An area of low pressure is tracking from east to west across the Southwest and is now located offshore. Monsoonal moisture responsible for recent heavy rains across Arizona has now shifted westward and surged into Southern California.

As a result, normally dry regions of SoCal will see elevated shower and thunderstorm activity, even extending to coastal regions around LA. Significant moisture levels will result in wetting rains as opposed to "dry" thunderstorms, which is good news for fire danger. 

As of late Monday morning, a large area of rain and thunderstorms was moving across the Mojave Desert and approaching the Southern Sierra Nevada Range.

Lightning activity has been impressive across this region of Nevada and California as well, while storms have also developed over Southern Utah and Northern Arizona.

By late Monday afternoon, thunderstorm coverage should be widespread across the mountains of Southern California, Sierra Nevada Range, Great Basin, and Southwest Utah.

Current Radar

Lightning Density

Forecast Radar

Flash Flood Watches are in effect for the portions of the SoCal Mountains, Mojave Desert, and Sierra Nevada Range extending northward to Lake Tahoe. Southwest Utah, including Zion National Park, is under a Flash Flood Watch as well.

As we head into the middle of the week, two areas of high-pressure setting over the Great Plains and Colorado will result in a southerly clockwise (anticyclonic) flow that directs the bulk of the monsoonal moisture into the Central and Northern Rockies.

Areas outside of the typical Four Corners monsoon region will including Northern Utah, Wyoming, Eastern Idaho, and Southwest Montana will see enhanced thunderstorm activity along with the potential for heavy rain and flash flooding as a result.

Precipitable water – a measure of the amount of water vapor in the atmosphere – will be around 200% of average across much of this region by Wednesday afternoon, a sign of how impressive the moisture surge will be. This pattern will remain in place over the Central and Northern Rockies from the middle of this week through early next week.

Despite the active monsoon pattern, only the far southwest will see cooler than average temperatures during the next 5 days, while warmer than average temperatures prevail elsewhere across the west. 

Generally speaking, the southwest will start the week much cooler than average (relatively speaking of course) before warming up as the week progresses, while the Central and Northern Rockies will start out well above average before gradually cooling as the week progresses.

Smoke continues to be an issue across the Interior Northwest and Northern and Central Rockies. Utah, Wyoming, and Eastern Idaho should see improvement as winds aloft shift to southerly and monsoonal moisture arrives, but further north and west, the problem isn't going away anytime soon.

Forecast Smoke (surface)

Forecast Smoke (sky)

Forecast for Monday, July 26th

Heavy rain and thunderstorms will favor SoCal, Nevada, and Southwest Utah on Monday as previously advertised. In addition, Southwest Colorado and much of New Mexico will see another active day of storms with locally heavy rain.

Unfortunately, we could also see some dry thunderstorms (i.e. storms that produce lightning but minimal rain) develop across Northern California and Southern Oregon – an area that is already extremely dry and is being impacted by large fires.

Forecast for Tuesday, July 27th

Remnants from Monday's moisture surge will help trigger thunderstorms across Nevada, Utah, Northern California, and Southern Oregon on Tuesday with a threat for dry thunderstorms extending northward into the Wallowas and portions of Southeast Washington.

New Mexico and the western ranges of Colorado will see elevated thunderstorm activity as well, while the leading edge of monsoonal moisture could lead to a few storms over Idaho and Western Wyoming.

Forecast for Wednesday, July 28th

A significant influx of monsoonal moisture will occur across the Central and Northern Rockies with widespread thunderstorms and heavy rain/flash flooding potential for the San Juans in Colorado, extending northward into the Uintas, Wasatch, Wind Rivers, Tetons, Gallatins, and Absarokas.  

The Sierra Nevada and Great Basin will remain under the influence of the monsoon as well with more thunderstorms expected.

Forecast for Thursday, July 29th

Thunderstorms will favor Northern Utah and much of Wyoming on Thursday with the potential for heavy rain and flash flooding. Outside of these areas, thunderstorm activity will be elevated on Thursday afternoon from Northern Montana to Southern Arizona/New Mexico, and westward into the Sierra Nevada and Southern Cascade Ranges.

Forecast for Friday, July 30th

Wyoming and Utah will continue to see the brunt of thunderstorm activity and heavy rain potential with localized flash flooding possible. Nearly the entire Rocky Mountain region will see an active thunderstorm day, as will the Southwest, Great Basin, and Sierra Nevada Range.

A few dry thunderstorms will be possible across the Pacific Northwest once again, which could potentially trigger new wildfires.

Outlook for Sat, July 31st - Wed, August 4th

A similar pattern will remain in place over the weekend and into early next week with elevated thunderstorm potential and locally heavy rainfall for the Central and Northern Rockies, while Arizona, New Mexico, and the Sierra Nevada Range will see plenty of action as well.

Toward the middle of next week, high pressure is expected to flatten and transition toward the Four Corners region, which could eventually result in a southward shift in the most widespread thunderstorm activity.

Thanks so much for reading! Next update on Wednesday (7/28).

ALAN SMITH

In addition to the wildfire smoke forecast maps, you can now view the real-time air quality index on OpenSummit to help you plan your next adventure.

The 'Air Quality Index' map overlay is the current US EPA PM2.5 air quality index from PurpleAir sensors. Shown as the 10-minute average. The higher the AQI value, the greater the level of air pollution and the greater the health concern.

Here is an example of the air quality index for the United States on July 26, 2021.

The map is updated every 10 minutes, with the average AQI value for the past 10 minutes.

View the air quality map on any device by upgrading to All-Access for $29.99/year (Single) or $39.99/year (Group). The subscription includes 5-day hourly forecasts, estimated trail conditions, and much more.

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Air Quality Map → OpenSummit.com/map