Subtropical Storm Andrea – The First Named Storm of 2019 Atlantic Hurricane Season (Credits: NHC, NOAA, Plymouth State Weather Center)

​The Atlantic hurricane season starts on June 1stof every year. Sometimes we can see storms before that date, or after that date – but subtropical storm Andrea was the first named storm of the 2019 season, and it came 11 days early! 

Andrea formed just southwest of Bermuda on May 20th, 2019. The National Hurricane Center (NHC) put out their first warning of the storm and said that it would never reach above 73 miles per hours sustained winds. The storm continued on its track for some time, but never developed into anything more than a storm. Two days after the NHC put out their forecast, the storm dissipated and turned into a tropical depression with sustained winds that were less than 39 mph.

Why didn’t Andrea form into a major storm? The most straightforward reason is that it formed well before the hurricane season started, which means that some of the “ingredients” for hurricane development weren’t present. 

​The “ingredients” needed for a hurricane to form include a combination  of the following conditions. First, we need a spot of convection off the equator. Convection can be explained by storms that produce severe weather, like heavy rain and thunderstorms.This is because if we want a storm to form, we need some sort of rotation, which comes from the Coriolis force. The Coriolis force doesn’t have any effect on storms (or weather) when it is isolated close to the equator the equator. This means that most storms form around 15 degrees’ latitude. When examining this condition in the context of Andrea, the the first ingredient is missing because she was located at 28 – 33 degrees North of the equator. 

The second ingredient necessary for a major storm to form is warm sea surface temperatures. The ideal temperature is about 26 degrees C. This allows the storm to use the warm waters and turn it into energy to intensify the storm. When looking at a sea surface temperature (SST) map of the ocean for Andrea, we can see that it formed in an area where the SST’s were around 27 degrees, and it quickly moved into colder water, which preventedt the storm from becoming stronger.

Third, th next element that is required for a major storm is relatively low wind shear. This means you want a little amount of wind shear in the area – too much will destroy the storm, not enough and the storm won’t form. When Andrea moved closer to Bermuda the stronger wind shear, the storm started to “crumble” or fall apart. It became asymmetric and ruined the formation of the storm. 
One other thing you would need is a moist environment. If there isn’t a lot of water vapor in the environment, then it is really hard for astorm to form. If the storm moves into an area where the environment is really dry, the storm will likely fall apart and weaken. In this case Andrea moved into a region of dryer air. 

​Overall the weather was just not ready to form a storm this early in the season, so Andrea ended up weakening. But now, we look forward to the start of the hurricane season!
 
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© 2019 Weather Forecaster Allison Finch