An unseasonably strong early season cold front will result in quite the pattern flip over the next 24-48 hours across the Northern Rockies.

Many towns in Montana and Wyoming experienced their hottest September day on record on Saturday. Now, on Monday night and Tuesday, some towns will tie or break the record for their earliest snowfall on record, with record low temperatures also possible in the days to follow. Incredible!

There are so many conflicting impacts leading up to and behind the cold front that need to be addressed. While significant weather swings are nothing new in the Northern Rockies and High Plains, this abrupt switch from summer to winter conditions is more extreme than a typical fall cold front.

Current temperatures as of late Monday morning show the cold front moving into Southern Montana:

Rain and snow is also becoming widespread across Montana behind the front. Most of the lower elevations are experiencing rain, while snow is filling in across the mountains.

Damaging Early Season Snow for Areas Outside of the Mountains in Wyoming and Southern Montana

As the trough of low pressure digs southward into the Southern Rockies and the cold front surges south, upslope flow will increase east of the Continental Divide and a widespread accumulating snow is now expected across much of the non-mountainous areas east of the Divide in Wyoming as well as higher elevation Southern Montana towns such as Red Lodge.

As for the mountains — Areas such as Glacier National Park and the Tetons in Wyoming will see lighter snow amounts as most of the precipitation will fall along and east of the Divide, while the Central Idaho ranges should only see a dusting.

The Continental Divide in Southern Montana and Wyoming will see the brunt of the heavy snow, with double digit amounts possible in the higher elevations of the Beartooth, Madison, Absaroka, Wind River, and Bighorn Ranges south to the Snowy Range in Southern Wyoming.

Here are the latest snowfall projection maps from the short-range HRRR model through 4am Tuesday. Keep in mind the totals will increase across Southern Wyoming beyond this timeframe.

Also a note, the lower elevation snowfall projections are likely a bit too high as warm ground temperatures will lead to some of what falls to melt on contact. Still, this is an impressive and rare snow event for so early in the year.

Unfortunately, the damage to fully leafed-out trees and other vegetation could be significant across Central Wyoming.

Hard Freezes and Record Low Temperatures over the Next Few Nights

Widespread below freezing temperatures can be expected across nearly all of Montana and Wyoming at various times from Monday night through Wednesday night, with many areas experiencing hard freezes.

The high elevation interior valleys in particular will see some of the coldest temperatures, with lows dipping into the teens. I expect we’ll see more than a few record lows on Wednesday morning.

Farther west, freezing temperatures are likely across the eastern half of Southern Idaho as well, including Idaho Falls and Pocatello.

High Winds to Develop Along and Behind the Cold Front on Monday Night

Another impact from this storm will be strong northeast winds that develop behind the front — a result of the significant pressure gradient and temperature differences ahead of and behind the cold front.

The strongest winds are expected across Wyoming and Southern Idaho on Monday night. Many valley areas will see gusts to 50 mph or higher, and High Wind Warnings are in effect as a result.

Here is the HRRR Model’s projected wind gusts (in knots) at 10pm on Monday evening.

How Will this System Influence the Drought and Wildfire Situation?

The most widespread drought conditions currently exist across Central and Eastern Wyoming. This system will bring beneficial moisture with precipitation amounts expected to range from 0.75-2.0” across the drought-stricken areas.

The moisture will also help with firefighting efforts on the Bridger Foothills Fire outside of Bozeman.

However, most of the active wildfires will miss out on the best moisture of this system. Idaho will see little moisture from this storm, and farther west, fire activity is picking up across Oregon and Washington (and of course, California) — areas that will not see moisture from this storm, nor any moisture to speak of for the foreseeable future.

Fire danger will continue to be high across areas that miss out on heavy precipitation once we break out of the cold snap, and we could see periodic smoke return in the coming weeks from wildfires across the western states.

Latest Western U.S. Drought Monitor:

Post-Storm Weather Outlook

Just briefly, I’ll mention that colder than average temperatures will prevail through Thursday or Friday across most of the Northern Rockies. Then, we’ll head back into a warm and dry pattern for the foreseeable future.

The “Indian Summer Weather” will be great for outdoor recreation, but the return of dry conditions is also not so great for fire concerns, especially in Idaho.

I don’t see any signs of weather systems arriving over the next two weeks (after this storm) as a blocking ridge of high pressure will remain parked in place near the west coast of the U.S.