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Western US Daily Summit

Cold storm system with snow for some mountain ranges

Summary

The first major storm system and cold front of the fall season will impact the Western U.S. this weekend. Heavy rain will fall across Washington, Oregon, and Northern California from Friday through Sunday. Colder air arriving will result in high elevation snow across the Cascades (above 6k), and more widespread snow across the Northern Rockies on Sunday & Sunday Night.

Short Term Forecast

The main weather feature over the next few days will be a powerful storm system producing heavy precipitation over the Pacific Northwest from Friday through Sunday, while colder temperatures and rain/snow showers associated with this system will reach the Northern and Central Rockies on Sunday and Monday.

A weaker "leading-edge" disturbance will also swing into the Great Basin and Central Rockies on Friday and Saturday, producing showers and thunderstorms ahead of the cold front. 

As of early Friday afternoon, widespread rain is falling across Washington and Southern Oregon, favoring the windward slopes of the Olympic and Cascade Ranges. 

The first significant cold front of the fall season will move across the Western U.S. over the coming days, starting with West Coast and Cascades on Friday night when precipitation rates could become intense for a few hours.

Check out the forecast progression of the cold front over the next 4 days.

Winds are going to be strong over the next several days throughout the Western U.S. ahead of and along the passage of the cold front, with gradually decreasing winds behind the cold front.

In general, temperatures during the 5-day period from Friday through Tuesday will be below average across the Northwest and above average in Arizona and New Mexico, but all areas will see a cooling trend during this time.

Precipitation totals from Friday through Tuesday morning will be heaviest across Western Washington, Western Oregon, and far Northern California. This rain will be very welcome for Northern California where active fires are burning, though the potential for burn scar flooding will also exist.

The Northern Rockies will see respectable precipitation amounts from this pattern as well with lighter amounts for the Central Rockies and Central California. Snow levels will drop to just above the valley floors across parts of the Northern Rockies on Sunday night/Monday morning.

Check out the projected precipitation totals from a blend of weather models through Tuesday morning:

Forecast for Friday, September 17th

Heavy rain will fall across the Washington and Oregon Cascades on Friday and Friday night with the heaviest totals across the Olympics and Northern Cascades. Lighter but still welcome rain will fall across Eastern Washington and the Northern Idaho Panhandle.

Across the Southwest, a weaker disturbance will produce some widely scattered showers and thunderstorms on Friday afternoon and Friday night. 

Forecast for Saturday, September 18th

The focus of the heaviest precipitation will shift southward into Oregon and Northern California on Saturday and Saturday night with fire relief expected for these areas. Heavy post-frontal showers will continue across the Washington Cascades behind the cold front as well with lower snow levels (6k feet or so). Thunderstorms are also possible across the Cascades as the atmosphere becomes unstable behind the cold front.

The weaker leading-edge disturbance will help to trigger more numerous showers and thunderstorms across Arizona, Nevada, Utah, Western Colorado, and Western Wyoming on Saturday. 

Forecast for Sunday, September 19th

The cold front will move across the Northern Rockies on Sunday with widespread precipitation developing along with falling snow levels. Post-frontal showers and thunderstorms will continue across the Cascades and Northwest Rockies (ID Panhandle, Wallowas, etc.) as well.

A few light snow showers could develop over the northern mountains of Utah and Colorado on Sunday night as the cold front moves through, but there is a greater degree of uncertainty in these areas.

Check out the projected snowfall across the Northern Rockies from Sunday through Sunday night:

Forecast for Monday, September 20th

The cold front will slide southward along the eastern slopes of the Continental Divide and adjacent plains on Monday with the best chance of showers expected along and east of the Divide as a result.

These areas will see lighter precipitation compared to what the western areas saw in previous days, but with colder air arriving, at least some snow can also be expected for the mountain ranges along the Divide. Confidence is higher that precipitation will fall across the Wyoming mountains with lower confidence farther south in the Colorado Front Range and Sangre de Christo Range.

Forecast for Tuesday, September 21st

Conditions will dry out across the entire Western U.S. on Tuesday with crisp blue skies likely for most areas in the wake of the cold front. Get outside and enjoy!

Extended Forecast

Outlook for Wed, Sept 22nd - Sun, Sept 26th

A much quieter weather pattern will develop mid to late next week. A weak disturbance will be possible across the Northwest and Northern Rockies around Thursday/Friday, but impacts look minimal at this time. The overall trend across the West will be for high pressure to build with warmer temperatures.

The good news is that above-average temperatures in late September = comfortable and not searing unless you're in the desert.

Mostly dry conditions are expected during this time, save for a chance of light showers in the Northwest courtesy of the previously mentioned weak disturbance. Next weekend should be outstanding for hiking, climbing, mountain biking, fishing, hunting, etc. 

Thanks so much for reading! Next update on Monday (9/20).

ALAN SMITH

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