Atmospheric River Over Arizona

We had a very wet period from late afternoon Thursday through early evening Friday (12/23-12/24) as an atmospheric river (AR) moved across the southwest and Arizona. From the Wikipedia site:

An atmospheric river (AR) is a narrow corridor or filament of concentrated moisture in the atmosphere. Other names for this phenomenon are tropical plume, tropical connection, moisture plume, water vapor surge, and cloud band.

GOES-17 water vapor channel showing the moisture plume.
GOES-17 water vapor channel showing the moisture plume.

An approaching trough of low pressure was able to tap into tropical moisture and move it across the southwest into Arizona overnight. Interestingly, the moisture plume aloft initially moved above a drier layer of air. Precipitation falling into this drier air experienced strong wet bulbing and evaporative cooling which lowered snow levels to ~6500 feet—putting Flagstaff into the snow. As the plume continued, these cooling effects diminished and the snow turned into rain. So we had about 4-6″ of snow followed by day-long rain. What a mess.

The Center for Western Weather and Water Extremes provides forecasts for ARs moving into the West. Below are some of the products valid at 5 A.M. Friday (12/24/2021) that clearly show the plume of moisture.

Integrated water vapor (iwv): Note how the moisture is depleted as it crosses the Mogollon Rim with very little moisture available across NE Arizona.
Integrated water vapor (iwv): Note how the moisture is depleted as it crosses the Mogollon Rim with very little moisture available across NE Arizona.
Integrated vertical transport (ivt)
Integrated vertical transport (ivt)

 

Maximum observed AR scale.
Maximum observed AR scale.

It’s great to get all this moisture although it would have been much better if had been either all rain or all snow. Mixed precipitation is always a mess.

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