Hilary Makes Weather History in Southern California

August 30, 2023 Applied Geography Biogeography Climate Climate Change Environment Physical Geography Water

Here is a different story about the tropical storm that made national news and will live long in California history. Hurricane Hilary was exceptional only for the path it took and its fast pace. Otherwise, this could have been just another example of the major hurricanes that form off the southwest coast of Mexico from June through October each year. (Last year on this website, we summarized some tropical cyclone science and their history of mostly avoiding California, as we followed a near-miss tropical storm Kay. We wondered out loud which future summer season weather pattern might finally send such a storm directly to us.) This year, had Hilary eventually drifted west out to sea like so many of the more typical tropical tempests, most weather watchers wouldn’t have given it much notice. Even if it had drifted over Mexico when it was a powerful hurricane, most Californians may not have paid much attention, much less remembered it. It’s funny how our perceptions of weather events and natural disasters are shaped by our intimate experiences with them and our distances from their impacts. And here I am writing about the kind of storm I and others experienced for the first time after several long decades of observing California weather events. Our big summer of 2023 SoCal weather story might leave folks from Mexico and parts of the southeastern U.S., where hurricanes remain dreaded threats throughout their warm seasons, wondering why Hilary was such a big deal. (Think Hurricane Idalia as it slammed into Florida about ten days later.) We know that every storm is different, but Hilary stands out for a number of reasons besides the obvious one: we’re not supposed to get such storms in California.

Follow this series of NOAA maps from August 16, four days before landfall:

This 500mb map shows pressure trends and winds about halfway up through our atmosphere (around 18,000 feet or 5,600 m above sea level). The shafts point into the wind to show direction and their barbs (flags) show the strength of the wind. Most important is the location of high and low pressure systems that often steer weather systems and determine weather conditions and winds at the surface. Note how California is sandwiched between two rather stationary pressure systems. Winds blow clockwise out of the high pressure to our east and counterclockwise around the low pressure to our west, leaving California experiencing winds from the south.
Here are the estimates of where Hilary will spread tropical storm force winds. This is four days before Hilary made landfall. Southern California is in the projected path.
The cone is used on this map to show Hilary’s projected path, also four days before landfall. 
This map shows estimates of when tropical storm force winds might sweep into southern California. Though the forecast is four days ahead, the timing is very close to Hilary’s actual arrival.

It all started several long days before Hilary’s August 20 landfall, when the National Weather Service recognized yet another band of showers and thunderstorms forming off Mexico’s southwestern coast. Models forecast favorable conditions (including warm ocean water temperatures) supporting the rapid development of a tropical cyclone that would likely become a major hurricane. And this one would be different. Longer-term models suggested that the strengthening storm would drift northwest and up along the Baja coast toward southern California. Days later, by Thursday, August 17, the National Weather Service was already warning us about Hilary, which was churning over the warm waters south of Baja. Here is an excerpt from the Forecast Discussion issued by the National Weather Service on that afternoon, still three days before the storm swept through:

National Weather Service in San Diego at 152 PM PDT Thu Aug 17 2023:
“Hurricane Hilary will continue to progress north off the coast of Baja, bringing widespread rain and the potential for flash
flooding across Southern California late Saturday through Monday.”
AND…
“Hurricane Hilary is expected to rapidly intensify as it moves northward up Baja
California, then weakening as it hits colder waters in northern
Baja.”
AND…
“Regardless of the exact track and intensity of Hilary, which 
could continue to change in the coming days, it will bring a 
substantial surge in moisture into Southern California, with heavy
rainfall and a high potential for flash flooding, especially for 
the mountains and deserts. A Flood Watch has been issued with the
afternoon forecast package for all areas Saturday through Monday.”
AND…
“Current forecast rainfall amounts Saturday through Monday:
Coast: 2 to 2.50 inches 
Valleys: 2.50 to 3 inches 
Mojave Desert: 3 to 4.50 inches 
San Bernardino County Mtns: 4 to 6 inches, locally up to 8 inches
on the eastern slopes
Riverside and San Diego County Mtns: 4 to 8 inches, locally up to
10 inches on the eastern slopes 
Lower Deserts: 5 to 6 inches”

Follow these NOAA images from August 17:

This map shows how the location of high pressure to our east and low pressure to our west didn’t change much during one day, leaving California with continued winds streaming from the south. We are three days from Hilary’s landfall.
Three days before the storm hit California, the cone now indicates how Hilary’s remnants will stream north of us after the storm dies out.

