Issued Sunday January 19, 2025 8:05 P.M. MST

Tonight…Fair skies and cold. Patchy late night freezing fog and low clouds developing near larger lakes and rivers. Lows from near 20 below zero to 5 above zero with near 5 below zero around 5000 feet. Light wind except local northeast winds 5 to 15 mph along portions of Lake Koocanusa, through favored canyons and across ridge tops. Wind chill values of 10 to 20 below zero possible.

Monday…Patchy freezing fog and low clouds near larger lakes and rivers through mid morning otherwise mostly sunny and cool except for increasing clouds late in the day. Highs in the mid teens to mid 20s with near 10 above zero around 5000 feet Light wind.

Monday night…Considerable high cloudiness and not as cold. Lows in the mid single digits to mid teens with mid single digits around 5000 feet. Light wind.

Tuesday…Variable cloudiness early otherwise mostly sunny and warmer. Highs in the lower 20s to lower 30s with upper teens around 5000 feet. Light wind.

EXTENDED OUTLOOK (WEDNESDAY THROUGH SUNDAY):

Wednesday…Dry. Patchy late night through mid morning freezing fog and low clouds in the valleys. Lows in the single digits except locally near 5 below zero colder valleys with near 10 above zero around 5000 feet. Highs in the lower 20s to lower 30s with near 20 around 5000 feet.

Thursday and Friday…A chance of snow showers. Lows in the in the teens all elevations. Highs in the mid 20s to lower 30s with lower 20s around 5000 feet.

Saturday and Sunday…Dry with areas of freezing fog and low clouds in the valleys. Lows in the lower single digits to lower teens with mid single digits around 5000 feet. Highs in the 20s with upper teens around 5000 feet.

In-Depth Weather Discussion for Northwest Montana/North Idaho/Northern Rockies

ISSUED 01/19/2025 2:55 P.M. MST/1:55 P.M. PST

SHORT TERM (SUNDAY NIGHT THROUGH TUESDAY):

The ever present upper level ridge that has remained either over the Western US or off the coast for what seems like an eternity will remain in place throughout the short-term forecast period. It will retrograde a bit offshore, enough to allow the large and very cold Hudson Bay low to spread further to the west but will not allow any upper level disturbances rotating around this polar low to make it west of the Continental Divide so the forecast remains dry and chilly. Minimal amounts of freezing fog and low clouds will be present the next couple of days with a much drier boundary layer in place now. Will still see some localized areas of freezing fog and low clouds mainly around Priest Lake, Libby, Lake Koocanusa and the other typical fog and stratus valleys during the winter months. Whatever stratus and freezing fog does develop will scatter out fairly quickly. Elsewhere will see hard frosts and freezes which will likely require some significant scraping of windshields if a vehicle is parked outside overnight. Temperatures will drop some on Monday with all locations remaining below freezing and likely remaining there through at least Tuesday and likely beyond. Northeast winds blowing along portions of Lake Koocanusa, through portions of the Clark Fork River Valley, through the Purcell Trench and across higher terrain on Sunday will likely decrease and remain under 10 mph by nightfall which will allow for minimal impacts from wind chills.

LONG TERM (TUESDAY NIGHT THROUGH SUNDAY):

Quiet conditions dominated by the positive phase of the PNA pattern(Western US ridge, Central and Eastern US Hudson Bay low) will remain in place throughout the rest of this forecast period. The only changes of note will be the approach of a shortwave that will help deamplify the overall flow pattern across the North Pacific and North America. While the region will remain in the positive phase of the PNA pattern the Hudson Bay low will begin to flushed out and elongated further east as a stronger subtropical jet merges with the polar jet diving down the backside of it and helping to spin up a strong surface low over the Atlantic. The combined jet on the south side of the Hudson Bay low will be the feature that helps “pull” this feature further east and loose amplitude which in turn helps the Western US ridge loose amplitude aided further by previously mentioned shortwave moving across the North Pacific. This feature looks to begin pushing into the Washington Coast later Thursday into Friday and will bring our region the next best chance of snowfall. This time the event will be all snow as the primary lifting mechanism will be cold air advection instead of our more typical warm air advection ensuring that all precipitation that falls will be snow. Still too early for snowfall amounts at this time but this does not appear to be a large snowmaker for our region but a few inches especially in the typically wetter areas (Bonner County in North Idaho, Priest Lake, Bull Lake, Yaak, Heron) are likely. This system looks to dive south of our region over the weekend with a reinforcing shot of colder air moving south as the Hudson Bay low is showing signs of re-loading once again. Still a lot going on and much can change so will continue to watch. In addition will have to see what the MJO is doing as it appears to be stalled out in Phase Space 1 at this time with only very slow eastward propagation as evidenced by a strong tropical cyclone that has developed off the Northwest Coast of Australia. Stay tuned.