Updated August 2021 North Atlantic Tropical Cyclone Season Outlook

Updated seasonal activity forecast is similar but slightly more intense compared to the June outlook. Florida is primary target.

Executive summary: The Climate Impact Company North Atlantic tropical cyclone season forecast of seasonal activity for 2021 is updated. The outlook indicates 11 tropical storms, 7 hurricanes and 4 intense hurricanes for August 1-November 30 making the seasonal totals very similar to the original forecast at 16 tropical storms, 8 hurricanes and 4 intense hurricanes. The only change is an increase from 3 to 4 for major hurricanes. The accumulated cyclone energy index forecast is increased to 133. The new forecast remains above climatology but not as active as last year and slightly less than forecasts provided by CSU, TSR/U.K. and NOAA. The forecast is based on a low-shear environment associated with La Nina which is ideal for tropical cyclone development. However, a stronger than normal Bermuda high-pressure system keeping the deep tropics quiet in recent weeks may last well into August before easing. Additionally, the ocean surface in the tropical North Atlantic is lagging cooler than the 20-year climatology although some warming is indicated for September/October. The NMME model is provided to identify most likely tracks and indicates Florida is a prime target for hurricanes.

Discussion: Although the 2021 North Atlantic tropical cyclone season is slightly ahead of climatology for August 1^st, the basin has been extraordinarily quiet since Hurricane Elsa earlier this month (Table 1). Last year at this time, the 2020 season (which set a record with 30 tropical storms) had produced 9 tropical storms but (similar to 2021) only 1 hurricane. The most active part of the North Atlantic tropical cyclone season begins August 1^st and lasts through middle of October. Seasonal forecasts anticipate another big year with above to much above normal activity expected although not quite as active as last year (Table 2). The CSU and TSR forecasts were edged upward slightly with their July update.

Table 1: North Atlantic tropical cyclone season activity as of Aug. 1, 2021.

Table 2: Most seasonal forecasts for North Atlantic basin tropical cyclone activity will be updated in early August. The current operational forecasts are indicated.

Climate discussion: Each tropical cyclone season brings a unique climate character. This year, the Bermuda high-pressure system is unusually intense and during much of July (Fig. 1) propelled above normal trade wind speeds in the deep tropics which prevented tropical waves from organizing.

Fig 1-2: The July 2021 sea-level pressure anomalies (SLPA) across the North Atlantic basin and relative humidity at 600 MB (mid-atmosphere). Strong trade winds and a drier than normal mid-atmosphere have prevented July deep tropics development.

The above normal trade winds up-welled enough cooler water to keep the surface at normal sea surface temperature (SST) which is unique for the 20-year climatology which has featured mostly a warmer than normal deep tropics. Additionally, stronger than normal trade winds have propelled dust from the African Desert to stabilize the atmosphere. Finally, mid-atmosphere relative humidity has generally been drier than normal in the tropics (Fig. 2) indicating a less than ideal environment for development.

The North American Multi-Model Ensemble (NMME) rainfall anomaly forecast for August indicates the tropics continue to struggle to generate significant tropical cyclones. The drier than normal signature in the Caribbean Sea and eastward suggests the very dry relative humidity pattern in this zone in the middle atmosphere during July continues in August. Based on the NMME projection, there is tropical cyclone risk to the northwest Gulf States, very deep tropics into the north coast of South America and offshore from near Bermuda and northeastward (Fig. 3).

Fig 3: The NMME tropical rainfall anomaly forecast for August 2021 and annotated/related most likely tropical cyclone tracks.

Fig 4: The NMME tropical rainfall anomaly forecast for September 2021 and annotated/related most likely tropical cyclone tracks.

Fig 5: The NMME tropical rainfall anomaly forecast for October 2021 and annotated/related most likely tropical cyclone tracks.

The NMME anomalous rainfall forecast for September supports a very active tropical cyclone month. Either side of Florida are favored track areas reaching the Carolinas with Cape Verde storms having a tendency to turn northwest and north and stay out to sea (Fig. 4).

The month of October features a low latitude risk based on the NMME rainfall anomaly projection including a northeast-tracking system (or 2) moving across Florida (Fig. 5).

The remainder of the season activity forecast is based on a constructed analog propelled by the most likely upper-level shear patterns across the tropical/subtropical North Atlantic basin controlled by the El Nino southern oscillation (ENSO) and the sea surface temperature anomaly (SSTA) pattern across the North Atlantic basin represented by the Atlantic multi-decadal oscillation (AMO). The AMO analog is correlated with deep tropics SSTA pattern as identified by tropical North Atlantic index (TNA).

The analog years are for ENSO are 1996, 2008, 2011 and 2017. The ENSO analog years feature neutral ENSO and/or La Nina following a year when La Nina was present similar to the 2021 projection. The AMO/TNA analog years are 2007, 2014 and 2015. Analog year 2015 is eliminated due to the presence of El Nino. The general AMO/TNA forecast indicates an environment not as warm as the (warm) 20-year climatology warming to the 20-year briefly in SEP/OCT before cooling-off.

Table 3: The August 1-November 30 North Atlantic basin tropical cyclone activity forecast by Climate Impact Company plus activity so far and the NEW total seasonal activity forecast is indicated.

The analog forecast equally weights each year and identifies tropical cyclone activity beginning August 1^st to the end of the season (November 30^th). Each analog year is similarly quite active except for the number of tropical storms in 2014. The weighted average indicates an additional 11 tropical storms, 7 hurricanes and 4 intense hurricanes. Combined with observed seasonal activity so far, the 2021 adjusted seasonal forecast now indicates 16 tropical storms, 8 hurricanes and 4 major hurricanes and an ACE index of 133. The adjusted forecast is slightly more intense (Table 3) with 1 more major hurricane and a higher ACE index compared to the initial forecast issued on June 2^nd.

Summary: The Climate Impact Company North Atlantic seasonal tropical cyclone season forecast is updated. The activity forecast is similar to the early June outlook but adjusted more intense with an additional major hurricane and high accumulated cyclone energy index. Since Hurricane Elsa, the North Atlantic basin has been quiet most likely due to an unusually strong Bermuda high-pressure system which has contributed to stabilizing the deep tropics. The NMME model indicates this pattern may last well into August. However, in September the environment should become ideal for producing tropical cyclones due to a low upper shear environment associated with La Nina and the warming of the deep-tropics oceanic conditions. The majority of intense tropical events during the 2021 season takes place in September. The NMME model targets the eastern Gulf of Mexico, Florida and the East Coast to the Carolinas as most likely target for coastal strikes certainly to include major hurricane(s). In October, the western North Atlantic basin can stay quite active and this scenario is expected this year. Once again, Florida is the primary target according to the NMME model. The 2021 season comes on strong later in August and September into early October could be unusually active. Florida is a primary target for several events this season.