A thunderstorm complex moved south through Illinois during the day of May 18th. Isolated wind damage occurred in Montgomery County, and isolated flooding occurred in Marion County.
Every event with a place to put it. Tap one for its entry below.
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.
A thunderstorm complex moved south through Illinois during the day of May 18th. Isolated wind damage occurred in Montgomery County, and isolated flooding occurred in Marion County.
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 May 18, 5:52 AM CST, 2 hours 15 minutes earlier. Its key messages:
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
652 AM CDT Mon May 18 2026
.KEY MESSAGES...
- Widespread showers and thunderstorms are expected throughout the
day with heavy rainfall in some locations potentially leading to
flash flooding.
- A few strong to severe thunderstorms are possible this afternoon
and evening with the main hazards being hail and damaging wind
gusts.
- Additional thunderstorms are forecast late tonight and early
Tuesday morning with another round possible along a cold front
Tuesday afternoon/evening.
&&
.SHORT TERM... (Through Late Tuesday Afternoon)
Issued at 402 AM CDT Mon May 18 2026
A complex of thunderstorms, that as of 09z, is currently positioned
near Quincy MO to Warrensburg MO. This activity initiated across
KS/NE on Sunday evening and has continued to push southeastward
early this morning, with arrival in central/northeast MO expect
around 09-10z. Within the last few hours, this complex has become
outflow dominant, which has led to a gradual weakening trend from
the severe wind gusts it was producing across northwestern MO around
midnight. Upstream observations indicate that the gust front has
been achieving winds gusts in the 30-50mph range. As a result, the
main severe hazard tied to these storms as they move into the area
are expected to be scattered wind gusts up to 50mph, along with
frequent lightning and heavy rain.
In fact, one of the main concerns regarding the short term is now
the threat for heavy rainfall and flash flooding, mainly across
central/south-central MO and points west. CAMs are in good consensus
that the southeastward progress of this thunderstorm complex will
continue to slow as it makes its way into the area this morning.
Additionally, there are signs that backbuilding of thunderstorms may
occur as the remnant outflow boundary/line of convection stall in a
west-east orientation, with strong low-level southerly warm/moist
advection overrunning this stationary boundary. Analysis of forecast
soundings unveils a plentiful amount of uniform CAPE (around 1,500
J/kg) throughout the column, along with warm cloud temperatures with
freezing levels around 12-14kft AGL. In addition, PWATs are
currently in the 1.4-1.5" range, with the latest HREF indicating
values increasing to 1.7-1.9" today, which is near the 99th
percentile. Lastly, the HREF LPMM precipitation reveals the
potential for a west-east oriented swath of 2-4" with localized
pockets of 4-6" possible. Given all of the variables mentioned
above, the risk of heavy rainfall has prompted us to issue a Flash
Flood Watch for central/south-central MO and portions of east-
central MO that is in effect from 12z today through this evening.
Regarding the severe weather potential throughout the day, the
threat remains for an isolated thunderstorm becoming strong/severe
with hazards being hail along with damaging winds. This potential
appears to be lower due to the lingering showers and thunderstorms
across a large portion of the area, inhibiting solar insolation, and
thus, robust destabilization this afternoon. Regardless, given the
warm/moist summerlike airmass in place and the continuation of
steepened mid-level lapse rates, maintaining sufficient instability,
the threat for an isolated severe storm remains. A few locations may
be able to clear by this afternoon, allowing the environment to
recover and destabilize, and potentially the threat for strong to
severe thunderstorms to increase in localized areas. With
uncertainty regarding the evolution/coverage of lingering
showers/storms, confidence is low regarding the extent of severe
weather that may be realized across the area this afternoon and
evening.
By tonight, the large-scale pattern features a broad mid/upper level
trough to the west, with an embedded shortwave ejecting
northeastward into the Great Plains. This mid-level feature triggers
lee-side surface cyclogenesis across the Central Plains/Midwest near
a quasi-stationary baroclinic zone. Convection is modeled by high
resolution guidance to initiate along this boundary in KS/NE, with
quick upscale growth ensuing with eastward progression. The surface
low is modeled to lift northeast into the Midwest on Tuesday, which
begins to slowly shift the quasi-stationary baroclinic zone
southeastward as a cold front. However, this convection is progged
by guidance to outrun this slowly advancing cold front out west, and
gradually weaken as it approaches the area. This weakening trend
appears reasonable since the best deep-layer shear remains displaced
to the north and west of the area and forcing for ascent will be
limited without convection tied to the boundary. Regardless, this
threat for severe weather late Monday night into early Tuesday
morning will include the potential for large hail and damaging wind
gusts. Additionally, if surges or bowing segments occur, then
severe outflow gusts or embedded circulations leading to brief
tornadoes may also be possible.
