textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Increasing winds continue today, especially across northwest Montana and the higher terrain.

- Scattered rain showers tonight into early Wednesday then slight warming trend into the remainder of the week.

- High confidence in warmer and drier conditions this weekend through much of next week (and maybe beyond).

Surface analysis as of early this afternoon indicates a broad area of low pressure over the Gulf of Alaska and an area of high pressure off the coast of California. Satellite imagery shows these two features forcing a plume of moisture onshore into Washington and southern British Columbia. The plume will reach the northern half of the Northern Rockies this evening and overnight tonight. While moisture will be available, forcing for ascent will be scarce as the upper level jet remains north of the border. Precipitation tonight through midday Wednesday will remain predominantly terrain-forced, though isolated light showers may spill into adjacent valleys. Valley rainfall totals will remain minimal (a few hundredths of an inch at most), while mountain locations across northwest Montana can expect 0.10 to 0.30 inches.

Upper level flow on Wednesday becomes west-northwesterly. The upper level jet will sag south of the border, bringing increasing west winds to the higher terrain of western Montana including the Idaho-Montana border.

High confidence (80-90% ensemble cluster support) exists for strong upper-level ridging to build over the Northern Rockies this weekend and persist into next week. While 850mb temperatures support daytime highs 10-20 degrees above seasonal averages, valley temperatures may be slightly tempered by autumn valley inversions and limited boundary-layer mixing. Conversely, overnight valley lows are expected to trend cooler than blended guidance such as the NBM suggests, as dry air, light winds, and clear skies facilitate efficient nocturnal radiational cooling. It's interesting to note that many ensembles point toward warmer and drier than normal conditions continuing through the entire first half of October.

AVIATION

Increasing clouds and wind will persist this afternoon ahead of an incoming moisture plume. West or southwest winds gusting 15-25 knots will be common this afternoon and evening. The incoming moisture will bring chances for precipitation to KGPI late this afternoon through Wednesday morning. Precipitation will be focused over northwest Montana, so aviators can expect precipitation to obscure terrain over the mountain ranges of northwest Montana and the Clearwater Range in Idaho this evening through about midday on Wednesday.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.