Monsoon math:
Traditional method of using dew point signals arrival of stormy season
According to the calendar, the Arizona monsoon started June 15, the date the National Weather Service has used since 2007. According to the old-school method, this year’s monsoon started Sunday, the first of three consecutive days when the average dew-point was above 55 degrees.
If the old-school method still held, Arizona’s monsoon season would have started on Sunday.
The old system, last used by the National Weather Service in 2007, looked at the dew-point average to calculate the start of the monsoon.
“We took an average of the dew-points in Phoenix and when they got above 55 degrees ... for three days, that is when we started the monsoon season,” said Travis Wilson, a National Weather Service meteorologist.
The average dew-point temperature was 60 degrees on Sunday, 58 degrees on Monday and 61 degrees on Tuesday, Wilson said.
The National Weather Service officially recognizes Arizona’s monsoon season from June 15 to Sept. 30. According to Wilson, every regional office had different methods of determining when the monsoon began, so they decided in 2008 to declare a season by calendar.
“Just because it’s not raining in Phoenix – it takes a little longer for it to moisten up down here – doesn’t mean we’re not in the monsoon season,” Wilson said.
For the past 30 years, the first measurable rainfall of the monsoon in Phoenix has typically fallen around July 4. That didn’t happen until July 10 this year.
Thunderstorms start with three ingredients: an unstable atmosphere, moisture and a trigger. Wilson said that the mountainous regions up north see thunderstorms first and these storms produce “outflows” that travel down to Phoenix, where, if conditions are moist enough, more thunderstorms can be produced. “You have to really have enough moisture in the low levels, like around Phoenix, to initiate more thunderstorms,” Wilson said.
Conditions were too dry in the Valley to produce new thunderstorms in midJune. But, Wilson said there was enough moisture around Phoenix Monday night to trigger new thunderstorms and bring rainfall to the Valley.
“Things are moistening up and we’re getting measurable rainfall down here,” he said.