Fears of damaging coastal winds and a possible storm surge would never be realized in California since it came ashore south of the border. However, final rain totals would confirm NWS predictions. By Friday, August 18, as predicted, Hilary was already a major Category 4 hurricane, but why were forecasters so confident that it would be steered toward the north-northwest and into southern California within just two days? We can partially blame that humongous, stubborn, and equally historic high pressure system that wobbled and expanded above the southern U.S., producing record high temperatures that tortured Texans and surrounding regions through most of the summer. Winds circulating clockwise from it were streaming from south to north over the southwestern U.S., on the far western edge of that high. These southerly winds were further enhanced by another unseasonably stubborn system: low pressure sitting just off the California coast. As winds were spinning counterclockwise into that low, southern California was positioned on its southeast edge, further encouraging winds to stream over us from the south and southwest. Sandwiched between these two pressure systems, southern California was located below an upper air high-velocity freeway, a perfect conduit that would rapidly push Hilary up the Baja coast and toward us.    

Follow this series of NOAA images from August 18:

Here are estimated rainfall totals two days before Hilary made landfall. Compare this to some of the final totals for individual locations, as they are listed in this story.
Look familiar? Two days before landfall, the 500mb map and upper-level winds haven’t changed much, as California is still positioned between dueling pressure systems to our east and west.
It is hard to believe that powerful Hurricane Hilary could make it all the way to California in just two days. Here, the storm draws in tropical air masses carrying tremendous amounts of moisture and energy. The system gets caught up in a southerly flow that will propel it toward us.
Two days before the storm arrives, the forecast track has not changed much. A late wobble or zig or zag will make the difference between landfall in Baja or a direct hit in Southern California.
This water vapor image shows a giant mass of dry air (yellow) associated with our summertime subtropical high pressure over the Pacific and that pesky persistent low spinning counterclockwise just off the California coast. It’s just two days before Hilary will arrive (late Friday, August 18, Cal time), but you can just see the eye of this powerful hurricane appearing on the very southeast corner of this image, as it drifts up the Baja California coast. Scattered summer monsoon thunderstorms are exploding over the Desert Southwest.

Those strong winds would also mold Hilary and change her behavior and impacts as she grew closer. First, the storm started moving very fast; unlike most other tropical cyclones, there was little time for it to completely dissipate as it raced over our colder waters that quickly kill most hurricanes. (Recall that tropical cyclones, powered by latent heat of condensation, require ocean water temperatures near 80° F (27° C) to develop and remain strong.) This made it likely that it would make landfall and then sprint into southern California as a dying tropical storm. Consequently, much of southern California was placed under the first tropical storm warning in history. But these strong steering winds also sheered the storm apart and tore it to pieces as it approached. The powerful updrafts, towering clouds, and tropical moisture were quickly swept to the north far ahead of the core of circulation, as it skimmed by the central Baja coast. Though moisture from the cyclone spread into Baja and southern California more than a day ahead, the eye of the deep low pressure and circulation into it became difficult to detect as it grew less organized and was torn to shreds.

Follow these NOAA images from August 19:

A little more than one day before the storm moves into California, the only major change in the pressure and wind patterns maps is that Hilary has appeared as it races up the Baja coast. Note how California is still situated below the southerly winds flowing between those big high (to the east) and low (to the west) pressure systems. 
By Saturday night, Aug 19, Cal time, Hillary spins just off the central Baja coast. But the storm’s bands of moisture and clouds are already being blown far to the north, ahead of the storm and into SoCal.
Also on the evening of August 19 (Cal time), the night before the storm slogged into California: This water vapor image shows still Hurricane Hilary beginning to show signs of weakening as it passes over cooler water and interacts with the land in central Baja.

