Hurricane Hilary, off the coast of Mexico, is forecast to move northward into the southwestern United States, becoming a rare instance of a tropical cyclone affecting California. The 1200 UTC imagery on 18 August 2023 above shows a well-defined eye, with warm sea-surface temperatures (SSTs) in the Pacific Ocean along the storm path until the storm reaches 25^oN, the latitude of central Baja California. (Sea Surface temperatures in the shallow northern Gulf of California are even warmer, near 90^oF, white in the enhancement used.). Steep topography along the Baja California peninsula and cold SSTs north of 27^oN latitude will both affect the strength of the storm as it moves northward.

A  Mesoscale Domain Sector was positioned over Hilary, providing imagery at 1-minute intervals — Visible images, below, revealed low-level mesovortices within the eye of the Category 4 hurricane. 

GOES-18 “Red”‘ Visible (0.64 µm) images, 1400 UTC on 18 August to 0000 UTC on 19 August (courtesy Scott Bachmeier, CIMSS) [click to play animated GIF | MP4]

Metop-C overflew Hilary at about 0500 UTC on 18 August 2023 (link), and Advanced Scatterometry winds from that time, below, show tropical storm-force winds extending about 200 miles to the northeast of the system center.

GOES-18 Airmass RGB imagery below, hourly from 0020-1420 UTC on 18 August, shows Hilary off the coast of Mexico, and also a slowly-moving cyclonic circulation off the coast of California, a circulation that will help guide Hilary towards a projected landfall in northwestern Mexico/the southwestern USA.

Imagery from the SSEC/CIMSS Tropical Weather website, below, shows the storm path, SSTs and current shear along the forecast path (favorable now, progressively less favorable into the weekend) and the 700-200 mb steering flow.

Interests in northwest Mexico, the Baja peninsula and the southwestern United States should pay close attention to this storm through the weekend into early next week. An important hazard will be flooding. MIMIC Total Precipitable Water fields, below, for the 24 hours ending at 1400 UTC on 18 August, show the moisture associated with the hurricane.

The latest information on Hilary is available at the National Hurricane Center and the National Weather Service Offices in San Diego, Los Angeles, Hanford (the San Joaquin valley), Phoenix and Las Vegas.

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