First storm of 2023 hurricane season formed in January, NHC says

NHC announcement here

Explanation from NWS here

  1. An extratropical cyclone forms. Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. These storms always have one or more fronts connected to them, and can occur over land or ocean. An extratropical cyclone can have winds as weak as a tropical depression, or as strong as a hurricane. Examples of extratropical cyclones include blizzards, Nor’easters, and the ordinary low pressure systems that give the continents at mid-latitudes much of their precipitation.

  2. If the waters under the extratropical cyclone are at least 21C (70F), thunderstorm activity will gradually build inside the storm and moisten and warm the lower levels. Over time, the core of the storm may gradually go from cold to warm, and the storm will start getting some of its energy from “latent heat”, which is the energy released when water vapor that has evaporated from warm ocean waters condenses into liquid water. Latent heat is what powers tropical cyclones. At this point, the storm is called subtropical. If the winds are already more than 39 mph (as happened in the case 2007’s Subtropical Storm Andrea), it is called a subtropical storm. If the winds are less than 39 mph, then it is called a subtropical depression. So, you don’t need to start with a subtropical depression in order to get a subtropical storm.

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Good example of how modern technology can detect systems like this that in the past were missed.