02 October, 2014

156 years later, the 1858 San Diego Hurricane still raises eyebrows

156 years ago today, a hurricane which formed off the coasts of Mexico and Central America several days prior took a very unusual course. Most of them either A, move westward, or B, move slowly over cold water, which thus has enough time to make them die out. Not this one. After missing the southern tip of Baja, this menace was picked up by a trough and swung rapidly northward. Having been a category 3 storm at peak intensity, it weakened down to a category 1, then briefly re-intensified to a 2 due to the pressure gradient influence of a ridge behind that trough (not to mention a patch of unusually warm El Niño water)... then back down to a 1, dangerously close to, where? None other than San Diego, California:

This monster hurricane caused damage that we can't even fathom today. It tore roofs off houses. It created a storm surge that overtopped Coronado Island, blasted across San Diego Harbor, flooded a good chunk of the city (well, actually, small town at the time), and shoved three large US Navy schooners, the USS Plutus, the USS Lovely Flora, and the USS X.L., completely aground. That's a key point: Schooners aren't just boats, they're massive ships. In order to completely beach (!) a 200-foot schooner with the massive keel that it has, much less three of them, you need at least a 15-foot storm surge. On top of all that, rain fell in buckets, enough to overtop rain gauges and cause normally dry, ephemeral riverbeds to rapidly overflow their banks. Due to the long time it took news to travel back in 1858, however, the folks on the east coast didn't even know about the damage until several months later.

When I bring up this storm, people are literally freaked out... and of course, I don't blame them. Why? Because if it happened before, it will happen again. What makes such a storm so destructive for only a category 1 hurricane is the sheer size. Remember what kind of storm surge Hurricane Sandy caused? Sandy was also a category 1, but the wind radius, just like the wind radii of most intense East Pacific hurricanes, was a good 500 miles out from the center. That is key to a storm surge catastrophe that could make Marie seem like a mere dress rehearsal.

See, when hurricanes are forced to move away from warm water at speeds too fast to dissipate in time, they begin to compensate for the lack of fuel by spreading out their wind diameters. A storm that's only 200 miles across at peak intensity can end up expanding to 1000 miles across when it gets to California (or New York)... and likewise, one that's 500 miles across at peak intensity can end up being 2000 miles across in that same case. The larger the storm's radius, the more water it displaces. Translation: it's a recipe for disaster. Something halfway between Sandy and Ike in terms of storm surge impacts.

Then, we come to the second major impact: rain. Rain that can amount to a staggering 2 inches per hour — on par with the kind of rainfall rates that Hurricane Irene brought to New York and New Jersey. When that gets dumped on mountainous terrain, guess what that causes? Extreme flooding. In 1976, our neighbors to the east — in Ocotillo — got a glimpse of that potential for catastrophic flooding when Hurricane Kathleen made landfall in Baja and moved north across the border as a tropical storm. As much as 14 inches of rain fell in a matter of hours, causing a dry creek bed that flows toward Ocotillo from the Baja mountains to suddenly explode into a 40-foot wall of water that blasted its way through the town, flattening everything in its path. When the storm was over, it looked as if a tsunami came through the town. Expect a repeat of that in multiple locations should a repeat of the 1858 storm occur.

And that's the thing: history does repeat itself. It's not a question of if, but WHEN SoCal will get hit again. Also, it's El Niño events like the ones in 1997, 2009, and, yes, 2014 that tend to result in far more powerful East Pacific hurricanes... not to mention, of course, that during the early fall months, the winter storms also begin to pick up strength and have more of a tendency to fling storms northward. Santa Ana pressure gradients also play a part, as do those whisps of forced evaporation that Santa Ana winds pick up when they hit the water: they not only instantly transform the Santa Ana air from dry to moist, but they also increase the salinity of the water and force it to temporarily downwell. Hopefully it's not too late. If it is, however, better hunker down...