As of Friday morning, Hurricane Hilary strengthened to a strong Category 4 storm in the tropical Pacific Ocean west of Mexico. For the first time in history, the National Hurricane Center issued a tropical storm watch for parts of Southern California, where unprecedented heavy rainfall could produce catastrophic and rare impacts in the coming days.  

Significant impacts are possible for the Baja California Peninsula in Mexico as well, in addition to the southwestern US later this weekend and early next week. Check out a recent satellite image of Hurricane Hilary below.

While remnants of tropical storms have impacted California in recent history, it has been 84 years since a tropical storm made landfall in California. It is not uncommon for storms that previously reached hurricane strength to track over the southwestern US bringing heavy rains. The latest guidance from the National Hurricane Center, seen below, keeps the possibility of a Southern California landfall. 

While Hurricane Hilary has continued to strengthen, eventually it will weaken as it moves over cooler water. You can see the sea surface temperatures cool off as you move north along the Baja California Peninsula coastline below.

By the time the storm reaches Southern California, it will likely have weakened to a tropical storm or tropical depression.

Initial landfall may occur over the Baja California Peninsula, but there is still uncertainty with the storm track. Regardless of the amount of strengthening and exact landfall location, there will be significant impacts from the Baja California Peninsula, up through Southern California, and perhaps in Nevada as well. 

The first major impact of this hurricane will be strong and potentially hurricane-force winds, which are possible on the western coast of the Baja California Peninsula of Mexico. With the storm weakening prior to arriving in Southern California, hurricane-force winds are unlikely. However, strong tropical-storm-force winds are possible, with the mountainous terrain in Southern California seeing more severe wind impacts.

The second and likely more significant impact from this storm will be heavy rainfall, as well as the secondary impacts from heavy rainfall including flooding and mudslides. Rain totals from hurricanes and tropical storms can be massive and devastating, with no shortage of extreme events in Texas and Louisiana with Hurricane Harvey, and even in the Northeast with Irene and Sandy over a decade ago. 

The most significant rain totals are currently expected in the northern Baja Calfornia Peninsula, southern California, and southern/central Nevada. NOAA’s Weather Prediction Center’s latest precipitation forecast is below, with an area of 6 - 10 inches of rain in the northern Baja Calfornia Peninsula, and a wide area of over 4”. In southern California and Nevada, a large area of 4 - 6” of rain stretches from the border through southern Nevada. Large swaths of 6 - 10” of rain are forecast as well, east of Los Angeles and west of Las Vegas, with some areas of 10 - 15 inches.

Most of the time, hurricanes and tropical storms produce massive rainfall totals simply due to the dynamics of these types of storms and have limited impacts due to terrain-enhanced precipitation. When this storm reaches southern California, the mountainous terrain in the region will produce enhanced rain totals even greater than what falls at low elevations. 

Many areas in the path of this storm aren’t used to this much rain in a short period of time. Certain parts of California and Nevada could see one to two years' worth of rain in just a couple of days. This could be the most moist airmass ever recorded in California.

This storm will likely fall into the strongest category of atmospheric rivers, an extreme to exceptional category 5. Just north of the border near San Diego, the magnitude is expected to be the highest possible category, exceptional, as shown below.

Away from the coast, in the foothills east of San Diego, the magnitude of the atmospheric river is an even strong exceptional category 5, shown below.

Even further inland, near the California border, the storm will bring exceptional category 5 atmospheric river moisture, shown below.

There is a sharp gradient in precipitation totals along the western edge of the storm, allowing small shifts in the track to change impacts drastically. You can see this sharp gradient in California in the forecast precipitation from the European model below.

With each day, we will further narrow the track of the storm, and the regions expecting the most extreme impacts. After dropping huge rain totals from California through Nevada, further impacts from heavy rain may be felt in Washington, Oregon, Idaho, and Montana as the storm gets absorbed into the jet stream. With yesterday's National Hurricane Center forecast, California, Nevada, Oregon, and even part of Idaho are in a tropical cyclone probability cone before Florida has been yet his year. This potential historic tropical system must be watched carefully in the coming days as we hope the trends during this time lead to less severe impacts.

Luke Stone