Hurricane Hilary stunned the West with a fast build up to Category 4, a rare track toward Baja California, record rain across the desert Southwest, and the first ever tropical storm warning issued for Southern California. Here is a clear, sourced timeline of what unfolded, what records fell, and the preparation steps that mattered most and will matter again.
Quick primer
- When: August 16 to 21, 2023.
- Peak intensity: Category 4 with 140 mph winds on August 18.
- Landfall: San Quintín area of northern Baja California as a tropical storm on August 20, then rapid weakening as the system pushed north and became post tropical.
- Why it was historic: The National Hurricane Center issued Southern California’s first tropical storm watch and then warning, and the event produced record single day rainfall in typically arid locations.
The timeline, day by day
August 16: Formation south of Mexico
A tropical wave organized south of Mexico and was named Hilary. Warm waters and favorable upper-level winds set the stage for intensification.
August 17: Rapid strengthening begins
Hilary reached hurricane strength while paralleling the southwest Mexican coast and kept turning more to the north northwest under the influence of an upper-level trough.
August 18: Category 4 peak and escalating alerts
Hilary rapidly intensified to a Category 4 hurricane with 140 mph sustained winds and a central pressure near 940 to 939 mb. The NHC expanded watches and warnings, including the first ever tropical storm watch for Southern California that later upgraded to a warning as the storm marched north.
August 19: Weakening trend sets in
Moving into cooler waters and encountering shear, Hilary began to weaken while curving toward the Baja peninsula. Moisture flooded northward, setting up the Southwest for a widespread rain event.
August 20: Landfall in Baja and record rain surges into the U.S.
Hilary made landfall near San Quintín, Baja California, as a tropical storm. Outer bands streamed into Southern California and Nevada, producing flash flooding, debris flows, and daily rainfall records. Death Valley National Park measured 2.20 inches at Furnace Creek, the wettest day on record there.
August 21: Post tropical over the interior West
By early August 21 the system lost tropical characteristics, with the NHC handing responsibility to the Weather Prediction Center as life threatening flooding continued into Southern California, Nevada, Oregon, and Idaho.
What the data shows about the impacts
Rainfall and flooding
Heavy tropical moisture produced widespread 2 inches plus rainfall totals across Southern California and southwestern Nevada, with 10 inches plus in the Transverse Ranges. Many sites set new August daily rainfall records. Streamflows surged above the 90th percentile, and Death Valley’s 2.20 inch day surpassed its previous all time daily record.
The NWS San Diego post event report catalogs double digit storm totals at mountain and foothill gauges. Upper Mission Creek topped 13.07 inches, Mount San Jacinto reached 11.75 inches, and numerous locations exceeded 7 inches.
The Weather Prediction Center’s storm summary traces Hilary’s path from Baja across Southern California and up through Nevada into Idaho and Oregon, and compiles the multi day rainfall footprint across the West.
Wind and infrastructure damage
Even as a weakening system, Hilary pushed tropical storm force winds across parts of Southern California. Observations included gusts in the 60 to 70 knot range at elevated sites, downing trees and power lines and forcing freeway closures in flood prone corridors like I 10 near Palm Springs.
Records and firsts that stand out
- First tropical storm watch and subsequent warning ever issued for Southern California.
- All time daily rainfall record at Death Valley’s Furnace Creek station.
- Dozens of August daily rainfall records from coastal basins to high deserts, with streamflow responses that surpassed typical summer behavior by a wide margin.
Preparation: What helped and what to do next time
Hilary’s warning cycle unfolded fast. Agencies moved quickly, but the storm exposed gaps that households and businesses can close before the next rare tropical remnant or warm season flood event. Use this checklist to harden your plan.
Know the products and where to look
- NHC advisories and key messages during the tropical phase. These explain track, intensity, and watch or warning changes. Bookmark the storm archive to study how the timeline evolves.
