Issued Wednesday November 20, 2024 7:50 P.M. PST

* WINTER WEATHER ADVISORY THROUGH 10:00 A.M. PST THURSDAY *

Tonight…Snow. Accumulations of 3 to 6 inches possible below 4000 feet with 4 to 8 inches possible above 4000 feet. Lows in the upper 20s to lower 30s with mid 20s around 6000 feet. Light wind except local northeast wind 5 to 15 mph through the Purcell Trench from Samuels to the Kootenai County line and across ridge tops.

Thursday…Rain and snow likely. Snow level rising to near 3000 to 3500 feet in the afternoon. Snow accumulations of 1 to 2 inches possible below 4000 feet and above 3000 feet with 2 to 4 inches possible above 4000 feet. Highs in the 30s with upper 20s around 6000 feet. Local northeast winds 5 to 15 mph through the Purcell Trench and across ridge top winds in the morning becoming light.

Thursday night…Rain and snow showers likely. Snow level near 3500 feet. Snow accumulations of 1 to 2 inches possible above 3500 feet. Lows in the 30s with upper 20s around 6000 feet. Light wind except northeast winds 10 to 15 mph developing overnight through the Purcell Trench from Samuels to the Kootenai County line and across ridge tops. Gusts to 20 mph across ridge tops.

Friday…Rain and snow. Snow level rising to near 4500 to 5000 feet in the afternoon. Snow accumulations of 6 to 12 inches possible above 5000 feet. Highs in the 30s to near 40 with near 30 around 6000 feet. Local northeast winds 10 to 20 mph with gusts to 30 mph across ridge tops.

EXTENDED OUTLOOK (SATURDAY THROUGH WEDNESDAY):

Saturday…A chance of rain and snow showers in the valleys and snow showers in the mountains. Lows in the 30s with mid 20s around 6000 feet. Highs in the upper 30s to lower 40s with upper 20s around 6000 feet.

Sunday…A chance of snow showers. Lows in the 20s except lower 30s along the immediate shoreline of Pend Oreille Lake with near 20 around 6000 feet. Highs in the 30s with lower 20s around 6000 feet.

Monday through Wednesday…A chance of snow showers. Lows in the mid teens to mid 20s except locally near 30 along the immediate shoreline of Pend Oreille Lake with mid teens around 6000 feet. Highs in the 30s with lower 20s around 6000 feet.

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

ISSUED 11/20/2024 9:20 P.M. MST/8:20 P.M. PST

SHORT TERM (WEDNESDAY NIGHT THROUGH FRIDAY):

First decent widespread snowfall of the season in currently ongoing across much of the region with the exception as of this writing in southern Sanders County in Northwest Montana however anticipate this area to begin to see a return to snow as well later. A push of cool, dense Canadian air east of the Continental Divide has allowed a stronger surface high to produce a tighter northeasterly surface pressure gradient which helps to hold surface dewpoint temperatures down and thus help develop an isothermal layer through the atmosphere and allow snow to fall through the column. The rain/snow line is rather pronounced around the Kootenai/Bonner County line in North Idaho and this is typically where the line often resides during the colder months. Snow will continue to fall overnight across the entire region before warmer air associated with another round of warm air advection finally begins to lift snow levels off valley floors sometime during the day on Thursday for all of North Idaho and Sanders County in Northwest Montana. Lincoln County in Northwest Montana could be a different story and would not be surprised if places such as Libby, Happys Inn, Eureka, Trego and Rexford remain as snow through the day on Thursday as the isothermal layer in this region is often tough to scrub out. Do not anticipate this region to see a changeover to rain until perhaps Friday when our next shortwave rotates northward and finally raises snow levels up to between 4000-5000 feet. The primary driver of the weather is not the intense surface low that has been talked about currently weakening and spinning off the Washington coast as this feature is simply a result of what was super typhoon Man-Yi that hit the Philippines about 5 days ago and merged with a cold and dynamic upper level low with a lot of jet energy spinning in the Gulf of Alaska. The parent upper level low still spinning over the Gulf of Alaska is the primary driver of the weather with a center near 522 decameters which is plenty cold. Combine that cold air aloft with an abundance of latent heat generated by a former super typhoon and the result is a rapidly intensifying jet that amplifies and spins up a very strong surface low offshore. With the Gulf of Alaska trough continuing to get reinforced it is also beginning to amplify which will force downstream amplification of the ridge centered over the Rockies. Our region will be in the favorable region of the strong jet stream being spun up between the upper level ridge and upper level trough with strong vertical motion through the atmosphere helping to generate plenty of precipitation. As for total snowfall amounts as of this writing here are the updated snowfall totals I anticipate before the snow changes over to rain (except of course in the mountains where it will remain all snow above 4500 to 5000 feet):

North Idaho valleys: 5-10 inches

North Idaho mountains(above 4000 feet): 18-30 inches

Northwest Montana valleys: 3-12 inches

Northwest Montana mountains(above 4000 feet): 18-30 inches

In addition to the snow, the shortwave on Friday has the potential to bring widespread rainfall of 0.50 to 1.50 inches across North Idaho below the snow line. For Northwest Montana rainfall amounts of 0.25 to 1.50 inches will be possible below the snow line. The lower end amounts for North Idaho will be found near Porthill to Bonners Ferry due to the Selkirk rain shadow and around Eureka and Rexford in the Tobacco Valley in Northwest Montana.

LONG TERM (FRIDAY NIGHT THROUGH WEDNESDAY):

The entire Gulf of Alaska upper level trough will begin to shift inland over the weekend which will also shift the best jet dynamics east and south of our region and allow a colder, unstable airmass to move in. The result will be more hit and miss snow showers, some peeks of sunshine and colder temperatures with snow levels falling back down to all valley floors by late in the day on Saturday. No significant accumulations are anticipated at this time across the region but this could change. The holiday week currently looks to start off on a quiet note with just some passing instability snow showers but no significant storms at this time. Am watching however a large amount of cold air that is currently expanding across much of Alaska and the Yukon and Northwest Territories in Canada. Models have been rather consistent towards the Thanksgiving holiday and the following weekend in showing a large chunk of this cold air eventually diving southward into British Columbia and into the Northeast Pacific as an amplifying upper level ridge tries to build in the Gulf of Alaska. This will need to be watched very carefully because if the cold air and the associated upper level trough digs further west across British Columbia and the northeast Pacific this could set up the POTENTIAL of a significant, if not major snowstorm across our region as this is the classic set up for very heavy snowfalls across our region as the Pacific becomes a source of strong vertical motion and cape to spin up storm systems despite a lack of so called “moisture”. This will bear watching especially with the upcoming holiday. A reminder that Schweitzer will open this Friday and remain open through Sunday with outstanding opening conditions not seen in several years so get up there and enjoy all the fresh snow!