Thunderstorms have been visiting the Tetons and Jackson Hole on a daily basis this week. Rainfall totals have been highly variable in this pattern, but the town of Jackson has picked up about 0.75" of rain over the past 3 days. 

Thursday's storms were on the stronger side and produced widespread small to medium size hail across Teton County, enough to cover the ground in some areas. We had a pretty intense burst of hail in town on Thursday with some of the larger hailstones around marble size, or about a half-inch in diameter. For hail to be qualified as "severe" it must be one inch or more in diameter, though.

An active pattern will remain in place as we head into the holiday weekend, with Friday and Saturday expected to be the two busiest days in the near term. The culprit will be a trough of low pressure across the Western U.S., or more specifically, an area of low pressure centered over the Pacific Northwest, with south/southwest winds ahead of this low directing moisture and energy into Wyoming.

High temperatures in the valley will reach the 60s each day through the holiday weekend (and possibly 70 on Monday) with overnight lows in the upper 30s to low 40s.

Snow levels will be around 11,000-12,000 feet with freezing levels of 12,000-13,000 feet during this pattern. So the tops of the highest peaks in the Tetons (Grand, Middle, South, Teewinot, Owen, Moran) will get dusted with wet snow during showers and thunderstorms. 

Forecast for Friday:

As of this writing (mid-AM Friday), a cluster of clouds and showers is approaching Jackson Hole from the south. This is part of the remnants of overnight showers and thunderstorms that impacted Utah and is associated with a shortwave disturbance approaching from the south that will be a driver for showers/thunderstorms in the Tetons.

The showers have been weakening over time but we still may see light rain develop during the morning hours. It remains to be seen how long cloud cover persists over our area from this morning activity, but it could act to delay the heating needed for thunderstorm development.

Short-range models have been ambitious to develop thunderstorms by early afternoon, but if the morning cloud cover persists for long enough, this could potentially delay development until mid-afternoon (2-4pm ish or so). However, this is uncertain, so be prepared for possible storms at all hours this afternoon. And once the sun comes out, that will be what is needed to destabilize the atmosphere for storms to eventually develop.

Instability does not look quite as impressive across Teton County compared to yesterday, but we could still see some stronger storms develop with frequent lightning and small hail.

Storm motions will be from south/southwest to north/northeast at 15-25 mph. The SSW winds will most likely mean that storms will initially develop in the ranges south of the Tetons (rather than over the Tetons themselves) as well as across Southern Yellowstone, with storms then tracking northward over time. 

Typical rainfall amounts of 0.05 to 0.25 inches total can be expected from Friday afternoon through early evening. However, if we see training of thunderstorms at all (multiple storms tracking over the same areas), then locally heavier rainfall amounts of 0.25 to 0.60 inches will be possible.

Most of the thunderstorm activity is expected to wind down by early Friday evening with a drying trend expected overnight.

Central/ Eastern Wyoming Severe Weather Potential:

East of the Continental Divide, there is a good chance we will see some severe thunderstorms develop on Friday afternoon as moisture, instability, and wind shear (changes in wind with height) will be greater. Stronger storms will be capable of large hail (1" in diameter or more) and wind gusts of 60 mph or higher.

The highest threat of severe storms will be across easternmost portions of the state, including the eastern slopes of the Big Horn Range, Black Hills, and the I-25 corridor from Casper down to Cheyenne and west into Laramie. 

Even further west, the potential for severe thunderstorms will extend toward the eastern slopes of the Continental Divide in the Wind River and Absaroka Ranges, including Lander, Dubois, and Riverton. 

Forecast for Saturday:

The center of low pressure over the PNW will work its way inland and a stronger shortwave disturbance associated with this feature will move through from the southwest, resulting in another active day with multiple rounds of showers and thunderstorms likely.

There will be a slight chance of showers in the morning, but this will be the best weather window before the afternoon turns wetter. Showers and storms are then likely to develop by early afternoon with multiple rounds possible through early evening. Storm motions will shift back to a more southwest-to-northeast direction with slower speeds of 10-15 mph.

The slower storm motions along with better potential for multiple rounds will result in heavier rainfall potential as well. Typical rainfall amounts of 0.1 to 0.3 inches can be expected, but areas that experience multiple rounds of stronger storms passing overhead could see locally heavier rain amounts of 0.3 to 0.8 inches.

Forecast for Sunday to Monday:

The low pressure system from Friday/Saturday will exit to our east on Sunday. We will remain in a southwest flow as another area of low pressure sets up off the California Coast, but thunderstorm activity will be a bit less in terms of coverage and potential rainfall amounts compared to prior days.

I would still expect scattered thunderstorms to develop both afternoons but with more spotty coverage. Still, don't let your guard down from a lightning perspective during the afternoons. Storm motions will generally be from southwest to northeast at around 10-20 mph.

Of the two days, Monday is currently projected to have a bit more thunderstorm coverage compared to Sunday, but confidence is low. 

Outlook for May 30 to June 4:

A persistent southwest flow pattern will remain in place throughout next week with a steady flow of moisture expected to keep thunderstorm chances going on a near-daily basis.

It will be a matter of timing individual shortwave disturbances to determine which days will be more active than others. But in general, it looks like the second half of next week (from Wed/Thu on) could be more active than the first half of the week with better thunderstorm odds and heavier rainfall potential.

Temperatures should warm back into the 70s during the early to middle part of next week in the valley (slightly above average) before cooling slightly late next week as moisture increases. Low temperatures will generally be warmer than average for this time of year and snowmelt will continue at a steady clip.

Thanks so much for reading and have a great weekend! Next update on Tuesday (May 30).

Alan Smith