An upper level shortwave trough passed through the area, bringing a wetting rain to a large portion of the area. There was just enough instability as this system moved through that a cold air funnel formed east of Great Falls.
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The episode as NWS TFX wrote it up.
An upper level shortwave trough passed through the area, bringing a wetting rain to a large portion of the area. There was just enough instability as this system moved through that a cold air funnel formed east of Great Falls.
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 Jun 20, 8:47 AM MST, 3 hours 23 minutes earlier.
Area Forecast Discussion National Weather Service Great Falls MT 947 AM MDT Sun Jun 20 2021 .UPDATE... A widespread area of light rain has now moved south of a line from around Choteau to Great Falls to Lewistown. Embedded showers are producing locally moderate rainfall rates. The forward edge of the rain showers reaches from around boulder to Bozeman. This activity will continue moving south through the afternoon. Scattered thunderstorms remain in the forecast, with one already observed near Townsend. Minor updates were posted to the forecast to better match position and timing of precipitation. && .SYNOPSIS... Widespread beneficial light rain spread over much of the region today, as northerly winds keep temperatures below normal. A few afternoon thunderstorms are possible. Skies will partially clear overnight in the wake of the system, which may allow patchy fog to form. Drying and warming conditions are then likely for much of this week, but another system may bring brief cooling with chances for showers and thunderstorms during the mid week period. && .AVIATION... 545 AM MDT Sun Jun 20 2021 (20/12Z TAF period) Widespread MVFR conditions in mountain-obscuring rain showers may lower at times to IFR conditions through 18Z or so along the east- and north-facing slopes of the north central Montana plains. However, the KHVR area will only receive showers during this time that may bring periods of MVFR conditions. The showers will move south over southwest Montana through 18Z, but it will mostly result in low VFR/periods of MVFR conditions with mountain obscuration. The showers will start decrease from the north over the plains after 15Z and mostly be out of the area after 20Z. However, the airmass will become weakly unstable after 18Z, mainly along and south of a KHLN- KLWT line, which may cause a few thunderstorms with gusty winds and brief heavy rain to occur with periods of MVFR conditions. Skies will partially clear after 00Z, which may allow patchy fog to form after 06Z. -Coulston Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 545 AM MDT Sun Jun 20 2021/ Today through Tonight...A shortwave trough moving south out of Canada this morning is on track to bring widespread rain showers to much of north central and southwest Montana today. Scattered showers have already spread south over the plains, producing up to 0.25 inches at Cut Bank. Total amounts along the east- and north- facing slopes over the plains (including Cut Bank, Great Falls, and Lewistown) should range between 0.25 and 0.50 inches, with amounts closer to 0.75 inches possible closer to the mountains. These showers should spread into southwest Montana later this morning, but with less of the northerly upslope precipitation enhancement expected, rainfall amounts will more so be in the 0.10 to 0.25 inch range. The airmass is also weakly unstable, so a few thunderstorms will also be possible, which may cause locally heavier rainfall. The airmass will likely be most unstable during the afternoon heating. A cold frontal passage also preceded the shortwave, which will keep temperatures up to 10 degrees below normal. The front will linger over southwest Montana throughout the day, which could also help generate thunderstorms there as the shortwave passes overhead this afternoon. Overall, though, any thunderstorms should remain weak and likely not cause strong wind gusts or large hail, but northerly winds could become gusty at times due to the passage of a secondary cold front. Showers and storms will move south out of the area through this afternoon and early evening as the system exits the area. This will allow skies to partially clear as winds decrease, which will cause overnight temperatures to fall below normal as well and allow patchy fog to form. Monday through next Sunday...A high pressure ridge over the southwest United States will build back north into Montana Monday into Tuesday, bringing mostly dry conditions as temperatures warm to as much as 15 degrees above normal for Tuesday and Wednesday. However, a weak disturbance moving through the ridge may bring a few thunderstorms to southwest Montana Tuesday afternoon and evening. A low pressure trough is then forecast to swing south out of Canada for Wednesday into Thursday, bringing a chance of showers and thunderstorms to much of the area. An associated cold frontal passage will likely also cool temperatures back closer to normal on Thursday. The high pressure ridge is forecast to build back into the area for Friday through the weekend, which will dry out the region and warm temperatures up to 15 degrees above normal once again. -Coulston && .PRELIMINARY POINT TEMPS/POPS... GTF 66 42 82 50 / 100 0 0 0 CTB 64 41 79 49 / 80 0 0 0 HLN 67 46 87 55 / 100 0 0 0 BZN 65 39 85 49 / 80 10 0 0 WYS 66 32 77 40 / 40 0 0 0 DLN 67 40 85 51 / 40 0 0 0 HVR 71 45 81 52 / 20 0 0 0 LWT 63 41 78 49 / 90 0 0 0 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && http://www.weather.gov/greatfalls
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
738 AM MDT SUN JUN 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0730 AM HEAVY RAIN 13 W CUT BANK 48.60N 112.61W
06/20/2021 M0.58 INCH GLACIER MT TRAINED SPOTTER
0.58 INCHES NEW RAINFALL OVERNIGHT.
&&PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
816 AM MDT SUN JUN 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0814 AM HEAVY RAIN CHOTEAU 47.81N 112.18W
06/20/2021 M0.41 INCH TETON MT PUBLIC
REPORTED MEASURED BY PUBLIC. STEADY RAIN
SINCE YESTERDAY LATE AFTERNOON.
&&
JAFThe Great Falls radar (KTFX), 38 km away, was scanning. The volume nearest the first event began at 19:11 UTC.
1 funnel cloud. Local standard time, as filed.
Cold air funnel reported 4 miles east of observers location. Photo provided on Twitter.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2021Every state in 2021This day in weather historyWarnings in Montana, June 2021
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: 965957.