Rain, thunderstorms coming; flash flooding, debris flows possible
KENNEWICK, Wash. — First we get high heat up to 109 degrees, then more smoke will move in, and now it looks like incoming precipitation may produce storm cells that could potentially dump rain onto burn scars, causing topsoil mudslides.
The National Weather Service reports a system off of California will pull monsoonal moisture from the desert southwest into eastern Washington and Oregon. Expect rainfall and thunderstorms.
Here’s a nice loop of mid-level water vapor–and sure enough, there are scattered showers over Oregon late this morning. pic.twitter.com/bfTDJFJgb4
— NWS Seattle (@NWSSeattle) July 30, 2021
There will be scattered showers on Saturday afternoon in Washington state but the main activity begins Saturday night.
“Any significant moisture Sunday comes with concerns of debris flow on Sunday to Monday,” said NWS Meteorologist Matthew Callihan.
Wildfire burn scars are particular areas of concern. Places downhill and downstream from burned areas are very susceptible to flash flooding and debris flows. The weather service says rainfall that would normally be absorbed will run off extremely quickly after a wildfire, as burned soil can be as water repellant as pavement.
“It’ll be wet, and where there’s no vegetation holding things together, everything can slide around,” Callihan said.
Here’s what meteorologists will be watching for: rainfall over 3/10 of an inch in 30 minutes, or 6/10 of an inch per hour. Those rates on burn scars have the possibility of triggering debris flows.
Some regions, such as southern Umatilla County, La Grande, and Enterprise in Wallowa County could get an inch of rain. The Cascades near Snoqualmie Pass could receive a half-inch of rain.
Rain and thunderstorms are expected over southeast Washington fire zones such as the Green Ridge Fire and Lick Creek Fire on Saturday afternoon into Sunday night and Monday.
Mary Wister, incident meteorologist for the Green Ridge Fire, said she prepares crews day by day. On Friday, she called attention to the lower relative humidity, higher temperatures of 96-101 degrees, and 5-10 mph wind with gusts up to 15, which later that day caused the fire to spread and put up more smoke. Come Saturday, she’ll prep crews for rain and lightning.
“If I anticipate thunderstorms, I’ll give a heads up during our briefing: the best option is go underneath a hard-top vehicle,” said Wister.
With many crews sleeping outside in tents with poles resembling lightning rods, Wister watches the radar and alerts crews when necessary.
“It’s a challenge because, you give them a heads up, but it’s discouraging to have to wake them up at 2 am,” Wister said.
Lightning, of course, can create new fire starts. Callihan said storms in central Oregon sparked 41 starts within the past 24 hours.
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