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Erin's TCR issued; CSU Tropical Activity Forecast

April 11, 2008

Erin's TCR
So, Erin's TCR was finally released (sorry I'm late- 3 1/2 days ago). The NHC decided that it wasn't a TS over land, as it had the characteristics of a TS for only 5 hours, and needs to maintain its convection for at least 6 hours to redesignate a TS. Here is the part about that problem: While the system's structure, particularly its convective organization as seen on radar, resembled and had some characteristics of a tropical storm for a few hours on 19 August, the prevailing view from the Hurricane Specialists at the National Hurricane Center (NHC) is that the system was not a tropical cyclone¹ over Oklahoma. The phrase “organized deep convection” in the NHC tropical cyclone definition has both spatial and temporal components, meaning that a tropical cyclone must produce deep convection over some period of time. While it is a subjective determination, in this case the deep convection is judged to have lasted an insufficient period of time to classify the system as a tropical cyclone. The limited duration of the convection also appears to be indicative of the physical mechanisms that caused the low to briefly strengthen. It is speculated that the upper-level shortwave trough forced the deep convection to increase via upper-level difluence, while briefly superimposed above the surface low that provided a focus for low-level confluence. The upper-level forcing was apparently a dominant mechanism, which is in contrast to tropical cyclones that are maintained primarily by extraction of heat energy from the ocean. Since the system was clearly non-frontal, designating it as an extratropical cyclone² is also not the most appropriate solution. In addition, the prevailing view among the NHC’s Hurricane Specialists is that the system’s duration over Oklahoma on 19 August was also too short to classify it as a subtropical cyclone³. Given all of the considerations described above, the system is simply designated as a “low” by NHC on 19 August.

1 A tropical cyclone is defined by NHC 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.”

2 An extratropical cyclone is defined by NHC as “a synoptic-scale low pressure system whose primary energy source is baroclinic.”

3 A subtropical cyclone is defined by NHC as “a non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones.”

Erin caused 16 deaths, as per NHC; 9 while a tropical cyclone and 7 while a remnant low. 