Heavy Snow — Jefferson, Montana

2020-10-16 to 2020-10-17 · Jefferson, Montana

Event narrative

Measured 7 inches new snow since midnight.

Wider weather episode

A deepening surface low positioned over southern Alberta on October 16th generated strong southwest winds winds across the region during the day on Friday. The strongest winds were along the Rocky Mountain Front. As of 3 PM, short-term model guidance was supporting the expectation that winds would spread onto the adjacent plains and across North-central Montana late in the afternoon into the evening, once the surface low began to pull away from the Rocky Mountain Front. Thus, high wind warnings remained in effect at least through early evening.

Areas of light rain and mountain snow were ongoing Friday afternoon. These were associated with weak warm advection aloft, mainly for areas mainly north and east of

a line from Great Falls to Cut Bank. This activity appeared likely to continue through the evening, along with continuing snow over the Northern Rockies.

As the surface low moved away to the east, the cold front would be drawn southward into North-central Montana. This was expected to occur by around 9pm for the HiLine, midnight for central MT, and early Saturday morning for south-central Montana. As the front moved by, light rain developed, which then changed over to snow. Initially wet roads were expected to freeze by morning, especially the bridges and overpasses. Wet snow sticking and freezing to power lines and branches had the potential to cause some isolated power outages. Rain and snowfall amounts were expected to be light across the North-central plains as the shallow front slid by. Some orographic enhancement in the northwest flow aloft had the potential to help maintain precipitation into Saturday for upslope areas along the Rocky Mountain Front and the upslope areas of central Montana. The cold air behind the front was shallow, and given northwesterly flow aloft, model guidance suggested the front would stall out somewhere north of I-90. Although this was not explicitly forecast at the time time, we would need to monitor for the possibility of the development of snow bands along this frontal convergence zone early

Saturday morning. Total snowfall amounts from this first wave had trended lower in Friday's updated forecast guidance. However, the forecaster noted that regardless of snow totals, any icing that developed with this early season event would be worth noting and extra precautions should be taken while driving.

The first set of Winter Weather Advisories remained in effect until 6 pm Saturday for north-central areas and midnight Saturday night for the southwest. A second disturbance would allow very little time to pass before the next round of snow and perhaps some mixed precipitation moved in late Saturday night and into Sunday. This system was a bit warmer but it it was a tricky forecast to know how much cold air would remain in the lower levels. There was potential that snow may become mixed with rain or even freezing drizzle at times, which will ultimately determine how much snow accumulation is realized. Most of this precipitation was expected to fall Sunday morning and afternoon but there was potential for some to linger through Monday morning. This resulted in another set of Winter Weather Advisories being issued from midnight Saturday through noon on Monday, for roughly the same areas as the first set that had been issued. Confidence was lowest in the Southwest due to the fact that warmer air may infiltrate before the most significant precipitation fell, but the forecaster decided to keep them in it since a little shift in timing could produce more impactful results. As mentioned earlier, forecast confidence in snowfall totals remain low but areas along the Divide and the central mountains had the potential to receive snow totals up to 6 inches or more in some cases. High temperatures for this afternoon were to be the coldest of the season thus far, with highs struggling to reach the 30s over much of Central/North-central Montana, with warmer 30s and 40s over the southwest. The cold temperatures would continue on Sunday but were expected to be slightly warmer than Saturday. More seasonal temperatures were expected by Monday afternoon after a brief period of patchy freezing rain.

Source: NOAA Storm Events Database, event_id 925584. Narrative written by the NWS forecast office that issued the report.