A highly-unstable atmosphere along an organized trough of low pressure allowed several strong thunderstorms to develop across the area, a number of which became severe through the evening and overnight hours.
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 GGW wrote it up.
A highly-unstable atmosphere along an organized trough of low pressure allowed several strong thunderstorms to develop across the area, a number of which became severe through the evening and overnight hours.
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 Glasgow, MT wrote its last area forecast discussion before the first event at May 17, 12:46 PM MST, 5 hours 44 minutes earlier.
Area Forecast Discussion National Weather Service Glasgow MT 146 PM MDT Thu May 17 2018 .DISCUSSION... Afternoon Update (2pm MDT): Not much is changed from this morning update besides keeping up with the current trends and utilizing the hi-res short-term models to keep up with the developing potential severe weather this evening. Recent model output still indicates storms firing to the west, around Phillips County, growing into a large MCS and then trucking to the east-southeast through Valley County and onwards towards North Dakota. While an isolated initial strong to severe storm or two may form out ahead of the boundary in the eastern and southeastern reaches of the region, the most likely scenario is for the strongest storms to form in the west and then move through the CWA to the east. Apart from tonight's event, the forecast was left relatively untouched. Bigelbach PREVIOUS DISCUSSION: The stage is nearly set for at least some marginally severe thunderstorms moving through NE Montana this afternoon through tonight. Earlier disorganized low pressure has now lined up into a low pressure trough through the northern Rockies region this morning. A warm and moist airmass becomes favorably-placed for thunderstorm development over the high plains beginning this afternoon. Instability remains high, even during these overnight and early morning hours with a few isolated thunderstorms persisting over our eastern zones. Expect CAPE values to re-surge this afternoon, well in excess of 2000 J/Kg. This model instability co-located with model QPF output would signal the best chance for severe storm possibility. Given the expected east and northeast winds up to 20 kts this afternoon through tonight, chose to issue a Lake Wind Advisory. When this storm system first showed up in the models several days ago, a large bulls-eye appeared along this axis of instability. Then over the last few days, the model storm seemed to weaken; but now with the last 2 or 3 model runs, a large and significant thunderstorm has returned in the GFS model. Hints of it show up in the NAM but the signal is not as strong there. This storm seems to form off the Cypress Hills of SW Saskatchewan this afternoon, then moves and strengthens across our Phillips and Valley Counties tonight. The model QPF bulls-eye signature for this storm would suggest that it could easily become severe, with large hail, damaging winds, frequent lightning, and heavy rain being the main threats. Next shift will want to re-investigate this storm in the model data for one more good forecast update today before storm initiation. Rain showers will become more widespread across the remainder of NE Montana through the late night hours tonight and into most of Friday. The associated cold front will only allow Friday's high temperatures to top out in the lower 50s for most locations. Through the remainder of the weekend, the upper level flow regime becomes benign between systems, with a high pressure ridge taking shape by Saturday night. BMickelson && .AVIATION... FLIGHT CAT: Expect mainly VFR ceilings. Some reduction in category to MVFR possible underneath strong thunderstorms this afternoon and evening. SYNOPSIS: An atmosphere favorable for showers and thunderstorms will dominate the region beginning late this afternoon and lasting through much of the overnight. Some of these storms late in the afternoon through midnight could become severe and would impact operations if they happen to move directly over a TAF site. WINDS: Expect continued winds from the SE at 10-15 kts, then increasing from the NE to around 15-20 kts later this evening. With a frontal zone slowly drifting north, and many thunderstorm outflows from this morning and those to come this afternoon and evening interacting with each other, winds should be expected to be erratic and blustery at times. Bigelbach && .GLASGOW WATCHES/WARNINGS/ADVISORIES... Lake Wind Advisory until 10 AM MDT Friday For Fort Peck Lake for Central and Southeast Phillips...Central and Southern Valley... Garfield...McCone...Petroleum. && weather.gov/glasgow
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GLASGOW MT
332 PM MDT THU MAY 17 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0255 PM HEAVY RAIN 1 SSE ZORTMAN 47.91N 108.52W
05/17/2018 E1.00 INCH PHILLIPS MT TRAINED SPOTTER
AT 255 PM SPOTTER ESTIMATES 1" OF HEAVY
RAINFALL IN HALF HOUR. RAIN ONGOING, THOUGH
LIGHTER AT TIME OF REPORT.
&&
BCBPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GLASGOW MT
709 PM MDT THU MAY 17 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0644 PM TSTM WND GST KING COULEE 47.80N 107.02W
05/17/2018 M57 MPH VALLEY MT MESONET
MESONET STATION KIGM8, KING COULEE.
&&
BCBThe Glasgow radar (KGGW), 83 km away, was scanning. The volume nearest the first event began at 01:29 UTC.
1 funnel cloud, 1 hail. Local standard time, as filed.
A trained spotter and their neighbor both observed a funnel cloud reaching about halfway down to the ground from the cloud base .
Public reported half dollar size hail up to 4 inches deep along with 2 foot drifts. They also reported more than 4 inches of rain in less than 2 hours. Siding of a mobile home was also damaged.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2018Every state in 2018This day in weather historyWarnings 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: 757477, 757470.