After a recent increase in tropical and subtropical activity in the month of May, the National Hurricane Center announced in February its plans to issue routine Tropical Weather Outlooks starting on May 15 in 2021.

It appears to have been the right decision. There are now two areas of interest being monitored by the National Hurricane Center for potential development: Invest 90L in the western Atlantic, which has a high chance of development, and Invest 91L in the Gulf of Mexico, which has a low chance of development. The first two names on the Atlantic naming list are Ana and Bill. Even if both invests develop into named storms, the record for the earliest second tropical storm will not be broken, as that record currently belongs to the second tropical storm of 1887, which formed on May 17.

As of 00:00 UTC Friday, Invest 90L was centered near 35.0°N 55.3°W. Maximum sustained winds were 45 knots (50 mph), with an estimated minimum pressure of 1007 mb. The NHC is giving 90L an 80 percent chance of development, both within 48 hours and five days. 90L is expected to turn to the west-southwest on Friday, and has a strong chance to become a subtropical or tropical cyclone on Friday or Friday night. It is interesting to note that recent model guidance has trended weaker with 90L, likely due to the marginal thermodynamic environment that will make sustained deep convection somewhat difficult. Sea surface temperatures are quite cool – near 20°C (68°F) – but subtropical cyclones can form over cooler sea surface temperatures than fully tropical cyclones do. Dynamical models suggest that 90L will likely become devoid of convection by late weekend, and the environment is expected to become more hostile for development by Sunday. The Bermuda Weather Service has issued a Tropical Storm Watch for 90L, but the NHC has not issued Potential Tropical Cyclone advisories so far. Interests in Bermuda should monitor the progress of 90L.

As of 00:00 UTC Friday, Invest 91L was centered near 24.3°N 93.7°W. Maximum sustained winds were 20 knots (25 mph), with an estimated minimum pressure of 1010 mb. The NHC is giving 91L a 20 percent chance of development, both within 48 hours and five days. 91L is likely to move northwestward to north-northwestward, and move inland into southeastern Texas. It is interesting to note that no tropical cyclone has ever been recorded to have made landfall in the state of Texas during the month of May. Although tropical cyclogenesis appears unlikely at the moment, the 00z run of the GFS-based SHIPS guidance indicates marginally favorable conditions for development. The SHIPS model predicts that shear will remain moderate – about 15 to 20 knots – for the next 24 hours, with sea surface temperatures marginal, near 26°C (78.8°F). 700-500 mb relative humidity values are also marginally favorable, near 55 percent. The 18z GFS model run does not develop 91L, but the 12z ECMWF, GEM, and UKMET models all showed some slight development with 91L. Even if 91L does not develop into a tropical cyclone, 91L has potential to produce heavy rainfall over eastern Texas this weekend.