Winter expected to be milder, drier than normal
Summer's unofficial end won't keep daytime temperatures from reaching 100 degrees, but September usually promises more bearable conditions when the sun sets, an especially welcome change this year after a season full of sweaty nights.
The meteorological summer - June, July and August - ended last week as the ninth-warmest on record in Phoenix, the National Weather Service reported.
The average daytime high of 105.3 degrees was almost normal, but overnight lows averaged 82.8 degrees, almost 3 degrees above normal and the fifth-warmest nighttime average in 114 years of record-keeping.
All of the 10 temperature records broken or tied during the summer months were high minimums, with the highest lows on July 16 and July 19, when the temperature never dipped below 94.
Phoenix's hot nights are largely the result of the urban heat island, scientists believe. The concrete, asphalt and buildings absorb heat all day and trap it, releasing it far more slowly than the open desert, where nights cool quickly.
The monsoon's moisture also traps heat on the nights it doesn't spawn rainstorms. Monsoon storms produced 2.35 inches of rain at Phoenix Sky Harbor International Airport, above the 30-year average of 2.02 inches. But there were relatively few nights of rain. Of the total, 1.33 inches fell July 31, a record for the day.
So far, most scientists would attribute the warm nights to the heat island rather than a permanent change in the climate.
"It's becoming more difficult to quantify a normal year," said Arizona State University climatologist Andrew Ellis. "I don't think people have a very good feel of what will be 'normal' over the next 10 to 20 years."
This winter, meanwhile, could produce fewer temperature extremes with the onset of La Niña, the cooling of the water in the Pacific Ocean. In Arizona, La Niña-tinged winters are often warmer and drier than normal, which is what forecasters are predicting.
Weather drivers such as El Niño, the warming of the ocean, and La Niña add "noise" to the climate record, Ellis said, producing more above- or below-average temperatures and precipitation and making it harder to find the trend.
La Niña conditions began to develop over the Pacific earlier this year and are expected to influence the weather through the winter and into the early spring. The federal Climate Prediction Center forecasts a better-than-even chance of drier, warmer conditions over Arizona and New Mexico through March.
The extreme shift in oceanic conditions - last winter was helped along by El Niño - can complicate efforts to monitor and forecast drought, Ellis said. But recent years have shown that a wet winter, even in the midst of several dry ones, can ease concerns by refilling reservoirs quickly.
"One wet year can bring us back to a good situation for water supply," he said. "The saving grace for us this year is that our reservoirs locally are in pretty good shape."
The big reservoirs on the Colorado River don't respond as quickly, but they hold more water and have so far weathered more than a decade of drought.
Source Link: azcentral.com