The first half of July has been very dry across the Western U.S. thanks to a delayed start to monsoon season. However, that is about to change for some areas as the seasonal North American Monsoon is beginning to take hold as of July 18th.

Monsoonal moisture has been building over Mexico for the past couple of weeks and is beginning to spill into the Western U.S. with a gradual uptick in moisture levels.

Weather Pattern – Next Seven Days (July 18-24):

Moisture is rotating clockwise (anti-cyclonically) around a high pressure center over the Southwest U.S. and reaching the Sierra Nevada Range, while stronger westerly winds across the Central Rockies are transporting this moisture into Utah and Colorado with a gradual eastward shift in moisture expected in the days to follow.

The result will be an uptick in thunderstorm chances and coverage from the Sierra into the Central Rockies during the middle part of this week. 

As we head into the later part of the week (Thursday-Friday), the center of high pressure will shift westward temporarily, allowing deeper moisture and resulting thunderstorm chances to increase across New Mexico.

Also, a cold front sliding down the east side of the Continental Divide will result in upslope winds and increasing moisture/thunderstorm activity across the Front Range and eastern ranges of Colorado and Wyoming. 

Heading into the weekend and early next week (July 22nd-24th), moisture will become more entrenched throughout the Four Corners region with a gradual uptick in thunderstorm coverage expected for ranges west of the Divide in Colorado, New Mexico, Arizona, and Utah.

A Look Back at the Dry Start to July:

Following an active June across the Western U.S., July has started out on a much drier note as the main Pacific storm track and associated moisture has retreated northward into Canada (typical for this time of year), while the monsoon has taken longer than usual to get going. 

Most areas in the Four Corners region have received less than a quarter inch of rain so far this month, with slightly higher totals across SE Arizona and SW New Mexico. The Front Range of Colorado has also received heavier rainfall, mainly due to occasional upslope easterly winds transporting Gulf of Mexico moisture toward the east side of the Continental Divide. 

Rainfall and Lightning Activity Will Increase During the Last 2 Weeks of July:

Despite the delayed start, the monsoon is getting going now and thunderstorm activity will be on the increase across mountain ranges in Colorado, New Mexico, Arizona, and Utah.

We will see varying levels of thunderstorm activity from July 18th-24th, then a further uptick in thunderstorm activity is likely from July 25th-31st. 

The main plume of monsoonal moisture will fluctuate day to day, but medium to long-range models are in good agreement that Western and Central Colorado will be the most favored region for thunderstorms and rainfall over the next 2 weeks.

In addition to the Four Corners states which typically experience the brunt of the monsoon, "fringe" monsoonal regions such as the Sierra Nevada Range, state of Nevada, Wyoming, Idaho, and Southwest Montana will see occasional intrusions of monsoonal moisture with increased thunderstorm activity as well.

Here is an image of projected rainfall totals over the next 2 weeks. This is from an average of 50 simulations of the European Weather Model. Keep in mind, this doesn't take into account the potential for localized heavy rainfall that can occur with thunderstorms, but gives a good general idea of what to expect.

The average of 50 simulations of the American GFS Model has a similar projection as the European Model. The main difference being that the GFS is lower resolution compared to the European.

Overall, the monsoon this season is not looking as strong compared to the past two summers, and rainfall amounts being projected for late July are not out of the ordinary for this time of year.

However, the pattern will be noticeably more active compared to the first two weeks of July, and this will impact outdoor recreation across the Southwest, Southern and Central Rockies, and at times the Sierra. 

How This Will Impact Outdoor Recreation:

Cloud-to-ground lightning is one of the main weather hazards associated with the monsoon from an outdoor recreation perspective. This is especially true for hikers, climbers, and peak baggers, including the Colorado 14ers and other ranges with above-treeline terrain and exposed summits.

Thunderstorms can also occur as early as midday once monsoonal moisture becomes established over an area for an extended period of time. 

And keep in mind lightning is not only a threat to peak-baggers, but also a threat to anyone who is spending time outdoors.

The other notable outdoor recreation hazard during monsoon season is flash flooding, especially in slot canyons and dry washes across desert/canyon regions of the Southwest. 

Moving forward, you can monitor lightning and rainfall potential using our Forecast Anywhere feature, where you can view daily and hourly forecasts for any location and elevation. 

Also, be sure to follow the Western U.S. Daily Summit for 3x/week weather discussions through the end of the summer. 

Alan Smith