Big Picture:

Hilary reached Southern California as a tropical storm on Sunday, where heavy rain fell causing flash flooding and mudslides. Hilary is now a post-tropical storm and has become absorbed into a trough of low pressure along the West Coast. However, the impacts are still being felt across the Interior Northwest and to a lesser extent further east.

Widespread soaking rains along with a flooding risk will exist from Northern California to Eastern Oregon, Idaho, and Northwest Montana on Monday and Tuesday. Another trough moving into Western Canada will tap into moisture from Hilary, bringing heavier showers and much-needed rainfall to Eastern BC.

Along the eastern periphery of this system, significant wind shear (change in wind speed and direction with height) along with warmer temperatures and greater instability will result in a chance of strong to severe thunderstorms across Arizona, Utah, Western Wyoming, and Southwest Montana on Monday and Tuesday. 

Fire and Smoke Outlook:

Somewhat overshadowed by Hilary in recent days has been the wildfire situation in BC and parts of Eastern Washington. Significant wildfire activity occurred in these regions during last week's heat wave, and conditions worsened over the weekend prior to the arrival of moisture due to strong winds.

Large fires have been impacting more populated areas as well, including Kelowna, BC, and Spokane, WA.

Fortunately, moisture and cooler air are beginning to reach these areas so we should see some relief in the coming days.

Fire activity has quickly tapered down across Idaho and Western Montana thanks to widespread heavy rain from Hilary. Smoke from fires in Northern California, Western Oregon, Washington, and BC will continue to impact portions of the Northwest through Monday evening.

On Tuesday, cooler air and higher relative humidity, along with soaking rains for some areas, will result in an overall reduction in fire activity and smoke, with an eastward shift in smoke also expected away from the coast due to a shift from offshore to onshore winds.

The smoke outlook is less certain for the remainder of the week as it will depend on how the fires respond to rainfall (some of which will be localized) and the cooler airmass. But in general, improvement is expected for the Northwest/Western Canada region as a whole.

Forecast for Monday Afternoon to Monday Night:

Widespread rain can be expected in a southwest-to-northeast trajectory from Northern California and Western Nevada to Northwest Montana with the heaviest rainfall expected in Eastern Oregon and West Central Idaho. All of these areas will see an elevated threat of flash flooding and excessive runoff.

Strong winds aloft (in the middle to upper atmosphere), combined with a warmer/more unstable atmosphere and significant moisture from both Hilary's remnants and the monsoon will result in thunderstorms across Utah, Arizona, Western Wyoming, and Southwest Montana.

Strong to severe thunderstorms will be possible across these areas with powerful straight-line winds and frequent cloud-to-ground lightning possible. There is also a threat of flash flooding across Arizona and Southern Utah, especially in slot canyons and dry washes. 

Forecast for Tuesday:

Hilary's remnants will begin to wane somewhat with a drying trend beginning in California and Nevada aside from light lingering showers. However, another wet day can be expected across Eastern Oregon, Idaho, and Northwest Montana with locally heavy rain and flash flooding possible.

Also, a trough of low pressure moving into Western Canada will tap into some of this moisture to result in more widespread showers and thunderstorms across Eastern BC and Western Alberta. Some of this rainfall should reach the Kelowna, BC area to help with the fire situation, though it's uncertain just how much will fall and the extent of relief expected.

Light showers are also possible across the Olympics and Cascades in Western Washington.

Further south and east, significant moisture, wind shear, and instability will remain in place across Arizona, Utah, and Western Wyoming. As a result, another active day can be expected with scattered to widespread thunderstorms, some of which will be strong to severe.

Gusty straight-line winds, lightning, and heavy rain/flash flooding (especially in slot canyons) will be the main threats with storms across these areas. However, a tornado also couldn't be ruled out in Utah.

Further east, thunderstorm chances will return to Western and Central Colorado, but severe weather is not expected.

Forecast for Wednesday:

Hilary's remnants will continue to wane with just some lingering lighter showers and thunderstorms across the Northern Rockies. Heavier showers will be possible north of the border in British Columbia, which will continue to help with the fire situation.

Further south, a more typical monsoonal pattern will take hold across the Four Corners states with scattered afternoon thunderstorms. Severe weather is not expected, except perhaps on a very isolated basis, but frequent lightning will be possible along with locally heavy rain and slot canyon flash flooding.

Forecast for Thursday to Friday:

Monsoonal moisture will increase across the Southwest and the Rockies during this period, and southwest winds associated with a trough of low pressure near the West Coast will transport abundant moisture into Wyoming as well.

There is also a chance we could see more widespread storms and heavy rain across Southern Wyoming and the Front Range of Colorado on Friday.

Moisture is also expected to reach portions of the Inland Northwest with an uptick in thunderstorm activity expected. The Sierra Nevada Range in California may see some isolated activity as well, but not much moisture is expected at this time. 

Outlook for Saturday (Aug 26) to Thursday (Aug 31):

An active pattern is expected across the West during this period with a consistent monsoonal moisture circulation resulting in regular rounds of thunderstorms across the Southwest and Southern/Central Rockies. Meanwhile, a slow-moving trough of low pressure is expected to bring some moisture into the Pacific Northwest.

Despite the overall active pattern, temperatures are expected to be warmer than average for most of the West as we finish out August.

Thanks so much for reading! Next update on Wednesday (August 23).

Alan Smith