The transition from summer weather to fall weather has been occurring across the Northern Rockies over the past several days. The first five days of September were much warmer than average with valley high temperatures in the mid to upper 80s.

However, since last Friday a cooler and wetter pattern has taken hold across the region. The town of Jackson has received 0.92″ of much-needed rain in the past four days.

Recent rainfall has put a significant damper on a late fire season that was just getting going over the past couple of weeks.

Take a look at the daily temperatures and rainfall for the town of Jackson over the first 9 days of September, which shows the recent pattern change.

DateHighLowRainfall
Sept 189320.00″
Sept 287350.00″
Sept 383370.00″
Sept 488370.00″
Sept 587420.00″
Sept 673530.39″
Sept 776430.09″
Sept 858390.35″
Sept 964450.09″

For the next couple of days, cool and wet weather will continue before warmer and drier conditions arrive late this week and into the weekend.


Showers and thunderstorms return Tuesday afternoon and evening

If we take a look at current satellite imagery and upper air analysis, we can see a trough of low pressure located to our west (image source: College of DuPage).

Water vapor satellite September 10 2019

Showers are already beginning to impact portions of the Tetons and Yellowstone as of midday Tuesday. Moisture and instability will continue to increase today ahead of the mentioned trough of low pressure.

This will result in more numerous showers and thunderstorms this afternoon, with rain showers continuing into the night. Highs Tuesday afternoon will be in the mid 60s in the Jackson Hole valley.

Currently, the freezing level is around 11,600 feet in the Tetons per the Lower Saddle weather station. Tonight, snow levels across the region should generally remain at 10,000 feet or higher.


Colder on Wednesday with more rain showers and some high elevation snow

On Wednesday, we’ll see the trough of low pressure move across Wyoming. As a result, we can expect a wet day with off-and-on rain showers and cooler air moving in.

The image below shows the projected position of the low pressure trough around noon on Wednesday per the GFS Model (image source: weathermodels.com).

GFS Model 500 millibar height anomaly September 10 2019
GFS Model projected 500-millibar height anomalies for noon Wednesday

Most weather models are projecting between 0.25-0.50″ of rain in the Jackson Hole valley on Wednesday, with higher amounts in the Teton mountains and across Yellowstone.

Temperature-wise, highs in the Jackson Hole valley will only be in the mid 50s.

Snow levels should start out around 10,000 feet or higher initially on Wednesday before falling to 8,000-9,000 feet Wednesday evening. Most of the precipitation should be ending across the Tetons Wednesday evening, while the eastern side of Yellowstone and the Wind River Mountains could see heavier snow amounts Wednesday night.


Dry and sunny weather returns Thursday through Sunday

By late Wednesday night, we should see precipitation ending across all areas. Thursday will see decreasing clouds and cool temperatures, but overall it should be a pleasant fall-like day with highs in the low to mid 60s in the Jackson Hole valley.

If you plan to hike or climb up high in the Tetons or Wind Rivers on Thursday, be prepared to encounter lingering snow cover above 9,000′ in the morning with good melting in the afternoon. I suspect snow accumulations will be light for most areas, except for perhaps the tops of the highest peaks.

Friday through Sunday, we should see outstanding weather conditions as the jet stream remains north of Wyoming, with high pressure building in from the south (image source: weathermodels.com).

GFS Model jet stream winds September 10 2019
GFS Model projected jet stream level winds for 6am Sunday morning

Look for mostly sunny skies each day Friday through Sunday with highs in the 70s in the Jackson Hole Valley. Any high elevation snow cover should melt off quickly, and trail conditions will dry out quickly. It will be a great weekend to get out and hike, climb, mountain bike, fish, etc.


Long Range Outlook – September 16-22nd

Next week, the weather pattern could become more unsettled again for a couple of days during the early to mid part of the week as a trough of low pressure approaches from the west.

At the moment, it looks like Tuesday through Thursday window will see the best chances of precipitation, with only a slight chance on Monday. Showers and thunderstorms, along with high elevation snow are all possible scenarios during this timeframe.

Latest trends point toward high pressure building back in by the end of next week with better odds of drier conditions and warm/pleasant temperatures.

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