Forecast Highlights:

• Significant rainfall for the Pacific Northwest Thursday-Sunday.
• Well-above average warmth for California, Southwest, Rockies Thursday-Saturday.
• Rain, thunderstorms, and high elevation snow possible for the Northern Rockies early next week.

Synopsis:

Minor disturbances will impact portions of the Northern and Southern Rockies on Wednesday. Attention then turns to a late-season atmospheric river event that will impact the Pacific Northwest from Thursday through Sunday with significant rainfall.

At the same time, a ridge of high pressure will strengthen over the Intermountain West, resulting in a trend toward well-above-average warmth with limited rain chances from California to the Rockies by late this week. 

On Thursday, the pattern will begin to turn very wet across the Northwest as a plume of subtropical moisture takes aim at the Cascades. The amount of water vapor in the atmosphere will be 150-200% of average for this time of year. 

The period from Thursday through Saturday looks particularly wet with rainfall amounts in excess of 2 inches projected across the Cascades and Olympics in Washington and Oregon.

Forecast for Wednesday:

We're looking at a relatively quiet day across most of the West compared to recent (and upcoming) patterns. Still, there are a couple of features that will bring unsettled weather to portions of the Rockies, while the Cascades and Northwest coast will enjoy a welcome break.

First, a minor disturbance moving across the Northern Rockies will bring widespread light to moderate showers to portions of Idaho and Montana, especially in the afternoon and evening hours. A few thunderstorms are possible as well, though instability is expected to be weak.

Further south, a cold front moving from north to south across the high plains will back its way into Southern Colorado and New Mexico. This feature will result in a chance of isolated thunderstorms near and east of the Divide with better chances of thunderstorms further east on the plains.

Our forecast radar for Wednesday afternoon looks unimpressive for mountain areas, but I still wouldn't rule out some afternoon lightning strikes along with brief showers if you're recreating near or east of the Continental Divide in Colorado or New Mexico.

Other models are projecting higher precipitation amounts across the New Mexico Rockies, but rain/thunderstorm coverage should remain spotty. 

Forecast for Thursday:

Southwest flow associated with a trough of low pressure approaching the Washington Coast will usher in significant moisture into the Cascades with widespread soaking rains developing across the Cascades.

The Olympics and Washington Cascades look to take the brunt of Thursday's rainfall and excessive runoff and rising streams are possible as these areas are already saturated from recent rain events. Moderate rain will work its way into Northern Idaho and Eastern British Columbia as well.

Fringe-level moisture and instability further east will result in a few isolated afternoon storms along and east of the Continental Divide from Montana to New Mexico.

Forecast for Friday:

Widespread rain will continue across the Cascades on Friday and to a lesser extent the Idaho Panhandle, with lighter showers extending into Northwest Montana as well. A threat for isolated thunderstorms will continue near and east of the Continental Divide across the Rockies.

Meanwhile, a strengthening ridge of high pressure will support warm to hot temperatures for much of the West (outside of the PNW) with temperatures running well above average for early/mid June, especially in California, Nevada, and Utah. Note the units in the map below are in ºC and not ºF.

Outlook for Saturday (6/11) to Tuesday (6/14):

There is some uncertainty with the position of the next low pressure system as we head into the weekend. But the latest trends are hinting that it begin to dip further south on Saturday with the core of the moisture plume taking aim at Oregon.

Showers and thunderstorms are also likely to increase in coverage across the Interior Northwest including Idaho and Western Montana in this pattern.

On Sunday, the system should start to move ashore from the Pacific with continued uncertainty as to the southward extent. The European Model seems to be the most middle of the road solution right now with the storm dipping into Northern California before swinging across the Northern Rockies on Monday.

Depending on the exact storm track, portions of the Northern Rockies could be looking at a good soaker on Sunday night and Monday, and if the system track far enough south then the Sierra, Northern California, and Utah could get in on the action as well. The Cascades should also remain in a wet pattern, though rainfall should generally be lighter and less widespread compared to prior days. 

Much colder air will also be arriving with this system early next week, and as a result, the potential exists for accumulating snowfall across the higher elevations of the Northern Rockies.

On Tuesday, confidence in the pattern further decreases, but rain (and high elevation snow) chances should continue to some extent across the Northern Rockies.

Outlook for Wednesday (6/15) to Wednesday (6/22):

The extended range pattern continues to look cooler than average across the Northwest, while California also looks quite a bit cooler during this period compared to the upcoming heatwave. The Four Corners region will be favored for the warmest temperatures.

A minor wet signal is showing up across the Northwest during this period, which makes sense as multiple troughs are likely to track across the region during this period. 

Interestingly enough, although it's a bit early for monsoon season, we are seeing wet signs of increasing moisture across the Southwest during this period as well. This could be a good thing (needed rainfall) or bad thing (lightning-triggered fires) depending on how much moisture arrives.

Thanks so much for reading! Next update on Friday (6/10).

ALAN SMITH