An upper level vort max moved through the region, triggering showers and thunderstorms. Some of the storms produced heavy rainfall that lead to flash flooding.
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The episode as NWS LSX wrote it up.
An upper level vort max moved through the region, triggering showers and thunderstorms. Some of the storms produced heavy rainfall that lead to flash flooding.
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 17, 11:55 AM CST, 2 hours 5 minutes earlier.
Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1255 PM CDT Thu May 17 2018 .SHORT TERM... (Through Late Tonight) Issued at 405 AM CDT Thu May 17 2018 Early this morning, a split flow regime prevailed at the upper levels across the central CONUS, with a weak NW flow aloft over our region. Of particular focus is a small upper LO located just northwest of Kansas City and is directly responsible for a few showers around Kansas City. Another small upper LO existed further to the south over south-central MO and is helping to maintain areas of showers and thunderstorms for much of that area. Outside of the cloudy skies found in the various clusters of showers and storms, a large veil of thinner cirrus clouds was spread over the entirety of our region. Temperatures were in the 60s with a light northeast wind at the surface. High dewpoint air continued to hang tough for much of the forecast area near and south of Interstate 70, with values in the low-mid 60s. The small upper LO will be our main focus for the forecast this period, and is expected to track slowly southeastward towards the Lake of the Ozarks by late this afternoon. While some showers and thunderstorms may intrude into the southwest and southeast peripheries of the forecast area until late morning, rain chances will really ramp up beginning late this morning as we head into peak heating and the small upper LO moves close enough to have an influence. Have boosted PoPs into likely category for a stretch from central to southeast MO with some widely scattered convective development possible across an expanded area that will reach into STL metro. Temps will struggle to rise most where the rain chances are highest, with 75-80 expected in parts of central and southeast MO. Otherwise, max temps from 80-85 are forecast, close to persistence values. The small upper LO will continue moving slowly, taking a more eastward turn tonight towards far southern IL, nearly reaching the MS river by 12z/Fri. Coverage of showers and thunderstorms downstream from this LO should decrease with loss of peak heating, but mentionable PoPs are expected to persist nonetheless, with showers and a few storms expected to increase in coverage across southern IL ahead of this LO and with some even building back in towards the STL metro area before sunrise Friday morning. Overnight min temps will be similar to persistence, ranging from the upper 50s to the mid 60s. TES .LONG TERM... (Friday through Wednesday) Issued at 405 AM CDT Thu May 17 2018 Unsettled pattern is expected through the weekend into the first half of next week. Global models are in relatively good agreement that upper low over Missouri and Illinois on Friday will open up and lift out over the weekend. This upper low will bring a chance of showers and thunderstorms to the area on Friday. Highs on Friday will be close to normal with the clouds and chance of rain under the upper low. A weak upper ridge will move across the area on Saturday once the upper trough lifts out. There will be a warm front move north into the area on Saturday and Saturday night bringing an additional chance of showers and thunderstorms. The upper flow will become southwesterly behind the weak upper ridge and a series of upper level disturbances will move over the area in the southwesterly flow. The upper flow will eventually become zonal by the middle of next week in response to a large upper trough dropping into the western CONUS and closing off. With a stalled front and unstable airmass over the area, will keep at least a low chance for showers and thunderstorms over the area each day. Expect temperatures to rise back 10-15 degrees above normal next week as the warm front moves back north into the area and 850mb temperatures climb into the 15-20C range. Britt && .AVIATION... (For the 18z TAFs through 18z Friday Afternoon) Issued at 1230 PM CDT Thu May 17 2018 TSRA continue along nrn periphery of MCV tracking slowly ewd south of the area. Believe TSRA will slowly expand/propagate newd this afternoon. Better chances remain south of KSUS/KCPS, however, still cannot rule out TSRA impacting these terminals. As upper low remains over the region tonight and into Fri morning, lower cigs are expected to move into the area from the east, impacting KSUS/KCPS early Fri morning. Confidence on the trends for these lower cigs is somewhat low and therefore kept in going TAF. SPECIFICS FOR KSTL: Believe bulk of TSRA will remain south of the terminal, but cannot rule out a stray storm impacting the terminal this afternoon. Otherwise, MVFR cigs expected to move into the area early Fri morning. Somewhat low confidence in these cigs and may need to be adjusted with future updates. Tilly && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && WFO LSX
Hazardous Weather Outlook National Weather Service Saint Louis MO 431 AM CDT Thu May 17 2018 ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027- 034>036-041-042-047>052-059>065-072>075-084-085-099-180945- 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- 431 AM CDT Thu May 17 2018 This hazardous weather outlook is for portions of eastern and central Missouri as well as west central and southwest Illinois. .DAY ONE...Today and Tonight There is a chance of thunderstorms across central and eastern Missouri as well as southwest and south central Illinois through tonight. .DAYS TWO THROUGH SEVEN...Friday through Wednesday There is a chance of thunderstorms each day. .SPOTTER INFORMATION STATEMENT... Spotter activation is not anticipated through tonight. Britt
The St Louis radar (KLSX), 116 km away, was scanning. The volume nearest the first event began at 19:58 UTC.
1 flash flood. Local standard time, as filed.
Up to 5 inches of rain fell in a short amount of time over southern portions of Ste. Genevieve County. Several roads were flooded and Saline Creek was out of its banks where it runs parallel to Boyd Road.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Missouri in 2018Every state in 2018This day in weather historyWarnings in Missouri, May 2018
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: 763694.