textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Cold temperatures through Sunday, with below zero wind chills along the divide overnight tonight.

- Light snow along the Divide and southwest Montana through Sunday morning.

- Dry, stagnant pattern Monday into Wednesday.

Temperatures in the Northern Rockies will remain a little below normal tonight and on Sunday as a large area of arctic air east of the Continental Divide moves east. A weak shortwave will move north to south along the Divide today, bringing snow along and just west of the Divide in northwest Montana. Webcams this afternoon show it snowing in West Glacier. Chances remain good (about 60%) for snow to continue into Sunday morning. Expect accumulations on the light side though, as ensemble solutions show about a 20% chance of one inch of snow by Sunday morning along US Highway 2 from Essex east through Marias Pass. Other roadways with snow in the forecast overnight include Homestake, Rogers, and MacDonald passes, I-90 Drummond to Butte, and MT Highway 200 in the Blackfoot valley.

A high amplitude ridge of high pressure will build over the West Coast on Sunday. The ridge moves inland, becoming centered over the Northern Rockies on Tuesday evening. This feature will bring dry, stable conditions that residents of the area have become accustomed to this January. There may be enough pressure gradient over the Northern Rockies to keep valley fog/low stratus at bay until Wednesday morning as a chunk of the upper level jet remains overhead. Agreement in model ensemble solutions begins to diverge later in the week, with some holding onto a strong ridge, while others depict a transition to westerly flow aloft. The latter solution would bring some weak shortwaves and enhance chances for light precipitation, while the former would result in ... well... you know... warm, dry, and stable with valley fog and low stratus.

AVIATION

GOES Satellite imagery this afternoon shows widespread stratus clouds entrenched across north-central Idaho and western Montana. Light snow has been observed along the divide as pulses of moisture continue to move southward from Canada. A shortwave trough will move southward this evening, with light snow becoming widespread along the divide and in southwest Montana. Medium- high confidence exists for snow to impact KBTM overnight into Sunday morning. The timing of snow starting at KBTM remains the main challenge, with the current TAF taking a middle of the road approach with 4SM light snow starting by 25/0200Z. Visibility reductions can also not be ruled out at KMSO, with a 20% chance of 5SM or lower visibility from 25/0600-1200Z. Light snow will generally begin to exit Southwest Montana after 25/1200Z Sunday as this system moves east of the divide.

MSO WATCHES/WARNINGS/ADVISORIES

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