JACKSON, Wyo. — Teton County received a significant amount of rainfall last week, and the Town of Jackson experienced its wettest two-day period in over a year. A break in the pattern is occurring early this week with warm and dry conditions, but another impressive surge of monsoon moisture will lead to an uptick in showers and thunderstorms with locally heavy rain from Thursday through Saturday.
August 2-8 Recap
Over the past week, we received some very impressive rainfall amounts for this time of year, making up for what was a very dry July. The Town of Jackson received a total of 1.94 inches of rain last week, which is more than the historical average of 1.23 inches for the entire month of August.
The culprit for the heavy rainfall was a very active North American Monsoon, which has been brewing over the Southwest U.S. since June. We only received fringe levels of monsoonal moisture in July, but two significant influxes of monsoonal moisture arrived during the first week of August and we finally received some beneficial rain.
The first monsoon moisture surge occurred last Tuesday (8/2), which supported a cluster of strong thunderstorms on Tuesday evening. While these storms produced beneficial rainfall (0.29 inches in Jackson), unfortunately, a lightning fatality also occurred with these storms in the Teton Wilderness east of Jackson Lake.
An even stronger surge of monsoon moisture arrived late last week with added support from a slow-moving trough of low pressure that tracked across the Western U.S. This pattern resulted in widespread cloud cover and wet conditions on Friday and Saturday.
The heaviest and most widespread rain fell during the overnight hours on Friday night and through much of the day on Saturday. The Town of Jackson received an impressive 1.54 inches of rain from Friday morning through Saturday night. First-hand reports and modeled terrain-based estimates indicate that Teton County received about 1-2 inches of rain in the valley and 1.5-3 inches of rain in the mountains.
In fact, the 1.54 inches of rain that Jackson received was the highest two-day rain total since May of 2021. The heavy rainfall could not have come at a better time and will reduce our threat of significant wildfires heading into the late summer period.
The Friday-Saturday event also turned out to be more of a widespread soaking rainfall with limited thunderstorm activity (which could have led to extremely heavy rainfall rates), and as a result, there were not any significant runoff or flash flooding occurrences.
The wetter pattern last week also resulted in cooler temperatures. We ended a 33-day streak of 80+ degree high temperatures last Tuesday, and during Friday and Saturday’s wet pattern, highs were only in the 60s in the valley.
High temperatures last week ranged from 68ºF on August 6 to 86ºF on August 4. Low temperatures were milder due to the higher moisture levels and cloud cover, ranging from 42ºF on August 4 and August 8 to 59ºF on August 2.
Forecast for Tuesday (8/9) to Monday (8/15)
A much drier airmass has moved into Western Wyoming early this week, resulting in 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 as a significant influx of monsoonal moisture arrives.
Tuesday will be our last day without any threat of thunderstorms for a while. Conditions will be sunny with highs heating up into the upper 80s. Skies will be a little hazy due to smoke from the Moose Fire in Idaho, but this is not expected to have a significant effect on air quality.
On Wednesday, another hot day is expected but we will also start to see some subtle changes to the weather pattern. Highs will reach the upper 80s to near 90, but skies should also be a bit clearer as southwest flow directs light smoke from the Moose Fire away from here.
There is a chance of isolated thunderstorms late Wednesday afternoon and evening as moisture begins to increase in advance of the next monsoon moisture surge. Any storms that develop 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 wildfire threat.
On Thursday, a trough of low pressure setting up off the coast of Washington will result in a south/southwest flow that will transport significant amounts of monsoonal moisture into Western Wyoming.
This will result in an active three-day stretch from Thursday through Saturday with scattered to widespread showers and thunderstorms each day, with stronger storms producing heavy rainfall and frequent cloud-to-ground lightning. Storm motions 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 after last week’s rainfall.
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 (referred to as “shortwaves” by meteorologists) will result in at least an isolated threat of showers and thunderstorms during the overnight and morning hours as well, so be prepared for this possibility if you have hiking, camping or climbing plans.
On Sunday and Monday, moisture will decrease a bit as the low-pressure system over the Northwest finally tracks east and moves across the Northern Rockies, placing Jackson Hole into more of a west/northwest flow. However, enough lingering moisture will result in a chance of isolated to scattered thunderstorms both days, with less coverage and lower intensity compared to prior days.
After a couple of hot days on Tuesday and Wednesday, temperatures will be cooler for the remainder of the week with highs in the upper 70s to low 80s most days. However, morning lows will also be milder than normal given the moisture in place.
Next week, we will likely see varying levels of monsoonal moisture across Western Wyoming, resulting in additional chances for afternoon thunderstorms from time to time. Rain amounts and thunderstorm coverage are not expected to be as extensive compared to this week, but confidence is low in exactly how the pattern will transpire.
Highs are generally expected to be in the 80s next week but could be cooler on any given day if a significant uptick in moisture occurs.
Alan Smith, Meteorologist
Town of Jackson Climatology for August 9-15:
Average High: 81
Average Low: 40
Record High: 95 (multiple dates)
Record Low: 22 (August 15, 1959)
Precipitation Since Oct 1st: 14.79″ (99% of Average)