Forecast Highlights:

• High elevation EF-2 tornado confirmed in Northeast Utah earlier this week. 
• Fast-moving thunderstorms with locally strong winds across the Central Rockies on Friday.
• Widespread thunderstorms with frequent lightning across Colorado, New Mexico, Arizona, and Southern/Eastern Utah from Saturday to Monday.
• Heavy rain and flash flood threat for Colorado and New Mexico from Saturday to Monday, especially near and east of the Continental Divide. 
• Well-above average warmth for the Pacific Northwest this weekend along with sunny/dry conditions. 

EF-2 Tornado confirmed in Northeast Utah above 9,000 feet on June 19

A rare "strong" high-altitude tornado was confirmed in Northeast Utah earlier this week, near Duchesne which is just south of the Uinta Range and north of the Book Cliffs. The tornado was on the ground for 2 miles with peak winds estimated at 125 mph, giving it a solid EF-2 rating.

Tornadoes rated EF-2 or higher (on a 0 to 5 scale) are considered strong tornadoes, and such occurrences in mountainous areas west of the Continental Divide are quite rare, though certainly not unheard of either.

The National Weather Service in Salt Lake City confirmed the tornado through a damage survey. Fortunately, the twister touched down in a rural area and no injuries or property damage were reported.

This was not the only unusual weather to occur this week either. An area of low pressure over California interacted with monsoonal moisture to result in a significant amount of cloud-to-ground lightning activity across Southern California earlier this week – much more so than is usual for this region. Unfortunately, one lightning fatality was reported near Los Angeles.

Synopsis:

A trough is moving across Montana on Friday while the area of low pressure that was located over California earlier this week has weakened into a smaller scale upper disturbance but is tracking across the Central Rockies. Both of these features will interact with monsoonal moisture to result in an uptick in thunderstorms across the Rockies on Friday afternoon. Meanwhile, a ridge of high pressure building along the West Coast will usher in (by far) the hottest temperatures of the year so far across the Pacific Northwest. 

On Saturday, a backdoor cold front will work its way into Colorado and New Mexico from the northeast. Behind the front, moisture will increase along the eastern slopes of the Continental Divide via upslope easterly winds, while the frontal boundary will also interact with monsoonal moisture arriving from the southwest.

The result will be an uptick in thunderstorms and rainfall across Colorado and New Mexico this weekend, especially near and along the eastern slopes of the Continental Divide.

A cooler airmass behind the cold front as well as an increase in cloud cover will result in below average temperatures over the next several days across the Southwest as well as areas east of the Continental Divide. Meanwhile, heat will build near the West Coast this weekend with highs rising into the 100s in lower elevations of California.

Across the Pacific Northwest, temperatures will be well above average with highs in the 90s in Western Oregon and well into the 80s across Western Washington.

Forecast for Friday:

On Friday, thunderstorm activity will become fairly extensive as a couple of upper air disturbances along with a surface cold front move across the region. However, storms will also be fast-moving which should limit the flash flood threat somewhat, though locally heavy rains will remain possible.

Isolated thunderstorms will also be possible across the Sierra, though any rainfall should be light. 

Some of the storms across the Central Rockies on Friday could produce strong outflow winds, and NOAA's Storm Prediction Center has highlighted much of this region under a Marginal Risk (level 1 out of 5) for severe wind gusts exceeding 60 mph. 

A Slight Risk (level 2 out of 5) for severe thunderstorms will exist over the Black Hills of WY/SD with potential hazards including winds gusts in excess of 60 mph and hail of 1" in diameter or greater.

A marginal risk of excessive rainfall and flash flooding will also exist across the Southwest, with dry washes and slot canyon flooding being the main areas of concern. 

Forecast for Saturday:

On Saturday, a noticeable uptick in thunderstorm activity is expected across the Front Range and Sangre de Christo Range of Colorado and New Mexico, and to a lesser extent the San Juan Range. Thunderstorms will be more widespread in coverage and could produce frequent cloud-to-ground lightning along with heavy rain.

Scattered thunderstorms are also expected across Arizona, Southern/Eastern Utah, Nevada, and the Sierra Nevada Range in California.

Northern New Mexico and Southern/Eastern Colorado will see the highest threat of excessive rainfall, will burn scars being particularly susceptible to flooding and runoff issues. Although not highlighted in this map below, slot canyons in Utah and Arizona will be at risk for flash flooding as well. 

Forecast for Sunday:

Moisture levels will remain elevated on Sunday with numerous thunderstorms, frequent lightning, and heavy rain expected across Colorado and New Mexico, with the most widespread heavy rains likely across Northern New Mexico. Arizona and Southern/Eastern Utah will see storms as well while a drying trend is expected across California.

Southern Colorado and Northern New Mexico will remain under an elevated risk for excessive runoff and flash flooding, while slot canyons in Utah and Arizona will remain at risk for flash flooding.

Outlook for Monday - Tuesday:

We will see one more day of elevated thunderstorm and heavy rain potential across Colorado and New Mexico on Monday, then the pattern will relax a bit on Tuesday with thunderstorms still expected – but less widespread and less intense.

A trough of low pressure along with a cold front will signal an end to the warm spell across the West Coast on Tuesday, and the North Cascades and Northern Rockies will also see some showers and thunderstorms return to the picture.

Outlook for Wednesday (6/29) to Thursday (6/30):

A similar pattern is expected during this period with daily rounds of thunderstorms expected across the Four Corners as the monsoon remains in place, while the Northern Rockies will also see chances of lighter showers/thunderstorms associated with an upper air disturbance moving through. 

Outlook for Friday (7/1) to Friday (7/8):

An active monsoon will continue across the Four Corners region through the first week of July, resulting in near to below-average temperatures. In addition, near to below average temperatures are expected along the West Coast where a trough is likely to set up for a period of time, while a ridge of high pressure further inland will result in warmer than average temperatures across the Northern Rockies. 

Thunderstorm chances should continue on a near-daily basis across the Southwest and Southern Rockies, with activity currently projected to favor New Mexico and Colorado. The Pacific Northwest (mainly Western Washington) could potentially see some shower activity associated with minor disturbances move through on occasion. 

Thanks so much for reading and have a great weekend! Next update on Monday (6/27).

ALAN SMITH