The Cold Smoke Report

After last weekend's storm, we're quickly thawing out and it looks like beautiful weather for this weekend. Hope you all are able to get out and enjoy it! The topic of today's post is something I've seen floating around the interwebs and a common question around town this time of year: When should I do my winter tire swap? I'm going to try to answer this with a bit of science and the latest weather forecasts.

We technically already had our first snowfall, with some road accumulation, in the last storm. However, road conditions only seemed dicey for maybe ~12-18 hours during the worst of the snow late Sun - early Mon. For those that held out and haven't swapped yet, you will be rewarded :). More on that in a bit. I should also add a couple of caveats here that may affect decision making. If you have to commute over a higher-elevation pass like Bozeman Pass, for instance, that probably bumps up these timelines a bit. Also, a lot likely depends on the traction quality of your current tire set. If you already have decent All-season tires, that allows for more risk tolerance. Okay, let's get into the #s!

For the Oct 10–12 storm you can see that we were right at the typical date for the first snowfall, but about 10 days ahead of schedule for the first 1-inch snowfall. Week-average highs dropping below 45ºF is used here as a rough threshold for when snow starts sticking to roads more easily, since colder road surfaces collect snow more effectively. Typically early November is when those temperatures are consistently colder.

Another consideration is the number of "warm days" left (from both a climatological and forecast standpoint). Basically, with the warmer weather that's expected over the next 5–7 days, we're still expecting another 10+ days with highs over 60ºF for the remainder of this year. More than half of these are this next week. With this information, if you're able to hold out, waiting at least another week, maybe 2 would prevent you from driving on warmer roads (and potentially wearing down your tires a bit faster). If you're not driving a lot, it's probably no biggie, but if you are, it's worth factoring in, in addition to increased fuel costs if you swap to winter tires too early. Overall, my recommendation would be the last week of October or first week of November for those that mostly do in town driving. For those with more mountain driving (or current tires aren't great), I'd recommend changing them out here sometime in the next week.

Finally, looking at the current forecast, it's likely we'll be above average in terms of temperatures for the next ~week before we may see our next storm system late next week. The next chance for colder weather and any potential snow looks to be Thursday night through Saturday (Oct 22–24). There’s a lot of uncertainty with the models right now: ECMWF and its ensembles are more aggressive, showing a 72% chance of >0.25” of water (44% chance of at least .25” falling as snow, which would be 2-3” in the Bridgers). The GFS ensembles, on the other hand, only show a 32% (16%) probability of decent precipitation. For those that aren't familiar with ensembles, I'll leave the details for a future post but they're essentially a model that's run with several slightly different combinations of either the initial state or model parameters. This is done because we know there are deficiencies in both the model and characterization of the initial atmosphere (a model's starting point). This allows us to get a more realistic spread of potential outcomes.

We’ll keep an eye on things heading into early next week as the forecast evolves. Hope everyone has a great weekend!

-Trey

What a beautiful morning looking up at the ridge! Overall, snowfall totals in the Bridgers hit on the upper end of forecasted ranges with 10–14" at mid-upper elevations and 1.5–2" of liquid water equivalent. Totals at Big Sky were a bit lower (4–7"), highlighting the very tight gradients of precipitation that made this forecast particularly tricky. The forecast in town was definitely a bust to the upside with 4–8" of snow. What happened? In my previous post you can see the stalled frontal boundary draped across the valley on Saturday. It pushed just far enough south to allow colder air to filter in about 18 hours earlier than was forecasted. Furthermore, the snow stuck better to the warm ground than expected. Hopefully you escaped any tree damage, since most of the in-town foliage has yet to fall.

If you want to try to get a couple of turns in, I think low-angle grassy slopes may work (tips up and rock skis only!) in the Bridgers with the high water content. You'll need to get after it today, though. Looking ahead, temperatures will be a bit below average for the next couple of days before we rapidly warm up into late week. Mid-60s by the weekend means most of the low-mid elevation snow won't last. At the upper elevations (especially north-facing) we're going to keep a wary eye out for faceting and the potential for a weak layer near the ground if we don't quickly either fully melt (hard to do this time of year but not impossible) or continue to get additional snowfall. Keep an eye on https://www.mtavalanche.com/ for updated backcountry forecasts. Thanks for reading!

-Trey

Hope you all were able to get outside the past few days with the great fall weather we've had! I was able to get in one last bike ride this afternoon (huge shoutout to SWMMBA, Crosscut, and Bridger for the awesome new trails up there!).

Hopefully this will all be covered in snow the next time we see it :)

Radar, temperatures and winds from this afternoon (the front's position is from 3 PM):

Currently a stalled frontal boundary is draped across SW Montana as two storm systems converge over the region: One from the west/northwest and the other, associated with Tropical Storm Rachel to the south. Precipitation should transition to snow at upper elevation sites overnight Saturday, and overall the forecast still looks to be on track, 6–12"+ above mid-mountain. The heaviest snow looks to fall Sunday afternoon through Monday morning. In Bozeman, expect 1–3"+ by Monday morning, with the heaviest snow Sunday PM into the overnight. There is still some uncertainty and a fairly strong gradient of moisture and precipitation, so there could be some big winners and losers. Do your snow dances!

The HRRR model's forecast of Rachel's moisture plume through Monday midday (star = Bridger Bowl):