The beginning of the peak part of hurricane season has arrived, and right on cue, multiple threat areas to monitor have developed in both the Atlantic and eastern Pacific. Both Texas and Southern California could see heavy rains from tropical systems early next week.

Three 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 moving into the eastern Caribbean is expected to enter the Gulf of Mexico by Sunday and interact with the remnants of an old cold front. Upper-level winds are predicted to be favorable for development, and with 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 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. Although some members of the Wednesday 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 western Gulf Coast is under drought, and the rains from this predicted tropical disturbance are likely to bring some beneficial drought relief. In their 8 a.m. EDT Tropical Weather Outlook, the National Hurricane Center, or NHC, gave two-day and seven-days odds of development of 0% and 20%, respectively, to the Gulf of Mexico.

NHC is also watching two tropical waves in the eastern Atlantic, which recent runs of the GFS and European ensemble models have been showing could develop over the next week. Both of these tropical waves are being given medium chances of development but are headed west-northwest, posing no foreseeable threat to land. Neither wave had been given an “Invest” designation by NHC as of Wednesday morning.

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

map of Hilary forecast cone showing the storm expected over Baja California by Sunday and potentially into Southern California by Monday
Figure 1. The first forecast from the National Weather Service for Hilary places San Diego, California, in the five-day cone of uncertainty.

Tropical Storm Hilary, soon to be a large and powerful hurricane, may affect California next week  

Tropical Storm Hilary formed off the southwestern coast of Mexico at 11 a.m. Wednesday and is expected to bring heavy rains and high surf to the Baja Peninsula this weekend. Heavy rains from Hilary will likely spread into the Southwest U.S. by early next week. In a rare occurrence, a portion of southern California was placed in Hilary’s five-day cone of uncertainty in NHC’s first forecast for the storm.

Forecast for Tropical Storm Hilary 

A trough of low pressure off the California coast and near-record-strength ridge of high pressure over the central U.S. will provide a well-defined steering flow for Hilary, taking the storm to the northwest along the coast of Baja Mexico this weekend. The storm has nearly ideal conditions for intensification over the next three days, with warm ocean waters near 30 degrees Celsius (86°F), low wind shear, and a very moist atmosphere (a midlevel relative humidity near 80%). Some of the models are very bullish with the intensity forecast, calling for Hilary to peak as a major category 3 or 4 hurricane on Saturday, when it will be making its closest approach to the southern tip of the Baja Peninsula. The 12Z Wednesday SHIPS model gave Hilary a 99% chance of rapidly intensifying by at least 65 mph in 48 hours – an unusually aggressive forecast of rapid intensification.

By Sunday morning, Hilary will be moving over much colder waters near 24 degrees Celsius (75°F), the surrounding atmosphere will become drier and more stable, and wind shear is predicted to increase. These factors are likely to cause rapid weakening of Hilary, and the National Hurricane Center is predicting Hilary will become post-tropical by Monday morning, when it will be nearing the Mexico-U.S. border. However, Hilary is expected to have a large circulation, with tropical-storm-force winds expected to span more than 480 miles on Sunday, so although rapid weakening is a safe bet, Hilary may not spin down quite as quickly as a smaller hurricane might. In their 11 a.m. EDT Wednesday wind probability forecast, NHC gave San Diego a 17% chance of experiencing tropical storm-force winds by Monday morning.

map shows potential rainfall from Hilary, with six to 12 inches possible in a narrow band of Baja California and Southern California
Figure 2. Total precipitation for the six-day period ending at 8 a.m. EDT Tuesday, Aug. 22, from the 8 a.m. EDT Wednesday, Aug. 16, run of the GFS model. The model predicted widespread rains of 2-4 inches over Baja and Southern California, with a narrow region of six to 12 inches. The exact location and amount of these heaviest rains are uncertain at this time, though. (Image credit: Tropical Tidbits)

A bigger concern for southern California than high winds will be heavy rains. Hilary will be dragging an impressive slug of moisture inland over the Southwest U.S. by early next week. Widespread rains of two to four inches are possible over much of Southern California, and rains of a half inch or more may extend to parts of Central California and Nevada. A swath of heavier rains of six to 12 inches is also a possibility, as predicted by the latest run of the GFS model (Figure 2).

There is also the potential for substantial lightning, which will be capable of igniting wildfires. In August 2020, the remnants of Tropical Storm Fausto led to a swarm of lightning strikes across California and eventually to wildfires that burned more than 1.5 million acres. Fortunately, drought conditions are nearly absent from California right now, compared to the widespread moderate to severe drought conditions that prevailed in 2020.

The first hurricane hunter mission into Hilary is scheduled for Friday afternoon.

Bob Henson contributed to this post.

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Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...