JACKSON, Wyo.– Teton County received a significant amount of precipitation last week and the city of Jackson experienced its wettest two-day spell in more than a year. A break in the pattern occurs early this week with hot and dry conditions, but another impressive surge of monsoon humidity will bring increased showers and thunderstorms with locally abundant rain from Thursday to Saturday.
August 2-8 Recap
Over the past week, we have received very impressive amounts of rain for this time of year, compensating for a very dry month of July. The city of Jackson received a total of 1.94 inches of rain last week, which is above the historic average of 1.23 inches for the entire month of August.
The culprit for the heavy rains was a very active North American monsoon, which has been brewing in the southwestern United States since June. We received only marginal levels of monsoon moisture in July, but two major influxes of monsoon moisture arrived in the first week of August and we finally received beneficial rains.
The first surge of monsoon moisture occurred last Tuesday (8/2), which supported a cluster of severe thunderstorms on Tuesday evening. Although these storms produced beneficial precipitation (0.29 inches in Jackson), unfortunately one lightning death also occurred with these storms in the Teton Wilderness east of Lake Jackson.
An even bigger increase in monsoon humidity came late last week with further support from a slow-moving low pressure trough that moved across the western United States. This pattern resulted in widespread cloud cover and wet conditions on Friday and Saturday.
The heaviest and most widespread rain fell overnight Friday evening and for much of Saturday. The city of Jackson received an impressive 1.54 inches of rain from Friday morning through Saturday evening. First-hand reports and modeled estimates based on the terrain indicate that Teton County received about 1 to 2 inches of rain in the valley and 1.5 to 3 inches of rain in the mountains.
In fact, the 1.54 inches of rain Jackson received was the highest two-day rain total since May 2021. The heavy rains couldn’t have come at a better time and will reduce our threat of fires by significant forest as late summer approaches. period.
The Friday-Saturday event also turned out to be more of a widespread torrential downpour with limited thunderstorm activity (which could have resulted in extremely heavy rainfall rates), and as a result, there were no significant occurrences of runoff or flash flooding.
Last week’s wetter trend also resulted in cooler temperatures. We ended a 33 day streak of high temperatures of over 80 degrees last Tuesday, and during the wet cycle on Friday and Saturday the highs were only in the 60s in the valley.
Last week, high temperatures ranged from 20°C on August 6 to 30°C on August 4. Low temperatures were milder due to higher humidity levels and cloud cover, ranging from 42°F on August 4 and 8 to 59°F on August 2.
Forecast from Tuesday (8/9) to Monday (8/15)
A much drier air mass moved across western Wyoming early this week, bringing a return to more typical summer conditions with sunny skies and warmer temperatures. However, another period of wetter than normal conditions is expected later this week with the arrival of a major influx of monsoonal moisture.
Tuesday will be our last day without any threat of thunderstorms for a while. Conditions will be sunny with highs warming into the upper 80s. Skies will be a little hazy due to smoke from the Moose Fire in Idaho, but this shouldn’t have a significant effect on air quality.
On Wednesday, another warm day is expected, but we will also start to see some subtle changes in the weather pattern. Highs will reach the upper 80s to near 90s, but skies should also be a bit clearer as the southwesterly flow directs light smoke from the Moose Fire away from here.
There is a chance of isolated thunderstorms late Wednesday afternoon and evening as humidity begins to rise ahead of the next monsoon surge of humidity. Any thunderstorms developing on Wednesday will be “dry” thunderstorms and will produce brief/light rain, gusty winds and occasional cloud-to-ground lightning. Despite the dry nature of the thunderstorms, recent moisture should limit the threat of wildfires.
On Thursday, a trough of low pressure off the coast of Washington will cause a south/southwesterly flow that will carry significant amounts of monsoon moisture into western Wyoming.
This will result in an active three-day period Thursday through Saturday with scattered to widespread showers and thunderstorms each day, with stronger storms producing heavy rain and frequent cloud-to-ground lightning. Storm movements will be from southwest to northeast each day.
There will also be a minor threat of flash flooding and excessive runoff in this pattern, especially now that ground conditions are more saturated following last week’s rains.
The highest threat of thunderstorms will occur during the afternoon and evening hours when instability is highest. However, a series of disturbances in the upper atmosphere (called “short waves” by meteorologists) will bring at least an isolated threat of showers and thunderstorms overnight and into the morning as well, so be prepared for that possibility if you have hiking, camping or climbing plans.
On Sunday and Monday, the humidity will ease a little as the low pressure system over the northwest finally heads east and moves across the northern Rockies, placing Jackson Hole in a more westerly/northwesterly flow. west. However, sufficient persistent humidity will bring a risk of isolated to scattered thunderstorms on both days, with less coverage and lower intensity compared to previous days.
After a few hot days on Tuesday and Wednesday, temperatures will be cooler for the rest of the week with highs in the 70s above 80 degrees most days. However, morning lows will also be milder than normal given the humidity in place.
Next week we will likely see variable levels of monsoon humidity in western Wyoming, which will occasionally bring additional chances of afternoon thunderstorms. Rainfall amounts and thunderstorm coverage are not expected to be as extensive compared to this week, but confidence is low as to how the pattern will unfold.
Highs are generally expected to be in the 80s next week, but could be cooler any day if a significant increase in humidity occurs.
Alan Smith, Meteorologist
Climatology of the city of Jackson from August 9 to 15:
Medium top: 81
Medium Low: 40
High record: 95 (multiple dates)
Record low: 22 (August 15, 1959)
Precipitation since October 1: 14.79″ (99% of average)