A deep upper-level low approaching the Pacific Northwest will bring widespread showers, significant rainfall totals, and temperatures 10 to 20 degrees below normal to the Treasure Valley and central Idaho mountains Wednesday night through Thursday. This system will provide meaningful relief to an active fire season across the region.
Cooler and wetter weather ahead in Scott Dorval's video forecast
Wednesday: Showers and Storms Before the Main Event
Wednesday will bring a 40 percent chance of showers and thunderstorms, mainly before 10 a.m., as a shortwave trough rotates across the area. Temperatures will drop to near 78 degrees, about 10 degrees cooler than Tuesday. Winds will be gusty Wednesday afternoon, with gusts of 20 to 35 mph expanding to most higher elevation locations. Locally low relative humidity near the Idaho-Nevada border and gusty winds will lead to elevated fire weather conditions Wednesday afternoon.
Wednesday Night Through Thursday: The Main Event
The most significant weather of the week arrives Wednesday night as moist southwesterly flow sets up across west-central Idaho ahead of the approaching upper low. Showers will become widespread and likely Wednesday night, with a 70 percent chance of precipitation.
Estimated Rainfall Totals:
| Location | Estimated Rainfall Wed Night Through Thursday |
|---|---|
| Treasure Valley (Boise area) | Less than 0.10 inches |
| West end of Treasure Valley | Up to 0.15 inches |
| West-Central Idaho Mountains | 0.50 to 1.50 inches |
| Southeast Oregon | 0.25 to 0.75 inches |
| Central Idaho Mountains | 0.50 to 1.00 inches |
Note to journalist: Please verify these rainfall estimates with the National Weather Service Boise at weather.gov/boi before publishing.
Thursday will bring a 50 percent chance of showers to the Treasure Valley with highs near 72 degrees, running 10 to 20 degrees below normal. Weak instability will support a 10 to 20 percent chance of thunderstorms Thursday afternoon, with locally heavier rainfall possible from any storms that develop.
How This Rain Will Help the Fire Situation
This week's precipitation event could provide significant relief to firefighters and fire managers across the West. The central Idaho mountains and west-central Idaho, which have seen some of the most active fire behavior this summer, stand to receive the most significant rainfall totals of 0.50 to 1.50 inches. This level of precipitation can:
- Significantly reduce fire behavior and spread rates across active fire complexes
- Lower fire danger ratings from critical to moderate or low levels
- Provide meaningful relief to firefighters who have been battling blazes in extreme heat
- Help improve air quality by washing smoke particles from the atmosphere
- Reduce the risk of new fire starts from lightning
Even the modest rainfall expected in the Treasure Valley will help reduce fire danger in the surrounding foothills and improve air quality across the valley.
Weekend: Brief Improvement Before Another Wet Period
Precipitation chances will decrease Saturday into early Sunday as the low pivots back to the west, with mostly sunny skies and a high near 79 degrees Saturday. Sunday will bring mostly sunny skies and a high near 82 degrees before another round of showers arrives Sunday night.
The low is then favored to move east late Sunday into Monday, increasing precipitation chances to 30 to 60 percent for Labor Day. Temperatures will cool once again early next week as showers increase, with Labor Day highs near 77 degrees.
Forecast confidence is lower during this period as tropical activity off the coast of Baja California will have downstream forecast implications. There is potential for entrainment of tropical moisture into the interior West, which would increase the risk for heavy rainfall.
Tonight
A 30 percent chance of showers and thunderstorms, mainly after 5am. Increasing clouds, with a low around 58. Northwest wind 5 to 7 mph becoming calm after midnight.
Wednesday
A 40 percent chance of showers and thunderstorms, mainly before 10am. Cloudy through mid morning, then gradual clearing, with a high near 78. North northwest wind 5 to 8 mph becoming south southwest in the morning.
Wednesday Night
Showers likely, mainly after midnight. Cloudy, with a low around 55. Southwest wind 5 to 8 mph becoming east after midnight. Chance of precipitation is 70%.
Thursday
A 50 percent chance of showers. Mostly cloudy, with a high near 72. East wind around 6 mph becoming west northwest in the afternoon.
Thursday Night
A 40 percent chance of showers. Mostly cloudy, with a low around 51. North northwest wind around 6 mph becoming calm after midnight.
Friday
A 30 percent chance of showers before noon. Patchy fog between 7am and 9am. Otherwise, mostly sunny, with a high near 74.
Friday Night
A 30 percent chance of showers after midnight. Partly cloudy, with a low around 52.
Saturday
Mostly sunny, with a high near 79.
Saturday Night
Mostly clear, with a low around 55.
Sunday
Mostly sunny, with a high near 82.
Sunday Night
A 40 percent chance of showers. Mostly cloudy, with a low around 57.
Labor Day
A 40 percent chance of showers. Partly sunny, with a high near 77.
Monday Night
A 20 percent chance of showers. Partly cloudy, with a low around 53.
Tuesday
Mostly sunny, with a high near 77.
