Colorado State University Raises Seasonal Tropical Activity Forecast. Could Be More Than Indicated!

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Colorado State University has raised the 2021 seasonal activity forecast for North Atlantic tropical cyclones to 20 tropical storms, 9 hurricanes and 4 intense hurricanes. The ACE index is now 160. The Climate Impact Company concern is amounts could be higher due to the lingering La Nina climate as defined by multi-variate ENSO index.

Highlight: Colorado State University increases 2021 seasonal activity outlook for North Atlantic tropical cyclones. CIC is concerned that higher amount could occur due to a lingering La Nina climate.

April 8June 3July 8
Tropical

Storms (14.40

171820
Hurricanes (7.2)

 

889
Intense

Hurricanes (3.2)

444
ACE

Index (123)

150150160

Table 1: Colorado State University issued a new seasonal outlook for North Atlantic tropical cyclones yesterday. There is a slight increase.

Discussion: Colorado State University updated their seasonal tropical cyclone outlook for the North Atlantic basin yesterday. The forecast has increased slightly. The new forecast indicates 20 tropical storms, 9 hurricanes and 4 intense hurricanes. The accumulated cyclone energy index is now 160. The outlook is not as active as last year (30 tropical storms, 13 hurricanes, 6 intense hurricanes and ACE index of 180). But! Twenty tropical storms are rare occurring only twice (last year and 2005). A very dangerous season is ahead. Early season hints are already occurring…Elsa is a top-5 July storm for number of tropical storm days (8).

Quick review…Colorado State University (CSU) is the creator of seasonal tropical cyclone forecasts (in 1984) by renowned meteorologist Dr. William Gray. Dr. Phillip Klotzbach is now in-charge of this research team. CSU has produced seasonal forecasts many years (decades?) prior to other providers nowadays

The slight increase in activity projection is based on increasing warmth of the deep North Atlantic tropics, already warm North Atlantic subtropics and neutral ENSO. These conditions all enhance seasonal activity. Emergence of Elsa as a hurricane this early in the season is also a catalyst to the increased forecast amount.

Colorado State University expects above normal risk of a major hurricane striking the U.S. coastline mid-to-late summer/early autumn. The risk is nearly 70% of a major hurricane striking the North Atlantic seaboard which is considerably higher than normal (Table 2). The risk is even (43%) for the East and Gulf Coast.

Risk of Major Hurricane Striking CoastNormal Risk
Entire U.S. coastline68%52%
East Coast43%31%
Gulf Coast43%30%

Table 2: Colorado State University probability forecast of a major hurricane striking the U.S. coastline in 2021.

Climate Impact Company comments: The potential for higher amounts of tropical storms and hurricanes compared to the CSU forecast is possible. Here’s why. Note that despite the oceanic index (Nino34) ending of La Nina in April, the multi-variate ENSO index remains in moderate La Nina territory (Fig. 1). The global atmosphere is locked-in a La Nina climate. (Following document “CIC ENSO forecast” explains why).

Fig. 1: Multivariate ENSO index maintains a La Nina global climate.