162
FXUS63 KDMX 081740
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1240 PM CDT Tue Sep 8 2026

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Hot and humid across central and southern Iowa this afternoon
  with heat indices near 100 degrees.

- Widespread storms and rainfall is expected across the state
  today. Locally heavy rainfall up to 2-4 inches may cause
  ponding and some risk of urban flash flooding. A few severe
  storms with damaging winds and hail are possible across
  central parts of the state.

- More seasonal with highs in the low 80s the rest of the week.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 253 AM CDT Tue Sep 8 2026

Storms are ongoing across northwest Iowa. Elevated instability and
the nocturnal LLJ are driving this convection and will likely
continue into the morning and perhaps linger until afternoon. Severe
threat is low, but it has been producing a lot of lightning, and
localized rainfall totals over 1" are possible in NW Iowa this
morning.

Pattern Evolution
The persistent 500mb ridge is currently sitting
over a large part of the south central US. A shortwave looks to
transverse the Dakotas on today with a cyclonically curved jet
streak clipping northern portions of the state through our active
period in the short term. A cold front will extend from the main sfc
low in northern MN down to and along the Missouri River around
midday and progress southeast across the state during the afternoon
and evening. This will be the primary forcing mechanism for diurnal
storm development this afternoon. However, storms may persist along
and behind the front this evening until Wednesday morning driven by
LLJ-nose convergence mainly in eastern and southern Iowa. Overall,
CAMs suggest storm development at 22z-00z in the central part of the
state, with upscale growth through the evening and possible LLJ
driven post-frontal elevated storms through the overnight, then
clearing by 15z Wednesday. Wednesday will have highs in the 80s and
be overall more dry and mild.

Hydrology Concerns
PWATs are expected to easily reach 2" in the zone
near the front this evening. NAEFS PWATs are even forecast to be >10
year return interval or even outside of climatology for this time of
year. Additionally, shear is expected to be largely boundary-
parallel and would encourage training thunderstorms even prior to
the strong nocturnal LLJ that sets in late evening. However, the
front should remain fairly progressive and lower the odds of
widespread rainfall totals over 3". 00Z HREF LPMM centers 3-5"
totals locally over east-central Iowa, and consensus across the CAMs
is that a swath of 2-4" is reasonable across portions of central and
eastern Iowa with widespread 0.5-1.5" totals and perhaps as high as
6" possible as a high end scenario. Note that these higher end
amounts will depend on the development of the more intense early
evening storms. Antecedent conditions would place eastern Iowa under
a lower risk of widespread flash flooding, with soil moisture fairly
dry in that region. Still, urban flash flooding risk is present
under the right circumstances.

Severe Potential
Initial storm development along the cold front will
pose the greatest risk of severe weather. 1500-3000 J/kg of MLCAPE
will be available to storms near the front. Decent hodograph
curvature may support a brief window for supercells and a hail
threat given mid-level lapse rates of 7-8C, but the boundary-
parallel shear would cause quick upscale growth. Still, SPC has
introduced a 2% tornado risk, likely due to potential for landspout-
esc funnels under powerful initial updrafts rooted on the front.
Damaging wind will be the primary risk, particularly where storms
can develop at the lead edge of the front and have access to better
mixed dry low levels in the warm sector. Storms may become more
elevated through the late evening, but wind can`t be ruled out if a
storm complex can become fairly organized.

Heat Discussion
Higher 850mb temps and ample sunshine may bring today
highs into the low to mid 90s in southern and central Iowa. NBM
populated with upper 70s Tds but this is likely several degrees too
high. The adjustment brings the heat index values to near 100 south
of I80 and perhaps up to 105 briefly in a few areas. 105 values
aren`t expected to be widespread or long lived, so have passed on
issuing a Heat Advisory. Still, it will be quite hot in central and
southern Iowa this afternoon.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 230 PM CDT Mon Sep 7 2026

The cooler temperatures will last through the end of the week and
upcoming weekend. Keep in mind, while these temperatures may feel
cooler compared to last week, they will typical for this time of
year. The average high for this time of year is 77 in Mason City, 80
in Des Moines, and 81 in Ottumwa. After a couple dry days a
shortwave embedded in zonal flow will pass across Iowa on Friday and
bring the next round of convection to the area. A more robust system
Sunday into early next week with bring additional shower and
thunderstorm chances to the area. These successive systems will
help to knock a few more degrees off high temperatures. At this
time range there is uncertainty on the timing and track of
these systems, but guidance does agree that cooler temperatures
filter in by early to middle of next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1240 PM CDT Tue Sep 8 2026

A line of showers is in place across north central Iowa ahead of
a cold front that will move across the state through the
afternoon and overnight. Storms may strengthen this afternoon
into early evening with a few stronger storms capable of gusty
winds. Otherwise heavy rain will be the main hazard. These will
sink south across the state overnight, lingering across
southern Iowa, including KOTM, thorugh Wednesday morning. In
northern Iowa patchy fog is possible Wednesday morning behind
front, mainly impacting sites KALO/KMCW but potentially reaching
west to KFOD.

&&

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

&&

$$

SHORT TERM...De Bruin
LONG TERM...Hagenhoff
AVIATION...Hagenhoff