JACKSON, Wyo. — Last Friday’s weather created quite the stir in Jackson Hole as a Pacific storm system moved. The pattern featured cooler temperatures, thunderstorms, wetting rains – and the sudden arrival of heavy smoke.
Oftentimes, when wildfire smoke is present and an organized weather system rolls through with stronger winds and precipitation, smoke ends up dispersing and clearing out, at least to some extent. Initially, that’s what happened in this case, too.
As the storm system approached, strong westerly winds aloft scoured out most of the smoke that was previously in place, and smoke transport from California fires shifted farther south into Nevada and eventually Utah. Rain from the storm system also led to a reduction in fire behavior across the Pacific Northwest.
Smoke levels on Thursday night were low across Jackson Hole.
Meanwhile, the weather turned very active in Jackson Hole starting on Thursday afternoon when frequent rounds of thunderstorms began tracking over the Teton Range. This pattern continued on Thursday night with thunderstorms impacting the Tetons throughout the night. Those who were camping in the backcountry probably did not get much sleep!
The pattern of showers and thunderstorms every one to three hours continued through Friday morning as a cold front arrived, and Jackson Hole experienced its coolest daytime high temperature (66ºF at the airport) in nearly two months. Rainfall was impressive as well, with rain totals across most of Teton County ranging from a quarter-inch to an inch on Friday.
The air quality index had fallen to low levels – well into the “Good” category – and we were able to finally breathe fresh air and enjoy the cool and wet conditions through the morning hours on Friday. It was a nice reprieve during what has been an otherwise hot, dry and smoky summer.
Then the smoke returned with a vengeance.
At first, skies started to become hazy again in between thunderstorm rounds early on Friday afternoon. Then, during the final round of thunderstorms later Friday afternoon, thick smoke arrived almost instantaneously. The smoke combined with the cloudy skies and recent moisture produced an eerie orange glow across the valley.
Many Teton County residents were concerned that new fires had started locally due to lightning. As it turned out, there were a handful of small fires that were started by lightning in and around Teton County, but they were all very small and posed a minimal threat for spread due to recent rainfall.
According to the Teton Interagency Fire Center, all of the small fires in Teton County are now out as of Monday, August 9, while one small fire in the Pinedale Ranger District is under control.
Instead, all of the smoke that arrived came from California where explosive wildfire growth occurred during the prior week – most notably from the Dixie Fire, which is now the second-largest fire in California’s recorded history.
As the storm system moved across Wyoming, winds aloft shifted to southwesterly and began transporting this smoke toward Jackson Hole.
The visible satellite image from 9 a.m. Friday morning shows cloudy skies (and at the time, clean air) associated with thunderstorms and rain over Jackson (circled in red), with thick smoke approaching from the southwest.
When smoke first arrived at the upper levels of the atmosphere early on Friday afternoon, air quality remained “Good” in the valley with AQI values in the teens. So why did it suddenly get very smoky during the final round of thunderstorms?
Thunderstorms are composed of both updrafts and downdrafts. Updrafts are the result of warm and moist air that rises and fuels thunderstorm development and growth (think of towering cumulus clouds that grow vertically). Downdrafts are the result of cool, dense air that rapidly descends with rainfall toward the surface.
When the final round of thunderstorms moved through, strong downdrafts likely transported the smoke from the upper atmosphere to the valley floor.
The air quality timestamp from a local EPA particulate matter (PM 2.5) sensor in Jackson reveals the rapid arrival of smoke at the surface. At 3:30 p.m. the air quality index (AQI) was 24 in Jackson. At 3:40 p.m., the AQI had risen to 74. And by 3:50 pm., the AQI had risen to to 156.
In other words, the AQI had risen from 24 (Good) to 156 (Unhealthy for All Groups) in only 20 minutes!
At 7 p.m. on Friday evening, the AQI had further worsened to 220 (Health Alert), which is the worst the air quality has been all summer.
Check out the visible satellite image from Friday afternoon, which shows smoke (brownish color) covering Jackson Hole while cloud cover (white colors) associated with rain and thunderstorms were still present. Essentially, the smoke had become entrained in the low-pressure system moving across Wyoming.
Over the weekend, we experienced fluctuating smoke levels. Fortunately, another system passed north of the area on Sunday night, and a stronger westerly flow behind it scoured out the smoke, leading to sunny skies and clean air on Monday.
For the remainder of the week through Friday, most of the California smoke should stay south of Jackson Hole with varying degrees of lighter smoke and haze from Northwest wildfires arriving at times.
This weekend and next week, we will likely see some of the California smoke return as winds aloft transition to a more southwesterly direction. In addition, hot temperatures will return late this week with highs of around 90 expected in the valley on Friday and Saturday.
Dry conditions will prevail through Friday, then on Saturday and Sunday, we will see a chance of isolated thunderstorms as some monsoonal moisture attempts to sneak in.
Any weekend storms will likely be of the “dry” variety and will not produce much rainfall. Winds will be on the increase as well, especially on Sunday, which will result in some fire weather concerns.
The pattern for early next week is looking more interesting as a Pacific storm system is projected to drop in out of the northwest, leading to cooler temperatures and a better chance of showers and thunderstorms – perhaps with some beneficial wetting rains, though confidence is low on the latter.
Alan Smith, Meteorologist