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So Far The 2020 North Atlantic Seasonal Tropical Cyclone Activity Is Lagging Behind The Very Dangerous Seasonal Forecasts

Discussion: So far the 2020 North Atlantic tropical cyclone season has yielded 11 tropical storms which is certainly above normal for middle August. However, only 2 minimal hurricanes so far lags behind the normal amount. Seasonal forecasts by Climate Impact Company, Colorado State University, Tropical Storm Risk U.K. and NOAA indicate near if not more than 10 hurricanes of which 3-6 could become major hurricanes (Table 1). Can that level of activity still happen?

 Tropical StormsHurricanesMajor Hurricanes
So Far in 20201120

Table 1: North Atlantic basin seasonal tropical cyclone forecasts for the 2020 season.

The climatology of North Atlantic tropical cyclones beginning in 1995 is reviewed for SEP/OCT/NOV tropical cyclone activity level. This 25-year period is selected due to the long-term cycle change of ENSO and the Atlantic multi-decadal oscillation (beginning in the mid-to-late 1990’s and still valid in 2020). On average, 54% of seasonal tropical storm activity occurs after September 1 since 1995. Interestingly, almost 2/3 of seasonal hurricane and intense hurricane activity occur after September 1st. The percent of seasonal activity after September 1 since 1995 generally indicates most of the season’s activity occurs in SEP/OCT/NOV (Fig. 1).

Fig. 1: The percent of seasonal tropical cyclone activity 1995-2019 occurring after September 1st.

Using the 1995-2019 climatology as a guide an expectation of about 12-13 additional tropical cyclones is expected in 2020 while additional hurricanes is likely near 4 and major hurricanes near 1 or 2. This projection would bring the seasonal activity to 23-24 tropical storms which is forecast but only 6 hurricanes and possibly 2 major hurricanes which is well below the seasonal forecast.

Each season is unique and long-term climatology is only a guide. However, the minimal amount of hurricanes so far and (likely) lack of hurricanes for the remainder of August does give rise to the question of why hurricanes are slow to form through the first half of the season. The primary reason is a persistent area of wind shear across the tropical North Atlantic still present basin-wide near 20N. Another possible negating factor has been the Saharan Dust which remains moderate across the subtropics of the North Atlantic basin reaching westward to the Gulf of Mexico. However, the ocean surface remains warmer than normal in the western North Atlantic and a La Nina climate is developing. Consequently, a dramatic uptick in number and severity of storms in September/October remains likely although number of hurricanes may be below the current seasonal forecasts. The 2020 season is likely to linger late – well into November.