Forecast Highlights:

• Widespread flooding across the Northwest & Northern Rockies, including historic flooding and major damage in and near Yellowstone National Park.
• Snow on Monday and Tuesday for the mid to high elevations of the Northern Rockies, heaviest in Glacier National Park.
• Critical fire danger across the Southwest with rapidly growing fires in Arizona and California.
• A relative break from significant weather on Wednesday and Thursday.
• An unusually early start to the monsoon late this week and over the weekend.

Synopsis:

A slow-moving trough of low pressure moving into the Western U.S. has resulted in significant and varying impacts including flooding, severe thunderstorms, new and expanding wildfires, and now even some late-season snow for the Northern Rockies.

High pressure will build in during the middle part of the week, bringing a welcome break to the pattern for most areas.

However, active weather will return this weekend as another slow-moving trough approaches the West Coast and taps into an early-season surge of monsoonal moisture to produce an uptick in showers and thunderstorms across much of the West. 

Take a look at the current National Weather Service Hazards to get an idea of just how active the pattern is across the West right now.

Across the Northwest and Northern Rockies, numerous Flood Warnings and Advisories are in effect. A Winter Storm Warning for Glacier National Park and surrounding ranges, while High Wind Watches and Warnings are in effect across the plains east of Glacier. 

Across the Southwest, Red Flag Warnings (for high fire danger) and High Wind Watches/Warnings are in effect, while the Great Basin and parts of Northeast California are under Freeze Warnings.

Ongoing Flooding Issues across the Northwest and Northern Rockies

It has been a wild past couple of days across the West, and not exactly in a good way. The combination of heavy rainfall associated with a late-season atmospheric river event combined with snowmelt from an above average spring/early summer snowpack is resulting in river flooding issues across Washington, Oregon, Idaho, Montana, and Wyoming.

Check out the rainfall totals from the past 72 hours across the Northwest, ending Monday morning. 

A zoomed-in view over the Northern Rockies shows where estimated rainfall totals in excess of 3-4 inches occurred near Yellowstone National Park and over the Beartooth Mountains. 

This area has seen a delayed snowmelt and above-average spring snowpack this year, so the recent heavy rains in combination with melting snow have resulted in catastrophic flooding in and near Yellowstone National Park.

Check out the Yellowstone River stream gauge from just north of the north entrance to the park. 

At this time, all of Yellowstone National Park is closed for at least the next three days with numerous roads having been washed out. Also, the town of Red Lodge, Montana is experiencing significant flooding downtown where Rock Creek has jumped its banks.

Severe Thunderstorms across the Northern Rockies and Black Hills

In addition to flooding, strong to severe thunderstorms have occurred across parts of the Northern and Central Rockies recently, with widespread large hail and wind damage reported just east of the Rockies across the Black Hills region of South Dakota.

Severe thunderstorms also caused wind and hail damage in Jackson Hole, Wyoming and over Southwest Idaho on Sunday, and in Colorado, a brief tornado touched down near Denver International Airport.

The threat for severe weather is significantly reduced across the Northern Rockies, but another round of severe thunderstorms are likely to develop across the Black Hills region of NE Wyoming and Western South Dakota on Monday afternoon and evening. 

The strongest storms over the Black Hills could produce hail of 2 inches or more in diameter along with damaging straight-line winds in excess of 60 mph. There is a minor threat of tornadoes as well. 

Rapidly growing wildfires in Arizona and California with smoke spreading into Colorado

Several new fires started in Arizona over the weekend, including the Pipeline Fire which ignited on Sunday just north of Flagstaff and has rapidly grown to 5,000 acres. Fires also grew across Southern California this weekend, forcing evacuations. 

Smoke from fires in Arizona and New Mexico have impacted a large portion of Colorado on Monday. The plume of smoke from the Pipeline Fire is most evident on our Smoke Forecast Map.

Ongoing fires are reportedly exhibiting extreme fire behavior due to strong winds and low relative humidity on Monday. A large portion of the Southwest U.S. is under Critical Fire Danger per the Storm Prediction Center.

On Tuesday, the strong southwest flow over the Southwest will begin to flatten somewhat and the smoke plume downwind of the Pipeline Fire will shift into Northern New Mexico and Southern Colorado with improving conditions over Northern Colorado.  

Don't forget to check out our new Air Quality Forecast Map, which is a valuable tool for projecting estimated smoke conditions globally beyond 48 hours. 

Snow for the Northern Rockies on Monday and Tuesday

A strong cold front is moving across the Northern Rockies early this week and a substantial late season snow event is expected over Glacier National Park with double-digit snow totals likely above 6,000 feet.

Snow is expected across mid to high elevations throughout the Northern Rockies as well as higher portions of the Cascades. Even flood-ravaged Yellowstone is expected to see snow across much of the (now closed) park on Tuesday with a few inches possible in the Tetons as well. 

Monday to Wednesday Precipitation Timing:

The heaviest precipitation has been falling (and will continue to fall) over the Northern Rockies near the Canadian border with lighter showers extending southward into the Central Rockies. The Cascades have also experienced another wet day, while thunderstorms over the Black Hills in South Dakota will produce locally heavy rains.

On Tuesday, heavy rain and snow will continue across Northwest Montana and Northern Idaho with lighter showers across the Cascades and Southwest Montana/Northwest Wyoming. Flood-ravaged areas near Yellowstone will remain wet but rainfall rates will not be as heavy compared to prior days at least.

At last, most areas will see a welcome break from precipitation on Wednesday, though Glacier and parts of Northern Montana could see some lingering rain/snow early in the day. The Pacific Northwest will also see a mainly dry day with increasing shower chances on Wednesday night as a disturbance arrives.

Outlook for Thursday (6/16) to Sunday (6/19):

Late this week and over the weekend, the pattern will begin to turn more active once again. A slow-moving trough of low pressure will approach the West Coast with embedded disturbances tracking across the Northwest and the Northern Rockies. 

Meanwhile, the North American Monsoon is going to make an early appearance this year as southerly flow ahead of the trough induces a surge of subtropical moisture into the Southwest and Southern Rockies. 

This moisture will be very welcome across drought and fire-stricken regions of the Southwest. Mid-June is also extremely early for a monsoonal pattern to be setting up. The start of the monsoon varies from year to year, but the average start time is around early July. 

We'll get more dialed on the timing of rain/thunderstorm chances moving forward. In general, Thursday looks fairly quiet with only some light rainfall across portions of the Northwest and Southern Arizona/New Mexico.

Thunderstorm and rain chances will then ramp up significantly across the Northwest and Southwest (California and the Great Basin excluded) from Friday through Sunday as monsoon moisture surges northward and disturbances containing Pacific moisture (and perhaps some remnant monsoonal moisture) track across the Northwest and Northern Rockies.

Outlook for Monday (6/20) to Monday (6/27):

Next week, cooler than average conditions will prevail across the Northwest as troughs continue to track across this region, while the Four Corners region should also see temperatures relatively close to average. California and areas east of the Continental Divide will be favored for above-average warmth.

Despite the cooler pattern over the Northwest, moisture is expected to be more limited with the disturbances moving across the area compared to recent weeks/months and we're actually seeing a dry signal across this region.

Meanwhile, the monsoon looks to hang on to some extent across the Southwest with New Mexico and Southern Colorado looking like the most favored regions at this time. And that means greater lightning potential as well for hikers and climbers.

Thanks so much for reading! Next update on Wednesday (6/15).

ALAN SMITH