JACKSON, Wyo. — If you’re tired of dry and smoky conditions, there is relief on the way. An impressive surge of moisture associated with the North American Monsoon will set up over Jackson Hole and Western Wyoming during the second half of this week.
While the Four Corners region typically sees the highest impact and most rainfall from the monsoon from late June through early September, we periodically see monsoonal moisture filter into Western Wyoming during the summer that leads to upticks in thunderstorm activity over a few days. That has already happened a couple of times this month.
However, this monsoonal moisture setup will be more robust and prolonged than what we usually see. In fact, there is a good chance that the next seven days will end up being the wettest mid-summer (July-August) period of weather for Teton County in several years. Not since 2014 or 2015 have we seen a monsoon pattern quite like this.
The monsoon fluctuates in intensity from year to year, but after several years in a row with a weak monsoon, the North American monsoon has been quite strong so far this summer with impressive rain totals across Arizona, New Mexico, Southern Utah and Southern Colorado in recent weeks.
Even California just experienced a monsoonal surge with heavy rain and localized flash flooding across the Southern California Mountains and Sierra Nevada Range.
Over the second half of this week and into the weekend, a ridge of high pressure will set up in an ideal location over the Central Plains, which will allow for monsoonal moisture (associated with an already active monsoon) to rotate clockwise around the High and surge northward into Western Wyoming.
Atmospheric moisture will be around 200% of average for our region starting on Wednesday and persisting through Monday. This amount of moisture will not only provide plenty of fuel for thunderstorm development, but also for heavy rainfall. Thunderstorms that develop will be more intense than what we are accustomed to in Western Wyoming.
Dewpoint values will be in the 50s in Jackson Hole during this weather pattern. This might not seem like much if you’re from the Midwest or Eastern U.S. but is impressive for Jackson Hole and Intermountain West standards and enough to produce intense thunderstorms when terrain-forcing from surrounding mountain ranges is factored in.
It’s one thing to have significant moisture in place to fuel thunderstorm development, but if wind speeds aloft are fast, then heavy rain may only fall for a short period of time. This is often the case in Western Wyoming.
However, in this pattern, winds aloft are going to be very weak. As a result, thunderstorms will be slow-moving, which further increases the threat of heavy rain. There is also potential for localized flash-flooding from strong, slow-moving thunderstorms that develop.
Timing of Thunderstorms and Rainfall
The leading edge of moisture will arrive late on Tuesday, but initially, only isolated thunderstorms with light rain are expected on Tuesday afternoon and evening. If thunderstorm activity manages to persist later into the evening, then some isolated downpours couldn’t be ruled out.
However, the main uptick in thunderstorms and rain from this pattern will occur from Wednesday (July 28) through approximately Monday (August 2). Numerous thunderstorms can be expected each day, although at this time, there are no clear indicators if any one day will be more active than the others. Stay tuned to daily forecasts as we work out potential day-to-day variations.
On any given day, thunderstorms will be most likely, and most numerous, during the afternoon and evening hours. However, this type of pattern sometimes results in showers and even thunderstorms at odd hours of the day, such as the early morning hours. Keep this in mind if you have hiking, climbing and/or camping plans.
Significant Lightning Risk Across the Higher Elevations
If you have plans to climb any of the higher peaks in the Tetons or surrounding ranges or to hike over high mountain passes, then you should really be on guard from Wednesday through Monday.
As previously mentioned, the thunderstorms we will experience in this pattern are not your typical run-of-the-mill storms. Aided by significant monsoonal moisture that is subtropical in nature, this week’s thunderstorms will be particularly intense and will produce frequent cloud-to-ground lightning.
Alpine starts will be mandatory if you plan to climb any of the Teton peaks, and even then you’ll want to keep a close eye on the skies for possible early-day thunderstorm development. Thunderstorms should be expected by noon each day, and on some days they may develop earlier.
Anyone planning to climb the Grand Teton between Wednesday and Monday should be particularly wary and have bail-out plans in place in case of early-developing storms.
The image below from the National Weather Service in Salt Lake City is a good visual tool for assessing lightning risk in the backcountry based on terrain.
Heavy Rain and Flash Flooding Risk
Intense, slow-moving thunderstorms will have the potential to produce heavy rainfall rates and localized flash flooding. Steep terrain and areas with poor drainage will be most susceptible to flash flooding. If you’re out hiking, be extra cognizant of loose, rocky terrain and also creek and stream crossings that could run high after a heavy thunderstorm.
Wildfire burn scars from recent years will be most susceptible to flash flooding and runoff so be extra cautious if you live or recreate near these areas.
July and August are the two driest months on average in Teton County with most areas averaging between 2 and 2.5 inches of rain during the two months combined.
However, this pattern will be much wetter than average – over the next seven days, the average rainfall projections of major weather models are showing between 1.5 and 2.25 inches of rain across Teton County and even higher amounts across the Salt River, Wyoming and Wind River Ranges. And this doesn’t factor in isolated heavier amounts that could occur under strong, slow-moving storms.
As a rule of thumb, thunderstorms on any given day will produce anywhere from 0.1 to 0.5 inches of rain, but rain amounts of up to an inch or more within 1-2 hours could not be ruled out under intense thunderstorms, which is where the flash flooding risk comes into play.
Fire Danger and Smoke Conditions
The good news is that this is certainly setting up to be a wet thunderstorm pattern rather than a dry thunderstorm pattern. So while our fire danger is high and lightning activity will be significant, the amount of moisture and potential rainfall should help our fire danger more than hurt it.
The only day we should be concerned about lightning-triggered fires is on Tuesday afternoon and early evening, when low level moisture will still be lacking and lightning could occur with gusty winds and minimal rainfall.
Also, once this pattern relents by the middle of next week, we could head back into an isolated dry thunderstorm regime. However, if we see significant rainfall in the days prior, this should at least temporarily reduce the fire risk next week.
Keep in mind that thunderstorm rainfall is still variable in nature, and we should not let off our guard in terms of fire danger during this pattern despite the rainfall forecast.
As for the smoke, a southerly flow will direct most of the smoke away from Jackson Hole, and we’ll finally be able to breathe some fresh air from Wednesday through Sunday or Monday.
Next week, winds aloft will shift back to southwesterly and eventually westerly, and as a result, smoke from fires in California, Oregon and Idaho will likely return.
Alan Smith, Meteorologist