From the ground, a few odd weather observations would only suggest that a tropical cyclone might be approaching the Southland. Scattered thunderstorms had built up over our mountains and deserts during afternoons before Hilary arrived, but they were typical of the summer monsoon moisture that sometimes slops into southeastern California from the Sonoran Desert. The only hints that a big storm might be coming were the progressively thicker high and middle-level clouds streaming off of Hilary and over our region. Though unusual, Saturday’s gradually increasing cloud cover was not unheard of for August. After all, these could have been debris clouds drifting off the previous day’s monsoon thunderstorms over the Sonoran Desert. But, as dew points increased into the low 70s and precipitable water (the amount of water that could be drained from one entire column of air) increased toward 2 inches, we were entering uncharted summer weather territory. We can thank NWS forecasters, using their forecasting technologies, for encouraging local officials to prepare our infrastructures to manage California’s first tropical storm in more than 80 years.  

Hilary raced north to make landfall on the Baja coast just south of the border on Sunday, August 20. This, along with regional wind conditions previously mentioned, helped to save the Southern California Bight from the high waves and storm surge we associate with big hurricanes that strike Florida and U.S. Gulf states. Our beaches and coastal flatlands were spared such destructive drama. While officials in coastal communities had played it safe rather than sorry, some were wondering if this big tropical storm talk was just big hype. By the time Hilary got to southern California, it was difficult to identify any organized eye or other center of circulation. But this had become two different storms. One showed up as a day of steady rains and mostly gentle swirls of winds around the coastal plains. The other Hilary ravaged slopes farther inland over the mountains and into the deserts with torrential rain and high winds. As the moist air was forced up mountain slopes, copious amounts of orographic precipitation were drained out in the form of heavier cloudbursts. So the mountains and deserts experienced the brunt of this storm that moved inland.     

Follow these NOAA images from August 20 as Hilary swept into SoCal:

National Weather Service forecasters gave us a good idea of where the flooding would be most serious and how Hilary’s moisture would continue streaming north as the storm dissipated.
This water vapor image shows now Tropical Storm Hilary making landfall just south of the border and breaking up into disorganized eddies and local downpours embedded in the remaining bands of steady rain. Noticed the moisture plume stripped off of the storm and spreading north.
As Hilary passed over Southern California and collapsed into bands of rain, it became difficult to identify an eye or any other well-organized circulation as the storm was increasingly stretched toward the north.
Here’s what the tropical cyclone looked like on a surface weather map just before it hit Southern California: a weather map to remember.

When Hilary swept across those massive mountain barriers that extend north from the Mexican Border, the storm became further disorganized. Sustained winds peaked at over 50 mph with gusts over 60 mph within the updrafts, downdrafts, and turbulent eddies that formed along mountain slopes. Rain totals over 6 inches were recorded from San Diego County mountains north into Canyon Country in L.A. County. Desert slopes adjacent to those mountain ranges (from near Borrego Springs to Palm Springs to Whitewater to Palmdale) received nearly as much rain, but on surfaces with little protective vegetation to absorb the water. Mud and debris flows with nowhere else to go buried roads and some neighborhoods (such as in Cathedral City), even blocking major Interstates I-8 and I-10. Within one day, Hilary’s unprecedented moisture plume continued streaming north across the Mojave and into the Basin and Range. Death Valley recorded its wettest day in history. And though officials anticipating the destruction had closed the park to visitors, about 400 people were stranded within park boundaries when roads were washed out or destroyed. Squeezed by winds streaming between those two massive pressure systems, the now disorganized moisture plume continued flowing north, delivering some precious water to drought-stricken regions northeast of Nevada. By Tuesday, August 22, Hilary was already history. Though several mountain and desert communities had plenty of digging out to do, Hilary’s quick hit impacts weren’t as catastrophic as a tropical cyclone that could have wobbled around and dissipated over us for a prolonged period.

