Iselle (2020)

A small tropical cyclone formed from a westward-moving tropical wave and an associated low that developed within a larger monsoon gyre. The system became a tropical depression by 1200 UTC 26 August 2020 about 600 nautical miles southwest of the southern tip of the Baja California peninsula, and strengthened into Tropical Storm Iselle six hours later. Iselle moved generally north-northeastward around the eastern edge of the monsoon gyre, reached its peak intensity on 28 August while centered about 380 nmi southwest of southern Baja California, then weakened steadily and became a remnant low by 1800 UTC 30 August. The remnant low dissipated by 1800 UTC 31 August less than 60 nmi west of western Baja California Sur.

Iselle did not make a landfall on any major populated coast. The only land observation of tropical-storm-force winds occurred at Clarion Island, Mexico, when the cyclone was about 40 nmi west-northwest of the island on 28 August; Clarion Island reported a sustained wind of 35 kt (40 mph) with a gust to 42 kt (48 mph) around 1930 UTC that day. No coastal watches or warnings were issued.

The storm’s maximum sustained winds peaked at 50 knots (about 58 mph) around 0600 UTC 28 August, with an estimated minimum central pressure of 997 mb. At peak intensity Iselle was equivalent to a strong tropical storm (below hurricane strength). The peak wind estimate is based on scatterometer surface winds, satellite intensity estimates, and a brief mid-level eye feature observed in satellite imagery.

Storm surge impacts were minimal and no specific surge heights were reported in the observations. Rainfall totals in the official report are not extensive for populated areas; the report’s notable observational data focus on winds at Clarion Island rather than large rainfall amounts on the mainland. There were no ship reports of tropical-storm-force winds associated with Iselle.

There were no reported deaths, injuries, or damage attributed to Iselle. Forecasts initially underpredicted the system’s genesis because forecasters expected continued strong wind shear, but NHC correctly anticipated the timing and magnitude of Iselle’s peak intensity and its subsequent weakening; official intensity forecasts were more accurate than many guidance models. No coastal impacts or casualties were recorded.