textproduct: Des Moines

KEY MESSAGES

- Heavy rainfall shifts into southern Iowa and northern Missouri tonight into Wednesday morning. Flood Watch remains for areas along and south of I-80 in much of central Iowa.

- Many rivers and streams south of Highway 30 and along/north of US 34 have seen rises, mostly within bank, but a few locations have reached or exceeded minor flood stage.

- Diminishing showers/storms through the day Wednesday into the evening before coverage of storms increases after midnight into Thursday morning.

- Trend towards the heaviest rainfall late this week pushing over northern Iowa and north, but saturated soils farther south leave conditions susceptible for flash flooding and potentially further stream rises.

SHORT TERM /THROUGH THURSDAY/

Issued at 253 PM CDT Tue Sep 15 2026

While northern Iowa has enjoyed a dry day with some peeks of sunshine and far southeastern Iowa is warm and humid with some sunshine, that is not the case in between. Off and on showers have persisted as the low level jet has weakened along with weakening low level thermal lift. There is also a disparity in dewpoints with 50s and even a few upper 40 dewpoints in northwestern Iowa, which is behind a surface boundary that has pushed south of much of Iowa. The question this afternoon becomes whether storms of any severity can develop over our far southeastern counties. Forecast soundings show a bit of initial dry air for gusty wind potential, but that is quickly removed as the profile saturates. Convective allowing models (CAMs) have shown a trend southward with some of the stronger updraft helicity tracks with many near if not south of the state line. If there are to be any strong storms, it will be a narrow window from late this afternoon until very early this evening. Thereafter, the concern will evolve to heavy rainfall as the environment is still primed south of the elevated front, which will be over the I-80 counties. Southern Iowa into northern Missouri will have in place deep warm cloud depths for efficient rainfall production and unseasonably high precipitable water values (PWATS) around or above 2 inches. This combined with 850-300mb flow around or under 20 knots, which is roughly parallel to the boundaries, point to another round of heavy rainfall. Compared to last night (Monday night/Tuesday morning), the kinematics are much less impressive with 850mb low level jet no more than 25 knots. However, the low level theta-e advection is sufficient for widespread showers and storms over southern Iowa into northern Missouri, which may start to lift back into Iowa toward sunrise Wednesday. Much of the deterministic guidance paints overnight rainfall totals of 2 to 5 inches over northern Missouri, though the 12z NAM and ECMWF have those type of amounts over far southern Iowa. 12z HREF pushes the highest rainfall amounts into Missouri while the REFS points to a far southern tier of Iowa counties scrape of the highest rainfall of 2 to 4 inches. National Water Model soil moisture analysis shows the bottom row of counties is at 50 to 90% of saturation while north of that it is generally above 80% pointing to limited capacity, especially from the Highway 34 through the I-80 counties. Will keep the flood watch going for the areas it is already in effect leaving our southeastern counties out due to having the most capacity. For the northern side, trimmed off the Highway 20 and 3 counties this morning and now the Highway 30 counties.

As the thermal lift moves away from the area along with the weakening low level jet, showers and whatever storms look to become spotty and eventually diminish through the day on Wednesday, particularly in the afternoon into the evening. Some increase in coverage is expected after midnight into the early hours of Thursday when the still weak nocturnal low level jet of 25 to 30 knots moves back over. Rainfall amounts do not look concerning with generally 1 to at most 2 inches. This rainfall, though, may fall over areas that have little time to recover any capacity with soils remaining saturated. Thus, it is possible a new flood watch may be needed as some point, but with a break from this (Tuesday) afternoon through Wednesday evening, have opted to clear the watch north of I-80 as mentioned above.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

Issued at 253 PM CDT Tue Sep 15 2026

Similar to Wednesday, whatever showers and storms early Thursday look to become less common through the day as forcing weakens. This will be in part as the ridge over the southeastern US bulges a bit poleward, which pushes the upper level kinematics and track of the nebulous vorticity piece away from Iowa. Further, low pressure over the High Plains look to surge the elevated warm front well north of the state with the highest rainfall amounts now trending northward to northern Iowa if not points farther north over South Dakota and Minnesota. This focus of rainfall may remain north Friday night into Saturday. Meanwhile, a substantive trough over the interior West will push eastward on Friday, which will finally shunt the ridge southeastward, but also bring the return of favorable heavy rainfall parameters this weekend. This front should bring drier conditions at some point, eventually, next week in what is an overall wet and active pattern until then.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1213 PM CDT Tue Sep 15 2026

Showers continue to sputter over central Iowa, but should see these continue to sink slowly southward through the afternoon into this evening. For FOD, MCW, and ALO, VFR conditions will prevail with the rain to the south of these terminals through the period. For DSM and OTM, showers will be off and on through the rest of the day, but will focus over far southern Iowa/OTM tonight into Wednesday morning. There is a short window of thunder potential there, but confidence is sufficient only for PROB30 at this time. Ceilings will also lower over far southern Iowa toward sunrise Wednesday and lift northward through the end of the period.

HYDROLOGY

Issued at 253 PM CDT Tue Sep 15 2026

Although flash flood and river flood concerns persist this week, the overall threat decreased from Monday. In the short term (through tonight), flash flooding remains a threat mainly along and south of US Hwy 30, and along and north of US Hwy 34, where top-layer soil moisture saturation percentages are highest. Additional heavy rain through tonight may lead to ponding and lower end flash flooding as far south as the MO border, where amounts and rates may be higher. At this time, significant flash flooding is not anticipated.

Some rivers and streams have increased significantly from last night's rain. These elevated stages and flooding will remain a concern through at least the end of this week. The NBM-forced National Water Model (NWM) with 10 days of QPF agrees, suggesting significant within-bank rises on many streams mainly between US Hwy 30 and US Hwy 34, with some minor flooding possible. The GEFS-driven Hydrologic Ensemble Forecasting Service (HEFS) basically agrees as well, however its probabilities decreased from Monday, reflective of lower trending QPF. The 50% exceedance probabilities show multiple streams reaching action stage or minor flood stage in central IA. The 30% exceedance probabilities show more pervasive minor flooding with a few locations at moderate flooding. The 10% level shows more pervasive moderate flooding with a few locations at major flooding. Variations in QPF location and amounts may impact the probabilities, so people should continue monitoring for river flood potential.

DMX WATCHES/WARNINGS/ADVISORIES

Flood Watch through Wednesday afternoon for IAZ057>062-070>074- 081>084-092>095.