Where are the giant waves and storm surge? Hilary made landfall in northern Baja before romping inland through SoCal. California was spared the coastal destruction that often accompanies tropical cyclones. But we were ready for the heavy rains that did arrive.
Santa Monica Beaches were closed but only locally flooded by runoff from Hilary’s rainstorms.
Instead of the dramatic ragged dark thunderheads you might expect from severe storms, blankets of gray clouds gradually thickened and darkened the day before Hilary arrived. They finally dropped steady curtains of rain and occasional cloudbursts as the storm passed by on Sunday, August 20. But there were no destructive winds and few other severe impacts along the coast, except for the unprecedented rainfall totals for August.

The big rain winners were mostly at higher elevations along and near the spine of the Peninsular Ranges and up into the Transverse Ranges. The official gauge at 8,616 feet asl, near the top of the Palm Springs Tramway, recorded 11.74 inches from the storm, while surrounding regions that included Idyllwild topped 7 inches. The Baywood Flats gauge at 7,097 feet in the San Bernardino Mountains recorded 11.73 inches, while surrounding locations that included Lytle Creek also accumulated more than 7 inches. Mt. Wilson caught more than 8.5 inches. Some higher terrain in San Diego County (including Mt. Laguna) also got more than 7 inches of rain from Hilary. As mentioned, L.A. County’s Canyon Country topped out at about 7 inches. Surrounding vulnerable desert terrain and recent burn scars delivered the most damaging mud and debris flows. The next day, as Hilary’s remnant moisture sprinted farther north, the National Weather Service Las Vegas Office issued this message after debris flows sprawled across the desert floor:
“Yesterday (August 20, 2023), Death Valley National Park observed 2.20″ of precipitation at the official gauge near Furnace Creek. This breaks the previous all-time wettest day record of 1.70″, which was set on August 5, 2022.”    

Follow these images from August 21, as what remains of Hilary slop far to our north:

On the day after Hilary passed and died out, that stubborn low anchored off the California coast finally made its move inland. This squeezed remnant moisture from Hilary farther to our northeast around that giant high pressure still plaguing the middle of the country.
Monday, August 21 was an odd recovery day. Clouds broke into mostly scattered fair-weather stratocumulus over southern California as that unusually persistent low pressure system finally spun inland and lifted out of northern and central California. Some shredded remains of Hilary appear as scattered showers and thunderstorms far to our north and east.
Hilary’s record August downpours (such as more than 3.5 inches in Santa Monica) tested infrastructures across the Southland. The day before this photo was taken, flooding crested a few feet over the bike path at the Pico storm drain, damaging drainage channels, taking out fences, and carving a wide canyon into the beach sand.
The day after Hilary, a cast of fair-weather stratocumulus is breaking up over the Santa Monica Mountains behind Santa Monica Pier. If it wasn’t for the stranded debris, damaged fences, and soaked sands, you’d never know such a tropical rainstorm had just swamped this beach.
Unfortunately, the floodwaters carried tons of garbage and other pollutants off city streets and into the ocean. This powerful “first flush” overwhelmed diversions that would otherwise catch pollutants during the dry season.
The first flush usually appears after the first big storm in autumn, but it came very early this year. All the trash and pollution left in and around city streets accumulates during the dry season and is then coughed out if the first rain is too heavy to divert its discharge or capture the tons of trash. We stay out of the water for a few days until bacteria levels return to safe standards. This is also the day after Hilary. More fair-weather stratocumulus are seen breaking up behind Santa Monica Pier. Within a week, the water away from the pier was safe to jump in again.

You might say the rain losers were in the coastal flatlands of San Diego and Orange Counties (many locations didn’t make it to 2 inches), where Hilary’s moisture lacked the final lift required to ring out her tropical air masses as she simultaneously collapsed and sprinted by. Nevertheless, numerous weather stations in southern California’s coastal plains received record August rainfall in just one 24-hour period. (See the record totals in our image from the L.A. National Weather Service Office.) More than 3.5 inches of rain in one 24-hour period would make history at any time of year here in Santa Monica. But such rainfall totals in August were unheard of. Until now. 

