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Hilary races toward western U.S. as a potentially epic rainmaker » Yale Climate Connections

Former category 4 Hurricane Hilary was losing structure on Sunday as it swept north atop cool Pacific waters near the rugged terrain of Baja California. Though downgraded to a tropical storm on Sunday morning, Hilary remained on track for a historic swing through the western U.S., as the storm is predicted to haul some of the richest troves of atmospheric moisture on record from the Desert Southwest to the interior Northwest. Widespread and potentially catastrophic flooding is expected.

Hilary made landfall just before 2 p.m. EDT Sunday on the coast of Baja California about 215 miles south-southeast of San Diego. Hilary was pushing north-northwest at 25 mph, roughly parallel to the coast, with top sustained winds of 65 mph. Torrential rains have been falling over and just east of the peninsula since Saturday (see video below). Pockets of disastrous flash flooding will be possible in far northwest Mexico on Sunday, with localized rainfall totals of up to 10 inches.

As one half of Hilary moves over increasingly chillier water and the other half butts up against mountains, the storm will continue to weaken quickly on Sunday. A Tropical Storm Warning (the area’s first on record) remained in effect from San Diego to west of Los Angeles on Sunday morning. Hilary was forecast by the National Hurricane Center to move into Southern California just east of San Diego by Sunday evening, most likely still as a tropical storm but with only hours left at that status. Increasingly strong steering currents will whisk the diffusing center of Hilary to western Nevada by early Monday before the cyclone dissipates.

This forecast of weakening fails to convey the extremely serious threats posed by Hilary’s rains over a broad corridor. Regardless of its status as a tropical cyclone, Hilary will deliver one of the most dramatic infusions of tropical moisture in modern records to a corridor stretching from Southern California to western Nevada, eastern Oregon and Washington, much of Idaho, and western Montana.

Light to moderate rainfall was already widespread from Southern California into Nevada and southern Idaho by Sunday morning, well ahead of Hilary itself, dropping scattered totals of up to 0.50 – 1.0 inches. These rains were intensifying by midday Sunday and will quickly ramp up when the core of Hilary’s moisture arrives on Sunday evening. Heavy rain often breaks out hundreds of miles ahead of a landfalling tropical cyclone in what are known as predecessor rain events, or PREs.

The National Weather Service’s Weather Prediction Center has maintained a unusual High Risk area for excessive rainfall through 8 a.m. EDT Monday across much of interior southeast California into far southwest Nevada. The center noted:

It’s looking increasingly probable that Hilary will be the wettest known tropical cyclone, post-tropical cyclone, or tropical cyclone remnant to impact Nevada ([record 4.36″] set in 1906), Idaho ([record 2.20″] set in 1982), and Oregon ([record 1.35″] set in 1976). Some locations within this arid region are slated to get 1-2 years worth of rain in one day. If a 7″+ maximum materialized over Mount Charleston Sunday into early Monday, which is now explicitly forecast, it would challenge Nevada’s 24 hour rainfall record, set in 2004.

See this page from the Weather Prediction Center for state-by-state record rain totals from tropical cyclones.

Strong winds are also possible as Hilary races through Southern California into Nevada late Sunday. Although the surface cyclone will be rapidly weakening, the flow at upper levels may take longer to spin down, so mountainous areas in particular could get sustained winds of 30-40 mph, with gusts into the 50-70 mph range. Some parts of the Los Angeles area could get a rare “wet Santa Ana” event, as strong northeast winds wrapping around Hilary push downslope much like the hot, dry Santa Ana winds typical of autumn. Given the quickly saturating soils, trees and power lines could come down in some areas.

With Hilary moving more quickly than predicted, the worst conditions should be winding down across Southern California and Nevada by Monday morning, as the heaviest rains and strongest winds push farther north. High wind warnings are in effect as far north as southern Idaho, and flash flood watches extend all the way to the Idaho panhandle.

NHC monitoring five systems in the Atlantic

The Atlantic continued to hum with activity on Sunday afternoon, with the National Hurricane Center (NHC) tracking five systems. Among these are a new tropical depression and tropical storm – both of which are expected to remain safely in the open Atlantic during their brief lives – as well as two other systems that may strengthen and pose threats to land this week.

On Saturday at 5 p.m. EDT, Tropical Depression Six formed in the remote central Atlantic, far from any land areas. Satellite images on Sunday afternoon showed TD 6 was an unhealthy system, with a surface circulation center exposed to view because of strong wind shear and dry air. TD 6 is headed into an even more hostile environment with higher wind shear and drier air, and is expected to dissipate by Monday without ever becoming a tropical storm.

A tropical wave located in the eastern Atlantic, a few hundred miles west-northwest of the Cabo Verde Islands, was upgraded to Tropical Storm Emily at 11 a.m. Sunday EDT. Emily’s sustained winds were estimated at 50 mph based on satellite-derived data, and satellite images on Sunday afternoon showed 98L was was increasingly organized. However, Emily is headed west-northwest out to sea into an area of high wind shear and is not likely to last long enough to trouble any land areas. Emily is projected to be angling toward the north-northwest as a post-tropical cyclone by midweek, remaining well east of Bermuda.

