JACKSON, Wyo. – The National Weather in Riverton has issued a Flash Flood Watch for Jackson Hole and the Tetons, along with much of Western Wyoming, from Friday morning through Sunday evening. The Watch does NOT include Yellowstone at this time, though isolated flash flooding could not be ruled out there either.
A significant plume of monsoonal moisture has set up across Western Wyoming with precipitable water levels (the amount of water vapor in the atmosphere) running well above normal for this region. Winds aloft are also relatively weak, which will result in slow-moving thunderstorms.
Strong and slow-moving thunderstorms that develop and are able to sustain themselves will have the potential to produce locally heavy rainfall, especially in the mountains.
Excessive runoff and isolated flash flooding will also be possible each day from Friday through Sunday, with small creeks and streams, low-lying areas and steep/narrow terrain with poor drainage being the most vulnerable areas for runoff.
Keep in mind that this pattern will feature hit-or-miss thunderstorms, so some areas could see heavy rainfall while other areas just a few miles away could see little to no rain on any given day.
On Friday, the highest potential for heavy rain will occur south and east of Jackson across Star Valley, the Salt River Range, Wyoming Range and Wind River Range due to better support and more widespread coverage associated with a disturbance moving into Southwest and West Central Wyoming.
Locally, the Southern Tetons and Jackson Hole Valley will be more favored for thunderstorms compared to areas further north, and heavy rain is a possibility here as well.
On Saturday, storm coverage looks to be a bit more widespread throughout Western Wyoming as a disturbance arrives from the West. As a result, this should be the most active day of thunderstorms across Teton County and Yellowstone, and will also feature the greatest potential for heavy rain and localized flash flooding. Storm coverage should be greatest during the afternoon and evening hours.
On Sunday, the bulk of the thunderstorm activity is expected to shift east and south a bit with Star Valley and the Wind River Range being the most favored for heavy rain. However, isolated to widely scattered thunderstorms will remain possible across Teton County, and any stronger thunderstorms that develop will be capable of producing heavy rain.
From Monday through the middle of next week, moisture will decrease across the area with lessening thunderstorm potential. Isolated afternoon thunderstorms will remain possible, especially over the higher elevations, but rainfall will be lighter.
Toward the end of next week, there are some hints we could see another uptick in monsoonal moisture with increasing thunderstorm chances.
High temperatures over the next 7-10 days will generally be in the low 80s, but could be warmer or cooler on any given day depending on the extent of cloud cover and timing of thunderstorms.
Note: I will be traveling next week and will not be writing a weekly weather outlook. Check back in for my next outlook on Tuesday, August 23.
Alan Smith, Meteorologist