Jeff Masters Hurricane Blog

Hurricane Hilary weakening – but risks growing

Heavy rains have already begun in the Southwest U.S. as tropical moisture streams northwards in advance of weakening category 3 Hurricane Hilary, which is expected to slam much of the U.S. Desert Southwest from Saturday into Monday with some of the region’s heaviest rains on record. Widespread and potentially catastrophic floods are expected from Southern California into parts of Nevada, in normally arid regions where it is uncommon to have flood insurance.

Now accelerating north-northwestward, Hilary is predicted to make landfall in Baja California on Sunday morning, then weaken to a tropical storm as it enters Southern California on Sunday afternoon. As a result, coastal areas from San Diego to west of Los Angeles are under the first Tropical Storm Warning for a Pacific Coast state since the modern watch/warning system was introduced in the mid-20th century.

Hilary could bring sustained winds of 25-45 mph with higher gusts to San Diego and the Inland Empire of California, potentially bringing down some trees and power lines. Even though Hilary will pass well east of Los Angeles and the San Fernando Valley, the circulation around Hilary could force equally strong winds in coastal areas from an unusual “wet Santa Ana” effect. Wind gusts could be even higher across the higher terrain across Southern California, especially because Hilary will spin down more quickly near the surface than at altitude.

Figure 1. Meteorologists Samantha Connolly (left) and Elizabeth Adams (right) prepare the first local tropical cyclone products ever issued in the modern operations history of the National Weather Service office in San Diego. (Image credit: NWS/San Diego via Matt Moreland.)

Track forecast for Hilary 

As of 2 p.m. EDT Saturday, Hilary had top sustained winds of 115 mph and a central pressure of 958 mb, and was located about 710 miles south-southeast of San Diego, headed north-northwest at 16 mph. At 7:40 a.m. MDT, Hilary brought sustained winds of 23 mph, gusting to 43 mph, to Cabo San Lucas on the southern tip of the Baja Peninsula. Heavy rain showers were affecting much of the southern portion of Baja Mexico, as seen on Los Cabos radar and a long radar loop saved by Brian McNoldy.

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, taking the hurricane on a course roughly parallel to the coast of Baja Mexico, likely resulting in a landfall Sunday south of Tijuana, Mexico, not far from the California border. If this occurs, Hilary would not officially count as California’s first landfalling tropical cyclone since 1939. However, the impacts from Hilary – particularly its rains – are expected to be severe and locally catastrophic in the Southwest U.S., and Hilary may end up being the most expensive eastern Pacific hurricane ever recorded in the United States.

Intensity forecast for Hilary

Hilary has begun its anticipated weakening process. Satellite images Saturday afternoon showed a notable erosion of the hurricane’s northwest eyewall. This was likely caused by drier, most stable air encroaching from the north, driven by moderate wind shear of 10-20 knots. Ocean waters around the core of the hurricane were near 29 degrees Celsius (84°F) – about a degree cooler than on Friday – though these waters were still some 2°C to 4°C warmer than average for mid-August. 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.

As Hilary heads north-northwest and then north between now and landfall, the hurricane will encounter much chillier 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 and dry air will increase as the storm heads north, and the storm will be interacting with the high terrain of the Baja Peninsula on Sunday morning. These factors should cause rapid weakening, and it is expected that Hilary will be below hurricane strength when it crosses into Southern California on Sunday afternoon.

Floods and mudslides the biggest threat from Hilary

The National Weather Service’s 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 much of the Southern California desert region, where three to six inches of rain can be expected, with isolated higher amounts of 10 inches in mountainous areas. This includes most of the area from the San Berardino Mountains to Death Valley and southward to the U.S./Mexico border.

The discussion for the forecast warned of 1-in-100-year storm total rainfalls in some areas, along with other startling possibilities:

“The 00z Canadian Regional shows local amounts of towards 10”, which would be exceeding rare for the region from a tropical cyclone, potentially unique for Nevada…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, it would challenge Nevada’s 24 hour rainfall record, set in 2004….Debris flows and rock slides are a given considering the volume of rainfall expected. The overall combination of effects could block and undermine roads, particularly sensitive areas such as sections of U.S. 50 in NV. Towns could get cut off.

Although cities from San Diego to Santa Barbara could get 2 to 4 inches of rain, the heaviest rains in coastal Southern California will likely be along the east slopes of the coastal mountains, where moisture will be squeezed out from strong easterly winds as Hilary approaches. The region’s powerful winter storms typically bring winds from the west rather than the east, so the easterly slopes are less accustomed to torrential rains and high winds.

The largest urban areas at risk from Hilary’s rains and floods are the Inland Empire just east of Los Angeles, including the Riverside–San Bernardino–Ontario metropolitan area (pop. 4.6 million), and the Las Vegas metro area (pop. 2.2 million). Rainfall will be exacerbated in the Inland Empire as Hilary’s strong southeast winds are forced upslope against the east-west Transverse Range. In Las Vegas, the storm total from Saturday to Monday could challenge the city’s all-time three-day rainfall record of 2.59 inches on Aug. 20-22, 1957.

