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5-Day Temperature Outlook (June 19-23):

A strong cold front will move across the West late this week and over the weekend, leading to a significant cooldown from West to East.

The West Coast and portions of the Interior West can expect below-normal temperatures over the next 5 days, while the Southern Rockies will be above-normal over the next 5 days where the cold front will be arriving later. 

Thursday (June 19):

A trough will push into the Northern Rockies with increasing moisture and instability, resulting in scattered afternoon and evening thunderstorms. 

Most storms will produce light to moderate rain, but locally heavy downpours and strong wind gusts are possible under stronger storms. Showers are also projected to redevelop overnight across Central Idaho and Western Montana. 

The remainder of the West will largely stay dry on Thursday, but a few isolated thunderstorms may pop up over the New Mexico Rockies and Far Southern Colorado. Any storms that develop will likely do so over the higher peaks and will be short-lived, producing brief light rain and gusty outflow winds.

Friday (June 20):

A deeper trough will move into the Northwest, and a strong cold front will track across the area with Pacific moisture arriving behind the front.

Widespread rain showers will develop along the West Coast and over the Olympic and Cascade Ranges, with Western Oregon favored for the heaviest rainfall as this area will be in the favored left exit region of the jet stream.

Thunderstorms are also possible along and west of the Cascade crest on Friday afternoon, especially in Oregon, where instability will be higher. 

Temperatures will also drop substantially behind the cold front, with snow levels ranging from 5,000-6,000 feet across the Cascades on Friday and Friday night. 

Across the Interior Northwest, showers will also be possible during the morning hours and the afternoon and evening hours, but this region will also see a higher threat of thunderstorms in the afternoon. 

High levels of instability and wind shear will support the potential of strong to severe thunderstorms across Montana, with quarter-size or larger hail and 58 mph or higher wind gusts possible. Heavy rainfall rates are also expected with stronger storms. 

The Southwest and Central Rockies will remain in a hot and dry pattern overall, but once again, a few isolated thunderstorms could develop over the higher terrain of New Mexico and far Southern Colorado. Any storms that develop will produce brief light rain and gusty winds. 

Saturday (June 21):

The low pressure system will slowly work its way eastward on Saturday, with widespread rainfall expected throughout the Northwest and Northern Rockies, both day and night.

The cold front will also push eastward across the Rockies, with a more stable atmosphere behind the front resulting in a lower threat of thunderstorms, though isolated storms will still be possible in the afternoon.

Snow levels will dip to 6,000 to 7,000 feet over Idaho and Northwest Montana behind the cold front, which is close to the elevation of Logan Pass in Glacier National Park. 

Precipitation Totals (Friday AM to Sunday AM):

Rainfall totals will be significant throughout the Northwest, with the highest totals expected across Western Oregon and Northern and Western Montana. North of the border, the Canadian Rockies are expected to see substantial precipitation from this system. 

Accumulating snowfall is likely across the higher elevations in the Cascades and Northern Rockies. Over Southwest Montana and Northern Wyoming, snow levels will be a bit higher, ranging from 9,000-11,000 feet, which is still low enough for snow to possibly accumulate over Beartooth Pass.

North of the border, heavy snow is expected in the Canadian Rockies.

Sunday (June 22) to Monday (June 23):

Active weather will cover a larger portion of the West, with three main focus areas.

First, a trailing weaker trough will move across the Interior Northwest with lingering Pacific moisture, resulting in additional showers from the Cascades to the Northern Rockies.

Rainfall amounts over the Cascades and Olympics will be lighter, while the Interior Northwest and Northern Rockies will see heavier showers and thunderstorms as these areas will have greater instability and will be better positioned relative to the trough moving across the area.

Even so, isolated afternoon thunderstorms will also be possible over the Cascades. 

The second focus area is over the Eastern Rockies of Wyoming and Colorado, where a backdoor cold front will move through, with moisture increasing behind the front along and east of the Continental Divide.

A trough will also arrive from the southwest, setting the stage for an uptick in thunderstorms near and east of the Divide. Wyoming and Southern Montana will see storm chances ramping up as early as Sunday, while the cold front will arrive later in Colorado, where storm chances may hold off until Monday. 

The third focus area will over New Mexico, where monsoon season will kick off in impressive fashion. An increased southerly flow will transport remnant moisture from Hurricane Erik in the Eastern Pacific into New Mexico, setting the stage for numerous showers and thunderstorms with the potential for heavy rainfall and flash flooding.

Sunday looks like an appetizer day for now with isolated to scattered thunderstorms initially, with more widespread coverage and heavier rainfall expected on Monday. Still, the timing and coverage is subject to change this far out. 

Here is a rainfall projection from the European Model for Sunday-Monday...

Outlook for June 24-28:

Next week, a ridge will gradually rebuild over the West Central U.S., but a consistent southerly flow will reinforce a monsoonal-like pattern with steering winds, moisture favoring New Mexico and Colorado. More consistent afternoon thunderstorms are likely across these areas as a result. 

Temperatures are also expected to rebound to above-normal values across much of the West as summer heat returns, but New Mexico and Southern Colorado are expected to be cooler-than-normal due to the uptick in moisture and cloud cover.

The rainfall projection from the European Ensemble Model indicates the potential for significant rainfall across New Mexico and, to a lesser extent, Colorado. Lighter rainfall is also possible across the Northern Rockies and Cascades. 

Thanks so much for reading! Next update on Friday (June 20).

Alan Smith