October 2018 in Phoenix bucked the trend of recent years by feeling like, well, fall.
The remnants of two hurricanes made it the wettest October in Phoenix history with 5.35 inches of rain. The clouds and moisture from those storms and a few more typical low-pressure systems combined to keep temperatures lower than normal as well.
October’s average high temperature of 83 degrees was 5.5 degrees below normal and the lowest since an average high of 82.5 in 2000. Last month’s average high was 10 degrees cooler than the past two Octobers. (Phoenix set a record for the latest 100-degree day with a high of 100 on Oct. 27, 2016.)
The rain also helped ease Arizona’s short-term drought situation. The question now is what does the coming winter have in store for us?
Why was there more rain?
The big drivers for the record-setting rain (October 2018 was the third-wettest month on record for Phoenix) were the remnants of Hurricane Rosa that blew through the state starting Sept. 30 and Hurricane Sergio about 10 days later. And a more typical, seasonal low-pressure system on Oct. 7 dropped 0.41 inch of rain, which set a record for that date.
University of Arizona Climate Scientist Mike Crimmins said the rain totals were record-setting but the weather behind them isn’t that uncommon.
“This is the month that we tend to see the big rain events of record related to tropical activity, like (Hurricane) Octave in 1983 with the flooding in Tucson,” Crimmins said.
That storm dumped heavy rain on the state over a 10-day period and caused severe flooding around Tucson, Clifton and Safford as well as in Yavapai and Mohave counties. The flooding was connected with 14 deaths and 975 injuries in Arizona.
The remnants of hurricanes Joanne in 1972 and Heather and 1977 also soaked the state in October.
Why was it cooler?
The clouds that brought all that rain helped keep the temperatures a little below normal. That’s also not unprecedented, even though recent history would suggest otherwise.
“The combination of the tropical activity and abundant moisture to the south along with starting to see fall-type low-pressure systems all worked together to keep the precip coming and the temps down,” Crimmins said. “It wasn’t until the (high-pressure) ridge was able to build back in this past week that temps popped back up.”
Will El Niño affect this winter?
The National Oceanic and Atmospheric Administration has said there is a 70 percent chance of weak El Niño conditions developing this winter.
El Niño conditions are signified by warmer-than-normal waters in the eastern Pacific Ocean. Those conditions often result in warmer and wetter winters in the southern half of the United States.
The winter outlook released by the National Weather Service in Phoenix takes into account the potential impact of El Niño, but it isn’t a certainty.
According to the report, the odds are tilted toward a warmer-than-normal winter for the Phoenix area, but they aren’t tilted as strongly toward above-average precipitation. There is a 35 to 40 percent chance of above-average rainfall in the Phoenix area, but there is also a 25 to 30 percent chance it could be below normal.
El Niño aside, recent history isn’t favoring a wet winter either. Six of the past eight winters have featured below-normal precipitation.
October 2018 by the numbers
• According to National Weather Service records, the average high temperature for October was 83 degrees. That was 5.5 degrees below normal and 12.8 degrees below the record of 95.8 in 1952.
• The average temperature (taking the average high and low and dividing by 2) was 73.8 degrees. That’s 2.9 degrees below normal. The record for that statistic is 92.7 degrees in 2003.
• The average low temperature was 64.5 degrees. which was 0.3 inch below normal. The record for that statistic was 70.8 degrees in 2003.
• Phoenix saw 5.35 inches of rain during the month, which set a record for October and is the third wettest month in Phoenix history. The previous record was 4.4 inches in 1972. October 2018 trails only July 1911 (6.47 inches) and September 1939 (5.41 inches) on the all-time list.