Mid-layer Relative Humidity Suddenly Increases in Tropics Favoring Uptick in Activity

Highlight: 91L fighting upper shear. However, general central/eastern tropics environment is improving to support TC generation.

Fig. 1: Morning satellite view of the North Atlantic basin.

Discussion: Tropical Disturbance 91L continues to drift slowly west-northwest with a strong shear axis just to the northwest of this feature (Fig. 1). The question remains whether 91L can fight through the shear pattern over the next 1-2 days and survive to develop as operational models are indicating. If so, 91L should become a tropical storm early Friday and possibly a minimal hurricane early next week before turning northeastward and staying south and east of Bermuda later next week. Soon-to-be Tropical Disturbance 92L off the West Coast of Africa also has a reasonable chance of reaching tropical storm strength before weakening over cooler water early next week. The low relative humidity dominating the central North Atlantic tropics in August (Fig. 2) but easing recently (Fig. 3) suggests the environment is improving for tropical cyclone development except for the upper shear pattern in the Caribbean Sea and central North Atlantic tropics.

Fig. 2-3: 600 MB relative humidity anomalies for August and the last 7 days in August.