Tropical Cyclones and Record Rainfall

Increasing Records: Tropical Cyclones and Heavy Rainfall

While Tropical Storm Hilary pummeled the West Coast and southwestern United States with heavy rainfall and flash floods, the National Oceanic and Atmospheric Administration’s Weather Prediction Center reported that rainfall records were shattered in four states: Idaho, Montana, Nevada, and Oregon.

Nevada experienced an unprecedented record-breaking rainfall of 9.20 inches, more than doubling the previous record. Meanwhile, Montana, Idaho, and Oregon all received rainfall exceeding the prior records by nearly an inch.

The occurrence of a tropical cyclone like Hilary setting rainfall records in four states is a highly uncommon phenomenon. Only two other individual tropical systems have approached this feat by setting rainfall records that impacted multiple states. For instance, in 1961, Tropical Cyclone Carla established rainfall records across Iowa, Wisconsin, and Michigan, while Tropical Cyclone Kathleen achieved a similar feat in 1976, setting records in California, Oregon, and Idaho.

The widespread rainfall can be attributed to Hilary’s atypical path across the states. After making landfall in Mexico, the storm first struck California and then proceeded to travel almost directly northward, affecting the western region and the northern Rockies.

Meteorologists had foreseen that due to the substantial influx of rich tropical moisture into the area, the probability of establishing rainfall records was high. These records encompassed not only daily and monthly benchmarks but also extended to records at the state level.

Idaho’s earlier record was held by Olivia in 1982, while both Montana and Oregon’s records were traced back to the tropical system Kathleen in 1976. Among the four states, Nevada had the most ancient standing record, originating from an unnamed tropical system in 1906.

The state precipitation record for California, which stands at 17.76 inches and was also established by Kathleen in 1976, remains intact despite Hilary’s impact.

With Hilary setting records across four states, the count of newly established records has risen to nine within the past six years. Ongoing research attributes the escalation of rainfall rates and quantities generated by tropical cyclones to the effects of climate change. Consequently, occurrences of historically significant rainfall could become more frequent. Since 2017, five additional states have broken records due to the influence of tropical systems.

In 2017, Hurricane Harvey set the record for Texas by depositing 60.58 inches of rain across the state. In 2018, Hurricane Florence established records for both North Carolina and South Carolina. In 2019, Barry brought an unprecedented amount of rain, exceeding 16 inches, to Arkansas. Similarly, Minnesota experienced record-breaking rainfall from Cristobal in 2020, with remnants depositing 5.06 inches over the state.

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