- Weather Prediction Center updates for rainfall and flood risk once a system becomes post tropical.
- Local NWS offices for flash flood watches, debris flow guidance near burn scars, and road closure updates.
Prepare for water first, wind second
In the Southwest, the dominant risk is flooding and debris flows rather than extreme sustained wind.
- Check your flood exposure by neighborhood and by commute route. If you cross washes or low water crossings, identify alternate routes.
- Use official stream and river dashboards for real time gauges during events. They help you anticipate when roads will close and when water will recede.
- Keep flood insurance current if you live in a basin, canyon, or near an alluvial fan. Tropical moisture events can exceed historical norms.
Harden your property against debris flows
- Clear drains, gutters, and scuppers well before rain bands arrive.
- Stock sandbags and place them where runoff concentrates around garages and downhill door thresholds.
- If you live below burned slopes, stage vehicles and valuables out of the runout path and be ready to evacuate on short notice.
Build a timing-based action plan
The best preparation plan triggers specific actions as forecasts cross thresholds, not just when you “feel” ready.
- 72 hours out: Start tracking NHC advisories. Top off vehicle fuel, refill prescriptions, and verify emergency alerts work on your phone.
- 48 hours out: If a tropical storm watch or flood watch appears, secure outdoor items, test sump pumps, and inform employees or family of potential schedule changes.
- 24 hours out: Pre stage sandbags, move vehicles to higher ground, reschedule non-essential travel, and charge backup power banks.
- During the event: Do not enter flooded roadways. Monitor local NWS and transportation feeds for closures. Post event, wait for rivers to crest and recede before driving through canyons.
Fresh lessons from Hilary
- Intensity forecasts improved, but hydrologic communication remains the priority. Hilary weakened before reaching California, yet its moisture plume still produced destructive flooding. That gap between “tropical storm or not” and the reality of flood impacts underscores why residents should anchor decisions to rainfall forecasts and local hydrology, not just the cyclone label.
- Records can fall far from the coast. The event broke the all-time daily rainfall mark in Death Valley and set August records across mountain ranges and deserts. Desert soils, burn scars, and steep terrain amplify runoff, which turns intense but brief bands into damaging flash floods. Prepare well inland, not just near the shoreline.
- First time warnings are wake up calls. The first tropical storm watch and warning for Southern California sent a clear message about evolving risk profiles for the region. Even if a system transitions to post tropical, the hazards remain. Expect more instances where abundant warm season moisture teams up with upper-level troughs to wring out historic rain.
Bottom line
Hurricane Hilary delivered a rare and revealing stress test for the Southwest in August 2023. A Category 4 hurricane over the Pacific weakened before landfall yet still drove a plume of tropical moisture that shattered rainfall records from coastal ranges to the Mojave. The first ever tropical storm warning for Southern California and the subsequent post analysis nuance about Hilary’s status both point to the same practical takeaway. In the West, plan for flooding first. Track official rainfall forecasts, know your low water crossings, and script actions by timeline so you are ready long before the first band arrives.
Frequently asked questions
Was Hurricane Hilary still a tropical storm in California?
Operationally, yes at first, but the National Hurricane Center’s post season analysis concluded Hilary became post tropical over northern Baja before entering California. The flooding impacts in California remain the same in either case.
What rainfall totals did the mountains see?
NWS San Diego documented storm totals over 13 inches at Upper Mission Creek, nearly 12 inches at Mount San Jacinto, and numerous 7-to-10-inch readings across the San Bernardino Mountains.
What made Death Valley’s total so notable?
Furnace Creek recorded 2.20 inches on August 20, the wettest day on record there, surpassing the 2022 daily record and eclipsing the park’s average annual rainfall in a single day.
Where can I review the official post event reports and maps?
The NHC Tropical Cyclone Report provides complete meteorological history. The Weather Prediction Center hosts storm total rainfall maps. CW3E at Scripps has a detailed event summary with graphics on rainfall and winds.