Unhoused people – many living in tents or without any shelter near riverbeds, culverts, and normally dry washes – are in particular danger from Hilary’s flooding, especially since rainfall at this scale is so far beyond everyday experience and expectations. In a forecast discussion, the NWS office in Las Vegas warned:

Concrete washes throughout the Las Vegas Valley will be incredibly dangerous and potentially deadly. Absolutely do not play or venture into these washes. If you have communication with populations experiencing homelessness who may be inhabiting these washes, ensure they get the message to vacate immediately.

Stay away from Death Valley!

Another place to stay away from this weekend is Death Valley, California. August is always a terrible time to visit what is the hottest place on the planet, where high temperatures regularly exceed 120 degrees Fahrenheit. But if you do decide to go there to witness what promises to be a historic rainfall event in Death Valley – over three inches of rain, in a location that averages just 2.2 inches of rain per year – you may well end up being stuck there. Substantial damage to the park’s roads and infrastructure are likely from Hilary’s rains, and the park has partially closed in anticipation of heavy rains and flooding from Hilary. Park campgrounds, the Furnace Creek Visitor Center, North Highway, and Badwater Road were all closed beginning Saturday morning.

And onward to Canada…

A truly rare meteorological event is expected on Tuesday: moisture from an eastern Pacific hurricane impacting the Canadian Rockies. On Monday night and Tuesday, Hilary’s remnants are expected to move into northern Idaho and Montana, then into British Columbia and Alberta, Canada, potentially bringing helpful rains to quench record wildfires raging there. It is possible that Hilary’s rains could trigger isolated flash flooding and mudslides on recent fire burn scars on steep mountain slopes, though rainfall amounts are generally predicted to be less than 0.5 inches.

Four threat areas in the Atlantic

The Atlantic continues to bristle on Saturday morning, with the National Hurricane Center (NHC) tracking four systems. At least one of these may become a named storm in the next week, ending a quiet period for the Atlantic, although the chance of a hurricane appears low at present.

The area of most immediate concern for the United States is in the Gulf of Mexico. A tropical wave (not yet assigned an “Invest” number) was bringing locally heavy rainfall to South Florida and western Cuba on Saturday afternoon, and will move into the eastern Gulf of Mexico on Sunday, when it 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 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 Saturday morning runs of the GFS and European model ensembles do develop the Gulf system 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 this predicted tropical disturbance are likely to bring two to four inches of rain, providing beneficial drought relief. In their 8 a.m. EDT Saturday Tropical Weather Outlook, NHC gave two-day and seven-day odds of development of 0% and 50%, respectively, to the future Gulf of Mexico disturbance. The Hurricane Hunters are on call to investigate this disturbance on Monday, if necessary.

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. From west to east, these have been designated as Invests 90L, 99L, and 98L. Steering currents appear likely to keep all of these away from North America; this type of welcome avoidance happens often with systems that develop in the open Atlantic during a strengthening El Niño event.

Of these three, the system of most immediate concern is the westernmost one, Invest 90L. This tropical wave was bringing heavy rain showers and gusty winds to the Leeward Islands and northern portions of the South America coast on Saturday 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, but no well-defined surface circulation was apparent. Barbados reported two wind gusts of 40 mph in heavy rain showers Saturday morning. Conditions were somewhat favorable for development, with warm ocean waters near 29 degrees Celsius (84°F) and only moderate wind shear near 10 knots, but a somewhat dry surrounding atmosphere (mid-level relative humidity of 60%).

Moisture from this system will spread northwestward to Puerto Rico, the Virgin Islands, and the Dominican Republic by Monday. The disturbance is beginning to pick up more model support for development, with the 12Z Saturday runs of the GFS and HAFS-B model showing 90L affecting Hispaniola as a tropical depression or tropical storm on Tuesday. In their 8 a.m. EDT Saturday Tropical Weather Outlook, NHC gave 90L two-day and seven-day odds of development of 20% and 40%, respectively. The Hurricane Hunters are scheduled to investigate this disturbance on Sunday afternoon, with the NOAA jet on call to fly Monday evening.

Figure 2. Forecasts from the European model ensemble issued on Friday night, August 18, with storm tracks extending to Thursday night, August 24. The ensembles favor eventual recurvature for Invests 90L, 99L, and 98L (left to right, with 99L redeveloping late in the period in the central Atlantic), while a new system may emerge next week off the African coast. The ensemble also shows the system of interest in the Gulf making landfall along the coast of South Texas or Northeast Mexico. (Image credit:

A tropical wave located in the eastern Atlantic, a few hundred miles west of the Cabo Verde Islands – Invest 98L – has the highest odds of development: two-day and seven-day odds of 70%, implying that 98L’s best prospects are between now and Monday. However, 98L is headed west-northwest out to sea and is not likely to trouble any land areas.

Another disturbance midway between the Lesser Antilles and Cabo Verde Islands – Invest 99L – was given two-day and seven-day odds of development of 30%. As with 98L, this system is also headed west-northwest and does not appear to be a concern to any land areas.

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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