A review of the 1994 and 1995 western north Pacific Ocean tropical cyclone data revealed that the intensity estimates of a significant number of cyclones that recurved and moved out of the tropics were underestimated by the tropical satellite reconnaissance network which used the Dvorak (1984) method to determine intensity. In 1995, the only site in the network that used the Hebert-Poteat (1975) subtropical technique on these transitioning systems was Air Force Global Weather Central (AFGWC). This review was initiated to give sites in the network some guidance on the determination of intensity of transitioning systems to insure that the best tropical satellite reconnaissance data are provided to our customers in 1996 and beyond.
The application of the Dvorak technique to transitioning tropical cyclones resulted in the intensity being underestimated by as much as three "T" numbers below the actual intensity [reference Typhoon Seth, (32W)]. This is due to the reliance of the Dvorak technique upon persistent central convection or the proximity of convection to the low-level circulation center. The loss of central convection by transitioning systems and the distance that convection either regenerates or migrates results in unrepresentative "T" numbers by the Dvorak technique. In contrast the Hebert-Poteat subtropical cyclone intensity estimation technique takes these factors into account, and although quite subjective, provides a way of addressing the loss of the central dense overcast. The Hebert-Poteat method was developed as a technique to determine the intensity of systems transitioning from subtropical to tropical in the Atlantic. It has been used by the tropical satellite reconnaissance network in the reverse scenario of systems making the transition from tropical to subtropical in the Pacific, as well as the rare instance here, of systems that become tropical after genesis in the subtropics.
This extratropical transition technique replaces the Hebert-Poteat technique and the use of "ST" numbers for systems that are transitioning from the tropics. It borrows from both Dvorak and Hebert-Poteat in focusing on the application of the log 10 spiral to the primary outer cloud band and computation of translational speed. In addition, the degree of organization of the low level circulation is evaluated, and the extent of any residual or regenerative convection between the low level circulation center and the primary outer cloud band is determined. These four factors are defined and evaluated for wind speed contributions in the determination of a final "XT" number. The technique is an additive process with wind speeds derived from the applicable factors being added together to reach a cumulative evaluation of the intensity of the system. Instead of using "ST" numbers as in the Hebert-Poteat technique, it uses "XT" numbers. The Hebert-Poteat "ST" technique will be retained for systems that transition to tropical, and its application for these systems will not change.
The current intensity "CI" will still be computed as in the Dvorak technique after this transitioning technique has been used. This will provide for a smoother switch between the two techniques. The analyst will insert "XT" in the satellite fix bulletin in place of "T" when this technique is used. An additional remark will be made in the discussion section of the bulletin indicating the system is transitioning.
Note that there is a direct correlation between Dvorak "T" numbers and the "XT" numbers. That is to say that a 35 knot intensity result from either technique will be encoded as a 2.5.