When Are Tropical Cyclone Advisories Initiated?

While I was working at the National Hurricane Center (NHC) I found one of the difficult decisions was in deciding when to issue the initial advisory on a tropical cyclone. The National Hurricane Operations Plan (NHOP) lists definitions used by United States tropical cyclone forecasting centers such as the Joint Typhoon Warning Center (in Pearl Harbor, Hawaii), the Central Pacific Hurricane Center (in Honolulu, Hawaii) and the NHC (in Miami). The NHOP defines a tropical cyclone as “a warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center.”

The weak area of low pressure currently between the Bahamas and Bermuda had a closed surface circulation for the past several days based on ship and land reports. At times the system has exhibited a “well-defined center” although that determination is certainly subjective. Mid- and high-level clouds often obscure the low-level center making this difficult to observe on satellite imagery. The tropical cyclone definition also requires “organized deep convection.” One can argue what it takes to qualify as organized deep convection, and this too is a subjective determination. Curved bands of thunderstorms wrapping around the center would qualify. But today there are no signs of anything resembling organized deep convection around the weak low-level center.

The main reason this system is not developing today is because of wind shear. However, most computer models indicate this shear will weaken and the low pressure system will develop and eventually head back toward the east coast of the United States. At this time, the most likely scenario is for the system to threaten the Carolinas.


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