On Wednesday May 19, 2021, an upper level Pacific trough dove south through the Pacific Northwest into the Great Basin and then became nearly stationary for a couple of days. The southerly flow ahead of this system transported middle and upper level moisture into the state, while an upslope east to northeast flow developed at the lower levels in response to relatively strong surface high and low pressure systems respectively set up over Alberta Canada and Utah.  The upslope flow eventually led to cold air damming and the development of a low level barrier jet along the Divide.  This resulted in strong northerly surface winds along eastern slopes adjacent to the Rocky Mountain Front for periods of near blizzard conditions and subsequently sent the unseasonably cold air into the southwestern portion of the state.  Locations from Roger���s Pass northward into the East Glacier and Browning areas observed the worst conditions due to the combination of falling snow and gusty northerly winds. The impacts ranged from severe driving conditions on area roadways to isolated power outages. By Thursday morning, the rain began to change to snow at lower elevations over North-central Montana and this transition eventually reached the southwest valleys later that evening.

Heavy Snow in HILL, MONTANA
From 21-MAY-21 11:24:00 to 21-MAY-21 11:24:00 (MST-7)
Reported by: Public

A snowfall total of 12 inches reported at 9 SSW Havre.

Heavy Snow in CASCADE, MONTANA
From 22-MAY-21 06:45:00 to 22-MAY-21 06:45:00 (MST-7)
Reported by: NWS Employee

A 10.5 inch snow total was reported at 6 NE Craig.

Heavy Snow in GALLATIN, MONTANA
From 20-MAY-21 23:10:00 to 20-MAY-21 23:10:00 (MST-7)
Reported by: Trained Spotter

Trained spotter at 7 ENE Manhattan reported snowfall total of 9 inches.

Winter Storm in JUDITH BASIN, MONTANA
From 20-MAY-21 23:55:00 to 20-MAY-21 23:55:00 (MST-7)
Reported by: Department of Highways

Severe driving conditions due to wind and snow were reported on MT Highway 3 MP 0 to 23 due to snow and wind.

Heavy Snow in SOUTHERN LEWIS AND CLARK, MONTANA
From 20-MAY-21 14:00:00 to 20-MAY-21 14:00:00 (MST-7)
Reported by: Trained Spotter

Snowfall total of 14 inches observed over 48 hr period at 2 W Helena.

Heavy Snow in JEFFERSON, MONTANA
From 22-MAY-21 09:30:00 to 22-MAY-21 09:30:00 (MST-7)
Reported by: Trained Spotter

Trained spotter at 2 SE Montana City reported a 12.3 snowfall total.

Heavy Snow in HILL, MONTANA
From 22-MAY-21 06:00:00 to 22-MAY-21 06:00:00 (MST-7)
Reported by: Trained Spotter

Trained spotter at 7 SE Rocky Boy reported a 14 inch snow total.

Winter Storm in SOUTHERN LEWIS AND CLARK, MONTANA
From 20-MAY-21 13:00:00 to 20-MAY-21 13:00:00 (MST-7)
Reported by: Public

A snowfall total of 18 inches reported near Roger's Pass.

Winter Storm in CASCADE, MONTANA
From 20-MAY-21 08:15:00 to 20-MAY-21 11:00:00 (MST-7)
Reported by: Utility Company

Northwest Energy reported power outages throughout Cascade County, including the Great Falls area due to snow and wind. Snowfall amount totals up to 3 inches were observed in the area.

Winter Storm in NORTH ROCKY MOUNTAIN FRONT, MONTANA
From 20-MAY-21 09:10:00 to 20-MAY-21 09:10:00 (MST-7)
Reported by: Department of Highways

Severe driving conditions reported on HW 89 from HW 44 to Two Medicine Bridge due to snow and wind.

Winter Storm in NORTH ROCKY MOUNTAIN FRONT, MONTANA
From 20-MAY-21 12:00:00 to 20-MAY-21 12:00:00 (MST-7)
Reported by: Trained Spotter

Snowfall amounts of 12 inches reported near Heart Butte. Reduced visibility and flickering electricity was also reported.

Winter Storm in SOUTHERN ROCKY MOUNTAIN FRONT, MONTANA
From 20-MAY-21 14:00:00 to 20-MAY-21 14:00:00 (MST-7)
Reported by: SNOTEL

A snowfall total of 15 inches recorded at the Heart Butte SOTEL (24 W Bynum).

Heavy Snow in CASCADE, MONTANA
From 22-MAY-21 06:45:00 to 22-MAY-21 06:45:00 (MST-7)
Reported by: NWS Employee

A 10.5 inch snow total was reported at 6 NE Craig.

Winter Storm in SOUTHERN LEWIS AND CLARK, MONTANA
From 20-MAY-21 05:35:00 to 20-MAY-21 05:35:00 (MST-7)
Reported by: Fire Department/Rescue

Snow and slush packed roads and reduced visibility create dangerous driving conditions on highway 200 near Roger's Pass.

Winter Storm in SOUTHERN LEWIS AND CLARK, MONTANA
From 20-MAY-21 06:06:00 to 20-MAY-21 06:06:00 (MST-7)
Reported by: Utility Company

Northwest Energy reports scattered power outages due to snow and wind near the Bowman's Corner area.

Winter Storm in SOUTHERN ROCKY MOUNTAIN FRONT, MONTANA
From 20-MAY-21 00:00:00 to 20-MAY-21 13:30:00 (MST-7)
Reported by: ASOS

Military ASOS Foxtrot station K1FM observed visibility ranging from 1/4 SM to 1 SM in snow and wind for much of the am hours and early afternoon.

Winter Storm in SOUTHERN ROCKY MOUNTAIN FRONT, MONTANA
From 20-MAY-21 07:15:00 to 20-MAY-21 07:15:00 (MST-7)
Reported by: Utility Company

Northwest Energy reported power outages reported in the Chouteau area due to snow and winds.

Winter Storm in EASTERN TETON, MONTANA
From 20-MAY-21 08:15:00 to 20-MAY-21 11:00:00 (MST-7)
Reported by: Utility Company

Northwest Energy reported as many as 323 units without power in Fairfield area due to snow and wind. Snowfall amounts up to 5 inches observed in the Fairfield area.

Heavy Snow in GALLATIN, MONTANA
From 22-MAY-21 04:30:00 to 22-MAY-21 04:30:00 (MST-7)
Reported by: Trained Spotter

Trained spotter reported snowfall total of 11.5 inches, most of which fell in a two hour period.