textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Widespread rain continues today, followed by much cooler temperatures and a return to winter driving conditions over mountain passes by Thursday.

- A potentially impactful band of heavy, wet snow could disrupt the Thursday and Friday morning commutes across western Montana valleys.

- Cold overnight lows on Friday and Saturday mornings could impact sensitive vegetation.

Satellite imagery and radar returns this afternoon indicate widespread rainfall moving south to north over the Northern Rockies, with the main low pressure circulation over southwest Montana and Lemhi County. The leading edge of the precipitation is providing the most intense rainfall rates. Some measuring sites in Clearwater County and west-central Montana already picked up about one-half inch of rain as of noon today. This system is also spawning isolated weak thunderstorms, which will continue throughout this afternoon and evening. Gusty west or southwest winds along a cold front associated with this system will also persist today.

Widespread precipitation will continue into Friday as the Montana low moves east and another disturbance moves over the Northern Rockies from the north. Colder air associated with this system will bring snow levels down to mountain passes and some valleys. Expect wet, heavy snow over passes and higher valleys for Thursday morning. Motorists should expect winter driving conditions over Homestake, MacDonald, Lolo, Marias, and Lookout passes.

Although confidence in mountain and mountain pass snow is high, there is still some uncertainty involving winter impacts in the valleys of western Montana. High resolution models are indicating a band of snow showers dropping north to south on Thursday morning. If this band develops over the valleys, heavy wet snow will cause low visibility, slushy roads, snapped tree branches, and isolated power outages. A second band of snow may develop over west-central Montana Thursday night, impacting the Friday morning commute. Colder air from the northeast will help to enhance snow production on Friday morning. Regardless of precipitation amount and type, sub-freezing temperatures will be here for Friday and Saturday mornings, 20s in many locations (including Grangeville and Pierce Idaho). Some valleys (Butte, Stevensville, Seeley Lake, Ovando, Philipsburg,etc.) will get to the teens on Saturday morning. Cool, unsettled weather will stick around through the weekend and into next week while temperatures moderate back towards seasonable values.

AVIATION

An upper level trough will track over south-central Montana by 0000Z(6 PM MDT), then into eastern Montana by 0600Z(12 AM Thursday). Wrap-around rain and snow will persist through Thursday morning by 1200Z(6 AM MDT). General IFR flying is expected with mountain obscuration from fog and low clouds. Breezy westerly winds are more likely at KSMN and KBTM at times through this evening. A cold front moving south out of Canada will bring snow to valley floors across northwest Montana between 1000Z(4 AM MDT) and 1700Z(11 AM MDT) Thursday. Brief periods of LIFR flying is possible under the stronger snow showers, possibly impacting KGPI. By Thursday afternoon, snow or graupel showers above 3000 feet along with lower visibility and ceiling are possible across western Montana and higher terrain of north-central Idaho. Additional snow showers are possible Thursday night which could impact all the terminals.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from 6 PM this evening to noon MDT Friday for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...Potomac/Seeley Lake Region.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT Thursday for West Glacier Region.

ID...Winter Weather Advisory from 6 PM MDT /5 PM PDT/ this evening to noon MDT /11 AM PDT/ Friday for Eastern Lemhi County... Southern Clearwater Mountains...Western Lemhi County.