Guidance is in fairly good agreement that the cold front will
approach northeast MO/west-central IL Tuesday morning with it
exiting to the southeast late Tuesday evening. Locations that remain
pre-frontal for a majority of the day on Tuesday will have the
greatest severe weather threat. The big question that remains is,
how quickly will the atmosphere recover from the early Tuesday
morning convection by the time the front is working its way through
the area. As of now, it appears the severe weather threat will be
greatest from east-central MO and points southeastward, as these
locations will have the best chance to destabilize in the early
afternoon on before the arrival of the front. Regardless,
instability and mid-level flow are still forecast to be sufficient
enough to support organized storms with potential for large hail,
damaging winds, and tornadoes. Any lingering outflow boundaries from
morning convection may serve as areas of focus for thunderstorm
development in the afternoon ahead of the cold front.
Peine
&&
.LONG TERM... (Tuesday Night through Sunday)
Issued at 402 AM CDT Mon May 18 2026
Low-level cold air advection within the post-frontal airmass will
lead to much cooler conditions for Wednesday, with highs forecast to
be around 10-15 degrees below average. Quasi-zonal flow aloft
encompasses the region through the end of the week, which should
allow for a period of dry conditions until Friday. By the end of the
week, the general trend of LREF temperature IQRs is a gradual warmup
to temperatures right around climatological normals.
Peine
&&
.AVIATION... (For the 12z TAFs through 12z Tuesday Morning)
Issued at 652 AM CDT Mon May 18 2026
A complex of thunderstorms continues to slide eastward early this
morning, nearing KUIN just before 12z with arrival expected at the
St. Louis metro terminals around 13-14z. The leading edge of this
complex has already impacted KCOU, producing a 62kt wind gust, which
has been the highest wind gust observed in the region this morning.
The leading edge of this convection will be capable of scattered
severe wind gusts where bowing segments occur. Given the low
predictability for scattered winds gusts of this nature, mention of
winds to this magnitude were not included in downstream TAFs. Short
term trends along the leading edge will need to be monitored as this
complex approaches the metro with amendments as needed. In addition,
brief IFR visibilites are possible along the leading edge of this
convection within the stronger thunderstorms. Widespread showers and
thunderstorms are expected to linger throughout the morning at all
terminals with intermittent impacts of MVFR visibilities/ceilings
associated with the heavier showers and scattered thunderstorms.
Gradual improvements are forecast to begin later this morning into
the early afternoon, however, confidence in how long showers and
scattered thunderstorms linger remains low. In addition to MVFR
impacts, strong and gusty southerly winds should prevail with gusty
and variable winds associated with showers and thunderstorms. Dry
and VFR conditions are forecast this evening and early tonight
before another potential complex of storms moves in from the
northwest early on Tuesday.
Peine
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through this evening for Boone MO-Callaway MO-Cole
MO-Crawford MO-Franklin MO-Gasconade MO-Moniteau MO-
Montgomery MO-Osage MO-Warren MO-Washington MO.
IL...None.
&&
WFO LSXHazardous Weather Outlook National Weather Service St Louis MO 600 AM CDT Mon May 18 2026 ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027- 034>036-041-042-047>052-059>065-072>075-084-085-099-191100- 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- 600 AM CDT Mon May 18 2026 This Hazardous Weather Outlook is for portions of eastern and central Missouri as well as west central and southwest Illinois. .DAYS ONE THROUGH SEVEN...Today through Sunday. More detailed information can be accessed at: https://www.weather.gov/media/lsx/DssPacket.pdf The Area Forecast Discussion is available at: https://forecast.weather.gov/ product.php?issuedby=LSX&product=AFD&site=lsx .KEY MESSAGES... - Widespread showers and thunderstorms are expected throughout the day with heavy rainfall in some locations potentially leading to flash flooding. - A few strong to severe thunderstorms are possible this afternoon and evening with the main hazards being hail and damaging wind gusts. - Additional thunderstorms are forecast late tonight and early Tuesday morning with another round possible along a cold front Tuesday afternoon/evening. NOTE: Starting May 29, 2026, this outlook product will be discontinued. Please see the above products for more detailed forecast information. WFO LSX
The St Louis radar (KLSX), 103 km away, was scanning. The volume nearest the first event began at 14:05 UTC.
1 thunderstorm wind, 1 flash flood. Local standard time, as filed.
Thunderstorm winds downed multiple trees in Montgomery County. In Litchfield, powerlines were also downed.
Heavy rain of up to 7 inches resulted in street flooding due to persistent rainfall. Cars were reportedly stalled all over town.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Illinois in 2026Every state in 2026This day in weather historyWarnings in Illinois, May 2026
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: 1334135, 1334137.