textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Strong valley inversions and fog/stratus linger into Thursday, especially in north central Idaho.

- Coldest air of the winter thus far begins to filter into the region Thursday through Friday. Cold wind chills around 10 below zero in northwest Montana and along the Continental Divide Friday morning.

Fog and low clouds remain trapped in the valleys of northwest Montana and north central Idaho today. Overall we anticipate this cloud cover will begin to lift overnight tonight into Thursday morning as a colder airmass from Canada begins to spill over the Continental Divide. North central Idaho will be the last to see clearing, most likely not until later in the day Thursday.

Breezy northeast to east winds will develop Thursday with the beginning of the arctic spillover, though not overly strong with exposed areas generally seeing winds between 20 to 30 mph through Thursday. Due to the airmass dropping out of Continental areas in Canada, there is little moisture for this front to work with, thus conditions will be mainly dry.

Much colder air, the coldest we have seen so far this winter, will filter into western Montana and north central Idaho Friday into Saturday. While not record-breaking cold by January standards, the mild winter we have had of late will make this feel like a significant change. The coldest air is anticipated across northwest Montana and along the Continental Divide where single digits to even below zero temperatures will be observed. Falling temperatures will overlap with wind late Thursday night into Friday morning to produce wind chills as low as -5 to -15 degrees below zero. Winds will relax during the day Friday, with many valleys likely seeing the coldest temperatures Saturday morning, depending on cloud cover.

Temperatures will slowly moderate Sunday into early next week. Forecast models continue to show a weak disturbance overrunning the colder air in place in the Monday/Tuesday timeframe. Seventy- five percent of models show this as a very light precipitation event with minimal accumulations. However, around 25 percent show somewhat higher amounts (2 to 4 inches snowfall), which could lead to some minor impacts due to accumulating snow, especially over mountain passes along the Idaho/western Montana border. Eighty percent of forecast models show the ridge of high pressure building back over the region by Wednesday next week, allowing strong inversions to redevelop with the threat of fog and/or low stratus.

AVIATION

Low stratus and pockets of fog remain locked in across northwest Montana and the lower valleys of north central Idaho. Obscured terrain will be the main concern with the lower cloud deck. As an arctic airmass drops south out of Canada, it will begin to increase a northeasterly pressure gradient, first across northwest Montana after 22/1200z and then into the rest of western Montana between 22/1500z to 22/1800z. Wind gusting up to 25kts in exposed locations will be possible, including at KGPI. This increased wind should help raise the cloud deck, other than perhaps through north central Idaho, where the easterly gradient won't really materialize until after 23/0000z. This air mass is relatively dry and thus widespread precipitation is not anticipated.

MSO WATCHES/WARNINGS/ADVISORIES

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