Forecast Highlights:

• Flooding continues across the Northern Rockies.
• Pipeline Fire grows near Flagstaff, AZ with occasional smoke impacting the Southern Rockies.
• The North American Monsoon emerges weeks ahead of schedule with rain and thunderstorms increasing across the Southern Rockies this weekend.
• Another slow-moving low pressure system will bring rain and thunderstorms to the Northwest and Northern Rockies late this week through early next week. 

Synopsis:

A ridge of high pressure will build over the Intermountain West during the middle of the week, resulting in a rapid warm-up across the Rockies along with a drying trend across the rain (and snow) soaked Northern Rockies. 

However, another slow-moving trough of low pressure is approaching the West Coast, and a series of disturbances associated with the trough will move across the Northwest late this week, and eventually the Northern Rockies over the weekend.

The North American Monsoon will also get going this weekend, much earlier than usual, as subtropical moisture streams into portions of the Southwest and Southern and Central Rockies. This moisture surge will also be aided by a tropical storm in the Eastern Pacific.

The arrival of monsoonal moisture will result in an uptick in thunderstorms heading into the weekend, most notably across New Mexico and Colorado where locally heavy rain will be possible. 

Flooding continues across the Northern Rockies:

Historic flooding is occurring in Yellowstone National Park and surrounding communities outside of the northern and western boundaries of the park. This was a result of prolonged heavy rainfall over the weekend on top of an above-average snowpack that, prior to this event, had been much slower than usual to melt due to colder than average temperatures this spring and some heavy late-season snow events.

Flooding is also being reported west of Glacier National Park, due to heavy rainfall from the same system that produced substantial snowfall across the higher elevations of the park. 

The good news is that precipitation is subsiding and the second half of the week looks dry. However, a rapid warm-up is also expected on Thursday and Friday with temperatures rising to well above average levels. Substantial snowpack remains across the higher elevations, so the warm temperatures will lead to another period of acclerated snowmelt that could stress the already swollen and flooded rivers. Rain chances will also be on the increase this weekend.

Fires continue across the Southwest with varying degrees of smoke impacts.

Numerous wildfires are burning across New Mexico, Arizona, and Southern California at this time. The most active fire (in terms of growth and extreme fire behavior) in recent days has been the Pipeline Fire, which is located only about 6 miles north of Flagstaff and has consumed 25,000 acres in just two days (as of Tuesday).

Winds will subside a bit over the second half of the week, which is good news, and relative humidity will gradually increase as monsoonal moisture arrives late this week and over the weekend. However, the main plume of monsoonal moisture looks to stay just east of Flagstaff this weekend and only light rain is expected as a result. Also, winds will increase again this weekend which is not so great. 

Westerly flow on Wednesday will result in smoky conditions across New Mexico, who will be dealing with smoke from both the Pipeline Fire in Arizona and the numerous fires burning locally within New Mexico.

On Thursday and over the days to follow, winds aloft will shift to more of a southerly direction (winds blowing from south to north), and this will direct the smoke northward into Utah and Colorado, and even into Southern Wyoming.

Check out our Smoke Forecast Map (out to 48 hours) and Air Quality Forecast Map (out to 5 days) for the latest smoke projections. 

Forecast for Wednesday:

Wednesday will be relatively quiet in between systems as lingering rain/snow showers taper off early in the day around Glacier with activity spreading eastward out onto the plains. The Northwest will also be mainly dry during the day but showers will arrive on Wednesday night as a disturbance arrives.

Forecast for Thursday:

Showers will continue across the Pacific Northwest on Thursday, mainly impacting the Northern Cascades along with Northeast Washington with drier conditions further south into Oregon. 

Further south, the leading edge of the monsoonal moisture will reach New Mexico with scattered thunderstorms developing, while Colorado could see a few isolated storms as well, mainly over southern portions of the state.

Forecast for Friday:

The previously mentioned trough will begin to deepen along the West Coast on Friday and showers will pick up a bit in coverage and intensity across the Cascades of Oregon and Washington with lighter showers extending into Northern Idaho and Montana as well. A few thunderstorms will also be possible across the Northwest.

Strengthening southerly flow in advance of the trough will begin to transport monsoonal moisture further north with more numerous thunderstorms expected across New Mexico, Colorado, Eastern Arizona and Utah, and possibly Wyoming, with frequent cloud-to-ground lightning and locally heavy rain possible.

This particular model is projecting some very heavy rainfall across the San Juan Range, but this represents a higher-end scenario as other models are projecting lighter amounts for the San Juans. 

Outlook for Saturday - Sunday:

Get ready for another active weekend across the West. There are still some question marks and model discrepancies regarding the timing/progression of the West Coast trough, along with the timing and extent of the northward surge of monsoonal moisture.

However, the overall theme will be for increasing shower and thunderstorm chances throughout the weekend for just about all areas except for California and Nevada (though northern portions of these states could see some rain). 

Locally heavy rainfall and flash flooding will be possible across the Southern Rockies, and models are coming into fairly strong agreement in projecting heavy rain totals across the San Juans over the weekend.

Also, energy and wind shear arriving from the west in combination with the uptick in monsoonal moisture could potentially result in strong to severe thunderstorms across the Central and Northern Rockies this weekend.

We will also have to keep an eye on Glacier and Northwest Montana, where some models (such as the one shown below) are projecting substantial rainfall, which could cause some flooding issues given the above-average snowpack and heavy recent snow.

Flood-ravaged Yellowstone and surrounding areas will likely pick up some rain this weekend and locally heavy downpours will be possible under thunderstorms, but compared to last weekend, rainfall is expected to be much lighter and less widespread. 

Forecast for Monday (6/20) to Tuesday (6/21):

Wet and stormy conditions will linger across the West during this period, especially on Monday, while the Northwest and Northern Rockies should start to see decreasing rain chances by Tuesday. 

The Southern Rockies will remain in a monsoonal pattern during this period with additional thunderstorms and locally heavy rains possible, favoring New Mexico and Colorado and eastern portions of Utah and Arizona, with less rainfall projected for central and western portions including the Pipeline Fire. 

Outlook for Wednesday (6/22) to Wednesday (6/29):

Heading into late June, it looks like the monsoon will hang around the Four Corners region and this should have a moderating effect on temperatures. Meanwhile, the West Coast will strongly be favored for cooler than average temperatures during this period. Quite a difference from this same time last year when the Northwest was experiencing an all-time record-shattering heatwave! 

The big difference is that despite the cooler temperatures, a drying trend is projected across the Northwest compared to recent weeks. Whereas the Southwest, especially Colorado and New Mexico, will be favored for wetter conditions and near-daily rounds of thunderstorms should be anticipated across these areas.

Thanks so much for reading! Next update on Friday (6/17).

ALAN SMITH