MISSOULA - In a westerly flow behind a cold front, expect scattered light snow showers running into Tuesday across western Montana. Highs will top out in the mid 30s to around 40°.
Of greater concern: A long duration atmospheric river event that takes center stage Wednesday afternoon into early Saturday with significant wind and precipitation.
Right now, it looks like the strongest winds will be Wednesday evening through Thursday with High Wind Watches issued for the Kootenai/Cabinet & Butte/Blackfoot Regions. All other valley locations (regardless of wind highlight issuance) will experience gusts in the 35-50mph range. We also have Winter Storm Watches that have been issued for the Bitterroot/Sapphire Mountains (above 5,000') and the West Glacier Region from Wednesday night into early Saturday. Here, snowfall will be measured in feet when all is said and done with blizzard-like conditions at times. Take a look at possible accumulations just into early Thursday afternoon:
While a rain/snow mix is possible for most valleys Wednesday, eventually warmer air will win over the airmass (remember, atmospheric river = subtropical moisture = warm) with valley rain the likely type Thursday, Friday & Saturday. Here's a look (again, only through early Thursday afternoon) at possible liquid precipitation amounts:
To get a little nerdy, I'm posting a portion of the NWS Missoula's recent forecast discussion because I think it speaks volumes: "Confidence in this atmospheric river is high, with ensemble guidance suggesting >90% probability for Integrated Vapor Transport (IVT) values to reach 250 kg/ms along the Divide over a 36-48 hour period, centered on late Wednesday into early Saturday. Furthermore, the IVT values progged by model guidance are near the 95-99% percentile of climatology, pointing towards a significant atmospheric river event. These potentially historic (for March) IVT values are being driven by a significant jet stream aloft, with EPS ensembles pointing towards 700mb winds (winds near 10,000' MSL) of 65-75kts (75-85 mph), which would be near an all-time high for March. The combination of strong winds and high atmospheric moisture will lead to significant mountain precipitation, with widespread precipitation totals of 3-6" along the Idaho-Montana border and northwest Montana ranges. Favored crests within the Bitterroot, Clearwater & Swan Ranges may see precipitation totals approaching 6-8" over a 72-hr period ending on Saturday. Those heading into the backcountry should exercise extreme caution given the severity of this atmospheric river event and should monitor the latest avalanche forecasts".
