Procedures

The application of this extratropical transition technique begins with the decision that a system is in the process of becoming extratropical.  This technique provides quantifiable criteria for making the determination of transition.  The single most important determining factor for deciding if a system is becoming extratropical in nature is the  decrease or loss of persistent central convection.  Take note of the energy associated with the system as reflected in the curvature of the organized convective bands and the cloud top temperatures of the deep convection.  Cloud system size may increase as the system begins to lose its tropical characteristics with the convection displaced further eastward and poleward away from the low-level center.  These transitioning systems usually maintain their forward motion or accelerate.  If this doesn’t happen, and dissipation begins, continue to apply the Dvorak technique.  Interaction with other synoptic systems may occur as a tropical system recurves.  This interaction usually indicates that extratropical transition is occurring.

Observational experience has shown that during the extratropical transition an increase in the subsidence generates an irregular shaped "warm hole" near the low-level circulation center as seen on the IR imagery using the BD enhancement.  This feature will remain through the completion of the transition and it is possible to use this conservative feature for positioning and as an indication of transitioning.  Using these criteria, a satellite analyst can identify the transition from tropical "T" numbers to extratropical "XT" numbers. The factors listed below are used to determine if a system is transitioning "XT", or still tropical "T",  in nature.

It is important to apply the "XT" technique as soon as appropriate in order to avoid an artificial decrease and then an increase in the intensity of the system as the techniques are switched.  It is reasonable that in some cases there will be a slight increase in reported intensity as the switch is made between the two techniques.  Practice with this technique should alleviate much of these artificial minima when changing analysis techniques.
This technique should be used until the satellite picture indicates that the system has completed the transition to extratropical.  At that point it is recommended that the Smigielski and Mogil (1992) technique is an appropriate technique to use if continued analyses is warranted.  The following criteria should be used to make the determination that a system has transitioned to extratropical and the use of this technique should be ended:
Figure 1:  Cyclonic vortex that has completed extratropical transition process


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