JACKSON, Wyo. — An impressive early fall cold front and storm system moved through Jackson Hole on Monday evening, resulting in rapid temperature drop, heavy rain, lightning and a rare early September snowfall to the valley floor.
The Labor Day weekend pattern was full of extremes, as we experienced record high temperatures in the low 90s on Saturday before a taste of winter arrived barely 48 hours later.
Labor Day Storm Recap
A strong cold front arrived from the north on Monday afternoon and a rapid temperature drop followed. The temperature at the Jackson Hole Airport fell from 75 at 3:50 p.m. to 34 at 7:15 p.m. — a 41 degree temperature drop in less than three and a half hours!
The longest-lasting and most widespread precipitation from this storm was set to occur east of the Continental Divide, and that has held true to forecast, but the round of precipitation we received in Jackson Hole was more potent than expected.
As the cold front moved through, the upper level low pressure system moving from north to south rapidly intensified as it neared Jackson Hole, and as a result we saw a band of heavy precipitation develop and stall across Teton County and west into Idaho.
Heavy rain developed around 5:30 p.m. and eventually mixed with and turned to snow in the valley by later in the evening, with quite a bit of thunder and lightning activity occurring as well — which speaks to just how much atmospheric “energy” was present.
Diverging air in the upper levels of the atmosphere means that air in the lower levels of the atmosphere must surge upward to “fill in the void”, and this upward motion contributes to more intense precipitation.
The precipitation we received helped tremendously with local wildfires and fire danger. The Jackson Hole Valley recorded precipitation amounts of 0.8 to 1.1 inches with this system. Amazingly, we received more precipitation in about six hours on Monday than we received during all of July and August combined!
Higher elevation snowfall measurements are limited at this time of year, but it appears most areas in the 9,000-10,000 foot elevation range received a few inches or so. The banded nature of the precipitation did not necessarily favor higher precipitation amounts in the mountains versus the valley as the orographic influence was minimal.
Winds were strong throughout the night with 40-50 mph gusts reported at the Jackson Hole Airport. Reports of damage and downed trees throughout the valley suggest that localized gusts were much higher in some areas.
Was this the Earliest Accumulating Snowfall on Record in the Jackson Hole Valley?
Snow at this time of the year in the Tetons and surrounding mountain ranges is far from uncommon, but early September snow in the Jackson Hole valley is quite rare — especially accumulating snow.
Heavy snowfall rates allowed for some accumulations on grassy surfaces and elevated surfaces in the town of Jackson on Monday night with some areas in Teton Village and north up into the park reporting 1-2 inches of accumulation.
The question is, where does this stand in the record books? Unfortunately, snowfall at the climate stations in Jackson, Moose and Moran are only measured once per day and this has been the case since the early 1900’s. Therefore, it could snow and then melt during the 24-hour period before snowfall is measured.
Measurement limitations aside, the earliest accumulating snowfall ever reported in the three valley locations are as follows:
- Jackson – September 13, 1970 (records dating back to 1905)
- Moose – September 15, 1982 (records dating back to 1959)
- Moran – August 29, 1964 (records dating back to 1911)
Moran certainly will not break a record for their earliest snow. The question is whether or not Jackson and Moose record any measurable snowfall on today’s report. Typically measurements occur between 9-11 a.m. each morning and the climate for the day is reported in the afternoon.
Even if the records are “officially” broken, they should be taken with a grain of salt given the time spacing and potential for melt prior to measurements being taken.
Despite these caveats, I think we can all agree that this was a rare valley snow event for this time of year and likely one of the earliest accumulating valley snowfalls on record, but not the earliest.
The Week Ahead: Get Ready for Indian Summer After Our Cold Snap
We’re in for a stretch of quiet weather following the Labor Day storm. Temperatures will be unseasonably cold on Tuesday and we could come close to setting record lows on Tuesday night as temperatures plummet into the teens. How quickly the winds subside overnight will likely influence how cold it gets, as winds typically have a moderating effect on temperature.
A northeasterly flow will keep us cooler than average on Wednesday and Thursday with a mix of sun and clouds, though it will be warmer compared to Tuesday. Heading into the weekend, high pressure will strengthen and temperatures will warm up well into the 70s.
The warm and mostly dry conditions could hang around for quite some time based on long range trends, making for a nice stretch of “Indian summer”.
Unfortunately, the one wrinkle to the outlook is the potential for smoke to return by the end of the week. Numerous large wildfires were started in Washington State over the weekend, and as winds shift to northwest starting on Friday and Saturday, there is a good chance some of the smoke from these fires will reach Western Wyoming.
Alan Smith, Meteorologist
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