Multiple thunderstorms developed over north-central and southwest Montana during the afternoon and evening of May 9th, 2018 as a shortwave trough and accompanying Pacific cold front approached from the west. An ordinary cell thunderstorm produced a brief tornado six-miles southwest of Moore, MT in the early evening.
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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 TFX wrote it up.
Multiple thunderstorms developed over north-central and southwest Montana during the afternoon and evening of May 9th, 2018 as a shortwave trough and accompanying Pacific cold front approached from the west. An ordinary cell thunderstorm produced a brief tornado six-miles southwest of Moore, MT in the early evening. Multiple pictures indicated the tornado lofted dirt into the air, but no damage, injuries, or deaths occurred. Also, a flash flood occurred in Helena later that evening.
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 Great Falls, MT wrote its last area forecast discussion before the first event at May 9, 2:06 PM MST, 2 hours 22 minutes earlier.
Area Forecast Discussion National Weather Service Great Falls MT 306 PM MDT Wed May 9 2018 .SYNOPSIS... Showers and thunderstorms will expand in coverage Wednesday evening. A few thunderstorms could produce small hail and gusty winds. Fair weather returns for much of Thursday, with highs in the mid 60s. The next weather system brings widespread rain and mountain snow to the region Thursday night and Friday. && .DISCUSSION... Tonight through Friday...Instability associated with a compact but potent shortwave trough will continue to introduce showers and thunderstorms across the forecast area this evening. This activity will become more widespread and transform into a band of moderate rain moving west to east across N-central Montana overnight. Fair weather returns for most of the forecast area on Thursday as the departing trough moves away into northeastern Montana. A larger stronger closed low develops over the interior PacNW Thursday afternoon. Low level winds turn easterly Thursday night, supporting a widespread area of upslope enhanced precipitation backing into Rocky Mountain Front. Simultaneously, scattered thunderstorms are possible over southwest Montana and some of these may be strong. Latest model guidance suggests that around 0.75 to 1" of precipitation may be possible through Friday as this system rotates through the region. Snow levels drop to near 6000 feet by early Saturday, but hydrologic concerns continue to expand as detailed below. An unusually-cool and unsettled day on Friday may transition to moderating temperatures and an overall drying trend this weekend. The evolution of this low pressure system is even less certain Monday through Wednesday of next week. The latest run of the ECMWF shifts the system eastward, allowing high pressure aloft to become established over our area, which would result in drier conditions. The latest GFS has the disturbance retrograding northwestward and becoming absorbed by a separate low pressure center aloft approaching the Pac NW, resulting in wetter weather for Montana. Latest forecast is a compromise and therefore temperatures may tend to be near or above-normal next week Monday through Wednesday. Jaszka && .AVIATION... Updated 1800Z. Mainly VFR expected through the TAF period as CIGS become widespread. Mountain obscuration is possible. Simultaneously, flow aloft will tend southwesterly to westerly as high pressure ridging moves eastward and multiple disturbances begin affecting our region. Showers and thunderstorms will traverse the area generally from west to east through 12Z/Thur due to the aforementioned disturbances and accompanying Pacific cold front. The best potential for thunderstorms exists this afternoon and evening, especially along and south of a KGTF to KLWT line. Some storms may produce small hail and surface gusts up to 45 knots. The aforementioned front looks to near the Divide between 18Z and 21Z/Wed and then near a KHVR to KBIL line by 12Z/Thur. A period of breezy westerly to northwesterly surface winds is expected behind this front. Jaszka && .HYDROLOGY... Warm temperatures continue to bring overland flooding due to snowmelt. The situation will get more complicated through the late week as several disturbances could bring areas of increased rainfall across the area. Taking snowmelt, rainfall, and increase flows through area dams some river forecasts were increased today. The result is a Flood Watch for Simms and Vaughn along the Sun River, Big Hole River at Melrose, and the Jefferson River near Three Forks. An areal flood warning remains in effect for the Helena Valley and the Lincoln area. Numerous problems have been reported along Tenmile Creek, with impacts possible on Prickly Pear and Sevenmile Creeks. Problems have also been reported in the Lincoln area, with multiple homes having water running through them, especially in the Stemple Pass and Good News Lane areas. Thus, this warning will remain in effect until the impacts from flooding have diminished. As mentioned...a series of disturbances will bring widespread chances for rain through the end of the week. Rainfall amounts of a half inch to an inch are possible during the next three days. This could bring additional flooding concerns. Those in low lying areas and near rivers or creeks should be alert for additional flooding. Anglin && .PRELIMINARY POINT TEMPS/POPS... GTF 47 65 43 50 / 80 40 80 90 CTB 43 64 41 50 / 70 30 60 70 HLN 45 66 41 52 / 100 30 80 90 BZN 44 66 40 53 / 60 50 90 70 WEY 34 59 32 47 / 40 40 50 60 DLN 41 63 37 51 / 50 40 70 60 HVR 47 63 42 55 / 100 80 30 50 LWT 45 62 40 50 / 100 60 70 70 && .TFX WATCHES/WARNINGS/ADVISORIES... An areal flood warning remains in effect for the Helena Valley and the Lincoln area. Flood watches are in effect for the Sun River at Vaughn and Simms, and for the Big Hole River at Melrose, and the Jefferson River near Three Forks. && http://www.weather.gov/greatfalls
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
227 PM MDT WED MAY 9 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0220 PM HAIL MARYSVILLE 46.75N 112.30W
05/09/2018 E0.25 INCH LEWIS AND CLARK MT TRAINED SPOTTER
A LOT OF PEA SIZE HAIL ALONG WITH VERY HEAVY
RAIN OCCURRING AT THIS TIME.
&&
MARTINSpecial Weather Statement
National Weather Service Great Falls MT
405 PM MDT WED MAY 9 2018
MTZ012-092300-
Cascade MT-
405 PM MDT WED MAY 9 2018
...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL CASCADE COUNTIES UNTIL
500 PM MDT...
At 403 PM MDT, Doppler radar was tracking a cluster of thunderstorms
stretching from Cascade to Monarch. These thunderstorms are moving
northeast at 20 mph.
Heavy rainfall in a short amount of time and winds in excess of 40
mph will be possible with these storms.
Locations impacted include...
Sluice Boxes State Park, Eden and Monarch.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.
&&
LAT...LON 4736 11077 4725 11079 4710 11077 4709 11069
4702 11067 4701 11108 4703 11111 4708 11108
4709 11116 4701 11117 4700 11153 4698 11155
4697 11168 4724 11173
TIME...MOT...LOC 2203Z 240DEG 17KT 4710 11127
MartinThe Great Falls radar (KTFX), 136 km away, was scanning. The volume nearest the first event began at 23:27 UTC.
1 tornado, 1 flash flood. Local standard time, as filed.
KRTV received multiple photos of a tornado six-miles southwest of Moore, MT. Tornado moved southward and lasted about five-minutes per observer. Path length and maximum path width were estimated.
KTVH reported flash flooding at intersection of 11th Ave and N. Montana Ave in Helena.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2018Every state in 2018This day in weather historyThe tornado archive for Montana, 2018Warnings in Montana, 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: 745660, 745706.