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California’s first Tropical Storm Watch on record as Hurricane Hilary heads for Baja » Yale Climate Connections

With category 4 Hurricane Hilary churning off the coast of Baja California and heading northwest, millions of residents of Southern California from San Diego across Orange County found themselves on Friday in the area’s first tropical storm watch since modern hurricane warning practices began in the 1950s. Hilary is expected to turn north-northwestward and accelerate over unusually warm waters off the coast of Baja Mexico this weekend, enabling it to maintain enough strength to bring tropical storm conditions to parts of Southern California Sunday night and Monday morning. The storm’s impacts are likely to be severe and highly damaging over a large region of the Southwest U.S., with record rainfall amounts possible in some desert areas.

Track forecast for Hilary 

As of 11 a.m. EDT Friday, Hilary had top sustained winds of 145 mph and a central pressure of 939 mb, located about 360 miles south-southwest of the tip of Mexico’s Baja California Peninsula, headed northwest at 10 mph. Hilary had rapidly intensified by a very impressive 75 mph in the 24 hours ending at 5 a.m. EDT Friday. The first hurricane hunter mission into the Hurricane was en route to the storm early Friday afternoon to provide more detailed information.

Hilary was about 65 miles southwest of Socorro Island, where sustained winds of 46 mph, gusting to 68 mph, were observed at 12:40 p.m. EDT. Heavy rains from the hurricane were affecting the southern tip of the Baja Peninsula, as seen on Los Cabos radar.

A trough of low pressure off the California coast and a near-record-strength ridge of high pressure over the central U.S. will provide a well-defined steering flow for Hilary, turning the storm to the north-northwest on Saturday, taking the hurricane on a course roughly parallel to the coast of Baja Mexico. Models have come into increasing alignment that Hilary will make landfall between Los Angeles and a point south of Tijuana, Mexico, just south of the U.S.-Mexico border. If Hilary makes landfall in California while still classified as a tropical cyclone, it would be the first one to do so since 1939.

Intensity forecast for Hilary

Hilary has favorable conditions for maintaining its intensity through Saturday morning, with warm ocean waters near 30 degrees Celsius (86°F) – some 2°C to 4°C warmer than average for mid-August – as well as low wind shear and a very moist atmosphere. This area of unusually warm water off the southern Baja Peninsula likely developed in connection with the tongue of El Niño-warmed water across the eastern tropical Pacific. Warm waters do not extend to great depths around the hurricane, though, limiting the total amount of heat energy available to power it. Early Friday, Hilary completed an eyewall replacement cycle, which likely weakened the peak winds of the storm, but spread hurricane-force winds over a wider swath. The top hurricane intensity models generally show Hilary has already peaked in intensity or will intensify by at most another 10 mph by Saturday morning.

As Hilary heads north on Saturday, the hurricane will encounter chilly waters of 20-25 degrees Celsius (68-77°F), which lie along the central Baja coast. The strong winds of the hurricane’s spiral bands will stir even cooler waters below to the surface ahead of the storm, sharply reducing the sea surface temperature. Wind shear will begin increasing to the moderate range by Saturday night, dry air will increase, and the storm will likely experience weakening from interaction with the high terrain of the Baja peninsula. All of these factors should cause steady-to-rapid weakening of Hilary beginning on Saturday evening. All six of our most reliable intensity models show Hilary will be below hurricane strength when it reaches Southern California Sunday night or early Monday morning.

Rainfall amounts of three to six inches are possible throughout Baja California, with localized totals of 10 inches on the peninsula, bringing the risk of flash floods and mudslides in mountainous areas.

Impacts of Hilary in California

Though at first glance it may seem like no big deal for California to get by what is expected to be merely a tropical storm, the presence of high mountains will greatly amplify the rains and winds of Hilary. This is unlike the situation that exists for the typical landfalling Atlantic storm in the U.S., where the terrain is relatively flat. Moreover, Hilary’s heaviest rains are predicted to fall in desert areas unused to extreme rains, and the counterclockwise flow of air around the storm will tend to bring the heaviest precipitation to east-facing mountain slopes. These are generally drier and more prone to flooding, since the prevailing west-to-east moving weather systems during the winter wet season favor the heaviest rains falling on western-facing mountain slopes.

