Main Development Region for North Atlantic hurricanes cools at rapid pace in May!

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Highlight: Main Development Region for hurricanes in the North Atlantic basin cools at record pace in May. Midday GFS stronger with Gulf TC.

Fig. 1: The tropical North Atlantic (TNA) index for the past 12 months and month-to-month change. The -0.50 change in May is the strongest cool change for any month in the 1950-2021 climatology.

Cool tropical North Atlantic discussion: Between the eastern entrance to the Caribbean Sea and off the northwest coast of Africa between 5N and 23N is where the tropical North Atlantic (TNA) index is calculated. The TNA is a simple SSTA value for this region which is also known as the main development region (MDR) for North Atlantic hurricanes. In May, the TNA index declined -0.50 to -0.18, the strongest negative monthly drop in the 1950-2021 climatology. Implied is upper ocean heat across the MDR has declined rapidly and is currently below normal. Reversing this observation (and trend) to the more familiar warmer TNA values of the past 2+ decades for the hurricane season is not impossible but very hard to accomplish. The steep decline in the TNA index heading into the tropical cyclone season suggests that hurricane development in the deep tropics will be held down this season due to limited upper ocean heat. If upper ocean heat recovers sufficiently to generate powerful low latitude hurricanes late season is that timeframe.