This image from the NWS Office in L.A. is impressive for August, but the rainfall totals are only about half of what accumulated on nearby mountain slopes.

That was a quick summary of the life and times of tropical cyclone Hilary as her dying remnants staggered or rampaged through a well-prepared southern California. The good news is how this storm soaked our higher-elevation forests and lower-elevation Mediterranean plant communities that are normally dehydrating into the most dangerous fire seasons of the year. Quick deluges bumped up moisture content in our soils and vegetation and rejuvenated springs and streams that normally dry up this time of year. We can see formerly drooping and wilting plants in our gardens and on our slopes that are suddenly and surprisingly reawakened, while other species seem to not know how to respond to so much water in the middle of summer’s Mediterranean drought. Even parts of the Mojave Desert that were shielded from winter’s drought-busting atmospheric rivers were finally lifted out of official drought categories by this storm. But contrary to a few media stories, Hilary didn’t add significant new supplies to California’s water capture and distribution projects, since most of them are in northern California drainage basins not impacted by Hilary. In SoCal, a small portion of the runoff was pooled in spreading basins to percolate and recharge groundwater supplies, but much of the flash flooding was not captured.

After the normal seasonal drying out, Temescal Canyon’s springs, stream, and its falls were refreshed by Hilary, an unlikely rejuvenation that flowed for several days.
Just a few days after Hilary left, Mediterranean plant communities in the Santa Monica Mountains were still adjusting to record August rainfall. A smattering of green reemergence decorated these slopes above L.A. during what are usually the punishing drought days of late summer. But even the summer smog has returned.
The falls at Escondido Canyon was another water world recharged by up to five inches of rain that Hilary poured on parts of the Santa Monica Mountains.
Streams and riparian environments that were parched by summer drought were suddenly recharged by up to five inches of rain in one day from Hilary. This is nearly a week after the storm in the Santa Monica Mountains.
Cliff Aster (Malacothrix saxatilis) blooms throughout the summer, so it stands out during the Mediterranean drought season. It is found on southern California’s coastal slopes. Hilary gave these flowers a surprising out-of-season boost that showed six days after the rains.
Chaparral Bush Mallow (Malacothamnus fasciculatus) is a common shrub within coastal sage scrub plant communities from Baja up to the central California coast. It is another nonconformist that blooms throughout our dry summers. Six days after the storm, it displayed benefits from the one-day record downpours.  
Summer flowers of mallow and cliff aster combine with other species in the coastal sage scrub plant community to celebrate the unexpected deluge that occurred a week earlier. The pungent California Sagebrush (Artemisia californica, in the lower right foreground) usually turns into a dusty, dehydrated gray shrub and is even drought deciduous as summer progresses. But it has also perked up a bit. Since it is very common in coastal sage and chaparral plant communities from Baja into northern California, it is not accustom to such drought-interrupting rain during summer.
Dehydrated by summer drought and suddenly fed by Hilary’s downpours nearly a week ago, these coastal sage scrub and riparian plant communities adapt. But each species responds very differently (and some don’t respond out of season) to the brief soaking that punctuated the summer of 2023.

If you’re still interested in viewing the more dramatic videos showing flood damage and destruction from Hilary, you can click one of the links that follow this story.

It’s been a bizarre 2023 weather year. We are plunging into an El Nino event in the Pacific, which is forecast to continue into winter. On to the next weird and wild weather anomaly… and our new weather and climate publication scheduled to appear early next year.             

William Selby

William Selby is a native Californian who has explored, researched, and worked in every corner of the Golden State. William has accumulated over 3 decades of experience teaching Geography and Earth Science at Santa Monica College. Since retiring from teaching, his lifelong love and extensive knowledge of the Golden State is reflected in the book as well as in his continued service to academia, geography and the State of California.

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