Forecast Highlights:

• Strong/widespread thunderstorms associated with the monsoon will favor Southern New Mexico, Arizona, Utah, Nevada, Idaho, Wyoming, and SW Montana from Wednesday to Friday.
• Isolated to scattered showers & thunderstorms across Washington, Oregon, California, Western Colorado, and Northern New Mexico with a mix of wet and dry storms expected.
• Elevated flash flood threat continues for the canyon regions of Arizona and Southern Utah, with isolated flash flooding also possible in mountainous terrain throughout the Rockies & Interior West.
• Elevated fire danger and locally smoky conditions across the Inland Northwest and Western Montana.

Synopsis:

An area of low pressure located off the Washington/Oregon is resulting in a southerly flow across the Interior West. This southerly flow is transporting significant amounts of monsoonal moisture into Nevada and Idaho, and to a lesser extent Oregon and Washington as well. An active monsoon is also in place across Southern New Mexico and most of Arizona.

 

On Thursday, the jet stream will move inland a bit, and this will help to "push" some of this monsoonal moisture eastward into Wyoming and Southwest Montana, while New Mexico, Arizona, Utah, and Eastern Idaho will remain in a wet and stormy pattern.

Monsoon Update:

Taking a closer look, we can see anomalously high levels of moisture reaching Nevada, Utah, and Idaho on Wednesday. These areas, along with Arizona and Southern New Mexico, will see more numerous thunderstorms capable of producing heavy rainfall and isolated flash flooding in vulnerable areas.

The interior portions of the Northwest (Eastern OR/WA, Northern ID, Northwest MT) will see a mix of wet and dry thunderstorms, with fire danger expected to be elevated due to the potential for dry storms. 

Energy associated with the jet stream and the area of low pressure off the Washington Coast will also interact with a little bit of monsoonal moisture to produce showers and thunderstorms across Western Washington on Wednesday, especially during the morning hours.

On Thursday, the plume of anomalously high moisture will extend into Wyoming and Southwest Montana, where thunderstorms will become more widespread with the potential for heavy rainfall. 

Here are the projected 3-day rain totals from Wednesday through Friday. Arizona, Southwest Utah, and Southern New Mexico will see the heaviest totals (and thus elevated flash flood concerns in slot canyons & dry washes), while locally heavy rains can also be expected across Utah, Nevada, Idaho, Wyoming, and Southwest Montana.

Locally heavy rainfall will also be possible across Western Colorado, though coverage will be more isolated/scattered compared to areas further west.

Lightning Outlook:

Lightning activity will be most active across Arizona, Utah, Nevada, and Southern Idaho on Wednesday. Isolated to scattered storms capable of cloud-to-ground lightning can also be expected across Northeast Oregon, Northern Idaho, and Washington, as well as New Mexico and Colorado.

The Southern Sierra and SoCal Mountains should see some isolated thunderstorms as well, but with less coverage and intensity compared to recent days.

On Thursday, lightning activity will favor Arizona, Eastern Idaho, Wyoming, Southwest Montana, and Western Colorado. The Inland Northwest could also see some thunderstorms with active cloud-to-ground lightning. The Cascades will only see some very isolated lightning activity, while storms are unlikely across the Sierra.

Thunderstorm coverage and associated lightning activity on Friday will be fairly similar to that of Thursday, with storms favoring the Southwest, Great Basin, and Central/Northern Rockies. The Inland Northwest will see scattered thunderstorms as well with isolated storms possible across the Cascades. A few storms couldn't be ruled out over the Sierra either, though a higher threat will exist across the SoCal Mountains.

Temperature Outlook:

Although there will be day-to-day temperature fluctuations due to cloud cover changes and weak cold fronts, the general pattern over the next 5 days will favor below-average temperatures across the Southwest, Great Basin, and Northwest Coast, and above-average temperatures across the Inland Northwest, Montana, and areas east of the Continental Divide from Montana to Colorado. 

Fire and Smoke Outlook:

The most active fire behavior is occurring with the Moose, Wood Tick, and Elmo Fires in Central Idaho and Western Montana. Fire activity has picked up just north of the Canadian border as well. On Wednesday, we will enter a period with consistent south/southwest winds across this region, resulting in heavier and more widespread smoke across Western Montana.

This smoky period could persist into the weekend depending on moisture levels and rainfall with the upcoming monsoon moisture surge, and whether or not this helps to limit fire behavior. On the other hand, there is some concern that lightning could trigger new fires across the Interior Northwest and Western Montana since a mix of wet and dry thunderstorms is expected.

Fires are ongoing across Northern California and Central Oregon right now as well, but fire behavior and smoke transport have not been too intense recently.

Outlook for Saturday (8/13) to Tuesday (8/16):

A similar pattern is expected on Saturday with widespread thunderstorms capable of heavy rainfall from Arizona to Utah to Wyoming with more isolated to scattered storms across Montana, Colorado, and New Mexico.

On Sunday, the area of low pressure over the Pacific Northwest will finally move inland across Canada, and this will lead to a subtle shift with moisture decreasing across Montana and Northern Wyoming, while the Central/Southern Rockies and Southwest remain in an active thunderstorm pattern.

On Monday and Tuesday, monsoon moisture will begin to expand further west back into California with an uptick in thunderstorm activity expected across the Sierra Nevada Range. Moisture is also expected to creep northward again across Wyoming, Idaho, and Montana with thunderstorm chances increasing once again.

Outlook for Wednesday (8/17) to Wednesday (8/24):

A ridge of high pressure is projected to take hold along the West Coast during this period and extend well northward into Canada. This will result in another stretch of well-above-average warmth across the Pacific Northwest, with near to below-average temperatures across the Southwest and Central Rockies thanks to the monsoon.

An active monsoon will continue during this period with wetter than average conditions expected throughout the Southwest and Southern/Central Rockies, as well as the SoCal Mountains and Southern Sierra Nevada Range. 

Thanks so much for reading! Next update on Friday (8/12).

ALAN SMITH