Caribbean disturbance 90L a heavy rain threat for Hispaniola

In the eastern Caribbean, a tropical wave designated Invest 90L was on the verge of becoming a tropical depression or tropical storm on Sunday afternoon. 90L was bringing heavy rain showers and gusty winds to the Lesser Antilles Islands and northern portions of the South America coast on Sunday afternoon as it moved west to west-northwest at 10-15 mph. Satellite images and Barbados radar showed that 90L was growing more organized with increasing heavy thunderstorm activity and a broad rotation, and there were increasing signs of a surface circulation developing. Conditions were somewhat favorable for development, with warm ocean waters near 29.5 degrees Celsius (85°F), moderate wind shear of 10-20 knots, and a reasonably moist atmosphere (mid-level relative humidity of 65%).

Moisture from this system will spread northwestward to Puerto Rico, the Virgin Islands, and the Dominican Republic Sunday night and Monday. The disturbance has strong model support for development, with multiple models showing 90L turning north and affecting Hispaniola as a tropical depression or tropical storm on Tuesday. In the longer range, the GFS and European model ensembles suggest that 90L will continue northward and perhaps intensify by next weekend, so Bermuda may need to keep an eye on this one.

In their 2 p.m. EDT Sunday Tropical Weather Outlook, NHC gave 90L two-day and seven-day odds of development of 90% and 90%, respectively. NOAA Hurricane Hunters were en route to investigate this disturbance on Sunday afternoon, with another flight scheduled for early Monday.

Gulf of Mexico disturbance expected to reach Texas or Mexico early this week

The area of most immediate concern for the United States is in the Gulf of Mexico. A tropical wave designated as Invest 91L was bringing locally heavy rainfall to South Florida and western Cuba on Sunday afternoon, and will move westward at 15-20 mph through the Gulf of Mexico while interacting 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 South Texas and 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 on Tuesday.

Although some members of the Sunday morning runs of the GFS and European model ensembles do develop 91L into a tropical depression or weak tropical storm, none of the forecasts show the system reaching hurricane strength. Much of the coast of Texas and northern Mexico is in moderate to severe drought, and the rains from 91L are likely to bring two to four inches of rain, providing beneficial drought relief. In their 2 p.m. EDT Sunday Tropical Weather Outlook, NHC gave the system two-day and seven-day odds of development of 50% and 60%, respectively. An Air Force Hurricane Hunter flight will investigate 91L on Sunday afternoon, with additional flights on tap for Monday.

Figure 1. Forecasts from the European model ensemble issued at 6Z (2 a.m. EDT) Sunday, August 20, with storm tracks extending to Saturday morning, August 26. The ensembles show no threat to North America from the various tropical systems, except for the Gulf of Mexico disturbance (Invest 91L) making landfall along the coast of South Texas or Northeast Mexico on Tuesday, most likely as a tropical depression or weak tropical storm. (Image credit:

Yet another tropical wave, newly emerged from the coast of Africa, was given two-day and seven-day odds of development of 20% and 60%, respectively. This system has been getting attention from the models as being something that could develop in the Tuesday-Thursday time frame. There is no threat to land from with system for the remainder of the week.

2023 on a near-average pace for named Atlantic storms

Emily is the sixth tropical or subtropical storm of the year, following a belatedly recognized subtropical storm from January followed by Tropical Storms Arlene, Bret, Cindy, and Don. After a fast initial spurt of activity, followed by nearly a month of inactivity, the Atlantic is back on a near-average pace. The average date of formation of the Atlantic’s sixth storm (1991-2020) is August 29. The record-earliest date is July 9 (Fay in 2021), and second place belongs to Franklin of 2005 (July 21). Given the multiple systems of interest in the Atlantic this coming week, it’s possible several more names will be used in short order even if none end up becoming hurricanes. The next two names on the Atlantic list are Franklin and Gert.

Only one of the six 2023 Atlantic storms has hit hurricane strength thus far: Don, on July 22. The average date for the Atlantic’s first hurricane is August 11. Bret and the January 16-17 unnamed subtropical storm came very close to hurricane strength, peaking with 70 mph winds. Both storms made landfall; the unnamed subtropical storm hit the northeast coast of Nova Scotia on January 17 with 50 mph winds, while Bret passed over St. Vincent in the Lesser Antilles Islands on June 22 with 65 mph winds. Neither storm caused loss of loss or serious damage.

This season’s accumulated cyclone energy (ACE) through August 20 stands at 16.2, which is just below the 1991-2020 average of 18.9 for the date, according to the Colorado State Real-Time Tropical Cyclone Activity page. In contrast, both the Northeast and Northwest Pacific have been about 50% more active than usual so far this year, thus pushing the Northern Hemisphere’s ACE total to 306.4 compared to an average for the date of 198.4.

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