Synopsis:

A trough of low pressure will work its way across the Northwest and Northern Rockies from Friday through Sunday, and will interact with monsoonal moisture to result in an active stretch of weather from the Sierra to the Great Basin to the Central Rockies from Friday into the weekend. Widespread showers and thunderstorms capable of heavy rainfall can be expected across these regions, and overnight/early morning activity is possible as well. 

Monsoon Update:

On Friday, a shortwave trough (i.e. a "mini-trough") located south of the jet stream will interact with anomalously high levels of monsoon moisture to result in widespread showers and thunderstorms across California, Nevada, Northern Utah, Eastern Idaho, Western Wyoming, and far Southwest Montana.

New Mexico and Colorado will see some monsoon action as well with slower-moving thunderstorms capable of producing heavy rain.

A similar pattern is expected on Saturday with the main differences being that moisture will decrease across California and Nevada while Southern Montana will see an increase in moisture. 

On Sunday, the trough of low pressure moving across the Northern Rockies will dip southward a bit into Wyoming and eventually will move east of the Rockies and out into the Dakotas. Much drier air will work its way into Idaho, Utah, Wyoming, and Montana behind the trough.

However, a backdoor cold front associated with the trough will work its way southward along the east side of the Continental Divide into Colorado and Northern New Mexico. Easterly upslope winds will transport "recycled" monsoon moisture toward the Front Range and Sangre de Christo Ranges, resulting in an uptick in thunderstorms with heavy rain possible.

Monsoon moisture will also begin to creep its way back into Southern Arizona and California on Sunday.

Here are the projected 3-day rainfall totals via the European Model from Friday through Sunday. Notice the heavy amounts projected across the Southern Sierra, Great Basin, Wasatch Range, Greater Yellowstone Region, Eastern Wyoming, Front Range, and Sangre de Christo Range. 

Flash flooding will be possible in vulnerable areas this weekend. While storm motions will be relatively fast from California to Utah to Western Wyoming, the potential will exist for "training" thunderstorms, which is a term used to describe storms that continuously re-develop and track over the same areas. 

Lightning Outlook:

For what it's worth, this particular model may be "under-predicting" lightning density where the atmosphere will be highly saturated (meaning lots of moisture to work with in the low to mid levels of the atmosphere) from the Great Basin to the Wasatch to the Northern Rockies.

At any rate, widespread lightning is expected across a large portion of the west, with a noticeable gap across Northern Arizona and Southern Utah where thunderstorm activity is projected to be much more isolated than recent days.

Thunderstorm activity will become more isolated across the Sierra Nevada Range on Saturday while widespread showers and thunderstorms can be expected once again from the Wasatch to the Tetons and into Southwest Montana.

Saturday will also be an active lightning day across the Front Range and central mountains of Colorado, Eastern Wyoming, and much of New Mexico.

On Sunday, the Wasatch and the Northern Rockies will see a rapid drying trend with no thunderstorms expected. Meanwhile, the Colorado Front Range, Sangre de Christo Range, and much of New Mexico and Southern Arizona are likely looking an active thunderstorm day. 

Storm chances should also begin to creep back up across the Sierra and SoCal Mountains as monsoon moisture returns, though this particular model is not very excited about thunderstorm prospects just yet.

Temperature Outlook:

The trough moving across the Northern Rockies along with abundant monsoonal moisture will result in much cooler temperatures across the Northern Rockies, Inland Northwest, Great Basin, and California over the next couple of days.

Above-average warmth over the next 2 days will primarily be confined to Colorado and New Mexico, and also the Northwest Coast.

On Sunday and Monday, a significant warm-up is expected across the Pacific Northwest and Northern Rockies, though it will not be as bad as last week's heatwave. Meanwhile, areas east of the Divide in Wyoming, Colorado, and New Mexico will see cooler temperatures behind the passage of a cold front sliding down the plains.

Fire and Smoke Outlook:

A handful of moderate to large fires are burning across the Northwest and Northern Rockies, but areas of heavy smoke are primarily confined to areas near and just downwind of the fires at this time. Widespread heavy smoke is not expected through the weekend based on current fire activity.

The McKinney Fire burning in far Northern California was hit with a heavy rainfall producing thunderstorm earlier this week that resulted in flash flooding and mud/debris flow over a recent burn scar, even while the core of the fire continued to burn. Just another example of the multi-faceted complications that can occur with wildfires. 

The good news is that abundant and widespread monsoonal moisture and the subsequent uptick in relative humidity and wetting rains should temper fire danger to some extent across the West through the weekend. Cooler temperatures across the Inland Northwest and Montana will also temporarily reduce fire weather concerns.

Outlook for Monday (8/8) to Thursday (8/11):

There are some forecast challenges for next week regarding the next surge of monsoonal moisture. Early in the week, a ridge of high pressure will rebuild over the Rockies and become centered near the Utah/Colorado border. Meanwhile, an area of low pressure will set up just off the West Coast of California, which will help to draw in another uptick in moisture back into the Sierra and Great Basin.

Models are in poor agreement regarding the movement and timing of this low-pressure trough.

Eventually, it should move north/northeastward along the Northwest Coast and perhaps slightly inland, with south/southwest winds ahead of the low transporting moisture into parts of the Great Basin, Inland Northwest, and Northern/Central Rockies with increasing rain and thunderstorm chances. The Cascades could potentially see some showers and storms as well.

Stay tuned as we get more dialed in on this pattern next week.

Outlook for Friday (8/12) to Friday (8/19):

Heading into mid-August, above-average temperatures are favored for most of the West during this period with perhaps some slight moderation of temperatures around the Great Basin/Southwest depending on cloud cover associated with the monsoon.

A wetter than average pattern associated with the monsoon is expected to persist across the Four Corners, Great Basin, and possibly portions of California and Wyoming. In other words, the monsoon is not showing any signs of significant weakening yet. 

Thanks so much for reading and have a great weekend! Next update on Monday (8/8).

ALAN SMITH