During hurricane season in North America, attention is typically focused on the Atlantic basin, but this year category 4 Hurricane Hilary is also marching up the Pacific Coast of Mexico with plans to deliver a deluge to the southwestern US.
Friday morning, Hilary was recording sustained winds of 145 miles per hour with stronger gusts. The storm is forecast to weaken as it approaches Baja California this weekend.
“Fluctuations in intensity are likely over the next day or so,” reads the forecast discussion from the National Hurricane Center. “Weakening is expected to begin by Saturday, but Hilary will still be a hurricane when it approaches the west coast of the Baja California peninsula Saturday night and Sunday. Hilary is expected to weaken to a tropical storm by Sunday afternoon before it reaches southern California.”
For years now I’ve tried to stress that in the 21st century, with oceans that are significantly warmer on average than they were in previous centuries, we need to consider more than just the wind speeds of tropical storms.
The reality is that a hurricane can inflict the majority of its damage long after it has weakened to category one hurricane or even been reduced to tropical storm status. Increasingly over the past two decades we’ve seen storms churn offshore for days, soaking up biblical amounts of warm sea water that then gets dumped after it comes ashore, weakens, slows down and just sits there flooding communities.
This is thanks, in part, to warmer oceans that encourage strong storms to slow down and just soak up the sea like a sponge. As they disintegrate over land, that atmospheric sponge gets squeezed.
This happened in dramatic fashion with Hurricane Harvey dumping up to five feet of rain on the Houston area in 2017, and again with Hurricane Florence over the Carolinas the following year.
Now it looks as if something similar is about to play out, albeit on a smaller scale, in southern California and the southwest.
Hilary is forecast to drop up to 10 inches of rain on Baja California this weekend, possibly causing flash flooding. Heavy rainfall is expected to continue as the storm crawls north to more heavily populated areas north of the US-Mexico border.
“Rainfall amounts of 3 to 6 inches, with isolated amounts of 10 inches, are expected across portions of southern California and southern Nevada, which would lead to significant and rare impacts,” the NHC forecast reads.
Flood watches are in effect from San Diego to Los Angeles and Las Vegas. Thanks to abundant mountainous terrain and desert soils, the danger of flash flooding is particularly concerning in this region, which isn’t particularly used to dealing with such conditions. Runoff cascading down narrow drainages can easily take travelers and others by surprise.
So be careful out there, check with local weather forecasts and remember that in the 21st century, a downgraded storm doesn’t mean the danger has passed.
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