End of monsoon to bring warmer, drier weather to the southwest US
A shift in winds will end the North American monsoon across the Southwest, bringing drier, warmer weather and increased fire danger in the days ahead.
Flash flood and severe thunderstorm warnings erupted in Phoenix and Las Vegas on the afternoon of Sept. 26, creating a dangerous situation.
The flow of moisture responsible for drenching thunderstorms in the southwestern United States is at an end as a storm pushing onshore in the Northwest will break the pattern in the coming days, AccuWeather meteorologists say.
This week, there has been a significant downturn in the amount of showers and thunderstorms over the interior Southwest.
"The pattern represents a typical October setup with a storm bringing needed rain but also gusty winds to the Northwest," AccuWeather Senior Meteorologist Chad Merrill said.
Some rain will extend as far to the south as coastal areas of Central California and parts of the Sierra Nevada.
The most recent surge of tropical moisture from Mexico led to the most single-day rainfall of the year in Phoenix. During most of the summer, the big downpours had avoided the Arizona desert metropolis. During the nearly four-month period from June 1 to Sept. 25, 1.36 inches of rain fell scattered over many days. The rain came with a vengeance and deposited 1.64 inches for the calendar day and 1.85 inches spanning Sept 26-27.
“Phoenix had its 19th driest summer on record with 0.95" of rain, but then it has at least seen its fifth wettest September on record with 2.26" of rain," Merrill said.
The recent rains, where they occurred, are expected to provide only temporary relief from drought conditions.
During the late spring to early summer, a change in wind direction from the west to the south is known as the North American monsoon. This seasonal shift in wind direction, similar to the monsoon in India and Southeast Asia, directs more humid air and rich tropical moisture over the Southwest states and produces a marked uptick in thunderstorms.
When winds shift back to the west, it signals the arrival of colder-season storms that take hold over this part of North America. This process began weeks earlier, farther to the north in Canada.
But for the Southwest states, this pattern change this time of the year is a dry one and can bring substantial warmth.
"Much of the Southwest will experience an extended period of sunshine, higher temperatures and increased fire danger into early October," Merrill said.
Where winds strengthen, sparks could ignite rapidly spreading wildfires where the brush and forests are dry or have time to dry out following prior rainfall.
The atmosphere in the West may continue to produce some back-and-forth antics, as it often does during the autumn season.
It is possible that a return of tropical moisture and downpours may occur during the second week of October.
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