And Hilary will be no ordinary tropical storm. Hilary is a large and very intense hurricane that is embedded in a very moist air mass, with midlevel relative humidity around 80%. The storm is towing an impressive amount of moisture northward with it, and the amount of water vapor in the atmosphere may approach all-time records in some locations along or near Hilary’s path. The storm’s impacts are likely to be severe and highly damaging to the Southwest U.S., with damages likely running at least into the hundreds of millions of dollars. Hilary is expected to bring some of the heaviest summer rains on record to Southern California, as well as to portions of Nevada, western Arizona, and perhaps southwestern Utah.

The National Weather Service Weather Prediction Center took the unusual step of issuing a “high” risk outlook for excessive rains leading to flash flooding Sunday morning through Monday morning over parts of the far Southern California desert, where three to six inches of rain can be expected, with isolated higher amounts of 10 inches in mountainous areas. The discussion for the forecast warned of 1-in-100-year rainfall amounts, saying, “there is a very real potential for 3″ amounts in an hour in this environment. Some of the guidance shows local amounts of 7″+, which would be exceeding rare for the region from a tropical cyclone, potentially unique for Nevada. If a 7″+ maximum materialized over Mount Charleston Sunday into early Monday, it would challenge Nevada’s 24-hour rainfall record, set in 2004.”

Moisture streaming ahead of the hurricane will reach the Southwest U.S. on Saturday, contributing to a “moderate” risk of excessive rainfall Saturday morning through Sunday morning near the California-Mexico border. This moisture will not be a direct part of the hurricane – instead, it is more akin to the predecessor rain events that can develop hundreds of miles ahead of Gulf and Atlantic hurricane landfalls – but nonetheless, it will be capable of triggering damaging flooding. The excessive rainfall discussion for Saturday night and Sunday morning warned of near-record amounts of atmospheric moisture from Las Vegas southward, with the potential for over five inches of rain overnight Saturday into Sunday, most likely on the eastern slopes of the Peninsular Ranges of Southern California near the Mexican border. 

Though Hilary’s winds over the ocean surface are expected to be at tropical storm force when the storm reaches California, winds over land blowing downslope through the mountains will be capable of reaching hurricane strength. For example, the wind forecast for the 850 mb level (roughly 5,000 feet high) from the 6Z (2 a.m. EDT) Friday run of the HWRF model (see Tweet above from Tomer Burg) showed hurricane-force winds at that elevation at 6Z (2 a.m. EDT) Monday along the Southern California coast.

In their 11 a.m. EDT Friday wind probability forecast, NHC gave San Diego a 49% chance of experiencing tropical storm-force winds by Monday morning; Long Beach near Los Angeles was given a 28% chance.

Strong downslope winds will also be a danger to fan any wildfires that might get sparked by lightning from the storm – though Hilary’s rains will tend to dampen such fires. On Tuesday, Hilary’s remnants are expected to reach Oregon and Washington. Little precipitation is expected there from the storm, whose winds may act to fan the flames of large wildfires.

Hilary’s winds may also bring a storm surge of several feet to the Southern California coast if the center remains offshore; large waves and high surf can be expected to cause damaging coastal erosion.

One analog to Hilary from the vaults: Hurricane Kathleen in 1976     

Some of the most dramatic winds and rainfall from a tropical cyclone to affect southern California and southwest Arizona in the modern era arrived on Sept. 10, 1976, with the remnants of Hurricane Kathleen – arguably the region’s highest-impact tropical cyclone since a tropical storm struck San Diego in 1939 (discussed in our Thursday post).

Kathleen was only a category 1 hurricane with 80 mph sustained winds at its peak strength; it made dual landfalls on the Baja California coast well south of San Diego early on September 10. However, the fast-moving system was slow to weaken as it tracked inland at speeds of 35-40 mph. Kathleen passed over the deserts of Southeastern California, moving just west of Las Vegas before dissipating.

