Main1. Procedures
JTWC has developed an experimental checklist for intensity analysis and forecast in the Northern Hemisphere (Tables D-1a and D-1b ). The first part is for intensity analysis and consists of four elements. The elements are:
(1). Dvorak classification for current intensity,
(2). Synoptic influences and upper troposphere outflow patterns,
(3). Sea surface temperature influences, and
(4). Dvorak trend estimation and cloudiness.
The second part is an intensity forecast that assesses future tropical motion, mid-latitude wave disturbances, tropical upper troposphere troughs, monsoonal and cross equatorial flow patterns, current satellite cloud imagery and outflow patterns.
To complete the checklist, determine a yes/no answer for each of the criteria listed. Sum up all the points assigned for yes answers. Finally, look up an intensity assessment using the total score. Some terms used in the checklist are explained after the checklist.
2. Northern Hemisphere Intensity Forecast Checklist (Tables D-1a and D-1b ) Terminology
(1). CCC developing:
Central cold cover (CCC) occurs when the central dense overcast (CDO) becomes glaciated across the top, wiping out the thermal gradient normally observed with the CDO of an intensifying TC. The CCC indicates arrested development for a period of 12-24 hours. The Dvorak trend is D0.0 as opposed to the D1.0 for normal development. When a CCC develops, the visual imagery may indicate a circular exhaust cloud where a single cumulonimbus penetrates the CCC or CDO and produces a canopy of concentric gravity waves. This occurs in the T3.5 to 4.0 range.
(2). CDO developing:
Central dense overcast (CDO) occurs when TC wind speeds reach the 45-55 kt range (T number is in the range from 3.0 to 3.5), and is the precursor to the formation of an eye. The alternative to eye formation without a CDO is through the development of a banding type eye where the primary convective band curves around the center until it forms a closed convective ring. This also occurs at about T 3.5. The CDO frequently contains penetrative cumulonimbus that can be used to help locate the center. The CDO also has a circular temperature gradient with the coldest temperature near the center. As the CDO evolves, a warm spot appears. This is the first hint that subsidence is beginning to clear the cloud from the upper portions of the eye.
(3). Cross equatorial flow:
For the Northern Hemisphere, flow originates south of the equator and moves north across the equator. In this case, cross equatorial flow can contribute to convergence in the Northern Hemisphere.
(4). Neutral point:
Neutral point can be either col or saddle point. A neutral point on a surface chart is the location of a relative high pressure between two lows (i.e., the col point) or the location of a relative low pressure between two highs (i.e., the saddle point).
***** End of APPENDIX D *****
Main
Chapter 6
Appendix C
References
Appendix D