First-ever tropical storm watch issued for southern California, flash flooding likely

The National Hurricane Center issued its first-ever tropical storm watch for southern California on Friday as Hurricane Hilary continues to gather steam in the eastern Pacific Ocean.

The storm is closing in on scale-topping strength, clocking maximum sustained winds of 145 mph as of the agency’s mid-morning update. Hilary is a high-end category four hurricane on the Saffir-Simpson wind scale.

It’s this formidable strength—and a steady increase in its forward speed—that will give it the momentum needed to survive interaction with land and colder waters to survive into southern California as a tropical storm late this weekend.

Tropical storm watches are in effect for the Los Angeles and San Diego metro areas, as well as Orange County and many locations throughout inland southern California. Winds of 40+ mph are possible alongside the drenching rains, which will likely lead to tree damage and power outages throughout the region.

While the storm will weaken considerably by the time it enters the southwestern United States, it’s going to bring a tremendous surge of tropical moisture inland, fueling the risk for prolific rains and catastrophic flash flooding for many vulnerable areas.

We’re on track to see widespread rainfall totals of 3-5+ inches through early next week, much of which will fall over deserts and mountains that can’t handle downpours that produce a fraction that much rain. Significant and widespread flash flooding is expected throughout the region, along with landslides across vulnerable terrain.

I talked a bit more in yesterday’s post about why Hilary is so unusual and what’s behind its track toward southern California. If the forecasts hold, this will be a first in living history.

But the fact that this is a rare storm following a rare track is almost secondary to the mammoth flooding threat that’s likely to unfold whether or not Hilary makes it to U.S. soil at tropical storm strength. Even if the storm weakens, that vast reserve of moisture aloft isn’t going anywhere. This rain is coming. The bullseyes may shift with the storm’s track, but much of southern California, southern Nevada, and western Arizona is on track for a copious amount of tropical rains through early next week.

If you live in the region, please take the watches and warnings seriously, and heed the advice of local officials if they issue evacuations in flood-prone areas. Flooding is the single deadliest hazard in any landfalling tropical system.

Never try to drive across a flooded roadway. It’s impossible to tell how deep the water is until it’s too late, and the road may have been washed out beneath the floodwaters. It takes very little moving water to lift a vehicle and carry it downstream. The vast majority of flooding deaths occur in vehicles, and they’re almost always preventable if the driver didn’t attempt to ford the water.

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