Arizona’s 2018 monsoon season ends with Phoenix seeing less rain than usual

With the aftereffects of Hurricane Rosa dominating the Phoenix weather scene, it’s easy to forget about the just-ended 2018 Arizona monsoon season.

Monsoon season ended Sunday with a little bit more rain courtesy of Rosa. Remnants of the storm wouldn’t be considered classic monsoon moisture, but any rain that falls from June 15 to Sept. 30 counts.

Phoenix saw 2.63 inches of rain for the monsoon season (defined by the National Weather Service as June 15-Sept. 30 since 2008). That’s 0.08 of an inch below normal for that period. The record wettest monsoon saw 9.56 inches in 1984 while the driest was 0.35 of an inch in 1924.

Light rain, cool temps on track before heavy storms move in Monday night

Statewide monsoon totals

While Phoenix was a bit below normal, Flagstaff, Kingman and Tucson were all on the wetter side.

Flagstaff saw 9.97 inches of rain for the monsoon, 1.66 inches above normal. Kingman recorded 3.07 inches for the season (2.82 inches is normal) and Tucson picked up 7.02 inches compared to the normal of 6.08 inches.

Yuma was also below normal with 0.89 of an inch compared to that city’s normal total of 1.29 inches.

Elsewhere in the Southwest, Albuquerque (4.83 inches versus 4.61) and Palm Springs (0.95 of an inch versus 0.65) were a bit above normal for the season while Las Vegas 0(.87 vs. 1.01) and El Paso (4.04 vs. 5.14) came up a little short.

Drought impact

Monsoon rains could start to ease some of the short-term (surface) drought conditions that have ratcheted up in the past year.

University of Arizona climate scientist Mike Crimmins explained that it would take a while to make that determination.

“I think we’re now at the point where we’ll start to re-evaluate the current drought status around central Arizona,” said Crimmins, who also works with the U.S. Drought Monitor. “The parts of Arizona that had such good, solid monsoons and are now under the track of Rosa are going to start to see improvements in the short-term drought conditions. The far northeast part of the state, it’s going to take a little longer because it didn’t have much of a monsoon.”

More hurricanes?

With the possibility of El Niño conditions developing in the eastern Pacific Ocean, there may be more storms like Rosa that could impact Arizona’s fall weather.

El Niño conditions mean there are warmer than normal waters in that part of the Pacific Ocean. Storms thrive in those warmer waters.

El Niño predictions show we might be on track for a wet, snowy winter

“(The Climate Prediction Center) was expecting an above average (hurricane) season and that’s certainly what happened,” Crimmins said. “Now that we’re in the transition from the summer monsoon into fall, that area is still very warm and kicking off storms about once a week. It’s possible, at least into the first few weeks of October, to have another event like that occur.”

Sergio, a tropical storm as of Monday afternoon, could develop hurricane strength. Its current path isn’t predicted to impact Arizona, but that could change.

“It’s a little early to see if that will happen,” Crimmins said. “But it could follow a similar path to Rosa and we could see another slug of moisture like we’re seeing now.”

September 2018 by the numbers

The average high temperature of 103.5 degrees was the fourth hottest in that category and 3.7 degrees above normal. The record for highest average temperature is 104.6 in 2001.

The average temperature (taking the average high and low and dividing by two) was 91.9 degrees. That’s 3.5 degrees above normal and the second warmest ever. The record is 92.2 degrees set in 2001.

The average low temperature was 80.2 degrees, second warmest ever for overnight lows.The normal is 76.9 degrees while the record is 80.6 in 1983.

While September was one of the warmest on record for the city, it was not one of the wettest.

Phoenix only saw 0.43 of an inch of rain for the month (62nd driest). That’s 0.21 of an inch below normal for the month.

Through the end of September, Phoenix is well behind the pace for total rainfall for the year at 3.40 inches. Normal for the first nine months 5.92 inches.

October got off to a good start with rain from the remnants of Hurricane Rosa, however, and could make up some of that deficit.

100-degree days

So far the thermometer at Sky Harbor Airport (the official weather station for the city) has recorded 128 100-degree days in 2018. Right now that’s third highest on the all-time list for most 100-degree days.

The record for 100-degree days in a year is 143 set in 1989. But if the city sees at least one more triple-digit high temperature it would move into a tie for second on the list with 2003.

While Phoenix typically sees its last 100-degree day on Sept. 29, it wouldn’t be out of the question to get another such day this fall. The record for the latest 100-degree day was Oct. 27, 2016.

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