Issued Sunday December 22, 2024 8:10 P.M. MST
Tonight…Mostly cloudy with rain and snow becoming likely overnight west of Lake Koocanusa and the Fisher River and from Thompson Falls north. Scattered rain and snow showers south and east of Thompson Falls and from Lake Koocanusa and the Fisher River east. Snow level near 3500 to 4000 feet. Snow accumulations of 1 to 3 inches possible above 4000 feet. Mild with lows in the upper 20s to mid 30s with lower 30s around 5000 feet. Light wind.
Monday…Mostly cloudy. Rain and snow showers likely early in the morning along and west of the Cabinet Mountains and from Thompson Falls north with scattered showers elsewhere. Snow level near 4000 feet. Local snow accumulations of up to 1 inch possible in the morning above 4000 feet. Highs in the upper 30s to mid 40s with mid 30s around 5000 feet. Light wind.
Monday night…Mostly cloudy and mild. Areas of fog developing. Isolated rain and snow showers developing overnight along and west of the Cabinet Mountains, mainly along the Idaho border north of Trout Creek. Snow level near 4000 feet. Lows in the mid 20s to mid 30s with upper 20s around 5000 feet. Light wind.
Tuesday…Mostly cloudy with scattered showers. Snow level near 4000 feet in the morning lowering to near 3000 feet by late in the day. Local snow accumulations of 1 to 2 inches possible above 4000 feet. Highs in the mid 30s to lower 40s with lower 30s around 5000 feet. Afternoon southwest wind 5 to 15 mph.
EXTENDED OUTLOOK (WEDNESDAY THROUGH SUNDAY):
Christmas Day through Sunday…Continued unseasonably mild with a chance of valley rain or snow showers and mountain snow showers. Lows in the mid 20s to lower 30s with upper 20s around 5000 feet. Highs in the mid 30s to lower 40s with lower 30s around 5000 feet.
In-Depth Weather Discussion for Northwest Montana/North Idaho/Northern Rockies
ISSUED 12/22/2024 8:55 P.M. MST/7:55 P.M. PST
SHORT TERM (SUNDAY NIGHT THROUGH TUESDAY):
Sorry to sound like a broken record here but the overall pattern that has largely been in place since the end of October still remains in place across the region with relatively mild conditions, an abundance of low and mid elevation rainfall and high elevation snowfall. The mean ridge position has remained largely unchanged and fixed across much of the intermountain west with positive height anomalies remaining centered from just off the West Coast extending eastwards into the Midwest with mean negative height anomalies centered over the Aleutian Islands and over far Eastern Canada and Greenland. This results in a mild and unsettled pattern across our region but no real big storm systems as the surface lows a forced to weaken as they travel towards our region and move away from the upper level cold air anchored well out over the Pacific that helps to spin up the surface lows and associated jet stream. With the position of the associated ridges and troughs as they are the incoming storm systems are entirely warm air advection driven which simply means that the lifting process to generate vertical motion through the atmosphere and precipitation is being caused by warm air moving into the region. For those of you who want to see snow at this time of the year as does this forecaster you need predominately cold air advection to be the main driving force to deliver low elevation snowfall. The end result of this relatively mild winter is little to no ice on area lakes for ice skating or hockey and an unbearably long (and dirty) mud season with very large potholes on our gravel and dirt roads. Some subtle changes will slowly start to take place this week as the overall flow slowly begins to flatten somewhat with very slow, and I do mean very, almost painfully slow lowering of atmospheric heights and thickness values which will correspond with colder temperatures through the atmosphere and lowering snow levels. It is possible that by Christmas Day itself the atmospheric column will finally be cold enough to support snowflakes down to valley floors although they may be very wet snowflakes and the overall chances of a White Christmas below about 3000 feet are extremely low if you do not currently have snow on the ground at your location. The endless stream of weak shortwaves moving in and providing cloudy skies and rounds of mainly light valley rain and mountain snow will continue with perhaps a slightly stronger shortwave approaching late on Christmas Eve. This feature looks to dig a bit further south with the help of some jet stream energy diving south on the backside of it to help force the upper level ridge that remains in place across the Western US further east with another ridge trying to amplify some well to the west over the Central Pacific. For our region to see better storm trajectories into our region we want to see the mean ridge position to be centered out around 150 to 160 West longitude.
LONG TERM (TUESDAY NIGHT THROUGH SUNDAY):
Weak and transient upper level troughing looking to pass overhead during the day on Christmas Day with a few snow showers or possibly rain and snow mixed across valley locations though any accumulations are likely to be very light and restricted to elevations above 3000 to 3500 feet. From here on out the flow will slowly deamplify with upper level ridging remaining offshore with fairly weak and somewhat splitty westerly flow continuing to allow weak shortwaves to move into the region. Each of these will largely bring mainly low elevation rain and higher elevation snow. Should slowly begin to see snow begin to accumulate in the mid slopes and perhaps towards next weekend or the start of the next week could see those snow levels finally lower down to valley floors as upper level ridging slowly begins to build out around 150 to 160 West in response to deeper upper level troughing off the coast of Japan which allows deeper troughing to become anchored across the Western US with the corresponding ridging out in the Central Pacific. There is slowly increasing confidence in an overall large scale pattern change around the start of 2025 especially if the MJO moves into Phase Space 7 or 8 but will wait for the update coming out on Monday. In the meantime let’s hope that the pattern changes as we really don’t have a long season where snowfall can accumulate to any significant depths. With the days now starting to get longer already the window for accumulating snowfall begins to slowly decrease at lower elevations although that really doesn’t start to really appear until mid to late February. There does appear to be a slow transition to more troughing and colder air beginning to slide southward into Western Canada if stronger positive height and thickness anomalies begin to develop out across the Central Pacific. The jet stream would then be forced to take a more northwest to southeast trajectory into our region which greatly increases the likelihood of storm systems with cold air advection being the main lifting mechanisms and drivers of precipitation to help build our snowpack across all elevations. Whether we like it or not, an abundance of low, mid and high elevation snowpack across our region greatly benefits the environment that makes up the Northern Rockies despite the inconveniences it may cause.