Dr. William Gray's April 2005 Predictions

This article may help you decide what to call the Low off the New England Coast.

..."James Franklin of TPC/NHC pointed out the obvious but sometimes troublesome fact that there are no well-defined boundaries between cyclone types. With regard to energy sources, cyclones come in a continuous spectrum, but forecasters (at least at NHC) have to pigeonhole them into three categories: tropical, subtropical, or non-tropical (extratropical). In deciding whether a particular storm is tropical or subtropical, James indicates that he would first look at the structure of the wind field. If the highest winds appear to be the result of central core convection, then it would be tropical. If they result from synoptic-scale gradients or forcing, then the system would be subtropical. Thermal structure is also important, but the data to definitively answer that question is often not available. (NOTE: It should be pointed out that, while in some circles the terms subtropical and hybrid are often used synonymously, this tends to not often be the case at NHC. Since its inception in public warning terminology in 1972, the term "subtropical storm" has become increasingly restricted in its application to hybrid-type marine cyclones. In other words, a subtropical cyclone is a hybrid between classical extratropical and tropical cyclones, but not all hybrids are considered subtropical storms.)

Jack Beven, also of NHC, states that he considers three main characteristics in deciding whether or not a given system is tropical or subtropical: satellite appearance, amount and behaviour of central convection, and any available information on how frontal a system is. He admits that all three are rather subjective quantities. Chris Landsea of AOML/HRD feels that a tropical cyclone should be called such when there is convection near or over the center of the system (within about one degree of latitude/longitude), it is warm core in the lower troposphere, is non-frontal, and has a relatively small radius of maximum winds (smaller than about 2.5 degrees of latitude/longitude).

David Roth of HPC feels that for classification as a tropical cyclone, a system should have no large dry slots, no cold fronts or stratus clouds, and should have deep central convection. In David's opinion, the (usually) small cyclones sometimes seen in the Atlantic (and also often in the Mozambique Channel) which may contain eye features but have shallow convection and shallow warm cores with cold cores aloft should be classed as subtropical rather than as tropical cyclones.

Commenting also on the topic of classification of tropical cyclones, Rich Henning, a meteorologist at Eglin AFB and a member of the Hurricane Hunters squadron, suggested that perhaps there should be a sliding scale based on the latitude of the system and the time of year.
Rich writes, "For example, for a system at a high latitude in November, there had better be a burst of deep convection at or near the center of the vortex that creates some evidence of a warm core and the establishment of a tighter pressure gradient near the center that can be traced to the convective event, i.e., that can be distinguished from the larger-scale mid-latitude cyclone gradient in which it may be embedded. For lower latitudes and/or when formation is from July to October, this may not be as strictly enforced, especially when cyclogenesis occurs over water that is warmer than or equal to 26.5 C. For cooler water temperatures, I am always skeptical about a system in the absence of deep, persistent convection at or near the vortex core."...

http://mpittweather.com/txt/jun01sum.txt

http://www.aoml.noaa.gov/hrd/iwtc/Evans4-3.html

Edited by danielw (Sat May 07 2005 08:26 PM)

Post Extras

Flhurricane.com - Central Florida Hurricane Center - Tracking Storms since 1995Hurricanes Without the Hype! Since 1995

NEWS

CURRENT STORMS

COMMUNICATION

STORM DATA

CONTENT

FOLLOW US