Where We Stand On The Tropics As Peak Of Season (September 10) Arrives

Table 1: Tropical cyclone season activity so far, normal for this time of year and the seasonal forecasts from primary providers. Source: Colorado State University.

Discussion: So far, the 2021 tropical cyclone season across the North Atlantic basin has produced 12 tropical storms, 5 hurricanes and 3 intense hurricanes (Table 1). The activity level is near or slightly more than twice normal. The activity level (so far) is very close to the 50-year normal. The accumulated cyclone energy index is 65.8 which is more than 50% above normal for this time of season. The seasonal forecasts from Colorado State University, Tropical Storm Risk U.K., Climate Impact Company and NOAA certainly indicate there’s more to come and quite possibly more than forecast.

A check on the SSTA conditions across the Gulf of Mexico indicate mostly near normal temperatures except a robust warm anomaly in the northwest Gulf States and in the vicinity of the Florida Keys (Fig. 1). The 30-day trend indicates moderate cooling to the Gulf basin (Fig. 2).

The North Atlantic basin is somewhat warmer than normal near the Gulf Stream especially east of the Northeast U.S. (Fig. 3). Anomalous warmth is also developing fast off Western Europe. The 30-day change is marginally warmer lead by the very warm trend off the West Europe coast (Fig. 4). SSTA is warmer than normal and trending warmer just east of the Windward Islands to northeast and well north of Puerto Rico – a popular hurricane route.

In the Main Development Region (MDR) for hurricanes in-between the Caribbean Sea and Northwest Coast of Africa the SSTA is near +0.11C which is neutral phase and unusually cooler than recent years at this time (Fig. 5). The 30-day trend is slightly cooler (Fig. 6).

For the rest of the season the regions of warm SSTA to be aware of for possible rapid development and/or intensification is the western Gulf of Mexico, near and southeast of Bermuda toward the Windward Islands and just-off the East Coast.

Fig. 1-2: Gulf of Mexico SSTA and 30-day change.

Fig. 3-4: The North Atlantic basin SSTA and 30-day change.

Fig. 5-6: The tropical North Atlantic index and 30-day change. The TNA index is measured across the main development region for hurricanes (MDR).