Area Forecast Discussion

KEY MESSAGES

- Active pattern this week with numerous rounds of showers and storms with heavy rainfall at times.

- There continues to be a Marginal Risk (Level 1 of 5) for severe storms this evening into tonight for portions of eastern Iowa. Primary hazards will be heavy rainfall and isolated damaging winds.

- An elevated flash flooding risk may develop mid-week with repeating rounds of storms. There is a medium to high chance (50-70%) for portions of eastern Iowa to see over 3 inches of rainfall, with localized areas of 5+ inches possible (5-15%), leading to rises on area creeks and streams.

SHORT TERM /THROUGH WEDNESDAY/

Issued at 239 PM CDT Mon Sep 14 2026

An active pattern will march on tonight throughout the week, with repeated rounds of showers and storms. Synoptically, we will continue to see a large upper-level anti-cyclone over the southeastern CONUS through much of the week, with large-scale troughing over the Pacific NW. This afternoon, an area of high pressure over the central Great Lakes has helped to bring southerly return flow/warm air advection over the area, with generally light showers over northeastern Iowa. In time, these showers will lift northward out of the area, leading to a brief respite from the precipitation late this afternoon through the early evening.

Chances of showers and storms (70-90%) will increase across our north late this evening and expand southward overnight tonight. This is ahead of a cold front that will be our main forcing mechanism for shower and storm activity for much of the week. Concurrently, a southwesterly 45 to 55 knot low-level jet (LLJ) will also help to enhance convergence and moisture transport ahead of the boundary, with Pwat values progged to exceed 2 inches tonight. Pwat values this high this time of the year is quite rare, placing above the the percentile per the NAEFS ensemble. Instability will be fairly modest, with MUCAPE values around 500-1000 J/kg and very weak mid- level lapse rates. Deep-layer shear is a bit more robust given the stronger kinematics, with 0-6 km shear around 40 to 50 knots. With all of this said, SPC continues to outline a Marginal Risk (Level 1 of 5) for severe storms late this evening through the overnight hours, mainly for a damaging wind threat given the stronger shear. Additionally, rainfall amounts will need to be watched closely overnight with the stronger LLJ and extreme Pwat values that could support some flash flooding. The 14.12z HREF ensemble PMM QPF values appear to range between 1-2 inches for a 6-hour period between 06-12z tonight, mainly over east-central Iowa, with isolated 2-3" possible. FFG values are right around these values, so some isolated flash flooding remains possible. One area to watch for is around the Cedar Rapids metro, which could support enhanced runoff.

As the cold front continues to dive southward Tuesday, and the nocturnal LLJ waning by sunrise Tuesday, the latest high-res guidance suggests a brief lull in the activity before things ramp up again during peak heating Tuesday afternoon across our south. The convective parameter space ahead of the boundary still shows more modest values, with SBCAPE values around 500-1000 J/kg per the HREF, with deep-layer shear slightly more favorable for stronger storms (around 30 to 40 knots). With a pretty weak low-level shear environment, damaging winds continue to be the primary threat, and SPC has maintained a Marginal Risk for areas along and south of a Keosauqua, IA to Walnut, IL line.

LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/

Issued at 219 PM CDT Mon Sep 14 2026

By Tuesday night, the cold front will stall out across northern Missouri and central Illinois and act as the focal point for additional rounds of showers and storm to redevelop mid- to late-week. A nearly stationary longwave trough over the western CONUS and ridging over the Southeast will keep our upper-level flow out of the southwest through Thursday as weak shortwaves are pulled northeastward into the Midwest. This will keep PoPs around Tuesday night into Thursday as scattered showers and storms fire along and north of the stationary front. With Pwats over 1.75 inches likely (70-90%) area-wide through Thursday, the flooding threat will remain elevated, especially after the heavy rainfall tonight and Tuesday. Per the latest NBM, there is a medium to high chance (50-70%) of total rainfall of 3 inches or more over much of eastern Iowa through Thursday with localized areas exceeding 5 inches possible (5-15%). Rises on area creeks and streams are likely.

Heading into Friday, a Front Range low in Colorado is forecast to form due to forcing from a lobe of mid-level vorticity pulled northward from the subtropics. This will introduce a more southerly component to the low-level flow as the stationary front converts to a warm front and moves northward into Iowa. The upper-level flow is expected to become zonal and force the Front Range low eastward Friday night into Saturday. The increased surface convergence will support another round of storms before the same boundary is left behind for Sunday, keeping PoPs in the area.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 104 AM CDT Tue Sep 15 2026

Moderate showers with isolated thunderstorms are expected at CID, DBQ, and MLI through 14 UTC with short breaks in between. A brief thunderstorms cannot be ruled out at BRL. With the heavier showers, visibilities could drop to MVFR conditions and briefly to IFR. The southwesterly LLWS at 34-45 knots currently over all TAF sites is forecasted to weaken/dissipate through sunrise from south to north. Ceilings are expected to remain above 5 kft also through 14 UTC before a MVFR cloud deck moves into CID, DBQ, and possibly MLI (30% chance) with an approaching cold front. The winds are forecasted to shift from southerly to northwesterly over the next 24 hours and weaken behind the front where scattered showers could redevelop late morning into the evening.

DVN WATCHES / WARNINGS / ADVISORIES

IA...None. IL...None. MO...None.