Last weekend, a heat wave intensified across the Western U.S., and Jackson Hole experienced its two hottest days of the summer so far. Then, on Monday night, a severe thunderstorm impacted Jackson Hole and produced widespread wind damage.
Last Weekend’s (July 22-23) Heat Wave
In terms of temperature, summer began gradually in June and early July as we did not hit 80ºF for the first time until July 1. And through the first eight days of July, we only topped 80ºF on three occasions.
However, mid to late July has been much hotter. A ridge of high pressure has become the dominant pattern across the Western U.S. in recent weeks and we have experienced above-average warmth since July 9.
Last weekend, a ridge of high pressure strengthened over the Central Rockies, and this corresponded with our two hottest days of the summer so far.
On Saturday (July 22), the high temperature reached 91ºF in the Town of Jackson, and on Sunday (July 23), the high reached 93ºF. High temperatures were the same on both days in Driggs, too.
Heading north of town, the Jackson Hole Airport hit 90ºF on both Saturday and Sunday. The Jackson Lake area was not as hot with a high of 87ºF at the Moran/Jackson Lake weather station on Sunday.
Fortunately, elevation helps us out and keeps our heat waves relatively tame compared to most places. For example, just to our west Idaho Falls recorded a high of 101ºF on Sunday!
We have seen a nice temporary reprieve from the heat during the middle part of this week. The high in Jackson on Tuesday was 83ºF, and as of late Wednesday afternoon, the high so far is 84ºF. These are more seasonal values for our area.
The average high temperature in Jackson in July is 82ºF and the average low is 41ºF. So far, temperatures in July are running 1.9ºF above average which is similar to what we experienced last July, but cooler than July 2021.
This is on pace to be only our second warmer-than-average month so far in 2023 (the other being May).
Monday Night’s Severe Thunderstorm
On Monday night (July 24), an intense thunderstorm impacted Teton County, producing substantial wind damage in Wilson, including downed trees and power lines, and more isolated damage around the Town of Jackson.
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The storm moved through between 10 and 11 pm, which is somewhat fortunate as more people were in the safety of their homes during this time compared to the daytime hours when more people would have been outside or driving on the roads.
Moisture associated with the North American Monsoon had been steadily increasing from Sunday night through Monday night across the Great Basin, Eastern Idaho, and Western Wyoming.
The day before (Sunday), a round of thunderstorms moved through Jackson Hole, producing lightning but little in the way of rainfall as moisture was not well-established.
On Monday, frequent intrusions of mid to high-level cloud cover helped to limit the amount of solar radiation, heating and instability needed for thunderstorms to develop during the daytime hours.
However, late Monday afternoon, a strong disturbance in the upper atmosphere known as a shortwave moved into Southern Idaho where instability was greater at the time and triggered a large cluster of thunderstorms.
This thunderstorm cluster intensified as it approached Jackson Hole from the southwest, coinciding with a continued uptick in moisture across our region.
As the storm moved across Jackson Hole, a wind gust of 61 mph was recorded at the Jackson Hole Airport. This officially qualified the storm as a severe thunderstorm, which is defined by the National Weather Service as a thunderstorm that produces wind gusts of 58 mph or higher, and/or hail of one inch or more in diameter.
Based on the damage observed in Wilson, it’s possible that peak wind gusts were even stronger there. However, there are no weather stations that report wind in Wilson, so there is no way to confirm exactly how strong the winds were there.
One benefit from this large cluster of thunderstorms is that it produced a widespread soaking rainfall. Rain totals ranged from 0.25 to 0.50 inches across Teton County with the heaviest amounts recorded around Wilson/Teton Village and also near Jackson Lake.
Upcoming Weather Pattern
We have experienced cooler and drier weather since Monday night’s thunderstorm. However, a weaker uptick in monsoonal moisture will result in a chance of showers and thunderstorms on Thursday.
We are on the northern fringe of moisture on Wednesday afternoon, and a few showers have popped up on radar across Southern Teton County and skies have also darkened.
However, no lightning activity has been noted (as of 5 p.m. Wednesday) and it’s likely that little to no rain is reaching the ground. Winds have been gusty all afternoon, though. This is due in part to outflow from these weak showers, as well as from the jet stream being located just north of our area.
On Thursday, a shortwave disturbance will arrive from the west/southwest during the morning hours along with an increase in moisture. This will result in scattered showers developing early in the day, and a thunderstorm also couldn’t be ruled out early.
Scattered showers and thunderstorms will remain possible into early/mid-afternoon Thursday with a drying trend expected late in the day. Weaker showers will only produce brief/light rain, while stronger showers and thunderstorms could produce brief moderate rain.
After that, Friday through the weekend is looking like a near-repeat of last weekend. Monsoonal moisture will retreat well to the south and a ridge of high pressure will re-strengthen over the Central Rockies.
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This will result in hot and dry conditions with highs reaching the upper 80s on Friday and low 90s each day from Saturday through Monday. Despite the heat, gusty winds can be expected each day with the jet stream just to our north and west, resulting in elevated fire danger in areas where fuels have dried out sufficiently.
The hot and dry spell will persist through Monday, then we will start to see some changes as a more substantial surge of monsoonal moisture will approach from the south – possibly reaching our area as early as Tuesday.
Here is the 7-day rainfall outlook from multiple simulations of the European Model. Any rainfall we see during the next seven days will occur on Thursday and/or Tuesday.
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Extended Outlook
The first several days of August (mid to latter portion of next week) are looking cooler and wetter. Confidence is increasing that a significant surge of monsoonal moisture will reach our area and could remain in place for a few days or more.
This will result in increased potential for thunderstorms, along with the potential for soaking rains to occur with thunderstorms. We should also see some heat relief with temperatures projected to cool off to near if not slightly below average, though this will be somewhat dependent on cloud cover.
Alan Smith, Meteorologist
