Another hurricane, Hilary, formed in the eastern Pacific and strengthened rapidly; it is now already a Category 4 hurricane. Hilary is moving northward and, after Mexico, will also hit the southwestern United States in a weakened form.
As expected, Hurricane Hilary brought an enormous amount of rain to Mexico as well as the southwestern United States in a short period of time. San Bernardino County received up to 342 mm in the last 72 hours, Riverside County received 297 mm, and Las Vegas also received around 200 mm. In some extreme cases, the amount of rain that fell during this period was equivalent to 1 to 3 years of precipitation! At countless stations, Sunday was the wettest August day on record, including Los Angeles and San Diego. The consequences were – as feared – mudslides, landslides and massive flooding.
Fig. 1: Rainfall amounts for the last 72 hours, given in inches (1 inch is 25.4 mm).; Source: NOAA
Hilary – a strong hurricane
The Atlantic has been relatively quiet lately, with no new tropical storms forming here for several weeks. In the meantime, however, there are some candidates, and it could slowly become exciting again. Currently, the focus is clearly on the Pacific. Storms in the eastern Pacific (east of the International Date Line) are called hurricanes, just as in the Atlantic. West of Mexico, Hilary, the 6th hurricane of the Pacific hurricane season, has now formed. After reaching hurricane intensity, it has strengthened rapidly in the past few hours and is now rated Category 4 – and it continues to intensify.
Fig. 1: Latest satellite image of Hurricane Hilary over the Pacific Ocean west of Mexico; Source: tropicaltidbits.com
Often tropical storms in the eastern Pacific move further west out to the Pacific, Hilary is now taking a special track and is likely to cause quite a few superlatives and problems further down the line. According to current models, Hilary will first hit Lower California (Baja California) and thus the two Mexican states of Baja California and Baja California sur during the weekend. Once it interacts with the mainland, it begins to weaken, but part of the system is expected to remain at sea. The farther north, however, the colder the water becomes. And tropical storms virtually "feed" on high water temperatures.
Fig. 2: Calculated track of Hurricane Hilary; Source: NHC
According to Mexico, it will reach the southwestern United States as a tropical storm, or at least as a tropical depression, during Sunday and early in the new week. This is the currently favored course, but a landfall only in the area of the USA cannot be excluded at present. The last time this happened was in 1939 during the Long Beach tropical storm. The only other known event was the San Diego Hurricane in 1858. So such an event is very rare!
Hilary will weaken in terms of winds, but will bring large amounts of rain in any case. And this is extremely unusual for the southwestern U.S. at this time of year! During the winter months, California can receive significant amounts of rainfall, such as this January and February. In August, however, rain rarely if ever falls in these rains! To put this in figures, here as an example the big city San Diego with nearly 1.4 million inhabitants in the extreme southwest. Here the series of measurements goes back to the year 1875. Since then, 94 years have seen no precipitation at all in August. In the other cases, August precipitation was low, with the absolute leader by far being August 1977 –, when 54 mm of rain fell at this station. In second place is August 1945 with 22 mm. This concerns the majority of the southwest, here the summers are basically hot and dry.
Fig. 3: Accumulated precipitation until 12 August 00 UTC (ICON); Source: MeteoNews, Ubimet
Now, however, Hurricane Hilary is coming with a lot of rain in a short time. Until next Tuesday, up to 100 mm – will fall regionally up to 250 mm. The parched soil cannot absorb these amounts, and the mostly sparse vegetation, damaged by forest/bush fires, offers no protection against erosion. Accordingly, flash floods, mudflows and mudslides must be expected. This affects densely populated areas such as San Diego, Los Angeles and its hinterland, but also mountains and deserts (for example, the Mojave Desert or Death Valley). In addition to Mexico and California, Arizona and Nevada (including Las Vegas) are also affected by this storm system.