A vigorous Pacific weather system and attendant cold front brought scattered areas of strong wind gusts throughout the forecast area. An area of 50 kt + H700 winds streamed ahead of a cold front, some of which was transferred to the surface.
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.
A vigorous Pacific weather system and attendant cold front brought scattered areas of strong wind gusts throughout the forecast area. An area of 50 kt + H700 winds streamed ahead of a cold front, some of which was transferred to the surface. One of the biggest impacts was the combination of these winds with critically low relative humidity. The Haystack Fire near Townsend grew to nearly 11,000 acres and prompted some evacuations. Later that evening, the cold front passed through the region and provided the strongest wind gusts of the day.
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 Sep 18, 10:27 AM MST, 1 hour 33 minutes earlier.
Area Forecast Discussion National Weather Service Great Falls MT 1127 AM MDT Sat Sep 18 2021 Updated Aviation Discussion .SYNOPSIS... Expect a windy afternoon and evening over much of the region today, as a Pacific cold front approaches our area. It will be warm ahead of the front, with afternoon highs in the 70s and 80s today, however it will be much cooler on Sunday, with afternoon highs only in the 50s and 60s. There will be a chance for rain and snow showers, tonight thru Monday night, with drier conditions expected for the middle portion of next week. && .UPDATE... The morning update has been published. Southwesterly flow continues to increase this morning ahead of a Pacific trough and attendant cold front. Relative humidity values are already well below 20 percent across much of the region, even over northern areas where some recovery was expected. The Red flag Warning was expanded to include zones 113, and 114 due to the drier than expected conditions. An area of 50+ kt H700 winds will move over the southwest this afternoon, likely yielding at least some wind gusts up to and possibly exceeding 60 mph, so no change was made to the High Wind Warning. Otherwise, pops, winds, qpf, and hourlies were adjusted to the current trends. No other updates are anticipated for this morning. -RCG && .AVIATION... 1127 AM MDT Sat Sep 18 2021 (18/18Z TAF period) Strong SW flow aloft is beginning to mix down to the surface across the region today with gusty southwest winds expected to impact most terminals through this evening. Strongest winds with gusts in excess of 40kts are most likely to occur early this afternoon with a few gusts to around 50kts possible across parts of SW MT and over higher terrain. A wind shift to the west moves east with a Pacific cold front from late this afternoon through this evening with west- northwest winds gradually decreasing overnight. Showers will develop initially along the continental divide overnight and increase over SW MT Sunday morning. Smoke from regional wildfire activity could impact visibility at times, especially across SW MT. Hoenisch Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .FIRE WEATHER... The Red Flag Warning was expanded to include zones 113 and 114 over North-central MT and continues through this evening. There was almost no recovery in terms of humidity overnight, and current readings are falling down to 10 percent already this morning. Expect winds to increase throughout the afternoon, making any new or existing fires difficult to contain. Higher humidity can cooler temperatures move into the region tonight. The best chance for precipitation looks to be over the southern districts on Sunday. && .PREV DISCUSSION... /ISSUED 414 AM MDT Sat Sep 18 2021/ Today through Sunday...The main concern today is the strong winds expected ahead of a Pacific cold front. The potential for damaging winds looks highest over Southwest MT in Jefferson, Beaverhead, Madison and Gallatin counties. Thus this region is now under a high wind warning. The remainder of the high wind watch has been cancelled, as wind speeds should remain below warning criteria. Expect well above normal temperatures today ahead of the front. The cold front will move from west to east across the region this evening. Colder air will move into the region overnight, with afternoon highs on Sunday about 20 to 30 degrees colder than today. Rain and snow will develop behind the cold front, with Southwest MT seeing the best chance for precipitation. Snow could fall down to about 6000 feet, with accumulating snow generally above 7000 feet. The heaviest snow accumulations will be above 7000 feet from Big Sky to West Yellowstone. Since most of the snowfall will be above pass level, no winter highlights will be issued at this time. Overnight lows will likely be near or below freezing in many areas on Sunday night. Monday through next Saturday...Expect showers to decrease on Monday, with below normal temperatures continuing. However, this cool snap will be short lived. A gradual warming trend towards above normal temperatures is expected for much of the remainder of the week. On Thursday, the GFS/EC differ, with the GFS bringing a weather disturbance through the region. The EC is dry...thus the NBM has a very low chance for some precip. Overall the confidence in any precipitation past Monday is very low at this time, and we could be entering another prolonged period of very dry conditions over the region. Brusda && .PRELIMINARY POINT TEMPS/POPS... GTF 46 62 39 62 / 20 40 20 20 CTB 41 60 39 59 / 30 20 20 20 HLN 49 61 41 62 / 20 80 40 20 BZN 45 57 36 58 / 30 90 70 30 WYS 36 48 28 49 / 30 100 80 40 DLN 43 55 32 55 / 40 90 60 10 HVR 48 66 40 64 / 10 10 20 30 LWT 45 59 38 58 / 10 70 40 30 && .TFX WATCHES/WARNINGS/ADVISORIES... High Wind Warning until 9 PM MDT this evening Beaverhead... Gallatin...Jefferson...Madison. Red Flag Warning until 9 PM MDT this evening Deerlodge/Western Beaverhead National Forest...Eastern Beaverhead National Forest. Red Flag Warning until 9 PM MDT this evening Central and Eastern Lewis and Clark National Forest Areas...Chouteau and Fergus Counties...Helena and Townsend Ranger Districts of the Helena National Forest...Hill and Blaine Counties...Lewis and Clark National Forest Rocky Mountain District-Rocky Mountain Front. Red Flag Warning until 9 PM MDT this evening Gallatin National Forest. && http://www.weather.gov/greatfalls
6 high wind, 1 wildfire. Local standard time, as filed.
The Haystack Fire was a lightning caused incident first discovered on July 31st, 2021. It was a relatively quiet event until September 18th, when strong southwesterly winds and critically low relative humidity fueled its growth to 10,800 acres. This led to pre-evacuation notice being issued for the Depot Hill and Boulder hot Springs area, according to KRTV.
MDT Norris Hill station measured a 51 kt (59 mph) non-thunderstorm wind gust.
KCTB measured a 54 kt (62 mph) non-thunderstorm wind gust.
Ledger station (6 NW Ledger) measured a 51 kt (59 mph) non-thunderstorm wind gust.
MDT Sunburst station measured a 51 kt (59 mph) non-thunderstorm wind gust.
KHLN measured a 52 kt (60 mph) non-thunderstorm wind gust.
Elkhorn RAWS station measured a 56 kt (65 mph) non-thunderstorm wind gust.
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, September 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: 986832, 986841, 986845, 986853, 986855, 986852, 986851.