JACKSON, Wyo. – It has been a dry summer across Western Wyoming. However, a significant monsoon moisture surge is expected during the week of August 24-31, which could result in near-daily rounds of showers and thunderstorms along with the potential for heavy rain.
Recent Weather Conditions
The first two weeks of August were exceptionally dry across the Tetons and Western Wyoming, but we finally saw some moisture late last week and over the weekend.
Thunderstorms from August 13-15 generally produced light and spotty rainfall in most areas. Then, on August 16-17, storms produced more widespread rain with locally heavy amounts in some areas, most notably around Teton Village (and likely in the core of the Teton Range, where there are no rain gauges).
The town of Jackson picked up 0.14 inches of rain on Sunday, which was the highest single-day total in nearly a month (on July 19, Jackson received 0.19 inches of rain). This is more indicative of how dry it had been previously.
Temperatures have generally been hot over the past week, including a high of 91ºF on Tuesday, but we did have a cooler day on Sunday with a high of only 79ºF.
Nights have also been cooler over the past few mornings, and it’s not uncommon to have huge temperature swings between day and night at this time of year when conditions are dry, since the nights are now getting longer.
Upcoming Weather Pattern
A weak disturbance passing north of our area has resulted in some clouds and a few isolated sprinkles on Wednesday.
Otherwise, a dry pattern will continue from Thursday through Saturday, though a stray afternoon thunderstorm couldn’t be ruled out on Thursday afternoon. High temperatures will reach the upper 80s each day. Average highs at this time of year are around 80 for perspective.
Starting on Sunday, a significant change to the weather pattern will occur, and we could potentially see our wettest stretch of weather in over three months.
The North American Monsoon has been weak and inconsistent across the Western U.S. this summer.
However, confidence is growing that the strongest and most consistent monsoon moisture surge of the season will occur in late August, with significant moisture reaching our area starting on August 24 and potentially remaining in place through Labor Day.
It’s too early to get into the specifics and day-to-day details, but medium-range models (including the more trustworthy European Model) are projecting water vapor levels in the atmosphere to be around 150% to 200% of average for much of next week.
This indicates significant moisture available for thunderstorms to develop, along with the potential for thunderstorms to produce heavy rain.
Starting on Sunday (August 24), we should expect daily to near-daily rounds of showers and thunderstorms each afternoon, and this pattern could continue through Labor Day weekend.
Also, models seem to be hinting at the potential for heavier and more widespread rain around the middle of next week (August 26-28, roughly speaking) as a low pressure trough is expected to move through. This would also increase the potential for overnight and morning showers.
The day-to-day details will come into better focus next week, but for now, be prepared for potentially wet conditions and elevated lightning danger if you plan on spending time outdoors next week.
Also, temperatures will trend cooler next week, with highs possibly only reaching the upper 60s to low 70s in the valley on some days, with cooler temperatures as you go up in elevation.
As for fire danger, this pattern should help a lot with cooler temperatures, increased relative humidity, and hopefully some good rainfall, though large variations are also possible given the hit-or-miss nature of thunderstorms.
Here is a 10-day rainfall projection from the European AI Ensemble Model, which represents the average of 50 simulations of this model.
So does this pattern change mean that summer is about to be over?
Maybe so in terms of “extreme” heat (by our standards, which means upper 80s to low 90s), but not in terms of mild and sunny weather.
September is arguably the nicest month of the year in Jackson with cooler but still comfortable temperatures (average highs in the 70s, average lows in the 30s) and typically some long stretches of sunny weather interspersed with occasional cool and wet periods.
Alan Smith, Meteorologist
