textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- High pressure will keep above-normal temperatures in the forecast through midweek

- Pattern change later in the week (next Friday - Saturday), bringing colder temperatures and snow

Visible satellite imagery this afternoon shows mostly clear skies across the Northern Rockies under high pressure with some notable exceptions. Strong inversions are keeping low stratus decks and, in some cases, reduced visibility due to fog across the valleys of Lincoln, Sanders, and Lake counties in Montana; and the Upper Salmon/Lemhi, Lower Salmon, and Lower Clearwater valleys in Idaho. Little pressure gradient to drive surface winds should keep these inversions in place through Sunday morning. A weak disturbance on Sunday night should help to break inversions that are still hanging on.

Mostly mundane weather persists through much of the upcoming work week as a high amplitude ridge sets up over the West Coast. The Northern Rockies will be under northerly flow aloft on the east side of the ridge. Some opportunity for moisture arrives on Tuesday afternoon for the Continental Divide under an embedded shortwave disturbance. Chances for measurable precipitation are low as the 90th percentile of ensemble distributions is only showing a few hundredths of liquid precipitation by Wednesday morning.

Ensemble solutions are trending towards a significant change starting Thursday night or Friday. Most solutions are favoring an arctic cold front breaching the Continental Divide from the east in northwest Montana on Friday. Uncertainty is stemming from the formation and position of low pressure descending south from Canada that would interact with the cold air behind the front. We'll have a clearer picture as the week unfolds. In the meantime, please stay on top of the forecast if you have travel plans for next weekend.

AVIATION

High pressure is keeping fog and low stratus over airfields at Salmon, Grangeville, and Ronan this afternoon. Visibility is still down below 1SM at Ronan in the Mission Valley early this afternoon. Fog has lifted at the other aforementioned terminals, but low stratus is keeping ceilings below about 400 ft AGL. Expect poor visibility and low ceilings to persist through Sunday morning at least. A weak disturbance moving north to south on Sunday evening should break the remaining valley inversions.

MSO WATCHES/WARNINGS/ADVISORIES

MT...ID...Air Stagnation Advisory until 4 PM PST Tuesday for Northern Clearwater Mountains...Orofino/Grangeville Region... Southern Clearwater Mountains.