Big Picture and Hurricane Hilary Update:

Hurricane Hilary is located just off the Baja Peninsula and will move northward this weekend, parallel to the Baja.

The center of the storm is expected to reach Southern California sometime on Sunday night, and could potentially maintain tropical storm strength by the time it gets to California – something that has only happened twice before in the past 100 years!

Here is the latest projected track of Hilary as of early Friday morning. The cone represents the range of possible tracks of the center of the storm.

Strong winds are certainly a possibility across Southern California and portions of the Southwest and Great Basin. But the main impacts will be heavy rain and flash flooding from SoCal all the way northward into Idaho and Eastern Oregon. Even LA and San Diego, two cities that typically see little to no rainfall in the summer, are expected to receive heavy rain.

As the storm moves northward, it will become entrained into a trough of low pressure near the West Coast and will follow the jet stream northward into the Inland Northwest.

Further east, the impacts of Hilary are not expected to be felt to the same degree across the Four Corners region and Central Rockies. However, varying amounts of monsoonal moisture will result in scattered thunderstorms across these regions from time to time.

Fire and Smoke Update:

Leading up to our weekend pattern change, fire activity and smoke coverage have increased substantially across the Northwest during the past week. This has coincided with a heat wave across the region, with hot and dry conditions and occasional dry thunderstorms supporting an environment conducive to wildfires.

On Friday, smoky conditions can be expected to varying levels across Northern California, as well as the Interior Northwest and Northern Rockies. 

As we head into the weekend, increasing south/southwest winds in advance of Hilary should shift the trajectory of smoke plumes into more of a SSW to NNE direction for many areas. 

Temperatures will also cool off this weekend and early next week with increasing moisture, which should help to reduce fire behavior in many areas unless winds are strong.

Significant rainfall is possible across portions of Idaho and the Inland Northwest, which could help as well. While areas closer to the coast across the PNW are less likely to receive significant rain. 

Forecast for Friday:

Showers and thunderstorms will largely be monsoon-related across the West with activity favoring the Four Corners states as well as portions of the Northern Rockies and the Sierra. Stronger storms will be capable of producing frequent lightning and heavy rain, while flash flooding will be possible in slot canyons and dry washes.

A trough of low pressure over Western Canada will also result in more widespread showers and thunderstorms across portions of Eastern BC and Western Alberta. This activity will favor the Northern Canadian Rockies (Jasper and Mt. Robson) and temperatures will be cold enough for snow to fall across the higher elevations.

Forecast for Saturday:

Moisture associated with Hilary will reach SoCal, Western Arizona, and Southern Nevada on Saturday and Saturday night, resulting in widespread rainfall.

Monsoonal moisture will also remain in place across Utah and Colorado with a bit of a northward surge also expected across Wyoming, Idaho, and Montana. Scattered thunderstorms capable of producing heavy rain and frequent lightning will be possible across all of these regions.

Across Northern California and Southern/Eastern Oregon, more isolated thunderstorms can be expected as these areas will be on the fringe of the moisture.

Forecast for Sunday:

Heavy rain will fall throughout Southern California and into the Southern Sierra Nevada Range and adjacent foothills/valleys. Nearly all of Nevada can expect rain as well, with the heaviest amounts through Sunday night expected across the southern half of the state. Significant flooding is possible across these regions.

Moisture will also reach Idaho and Western Montana where locally heavy rain will be possible. 

The main plume of heavy rainfall projected in the map below is subject to slight eastward or westward fluctuations, based on the exact track of Hilary and its interaction with the trough of low pressure located near the West Coast.

Further east, conditions will dry out across Eastern Utah, Colorado, New Mexico, and Southern Wyoming.

Forecast for Monday to Tuesday:

Confidence is a bit lower in this forecast as it will depend on the precise track and speed of Hilary, which will be a post-tropical cyclone by this point as it moves into the Inland Northwest.

Heavy rain is likely to continue across Nevada and Idaho with Eastern Oregon expected to see heavy rain as well. Montana is a bit less certain, but western portions of the state could see heavy rain as well. A little bit of moisture may also make it into the Oregon Cascades. Moisture will linger over California into Monday with an overall drying trend expected by Tuesday.

Further east, monsoonal moisture will return to the Four Corners region and Central Rockies by Tuesday with an uptick in thunderstorms expected following a temporary break.

A new trough of low pressure is also expected to move into Western Canada with increasing showers and thunderstorms across portions of BC and Alberta. Some light showers also couldn't be ruled out in the WA Cascades, but this system may miss them a bit too far to the north. 

Outlook for Wednesday (Aug 23) to Sunday (Aug 27):

A trough of low pressure will set up near the West Coast during this period and will interact with lingering moisture from Hilary as well as an abundance of monsoonal moisture to result in above-average rainfall across much of the West. 

No major heatwaves or deviations from average are expected during this period, though areas east of the Continental Divide will likely feel the heat more than other areas.

Otherwise, areas near the West Coast and near the Continental Divide are likely to be a bit warmer than average, while areas across the Interior West should be fairly close to average. 

Thanks so much for reading! Next update on Monday (August 21).

Alan Smith