JACKSON, Wyo. — An active period of thunderstorms is underway and will persist through the Fourth of July holiday weekend due to the arrival of monsoonal moisture from the south.
The seasonal North American Monsoon has taken hold over the Four Corners region over the past week. Arizona, New Mexico, Colorado and Utah typically see the highest impacts and heaviest rainfall during monsoon season in July and August, but other areas of the west including Jackson Hole will see varying degrees of impacts during the summer as well.
During the previous two days, we saw a preview of the pattern ahead with afternoon thunderstorms developing throughout Teton County on Tuesday and Wednesday. Moisture was only just beginning to increase both days, so rainfall amounts were on the lighter end of the spectrum.
However, the flow of monsoonal moisture is increasing into Western Wyoming on Thursday with dewpoint values projected to rise into the mid to high 40s – these values are high enough to result in wetting rains with thunderstorms, whereas dewpoints in the 30s typically result in only brief/light rain and “dry” thunderstorms.
On Thursday, thunderstorm development will also be aided by an area of low pressure moving into Western Wyoming, which will provide an extra source of “lift” for thunderstorm development, in addition to driving more moisture into the area.
As a result, more widespread thunderstorm activity is likely on Thursday. Storms will be slow-moving and erratic due to weak steering winds aloft, which could lead to locally heavy downpours along with frequent cloud-to-ground lightning. Storms will exhibit more of an atypical east to west movement due to winds aloft being out of the east.
Embedded disturbances rotating around the area of low pressure will also keep a chance of showers and thunderstorms going into the overnight and early morning hours on Friday, so heads up to keep a close eye on conditions if you have early morning climbing or peak-bagging plans on Friday.
Sufficient moisture will hang around through the holiday weekend and into early next week, resulting in daily rounds of afternoon thunderstorms. Winds will shift back to a westerly direction starting on Friday, and from that point forward we’ll see a more typical west-to-east movement of thunderstorms.
On Sunday and Monday (July 4 and 5), the latest medium-range model trends are suggesting another uptick in moisture along with a series of embedded disturbances, which could result in more widespread thunderstorms on both days.
Locally heavy downpours and frequent cloud-to-ground lightning should be anticipated on both of these days as well, although storms will start to move faster compared to prior days thanks to stronger winds aloft.
Here is the general summary of thunderstorm activity and behavior expected over the coming days:
Thursday (7/1): Widespread thunderstorms, locally heavy downpours, slow and erratic east to west movement, storms possible overnight.
Friday (7/2): Scattered (less widespread) thunderstorms, locally heavy downpours, slow west to east movement, storms possible morning through evening but unlikely overnight.
Saturday (7/3): Isolated thunderstorms favoring higher peaks, locally heavy downpours still possible despite fewer storms, slow west to east movement, storm activity mainly confined to afternoon/early evening.
Sunday (7/4): Scattered to widespread thunderstorms, locally heavy downpours, faster west to east movement, storms possible overnight.
Monday (7/5): Scattered to widespread thunderstorms, locally heavy downpours, faster west to east movement, storms possible starting in the morning.
Dewpoints are projected to remain well into the 40s throughout the next five days and could even approach 50 at times. It will be something to watch as dewpoints in the 50s in Jackson, while less common, typically result in our strongest and heaviest rainfall producing thunderstorms of the summer.
Perhaps the one limiting factor for the Jackson Hole Valley (but not the Tetons) is that steering winds aloft will not be coming from the south or southwest in this pattern, which is the direction that typically results in our deepest fetch of moisture and most widespread thunderstorm activity in the Jackson Hole valley without terrain interference from the Tetons.
Even so, there will be enough other factors to expect a stormy several days throughout Teton County. Be sure to plan summit attempts, high elevation hikes, or lake activities for early in the days, while also keeping a close eye out just in case any early day thunderstorms develop, which is a possibility in this pattern.
Quick Lightning Safety Reminder
When it comes to high elevation hiking/climbing and peak-bagging when thunderstorms are a possibility, the first thing you can do to reduce your chances of getting caught in a thunderstorm is to get an early start and be off the summit before storms develop.
Depending on your objective, that could mean a pre-dawn start to reduce your threat.
Also, be sure to constantly keep a close watch on the sky, looking for vertically growing “puffy” cumulus clouds. Thick and tall rapidly rising clouds high into the atmosphere with darkening bases are a clear sign of storm development, and if the cloud starts to take on a “mushroom” shape with a flat anvil extending outward from the top of the cloud, that is a sign of a mature thunderstorm.
If you start to see these obvious signs of a thunderstorm, and of course if you hear thunder or see lightning, then begin to descend immediately. If you are caught in a thunderstorm, rather than taking cover in a lightning crouch position, newer research and advice suggests the best course of action is to continue descending as efficiently as possible.
Below treeline, places that are safer to take cover include low-lying ravines or drainages, and forests with relatively uniform tree cover. What you want to avoid at all costs is isolated trees or small clusters of only a few trees in otherwise open areas. You are better off being caught in open treeless meadows than near a lone tree.
Drying trend expected mid to late next week
The upcoming pattern will feature higher odds of wetting rains with thunderstorms and that is a good thing, though at this time of year dry lightning and fire starts still can’t be ruled out. Even so, let’s hope we can pick up some beneficial moisture from thunderstorms.
Starting on Tuesday or Wednesday of next week, we should start to see a downtick in thunderstorm activity as drier air arrives and monsoonal moisture becomes suppressed well to the south.
It’s possible we could see a couple of days of “dry” thunderstorm potential, which is not good for fire danger, but then by late next week odds favor minimal thunderstorm chances with dry conditions and hot temperatures.
In fact, medium to long-range guidance suggests highs getting into the 90s by the end of next week.
June finishes much hotter and drier than average
Depending on your exact location, this past June was one of the hottest ever in Jackson Hole.
Records are tricky because the Jackson weather station has exhibited a cool bias in temperature reports during the past 10 years (meaning reported temperatures are likely cooler than reality) whereas the Moose weather station has exhibited a warm bias during this same time period – both based on temperature comparisons with surrounding weather stations.
Taking this into account, the more reliable Moran weather station recorded its second-hottest June ever dating back to the early 1900s. Only June of 1988 was hotter at this location.
In spite of the mentioned biases, the Jackson weather station recorded its 11th hottest June on record while the Moose weather station recorded its number one hottest June ever. Remember, recent temperature records should be taken with a grain of salt at these locations.
Elsewhere across the region, Pinedale experienced its hottest June on record, Afton experienced its second hottest June on record, and Alta experienced its third hottest June on record.
Rainfall was low across Teton County and surrounding areas in June with most stations recording between 0.2 and 0.4 inches for the month, which is well below average. Most of the rain occurred during thunderstorms late last week after a nearly dry first three weeks of the month.
Fire danger is high heading into July following a dry spring season and a hot and dry June. In fact, the Town of Jackson has canceled its fireworks for the Fourth of July this year due to fire danger.
On the bright side, at least we missed out on the extreme all-time record-breaking heat that has impacted the Pacific Northwest. Seattle (108ºF) and Portland (116ºF) both shattered their hottest temperatures on record by several degrees dating back to the late 1800s. Incredibly, just over the border, Lytton, British Columbia set an all-time record high for the entire nation of Canada this week with a high temperature of 121ºF. Wow!
Alan Smith, Meteorologist