Hernan (2020)

A low-pressure area in the eastern North Pacific organized into Tropical Storm Hernan on 26 August 2020 about 295 nautical miles south-southwest of Cabo Corrientes, Mexico. Hernan moved slowly eastward at first, then turned north and later north-northwest, passing near Las Islas Marias on 27 August. The system weakened rapidly after 28 August, became a tropical depression by 28 August 1200 UTC, degenerated to a remnant low about 6 hours later, and was absorbed by Tropical Storm Iselle by 0000 UTC 29 August. In total Hernan was a tropical cyclone for roughly 54 hours (26–28 August).

Hernan did not make a confirmed landfall on the Mexican mainland. Its center passed about 25 nautical miles west of Cabo Corrientes and moved close to Las Islas Marias on 27 August, when tropical-storm-force winds may have affected those areas. No coastal observations from Cabo Corrientes are available; an automated station on Las Islas Marias recorded winds below tropical-storm force.

The storm’s peak intensity was 40 knots (46 mph) with a minimum central pressure of 1001 mb, reached early on 27 August. This corresponds to a moderate tropical storm (not hurricane strength). Peak winds were determined mainly from satellite scatterometer (ASCAT) data.

Heavy rain and high surf were the main hazards. Cihuatlán, in the state of Jalisco, reported 23.24 inches (590.2 mm) of rain from 26–28 August; other locations in Jalisco, Colima, Michoacán, and Sinaloa reported totals exceeding 15 inches (380 mm). High surf impacted parts of the Michoacán coast. No specific storm-surge measurements along the coast are reported in the NHC write-up.

Media reports linked one indirect death in La Huerta, Jalisco (a man who fell from a roof while checking damage). Flooding and mudslides damaged at least 630 homes, washed out roads, and caused power outages affecting up to about 97,000 people across western and southwestern Mexico. No monetary damage estimates were provided.

Noteworthy items: Hernan’s formation was anticipated well in advance, though genesis timing in some forecasts was too early. Forecast track errors were larger than recent averages because the storm moved farther north than many models showed before turning west, while official intensity forecasts performed relatively well in predicting little strengthening. No tropical storm warnings were issued for the nearby coast in real time, though a warning would have been appropriate in hindsight for Cabo Corrientes and Las Islas Marias.