Unusual Heavy Rain Soaks Arizona

The Glossary of Weather and Climate defines a desert as "a region where precipitation is insufficient to support any except xerophilous vegetation; a region of extreme aridity."

With that said, parts of Arizona have seen a soaking to a degree that is quite unusual anytime of year, much less in late November.

Slow Low Taps Deep Moisture

The culprit for the soggy scenario is a slow-moving area of low pressure in the upper levels of the atmosphere.

This low is a slow-mover because it is temporarily cut off from, or left behind by, the jet stream, like a sailboat with insufficient steering winds. You can see this feature in our weekend jet stream forecast map in the Desert Southwest.

This so-called cutoff low tapped a plume of deep moisture from the Pacific Ocean and directed it into the Desert Southwest.

Ahead of the cutoff low, a roughly south-to-north band of moderate to heavy rain focused on the western half of Arizona through much of Friday. Rainfall will continue in parts of central and eastern Arizona through Saturday, but amounts should be lighter overall.

(FORECAST: Phoenix | Flagstaff | Tucson)

From late Thursday until 5 a.m. MST on Saturday, Phoenix Sky Harbor International Airport received 2.34 inches of rain. Friday's rainfall total alone was 1.60 inches, making it the second wettest November day since 1895 in Phoenix. The wettest November day was in 1923 when 2.24 inches of rain was recorded on Nov. 10.

Unusually heavy rainfall also occurred over the border in Southern California. The town of Imperial received 0.94 inches of rain on Friday. This is about 40 percent of Imperial's average annual rainfall total of 2.35 inches all in one day. 

Let's now examine how unusual this pre-Thanksgiving soaking really is.

How Unusual?

Mouseover each bar in the interactive graph above to see rainfall stats for Phoenix (Sky Harbor Airport).

How common is a 1-inch rain event in Phoenix, given they only average about eight inches of rain annually? It's more common than you might think.

Dating to 1895, Phoenix has had calendar days with one inch or more of rain in every month except May. (April through June, just before the North American Monsoon returns moisture for thunderstorms to the Desert Southwest, is typically the driest time of year, there.)

With that said, on average, Phoenix's Sky Harbor Airport only has one day a year of at least one inch of rain.

Furthermore, they've only had one such day since the beginning of 2012, on Jan. 26, 2013.

Examining November, in particular, Friday became the eleventh November day since 1895 with at least 1.02 inches of rain. This equates to a 0.28 percent chance of such an occurrence in any November. Not quite lottery odds, but still a remote chance.

Incidentally, the record wettest calendar day in Phoenix is 2.24 inches on Nov. 10, 1923. The record wettest month of November, there, is 3.61 inches in 1905. Average November rainfall is only 0.64 inches.

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