DRIGGS, Idaho – A supercell thunderstorm developed over Teton Valley, Idaho on Tuesday, producing hail up to 1.5 inches in diameter north of Driggs. A warmer and drier pattern will take hold late this week, before a low-pressure system arrives this weekend with widespread, significant rainfall possible.
Tuesday’s Thunderstorms
Tuesday was an eventful day across our region, in response to a upper level low pressure system located over Northern California. To the east of this low, a robust shortwave disturbance arrived from the southeast, supplying an uptick in moisture, instability, and atmospheric dynamics.
Multiple rounds of scattered thunderstorms developed on Tuesday afternoon and evening in response to this shortwave, and storms traveled in an atypical direction from southeast to northwest due to the strong southeast flow.
The map below shows upper-level wind speeds and directions on Tuesday afternoon, with winds aloft blowing counterclockwise around the low pressure center over Northern California.
Shortly after 2:30 pm, a cluster of developing thunderstorms traveled over Teton Pass into Victor, then one cell became dominant and quickly intensified into a supercell thunderstorm as it traveled northward through Driggs and Tetonia.
Supercell thunderstorms are storms that have a rotating updraft, and are more intense than regular thunderstorms, often developing and behaving in isolation from other thunderstorms, and are the type of thunderstorms most often associated with large hail and tornadoes.
They are more common east of the Continental Divide, and we typically only see 1-2 of these per year at most in the Greater Teton-Yellowstone region.
Here is a loop of the thunderstorm cell as it intensified, moving from south to north. This storm also exhibited deviant motion from the mean SE to NW flow, traveling in more of a due north to NNE direction. Deviant thunderstorm motion is a common trait of supercells, and indicates a more intense storm.
The Doppler Radar Base Velocity also indicated a clear rotation signature as the storm intensified. Red and green colors that display in a tight couplet indicate winds blowing in opposite directions, hence the rotation signature.
Red colors indicate winds blowing away from the radar site near Pocatello, and green colors indicate winds blowing toward the radar site.
First-hand reports include moderate to large hailstones that covered the ground in Driggs, along with torrential rainfall, while hail damage to vehicles was reported north of Driggs with hailstones measuring up to 1.5 inches in diameter, or about the size of ping pong balls.
The well-positioned webcam at the top of Grand Targhee captured a dramatic scene of this supercell as it moved through Teton Valley.
Aside from the Teton Valley supercell, two rounds of storms also moved through the Jackson Hole Valley from southeast to northwest, with storms producing locally heavy rainfall rates thanks to the uptick in low-level moisture.
Although the duration of rainfall was short within each storm, rainfall totals amounted to 0.3 to 0.4 inches around the town of Jackson and at Jackson Hole Mountain Resort.
Weekly Summary
It’s hard to believe that only a week ago, we were experiencing a final blast of chilly weather with highs in the 40s and even some wet snow reaching the valley floor.
Since picking up modest but needed rainfall last Wednesday, we have transitioned to a much warmer pattern with afternoon temperatures now getting into the 70s on a regular basis.
Isolated to scattered thunderstorms also produced light and spotty rainfall on Sunday and Monday, followed by heavier (but still somewhat scattered in nature) rainfall on Tuesday.
Below is a 7-day weather summary for the town of Jackson.
For perspective, the average high and low in Jackson on May 27 are 65º and 36º.
Upcoming Weather Pattern
The previously mentioned low pressure system has traversed a bit further south to near the California/Nevada border, where it will stall for the next couple of days.
Warmer, drier, and more stable air is pushing into our area out ahead of this low, so our weather pattern will quiet down a bit from Wednesday to Friday.
A substantial warm-up is also expected with highs peaking in the mid 80s on Thursday – the kind of temperatures we usually don’t see until later in the summer.
A few isolated thunderstorms couldn’t be ruled out on Wednesday afternoon, and again on Friday afternoon. But overall, the pattern looks more tame compared to the past few afternoons.
Heading into the weekend, the low pressure system will swing to the northeast and track across the Northern Rockies. Confidence is growing that we will see a more widespread, soaking rain event over the weekend in response to this system.
For now, the timing of the most widespread rainfall looks to be from Friday night through Sunday morning, with Saturday being the overall wettest day of the weekend. However, cut-off lows are notoriously tough to pin down, so don’t be surprised if the forecast and timing change a bit as we get closer.
Most models are projecting anywhere from 0.5 to 2.0 inches of rain/precipitation across Teton County (Wyoming and Idaho) this weekend, with the more reliable European Ensemble Model (physics-based and AI versions) being one of the wetter projections across our area.
While it’s a bummer for weekend plans, it would be great news for our area if the wetter model solutions we’re currently seeing were to pan out, as we could certainly use the moisture after a lackluster snow season and early start to spring.
While Saturday looks like the wettest day, cut-off low pressure systems like this one (meaning they are cut off from the main jet stream) can be notoriously stubborn and slow-moving, so I wouldn’t rule out pesky showers persisting for much of Sunday as well. Monday and Tuesday are uncertain, but for now, at least a chance of showers and thunderstorms can be expected.
Cooler temperatures will also accompany the moisture this weekend. Currently, snow levels are projected to dip to 9,000 feet or lower on Sunday morning, and some flakes couldn’t be ruled out on the pass either, though travel impacts are unlikely.
The 7-day precipitation projection below represents the average of 50 simulations of the European AIFS Model. This is a good look for our area!
Extended Range
Beyond June 3rd, confidence in the pattern decreases overall, but at the very least, we should see a trend toward warmer temperatures.
Also, a general troughing signal is expected in the upper level pattern in early to mid June with near to above-average atmospheric water vapor levels; a combination that would favor frequent or semi-frequent shower and thunderstorm chances.
Alan Smith, Meteorologist
