Tropical moisture from a Pacific hurricane could intensify late-season monsoon activity in Arizona, contributing to a rainy end to the week in Phoenix.
Hurricane Lorena, which rapidly intensified off Mexico’s coast on Wednesday, Sept. 3, is pushing even more moisture into an already active monsoon pattern. The storm should peak at hurricane strength before weakening into the weekend, but its moisture could push inland even if the storm doesn’t.
“We were already seeing some moisture being pumped up into our area because we finally got the high pressure over the right area,” said Alicia Ryan, a meteorologist for the National Weather Service in Phoenix. “But the moisture from Lorena will enhance the moisture we’re already expecting.”
Rain chances in Phoenix and across southern Arizona were highest on Thursday, Sept. 4. The National Weather Service has issued a flood watch in portions of south-central Arizona from the afternoon to the evening on Sept. 4.
When will it rain?
Lorena reached Category 1 strength hurricane on Wednesday, Sept. 3, but was expected to lose steam quickly on Thursday and keep dissipating into the weekend. Its exact track will determine how much moisture makes it into southern Arizona, New Mexico and western Texas, but forecasters say the setup is already primed for storms.
For Phoenix, storm chances were expected to peak Thursday night. Dew points have hovered in the mid-60s, a sign of sticky conditions that could fuel downpours. Storm odds will taper off over the weekend. No rain was in the forecast for Sunday.
Ryan said the storm will weaken as it heads inland, but even its remnants could bring more rain than originally predicted.
How do hurricanes impact Arizona?
The Pacific hurricane season overlaps with Arizona's monsoon in August and September. When the monsoon ridge shifts over the Four Corners region, winds aloft summon tropical moisture north, sometimes bringing the remnants of Pacific hurricanes with it.
“Even without the tropical systems, we get a lot of moisture across Mexico through the Gulf,” Ryan said. “More moisture into our region along with higher temperatures and convection just pops off storms, and that’s why we get our summer monsoon.”
When these systems align, the resulting weather can be dramatic.
In 2014, Hurricane Norbert fueled a storm that dropped 3.3 inches of rain at Phoenix Sky Harbor International Airport in a single day — the city’s wettest day in almost 115 years of record keeping.
Interstate 10 was flooded, with cars poking out from the water. Homes in Mesa flooded, and Chandler was hit the hardest with 5.5 inches of rain. Two thunderstorms had merged, dropping record-breaking precipitation across Phoenix.
While this week’s forecast isn’t expected to rival that day, Lorena is another reminder that even though Arizona is hundreds of miles from the coast, it can still feel the effects of tropical storms when summer heat, monsoonal instability and Pacific storms collide.
Flood safety tips
Hayleigh Evans writes about extreme weather and related topics for The Arizona Republic and . Email her with story tips at [email protected].