textproduct: Des Moines

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 /12Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 646 AM CDT Tue Sep 8 2026

VFR conditions largely prevail across the terminals currently, though not without plenty of complex conditions. Thunderstorms are ongoing across the northern parts of the state, impacting KMCW and brushing KFOD/KALO. Similar conditions are expected for the next several hours before eventually drying out. KDMX radar shows WS020 of 35kts, likely impacting KDSM and perhaps KALO. Shear may decrease slightly in the next hour as a surface windshift boundary passes. Another round of thunderstorms forms this afternoon and evening, impacting northern sites first, and KOTM tonight. Additional shear is possible during the overnight, though may be <35 knots. Lastly, CIGs look to lower towards the end of this TAF period, perhaps into MVFR or IFR, but confidence is too low to include at this time.

DMX WATCHES/WARNINGS/ADVISORIES

None.