The rain in North Phoenix and across much of the Valley today is not likely to presage a wet winter, climate researchers say. La Niña is in place and expected to bring drier-than-normal conditions and above-average warmth across the Southwest through February (see graphics below).
The chance of showers at Deer Valley Airport will diminish to 20 percent tonight, according to the National Weather Service, and clear skies and moderate to above-normal temperatures are in the forecast tomorrow through the end of the week.
“Our second La Niña year in a row is in full swing now, and is forecast to last through the winter,” said Emily Becker, a scientist who does climate diagnostics for NOAA’s Climate Prediction Center. La Ninas frequently last through two winters, and sometimes three.
What’s Going On
During La Niña, the cooler “little sister to El Niño, ocean temperatures in the eastern equatorial Pacific Ocean, off South America, are cooler than normal. This shift affects air temperatures and large-scale circulation patterns, and the impacts are global.
In the Pacific Ocean, warm air generally rises along the equator and travels toward the poles, sinking at mid-latitudes and fueling the jet stream, an upper-atmosphere river of air that tends to brings storms to the U.S. West Coast, Becker explained.
During La Niña, there is more of this convection in the far western Pacific (around Indonesia) and less in the central and eastern Pacific (because the water is cooler than normal), so the jet stream retracts westward.
“The retracted jet means fewer storms reach the west coast of the United States, tending to leave the southern tier of the country warm and dry,” Becker said in an analysis Dec. 14. This doesn’t mean no storms, it just reduces the chances overall.
The Valley normally gets more than half its annual rainfall between November and March, with about 6 of the annual 10.8 inches falling at Deer Valley Airport during this stretch. December normally brings 1.1 inches of rain and an average high of 63 degrees.
How Does 2017 Compare?
- Phoenix’s longest dry spell in for years—103 days— ended Dec. 5 when minor showers dropped a mere 0.07 inches of rain at Sky Harbor. However, only traces fell in North Phoenix — nothing measurable.
- November 2017 was the warmest November on record in Phoenix (dating back to the 1890s).
- October 2017 was rain-free in the Valley for the first time in 18 years.
- The 2017 monsoon brought below-normal precipitation to North Phoenix and the Valley.
- Prior to the current storm, Sky Harbor had received 4.7 inches of rain this year (normal for a year is about 8 inches).
Arizona is in the midst of a 21-year drought, according to the state climate office, with most watersheds having seen just seven or eight wetter-than-normal years during that span. Most of the state is currently in moderate drought conditions (see graphic below).