Kathleen was classified by the National Hurricane Center as a tropical depression throughout its California/Nevada track, but its winds were notably stronger. As usual with tropical cyclones in North America, the strongest winds were on the right-hand (east) side of Kathleen’s decaying center. Yuma, Arizona, experienced sustained winds of 57 mph, solidly in the tropical-storm range, and winds gusted as high as 76 mph before the National Weather Service office in Yuma lost power. A post-storm study from the National Weather Service attributed Kathleen’s unusual strength in the U.S. Southwest mainly to its rapid rate of motion and higher-than-average sea surface temperatures.

map of rainfall from Hurricane Kathleen, including maximums of 14.76 inches in San Gorgonio, California, and 6.52 inches in San Antonio, Mexico
Figure 1. Rainfall totals from Hurricane Kathleen. (Image credit: NOAA/NWS Weather Prediction Center.)

Total rainfall from Hurricane Kathleen and its remnants included widespread one- to three-inch amounts over southern California and Nevada, with pockets of three to seven inches, roughly comparable to what Hilary might produce (though localized totals could be even higher in Hilary). Palm Desert received 3.57 inches of rain, almost double its annual average. Peak rainfall from Kathleen was 14.76 inches at San Gorgonio in California’s Transverse Mountains, the east-west range north of Los Angeles that may get hit hard by Hilary as well. Six deaths were reported from major flash flooding in Ocotillo, California, where many homes were inundated. Losses from Kathleen in California, including most of the year’s raisin crop, topped $100 million in 1976, or more than half a billion in 2023 dollars.

Four threat areas to watch in the Atlantic

In the Atlantic, the area of most immediate concern to monitor is in the Gulf of Mexico. A tropical wave currently over the Bahamas will bring locally heavy rainfall to Florida on Saturday, then enter the Gulf of Mexico on Sunday. On Sunday and Monday, the wave will interact with the remnants of an old cold front. With favorable upper-level winds and record-warm ocean temperatures in the Gulf, a tropical depression could form by Monday.

Anything that forms will be steered mostly to the west, resulting in the greatest threat to Texas, Louisiana, and/or the coast of northern Mexico, south of the Texas border. However, the system will likely not have enough time over water to intensify much, since landfall is expected in Texas on Tuesday, with South Texas being the most likely landfall location. Although some members of the Friday morning runs of the GFS and European model ensembles do develop something in the Gulf early next week, none of the forecasts show the system reaching hurricane strength. Much of the coast of Texas and Louisiana is in severe drought, and the rains from this predicted tropical disturbance are likely to bring two to four inches of rain, providing beneficial drought relief. In their 8 a.m. EDT Friday Tropical Weather Outlook, the National Hurricane Center, or NHC, gave two-day and seven-day odds of development of 0% and 30%, respectively, to the future Gulf of Mexico disturbance.

The National Hurricane Center is also watching three areas of disturbed weather in the tropical Atlantic, which recent runs of the GFS and European ensemble models have been showing could develop over the next week. The easternmost wave, designated Invest 98L by NHC, was located a few hundred miles west of the Cabo Verde Islands. This disturbance has the highest odds of development: two-day and seven-day odds of 60% and 70% respectively. However, it is headed west-northwest out to sea and is not likely to trouble any land areas.

A second disturbance midway between the Lesser Antilles and Cabo Verde Islands (Invest 99L) was given two-day and seven-day odds of development of 40%. This system is also headed west-northwest and does not appear to be a concern to any land areas.

Finally, residents of the Lesser Antilles will want to monitor the progress of an area of disturbed weather located a few hundred miles east-southeast of the islands. Dry air and wind shear will impede development of this system, which was given two-day and seven-day odds of development of 10% and 30%, respectively. Moisture from this system will affect the Lesser Antilles beginning on Saturday night, spreading northwestward to Puerto Rico and the Virgin Islands by Monday.

The next name on the Atlantic list of storms is Emily. It has been almost a month since Category 1 Hurricane Don, the Atlantic’s only hurricane of 2023 thus far, became a post-tropical cyclone on July 24.

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