The Treasure Valley and central Idaho mountains will see a significant weather pattern change beginning Thursday as monsoon moisture arrives ahead of an upper-level trough, bringing thunderstorm chances, fire weather concerns, and the potential for heavy rainfall and flash flooding through the weekend.
Increasing moisture and storm threat as temps drop in Scott Dorval's video forecast
Smoke and Air Quality Through Wednesday
Wildfire smoke will continue to impact the Treasure Valley through Wednesday. Nighttime surface-based temperature inversions will continue to cap mixing and allow smoke to settle thickly into valley locations overnight, with brief improvements each afternoon as mixing conditions improve. Residents with respiratory conditions, the elderly, children, and others sensitive to smoke should continue to limit outdoor activities during the morning hours. Check AirNow.gov or the Idaho Department of Environmental Quality for real-time air quality conditions.
Warm and Dry Wednesday Before the Pattern Changes
Wednesday will bring another warm and dry day to the Treasure Valley with highs near 94 degrees and patchy smoke. A slight 15 percent chance of isolated thunderstorms will exist near the southern Idaho border Wednesday afternoon, with any storms producing lightning on dry fuels being the primary concern.
Fire Weather Concerns Increase Thursday
Monsoon moisture will arrive Thursday afternoon, increasing the threat of thunderstorms and raising fire weather concerns. Forecast soundings show enough mid-level moisture to fuel thunderstorms producing lightning as early as Thursday afternoon. Critically, the lower levels of the atmosphere are expected to remain dry initially, which will limit rainfall potential while increasing the risk of new fire starts due to dry lightning. A 20 percent chance of showers and thunderstorms will develop after 3 p.m. Thursday, with a 30 percent chance continuing into Thursday evening.
Heavy Rain and Flash Flood Threat Friday
Friday will bring the most significant precipitation threat of the period as southwest flow aloft continues to transport substantial monsoon moisture into the region. Forecast moisture values are near the 80th to 90th percentile of normal, indicating high potential for heavy rainfall in any showers and thunderstorms that form. The Treasure Valley will see a 40 percent chance of showers and thunderstorms Friday, while the central Idaho mountains and areas near the Nevada border will see a 50 to 70 percent chance of precipitation.
Gusty outflow winds up to 35 mph, lightning, and heavy rain are the main hazards with these storms. Burn scars and steep and rocky terrain in the central Idaho mountains will be particularly susceptible to flash flooding and debris flows with any heavy rainfall. Anyone planning to be in the mountains Friday should be aware of this threat and have an evacuation plan. Monitor the National Weather Service Boise at weather.gov/boi for any flash flood watches or warnings.
Temperatures will drop 5 to 10 degrees below normal Friday and Saturday, with highs in the mid to upper 80s providing welcome relief from the recent heat.
Weekend and Beyond
Saturday will see mostly sunny skies and a high near 90 degrees as the moisture begins to taper off. A ridge of high pressure will build back into the region Sunday and Monday, bringing a brief warming and drying trend with highs climbing back to the low 90s. Lingering moisture will keep a 10 to 20 percent chance of precipitation over higher terrain through Monday. Another quick-moving low will bring another push of showers and thunderstorms late Tuesday.
Tonight
Patchy smoke. Partly cloudy, with a low around 62. Light wind.
Wednesday
Patchy smoke. Sunny and hot, with a high near 94. Light wind.
Wednesday Night
Patchy smoke. Mostly cloudy, with a low around 62. Light wind.
Thursday
A 20 percent chance of showers and thunderstorms after 3pm. Patchy smoke before 7am. Partly sunny & cooler, with a high near 87. Light wind.
Thursday Night
A 30 percent chance of showers and thunderstorms, mainly before 9pm. Mostly cloudy, with a low around 61. Light wind.
Friday
A 40 percent chance of showers and thunderstorms. Partly sunny & warm, with a high near 86.
Friday Night
A chance of showers and thunderstorms before midnight, then a slight chance of showers. Mostly cloudy, with a low around 59. Chance of precipitation is 30%.
Saturday
Mostly sunny and hot, with a high near 90.
Saturday Night
Partly cloudy, with a low around 60.
Sunday
Mostly sunny and hot, with a high near 93.
Sunday Night
Partly cloudy, with a low around 63.
Monday
Partly sunny and hot, with a high near 92.
Monday Night
Mostly cloudy, with a low around 62.
Tuesday
Partly sunny, with a high near 88.
