The first significant fall storm system of the season will impact the Northwest from Friday through Sunday. Strong winds can be expected across a large portion of the West in advance of this system.

By Friday afternoon, a large trough of low pressure will be located just off the British Columbia and Washington Coasts, with significant moisture and energy downwind of the trough impacting the West Coast and Cascades.

In fact, this is setting up to be an Atmospheric River Event with a plume of mild, subtropical air setting up over the West Coast and Cascades and extending toward the Idaho Panhandle and NW Montana.

Heading into the weekend, the trough will gradually move inland and a strong cold front will move through the Northwest, causing snow levels to fall.

On Sunday and Sunday night, shower potential will increase across the mountain ranges of Southwest Montana, Western Wyoming, Northern Utah, and Western Colorado with snow possible for some of these areas on Sunday night depending on the timing of the cold front.

Over the next 5 days (Wednesday through Sunday), below-average temperatures can be expected across the Pacific Northwest and California, with above-average temperatures for the Southern/Central Rockies and desert regions. However, these latter areas will see a cooling trend by late in the weekend.

Forecast for Wednesday, September 15th

A weak disturbance moving across the Northwest could produce a few light showers near the Canadian border in Idaho and Montana. Otherwise, generally quiet (and for some, windy) conditions can be expected across the West. A few stray showers/thunderstorms will be possible in New Mexico. 

Forecast for Thursday, September 16th

Post-frontal upslope winds (behind a weak cold front) could trigger a few showers along/east of the Divide in Montana and Wyoming with dry conditions prevailing elsewhere. Late Thursday night, showers will begin across the Olympic Peninsula and Cascade Range in advance of an approaching storm system.

Forecast for Friday, September 17th

Heavy rain will fall across the Washington Coast, Olympics, and Cascades on Friday with (relatively) moderate rain amounts extending southward across the Oregon Cascades and northeast into the Northern Idaho Panhandle and far NW Montana. Snow levels will remain relatively high (8,000 ft. or so in the Cascades) as this area will be in the "warm sector" prior to a cold front arriving late Friday night/early Saturday morning.

Forecast for Saturday, September 18th

Terrain-enhanced moderate to heavy showers will continue on Saturday behind the cold front with snow levels dropping to 6,000 feet across the Cascades. There will also be a chance of thunderstorms over the Cascades and West Coast as the atmosphere becomes unstable behind the front.

As the storm system gradually moves inland, shower coverage will expand into Northern California, Eastern Oregon, Central Idaho, and Western Montana. A few showers/thunderstorms could develop across Utah, Arizona, and Western Colorado on Saturday afternoon as well.

As colder air arrives, snow can be expected across the mid to high elevations of the Cascades, Wallowas, Sawtooths, and Bitterroots with more widespread and heavier snow north of the Canadian border.

Forecast for Sunday, September 19th

Showers will continue across the Cascades on Sunday, and will pick up in coverage and intensity across the Northern and Central Rockies on Sunday afternoon and Sunday night. There are still some question marks regarding the timing and southern extent of this inland push, however. 

Depending on the timing of the cold front as it moves inland, snow levels will continue to drop moving eastward on Sunday night with accumulating snow likely across the higher terrain of the Northern Rockies.

Outlook for Mon, Sept 20th - Fri, Sept 24th

The storm system will continue to impact the Northern Rockies on Monday (and possibly Tuesday) with a drying trend expected across the Cascades and West Coast. Another system is then possible during the second half of next week across the Northwest, but right now it looks much weaker than the previous system.

In general, temperatures will be below average across the Northwest and above average across the Southern Rockies and desert areas next week.

The Northwest and Northern Rockies will see the best chances of precipitation next week, though the Central Rockies (Northern Utah, Northern Colorado) should at least see some light showers early in the week.

Thanks so much for reading! Next update on Friday (9/17).

ALAN SMITH

P.S. Could you take a moment to rate the OpenSummit app with a 5-star review? These reviews help us to be seen by more people, and a larger audience provides the support needed for us to build more features and value for you.

Review iPhone app

Review Android app

Simply click either link above and scroll down to the section titled 'Ratings & Reviews'.