday over central Iowa.

High pressure will transit across the region Tuesday night into
Wednesday providing a less active period of weather with seasonable
temperatures. However, this is short-lived as a shortwave trough off
the west central coast moves eastward flattening the Rockies ridge
as it nears the state by late Thursday. There will be some phasing
of the thermodynamic and kinematic fields and a favorable
instability and deep shear space as the shortwave lifts through the
region. This will make for periods of thunderstorms from Thursday
night through Friday night over Iowa. Strong to probably a few
severe storms will be possible in this period per the pattern and
supported by various AI and ML guidance. Efficient rainfall
processes may be in play with deep warm cloud depths and
precipitable water values of 1.5 to around 2 inches, though the
heavy rainfall concerns may be mitigated depending on storm
speeds. While ensemble means have just a 30% chance of over an
inch in 24 hours, the deterministic 0 or 6z GFS, ECMWF, and CMC
have between 1 and 3 inches in some 24 hour period between late
Thursday and late Friday. Early week soil moisture percentiles
are above the 80th level north of roughly Highway 30 and while
there will be some drying between later Tuesday through late
Thursday, this area may be more vulnerable to renewed water
issues if those rainfall amounts fall over the more vulnerable
basins.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1251 AM CDT Tue Jul 8 2025

A line of storms will move across portions of south-central Iowa
early on in the period with DSM most likely to be impacted. Brief
MVFR/IFR is possible at DSM with a period of heavy rain likely
roughly between 06-07Z/Tuesday. Additionally, gusty erratic winds
(40+ kts) are possible as the thunderstorm rolls through. Confidence
is much lower to the southeast near OTM as the line will be
gradually weakening, so have included a PROB30 for thunder in the
TAF. Aside from the potential for MVFR ceilings at MCW for a brief
time Tuesday AM, VFR should prevail at the terminals for the
majority of the day and winds will remain light.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Ansorge
AVIATION...Uttech