Showers and thunderstorms were widespread across the Jackson Hole and Greater Teton region on Tuesday afternoon, and another round of heavier showers moved through on Tuesday night. 24-hour rainfall amounts on Wednesday morning have ranged from 0.15 to 0.30 inches at weather stations across our region. 

Get ready for a rinse-and-repeat pattern for the remainder of this week and over the holiday weekend. A trough of low pressure will remain in place over the West Coast during this period as a strong ridge of high pressure just east of the Continental Divide will prevent the trough from progressing inland.

The result will be a consistent southwest flow that will transport and reinforce an abundance of moisture into our region. A series of embedded shortwave disturbances on the downstream side of the trough will track across our area, and we will also be in a relatively warm airmass that will support high instability (i.e. thunderstorm "fuel"). All of these factors will result in an active thunderstorm period for the Tetons, Yellowstone, and surrounding areas.

While I wouldn't rule out an occasional overnight/morning shower or even a thunderstorm, generally speaking, the morning hours will offer the best weather windows with more inclement conditions in the afternoons. Even the afternoons won't be all-day rainouts with sunny breaks expected, but you'll want to time your activities appropriately to correspond with lightning threats along with the potential for locally heavy rain. 

High temperatures in the valleys will generally be in the 65-70ºF range over the next several days with lows in the low to mid 40s. The average high and low in Jackson right now are 67º and 33º.

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 these storms. 

Forecast for Wednesday:

Cloud cover early on Wednesday morning is showing some signs of clearing out already, which will allow for good daytime "heating" as the day progresses to help destabilize the atmosphere.

Also, temperatures are starting out relatively mild in the valley in the mid to upper 40s, which is a result of our elevated moisture levels as dewpoints are also in the mid to upper 40s (sufficiently high by our standards, for us to see heavy rain).

Another shortwave will move through from the southwest today, which along with orographic/terrain-enhanced lift will result in thunderstorms initiating over the highlands of Southeast Idaho. Storms will then track across the Tetons and Greater Jackson Hole/Yellowstone region with multiple rounds possible.

Storms will be a bit faster moving compared to previous days with storm motions from SW to NE at 20-30 mph. As far as timing goes, the best chance of storms will be from early afternoon through early evening.

Stronger storms will be capable of producing frequent cloud-to-ground lightning, locally heavy rain, gusty winds, and small hail.

Typical rainfall amounts will range from 0.1 to 0.3 inches with heavier amounts of 0.3 to 0.8 inches under stronger thunderstorms or training of thunderstorms (i.e. multiple storms tracking across the same area). A low-probability/high-end "boom" scenario would be for stronger storms to produce an inch or more of rain within a few hours, therefore localized flooding of snowmelt-swollen streams couldn't be ruled out.

Here is a rainfall projection from a blend of high-resolution weather models:

Forecast for Thursday:

A similar day is expected with scattered to numerous afternoon and evening thunderstorms. Storm initiation is likely across the SE Idaho highlands again with storms then tracking across the Greater Teton and Yellowstone region.

Storms on Thursday will also move from SW to NE at 20-30 mph. Instability even looks a little higher on Thursday compared to Wednesday based on recent model runs, which would support a threat of more intense lightning along with larger (pea to marble size) hail under stronger storms.

Moisture levels may be slightly lower on Thursday compared to Wednesday, but not by much. I'll go with typical rainfall amounts of 0.05-0.25 inches with locally heavier amounts of 0.25 to 0.60 inches.

Forecast for Friday:

Rinse and repeat! The overall pattern will remain similar with scattered to numerous thunderstorms, especially in the afternoon and early evening hours. Once again, stronger storms will be capable of producing frequent cloud-to-ground lightning, heavy rain, gusty winds, and hail.

Storm motions may be oriented a bit more from SSW to NNE with similar storm speeds of 20-30 mph (relatively fast-moving).

My 3-day lightning chart is the same for each of the next 3 days. The Central/Southern Wind River Range looks a bit less favored compared to areas further north. For the northern geographical extent, the southern half of Yellowstone looks a bit more favored compared to the northern half, though storms are certainly possible throughout the park.

Outlook for Memorial Day Weekend (Saturday to Monday):

An active pattern will continue right through the holiday weekend. A stronger shortwave associated with the parent trough near the West Coast is expected to move across the Central Rockies on Saturday with scattered to widespread afternoon/evening thunderstorms expected (similar to prior days).

Behind this shortwave, we may see a relative lull in the pattern on Sunday with fewer/more isolated thunderstorms before a wetter/more active pattern returns on Monday. However, keep in mind these details could change moving forward.

Outlook for Tuesday (May 30) and Beyond:

The ridge of high pressure over the Great Plains is projected to shift a bit further west into the Northern Rockies, which would support warmer temperatures, possibly well into the 70s depending on afternoon cloud cover.

However, a trough of low pressure is expected to be the dominant feature in the Southern California vicinity, which would keep us in a persistent southwest flow with moisture and energy approaching from the south/southwest.

The exact positioning of the Northern Rockies ridge and West Coast trough will influence our rain and thunderstorm potential, but at the very least, we should see frequent chances of showers and storms though coverage/intensity is uncertain.

Thanks so much for reading! Next update on Friday (May 26).

Alan Smith