Wet September Saves Monsoon Season

Weather Service predicts strengthening El Niño could deliver wetter-than-normal winter


Rim Country’s monsoon season started late, but could finish strong.

Another weekend cloudburst, complete with hail, deafening thunderclaps and power outages, boosted the September rainfall total in Payson to 3 inches, compared to a long-term September average of just over two inches.

However, the nearly 11-inch total for the year so far still lags well behind the long-term average of just more than 16 inches for this point in the year.

With just over one week left in the official monsoon season, 2009 no longer threatens to produce the stingiest monsoon season in a century.

Moreover, the U.S. Weather Service is now predicting a somewhat wetter winter than normal in Northern Arizona, thanks to the continued warming of the water near the surface in the Eastern Pacific.

The surface temperature of water in the eastern Pacific right now stands at about 4 degrees above normal and is still heating up, thanks to a change in the pattern that takes place about once every four or five years known as El Niño.

That shift in trade winds triggers a climate chain reaction that changes weather patterns all over the world.

El Niño often shifts high altitude winds, so that winter storms that would normally hit Oregon and Washington instead wander down through Northern California and into Northern Arizona.

Turns out, weak El Niño patterns don’t have much impact on the American Southwest, which would include the last two El Niño events.

However, a strong El Niño can significantly increase rainfall. The last three strong El Niños doubled normal rainfall and snowfall in Northern Arizona, according to the Weather Service.

So far, the still strengthening El Niño looks moderate and the Weather Service is predicting normal to slightly above normal snowfall this winter.

If so, that would provide another welcome break from the drought that has gripped the region for the past decade and which seemed to reassert itself in a long, dry summer, before the skies opened up in September.

Runoff from the Rim remains well below normal, however, despite the short but intense thunderstorms of September.

On Monday, the flow of the Salt River into Roosevelt Lake stood at 219 cubic feet per second, about two-thirds of normal for this time of year. The flow of the Verde River at Tangle stood at 151, about 19 percent below normal. Tonto Creek at Roosevelt Lake was dry Monday, compared to a normal flow of 10 cubic feet per second.

Still, Roosevelt is 80 percent full, with the peak water use months past and winter still ahead. The reservoirs on the Verde River are at about 57 percent of capacity.

All told, the SRP reservoirs that serve Phoenix by capturing streams that run down off Rim Country and the White Mountains were 79 percent full, compared to 87 percent a year ago.


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