During the afternoon of August 12th, thunderstorms developed along an axis of weak surface convergence. The environment they existed in was conducive for gusty winds and heavy rain.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS LSX wrote it up.
During the afternoon of August 12th, thunderstorms developed along an axis of weak surface convergence. The environment they existed in was conducive for gusty winds and heavy rain. One of these thunderstorms became severe in western Madison County where it caused damage to power lines and structures and produced quarter sized hail.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS St. Louis, MO wrote its last area forecast discussion before the first event at Aug 12, 1:28 PM CST, 12 minutes earlier. Its key messages:
Area Forecast Discussion National Weather Service Saint Louis MO 228 PM CDT Tue Aug 12 2025 .KEY MESSAGES... - Scattered storms capable of locally heavy rainfall and gusty winds are expected to continue across portions of the area through this evening. - Potentially dangerous heat returns to the region this weekend into early next week. && .SHORT TERM... (Through Late Wednesday Night) Issued at 227 PM CDT Tue Aug 12 2025 Current water vapor imagery shows upper-level troughing digging into the Midwest with several shortwaves traversing the flow within it. One of these shortwaves is moving through northern Missouri as of early this afternoon, and a cold front can be seen in surface observations moving through northeastern Missouri. Ahead of the front, an area of surface convergence along the I-44 Missouri/I-70 Illinois corridor has led to convection developing early this afternoon - about an hour or two earlier than anticipated. If ACARS soundings out of KSTL are accurate, models have been overdoing instability by about 1,000 J/kg for SBCAPE. Between this and the slightly earlier convective initiation, updrafts may not be able to reach their full potential later this afternoon. Effective shear is around 20 kts across the CWA currently, leading to the poor storm organization we have seen so far. While PWAT is around 2" and an inverted "V" signature is noted in several soundings, the slightly lower instability and convective debris may inhibit updrafts from reaching a height that, when they collapse, would lead to severe- strength winds. Even so, several downburst signatures have been noted around the St. Louis metro area so far this afternoon, and confidence is high that at least isolated sub-severe downburst winds (up to 50 mph) will occur under stronger storms that collapse. The high PWAT and slower storm motion may also lead to isolated flooding issues, but the shorter lifespan of storms is expected to mitigate a more notable flood threat. With instability waning with the setting sun and convection easily becoming outflow dominant, a gradual weakening and decrease in coverage of convection is expected this evening. The front will gradually pass through the rest of the CWA this evening and tonight, clearing it to the southeast by Wednesday morning. In its wake, high pressure will spread into the Middle Mississippi Valley, characterized by generally northerly winds and temperatures running at to just below climatological normals. There is a low probability (20%) that there will be enough boundary layer moisture behind the front to lead to isolated to scattered convection across far southeastern Missouri and southwestern Illinois as a shortwave and the upper-level trough axis swing overhead during the afternoon, but most locations will remain dry. Elmore && .LONG TERM... (Thursday through Next Tuesday) Issued at 227 PM CDT Tue Aug 12 2025 The main focus for impactful weather in the extended forecast period is going to be the return of above normal temperatures. While guidance consensus is that upper-level ridging will still be gradually building into the Midwest on Thursday, the trend has been for it to do so more slowly. As a result, the surface high responsible for our more seasonable weather mid-week will hang around a little longer and inhibit southerly flow from returning to the CWA, with conditions on Thursday being similar to those forecast on Wednesday. However, the upper-level ridge will notably amplify on Friday, quickly pushing the surface high eastward and deepening southerly flow over the Middle Mississippi Valley. Temperatures at 850 mb are expected to see at least a 5 degree C jump, with values forecast to top out in the low 20s C. This climatologically favors surface temperatures in the mid 90s, which matches well with ensemble means on Friday. Guidance consensus is that the ridge will only continue to build into the region over the weekend, with potentially dangerous levels of heat and humidity occurring. NBM- based probabilities for high temperatures of at least 90 degrees top out at 80-100% for nearly the entire CWA Saturday and Sunday, and 60- 80% of at least 95 degrees for locations mainly along and south of I- 70 over the same period. With dew points from the NBM and ensemble mean hovering in the low to mid 70s, heat index values of at least 100 degrees are expected, and there is growing concern for Heat Advisory criteria or perhaps Extreme Heat Warning criteria for portions of the area over the weekend. How long this heat carries into next week is uncertain. Both deterministic guidance and ensemble clusters show the upper-level ridge retrograding westward as troughing develops over the eastern CONUS. However, guidance differs on the timing of this and the amplitude of the trough, leading to the spread among the global ensemble IQR increasing to 10 degrees next Tuesday. If the ridge is able to hold on over the region, we may at most see temperatures only decrease by a degree or two through the early week period compared to the weekend. However, if the ridge moves far enough west, the CWA will be beneath northwesterly flow aloft, allowing for cooler air and increasing rain/cloud chances across the region early next week. A majority of guidance favors the more stubborn ridge, and this is reflected in the current forecast. Elmore && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1218 PM CDT Tue Aug 12 2025 The main concern for impacts this TAF period is scattered showers and thunderstorms along a cold front sinking southward through the region. This showers and thunderstorms have the greatest chance of impacting KSUS, KSTL, and KCPS, though a brief period of impacts can't be ruled out at KCOU and KJEF. With showers or storms that make direct impact to terminals, heavy rainfall will reduce visibilities, and storms in proximity to local terminals could produce gusty, erratic winds. Behind the front tonight and tomorrow, winds will be very light and mostly variable. Elmore && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && WFO LSX
Hazardous Weather Outlook National Weather Service St Louis MO 236 PM CDT Tue Aug 12 2025 ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027- 034>036-041-042-047>052-059>065-072>075-084-085-099-131945- Greene IL-Macoupin IL-Montgomery IL-Bond IL-Fayette IL-Clinton IL- Marion IL-Washington IL-Randolph IL-Adams IL-Brown IL-Pike IL- Calhoun IL-Jersey IL-Madison IL-Saint Clair IL-Monroe IL-Knox MO- Lewis MO-Shelby MO-Marion MO-Monroe MO-Ralls MO-Pike MO-Boone MO- Audrain MO-Moniteau MO-Cole MO-Osage MO-Callaway MO-Montgomery MO- Lincoln MO-Gasconade MO-Warren MO-Saint Charles MO-Franklin MO- Saint Louis MO-Saint Louis City MO-Jefferson MO-Crawford MO- Washington MO-Saint Francois MO-Sainte Genevieve MO-Iron MO- Madison MO-Reynolds MO- 236 PM CDT Tue Aug 12 2025 This Hazardous Weather Outlook is for portions of eastern and central Missouri as well as west central and southwest Illinois. .DAY ONE...Tonight. There is a chance for thunderstorms through early tonight. Some storms may produce locally heavy rainfall and gusty winds. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. There is a low chance of thunderstorms for portions of the area Wednesday, Thursday and Monday. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. Elmore
The St Louis radar (KLSX), 54 km away, was scanning. The volume nearest the first event began at 19:39 UTC.
1 hail, 1 thunderstorm wind. Local standard time, as filed.
A trained spotter on St. Louis Ave in East Alton reported half inch to 1 inch sized sized hail.
Thunderstorm winds knocked down power lines and caused damage to structures near Alton.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Illinois in 2025Every state in 2025This day in weather historyWarnings in Illinois, August 2025
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 1278818, 1278819.