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.

ISSUED 12/17/2024 9:20 P.M. MST/8:20 P.M. PST

SHORT TERM (TUESDAY NIGHT THROUGH THURSDAY):

As was mentioned in the discussion from Sunday night a more robust surface high has moved into Alberta, Canada and resulted in a steady flow of low level colder and drier northeast winds across the entire region. Although winds are relatively light at the surface the source region for the airmass up to around 10,000 feet ASL is colder and this is resulting in steady snow falling across the entire North Idaho Panhandle and Northwest Montana. Snow advisories have been issued by both NWS Spokane and NWS Missoula to account for the anticipated snowfall accumulations. Unlike the last event which featured very underwhelming snowfall totals, this system is overperforming to the time of this writing with this forecasters location closing in on 4 inches of new snow since Tuesday afternoon. Not a lot but much better than what was forecasted. Again experience shows that this type of set up is common with a stronger surface high feeding in low level cold air. The atmosphere will continue to moderate overnight but the warming aloft will be much quicker than at the surface however the atmosphere looks to become isothermal overnight until around dawn on Wednesday when the approach of the “cold front” and leading edge of the incoming upper level trough plows into the region resulting in better mixing and warmer temperatures which is why the term “cold front” is in quotes. It will actually be a warmer day on Wednesday than what we saw on Tuesday due to better mixing. A deepening surface low will develop in the lee of the Continental Divide which will replace the surface high in a similar location and this will reverse the pressure gradient allowing for breezy southwest winds to develop. Overnight anticipate snowfall totals to range between 2 to 4 inches with local amounts up to 6 inches in favored locations below 3,000 feet. Above 3,000 feet anticipated 3 to 6 inches with local amounts up to 8 inches. By sunrise on Wednesday snow levels will rapidly climb up to near 5,000 feet or even perhaps a bit higher before they crash back down to 3,000 feet during the late afternoon on Wednesday but by then all lift, instability and surface convergence will be gone leading to a dry and colder night Wednesday night once winds decrease. Thursday could actually end up being a nice early winter day if valley stratus and fog doesn’t settle in with the potential there for some lengthy sunny spells but this remains to be seen.

LONG TERM (THURSDAY NIGHT THROUGH TUESDAY):

Transient upper level ridging will migrate across the region through Saturday before migrating east and allowing influence from an offshore upper level low pressure area to direct shortwaves into our region with, unfortunately for the snow lovers such as this forecaster, mild weather systems with higher snow levels. This means largely rain for elevations below about 4000 feet and snow above 4000 feet. With Tuesday being Christmas Eve and many hoping for a white Christmas unless the snow you receive overnight tonight doesn’t melt then you most likely will not be having a white Christmas this year at least in elevations below 4000 feet. The low and mid elevation snowpack is running well below seasonal average but the high elevation snowpack is faring much better and generally around 110-120% of average for this time of year. With frozen ground and a fair amount of low and mid elevation rain expected from Saturday through the rest of this period will need to watch for runoff issues along with rock slides in steeper terrain. The MJO remains active and continues to very slowly progress through Phase Space 6 and is anticipated to cross into Phase Space 7 around the first of the year. Pattern recognition and history show that once the MJO passes into Phase Space 7 this corresponds very well to a colder and wetter(snowier) pattern for the Northern Rockies. On the downside most models do show the MJO weakening as it crosses into Phase 7 but then show the potential of a resurgence in Phase 8 if it makes it that far. Phase 8 is also a wetter pattern for our region at this time of year. Time will tell.

ISSUED 12/15/2024 8:20 P.M. MST/7:20 P.M. PST

SHORT TERM (SUNDAY NIGHT THROUGH TUESDAY):

Shortwave upper level ridging continues to migrate across the region while a surface high slides down to the east of the Continental Divide which will cause surface pressure gradients to switch to the northeast overnight and likely clear out some of the fog and low cloud cover specifically across the Clark Fork River Valley and portions of North Idaho’s Bonner County. The areas that manage to see some clearer skies develop and winds remaining light will likely see temperatures drop down into the lower to mid 20s while areas that remain socked in will likely not see temperatures fall much from where they are as of this writing on Sunday evening. Next system will move inland with decent lift and some jet dynamics which will result in a widespread light to at times moderate snow event across the entire region during the day on Monday and lasting into Monday evening. The lifting process will be driven by warm air advection but with a lingering northeasterly surface pressure gradient from the surface high east of the divide filtering in cooler air the more likely situation is for the atmosphere to become increasingly isothermal with not much temperature difference between the valleys and mountain tops which will ensure snow as the precipitation type through this event. Overall amounts look to range in the 1 to 4 inch range below 4000 feet although could see some local amounts up to 5 inches in a few lucky locations. Above 4000 feet mainly 4 to 6 inches with local amounts up to 8 inches will fall. These amounts do qualify for very low end Snow Advisory Criteria and hence these have been issued for our entire region of North Idaho and Northwest Montana. Precipitation will wind down Monday night into Tuesday as another transitory upper level ridge moves over ahead of the next approaching system. In addition there are some signs that a stronger surface high will be sliding southeast through Alberta, Canada during the day on Tuesday following some colder air spilling southward into Central and Eastern Montana which would set up a bit stronger of a northeast wind across the region by later in the day on Tuesday. This is important as if this does come to pass as the new HI-RES NAM model is showing then precipitation by Tuesday night is likely to start as snow across the entire region. Read on for more information.

LONG TERM (TUESDAY NIGHT THROUGH SUNDAY):

Next system entering into the region on Tuesday night into Wednesday looks to be a bit stronger with better lift and a stronger surface low and also stronger warm air advection associated with it which will result in rising snow levels faster. The challenge with this set-up is that as mentioned above there are signs that a decent surface high will slide southward near the Montana/Alberta border which would cause winds to blow from a colder northeast direction thus delaying the changeover from snow to rain below 4000 to 5000 feet. This could result in some heavier snowfall totals regionwide if temperatures remain cold enough through the column. Experience shows that what often happens is the atmosphere becomes isothermal in these situations which has resulted in heavier snowfall totals. Not saying that will be the case this time but bears watching. Contrary to what some in the media and in some weather circles this next system IS NOT a so called “atmospheric river” as some want to call it, rather this is simply another warmer type system moving in from the southwest with good lift and jet dynamics to help generate the potential for heavier precipitation. There is no endless stream of moisture or “river” flowing into the region rather all precipitation will be tied to where the best lift and convergence sets up across the region. Upper level ridging will build across the entire Northern Rockies by late in the day on Wednesday and will last into Saturday which will result in a more settled period of weather and likely valley fog and low clouds developing. By the end of the weekend into the following week there are some signals that show the pattern becoming potentially much more active with a more consolidated jet stream across the Northeast Pacific with height rises beginning to take place further west in the Central Pacific and corresponding height falls along the West Coast and Western US. This would result in stronger, wetter and hopefully colder systems having a more direct impact into our region. From an historical perspective this would line up fairly consistent with past history and weather patterns across the North Pacific and Western US as the time period between Christmas and New Years in most years typically features very active weather and often many travel headaches. Time will tell if this happens again this year but will be watching these developments closely as well as seeing what the MJO is up to once the new report and analysis is released on Monday.