Monsoon season getting started across the Southwest

The North American monsoon is getting underway across the Four Corners region as upper level easterly and southerly winds transport moisture Mexico northward into the southwestern U.S.  This signals a shift toward a period in which afternoon thunderstorms become very common across the Four Corners states, along with an increased potential for heavy rainfall with thunderstorms.  Of course the monsoon ebbs and flows depending on the location of the subtropical High, and is influenced by a variety of moisture sources such as tropical activity in the eastern Pacific.

The start of this year’s monsoon is looking quite active, especially across Arizona and Utah.  We’ve already seen an influx of moisture this weekend into this region.  One “unofficial” way the monsoon is measured is when the average dewpoint in Tucson, Arizona is 54 degrees or higher for 3 consecutive days.  We are almost there right now.  Over the past three days, the dewpoint has averaged 51, 52, and 58.  Two more days of dewpoints above 54, and this loose criteria for the start of the monsoon will be met.  The image below was produced by NWS Tucson.



High pressure centered over Western Colorado this week is resulting in anti-cyclonic (clockwise) flow of moisture around the periphery of the High that will favor moisture in Utah and western Arizona the most early this week.  Nevada and the mountains of California will see enough of this moisture influx for some thunderstorm activity as well, as will the Wind River Mountains in .  The image below (courtesy of is the ECMWF Model projected precipitable water anomaly for Monday – a good depiction of the early week pattern, with significant moisture available to fuel thunderstorms over Arizona and Utah.  This is the type of pattern that could easily result in flash flooding in the canyon regions such as Zion, Grand Canyon, etc.



Later this week, high pressure will begin to shift farther east into the Central Plains.  This will allow for monsoonal moisture to shift farther east.  While Arizona/Utah will continue to see good moisture, we should start to see an uptick in thunderstorm activity across Colorado over the second half of the week, including the Front Range on Thursday and Friday.  Check out the moisture anomaly projected by the ECMWF model on Thursday over Colorado, compared to the much drier values from earlier in the week. 



It looks like a generally active pattern will continue through next weekend, with some models projecting healthy amounts of rainfall across portions of Colorado, the Southern/Central Rockies, and Desert Southwest through next weekend.  After that, longer range projections are more difficult.  There are some indications a trough of low pressure could develop across the West, which could act to either scour out the moisture for a period of time, or enhance southwesterly flow of moisture into the region.  Too early to tell how that will play out, but at any rate, the outlooks is beginning to turn wetter across the Southwest and southern half of the Rockies over the next week.  Welcome news for this drought and fire stricken region!

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