For this time of year, after a warmer day on the 9th, recent additional melting of the patchy snow pack and some wet ground around the region provided some additional moisture near the surface.  Early on the morning on the 10th, as temperatures dropped well into the teens, conditions were favorable for some areas of freezing fog development, which persisted off and on for a few days in this stagnant weather pattern.

Dense Fog in CENTRAL AND SOUTHERN VALLEY, MONTANA
From 10-DEC-20 10:44:00 to 10-DEC-20 17:53:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

At the Glasgow airport ASOS site (KGGW), visibility dropped to a quarter mile or less for 7 hours through the day on the 10th.

Dense Fog in RICHLAND, MONTANA
From 11-DEC-20 01:12:00 to 11-DEC-20 13:16:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

At the Sidney airport ASOS site (KSDY), visibility dropped to a quarter mile or less from the early morning through the early afternoon on the 11th.

Dense Fog in DAWSON, MONTANA
From 10-DEC-20 23:36:00 to 11-DEC-20 20:43:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

At the Glendvie airport ASOS site (KGDV), visibility dropped to a quarter mile or less beginning in the late night of the 10th, becoming more intermittent through the later morning and afternoon hours of the 11th, then ending in the evening of the 11th.

Dense Fog in CENTRAL AND SE PHILLIPS, MONTANA
From 10-DEC-20 05:00:00 to 11-DEC-20 04:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

The Saco US-2 DOT webcam showed visibility of a quarter mile or less from the early morning of the 10th to the early morning of the 11th.  Times are estimated.

Dense Fog in NORTHERN VALLEY, MONTANA
From 10-DEC-20 07:00:00 to 10-DEC-20 20:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter 12SSE Opheim reported visibility of a quarter mile or less beginning around sunrise on the 10th, becoming more intermittent through the day, then ending in the evening of the 10th.

Dense Fog in WESTERN ROOSEVELT, MONTANA
From 10-DEC-20 17:15:00 to 11-DEC-20 21:45:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

At the Poplar airport mesonet site (KPO1), visibility dropped down to a quarter mile or less beginning in the late afternoon of the 10th, then becoming more intermittent through the early morning of the 11th, then ending later in the morning of the 11th.

Dense Fog in MCCONE, MONTANA
From 11-DEC-20 08:25:00 to 11-DEC-20 13:55:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

At the Circle Town County airport mesonet site (K4U6), visibility dropped down to less than a quarter mile from the mid morning to the early afternoon on the 11th.

Dense Fog in SHERIDAN, MONTANA
From 10-DEC-20 21:25:00 to 11-DEC-20 09:10:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

At the Plentywood airport mesonet site (KPWD), visibility dropped to a quarter mile or less for nearly 12 hours from the night of the 10th through the morning of the 11th.

Dense Fog in GARFIELD, MONTANA
From 10-DEC-20 05:00:00 to 10-DEC-20 15:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

The Lufborough Hill MT-200 DOT webcam showed visibility of a quarter mile or less from the early morning to the afternoon of the 10th.