The Valley's annual cold spell hit a month earlier than usual, resulting in a string of nippy November nights, meteorologists said Sunday.
The trend is expected to continue this week--if not longer--with daytime highs in the 70s, nighttime lows in the upper 30s and northeasterly winds at 25 mph in the Valley's canyons and passes, said Bruce Rockwell, a meteorologist with the National Weather Service.
Windy nights are usually warmer ones, said meteorologist David Gomberg.
"The areas that stay calm will turn colder," Gomberg said. "That's because the wind tends to mix things up and brings warmer air down toward the surface."
Minimum temperatures have been about five to 10 degrees below normal for this time of year, Rockwell said.
A combination of clear, cloudless skies, longer nights and a sun lower on the horizon results in heat dissipating quickly. Thus, the colder nights, Rockwell said.
"Clouds keep the heat in," he said. "Without them, the heat is lost into space at night. And with longer nights, there's more opportunity to radiate the heat off. When the sun's low on the horizon, there isn't enough time to heat us up."
Winter begins Dec. 21, meteorologists said, but they don't expect the cold spell to last until then.