WDPN33 PGTW 041500
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 19W (PARMA) WARNING NR 28// 
RMKS/
1. FOR METEOROLOGISTS. 
2. 12 HOUR SUMMARY AND ANALYSIS. 
    A. TROPICAL STORM (TS) PARMA HAS WEAKENED OVER THE PAST SIX 
HOURS. A FAIRLY NEW UPPER LEVEL FEATURE (SHORTWAVE TROUGH) APPEARS 
TO BE RESPONSIBLE FOR THE RAPID DEPLETION OF PARMA'S DEEP 
CONVECTION. THE TROUGH ORIGINATED OVER CENTRAL CHINA AND HAS SINCE 
CONTINUED TO DIG EQUATORWARD. SKEW-T SOUNDINGS ALONG THE SOUTHERN 
COAST OF CHINA INDICATE THE TROUGH EXTENDS AS FAR SOUTH AS 23.5N, 
LESS THAN 4 DEGREES SHY OF PARMA'S LATITUDE. A VERY TIGHT SHEAR 
GRADIENT, AND RELATIVELY DRIER AIR MASS, ASSOCIATED WITH THIS 
TROUGH, CONTINUE TO WEAKEN THE SYSTEM. DESPITE THE CURRENT VOID OF 
DEEP CONVECTION A STRONG SURFACE CIRCULATION REMAINS. DVORAK 
ESTIMATES FROM PGTW HAVE FALLEN TO A 3.0/4.0. THE FINAL-T NUMBER IS 
BASED ON THE PATTERN-T, VICE A WRAP, BECAUSE THE CONVECTION IS TOO 
WEAK. THE SYSTEM HAS ALSO GENERALLY TRACKED FROM NORTHWESTWARD TO 
MORE NORTHWARD OVER THE PAST 12 HOURS WITHIN A WEAK STEERING 
ENVIRONMENT.  
3.   FORECAST REASONING 
    A. THE SYSTEM IS NOW CLASSIFIED AS A TROPICAL STORM BECAUSE THE 
INITIAL INTENSITY HAS DROPPED BELOW 65 KNOTS. 
    B. THE SYSTEM IS STILL EXPECTED TO MEANDER TO THE NORTHEAST OF 
LUZON IN THE SOUTH CHINA SEA FOR THE NEXT 5 DAYS. THE POTENTIAL FOR 
REINTENSIFICATION EXISTS ONCE THE TROUGH MENTIONED IN PARA 2.A. 
PASSES TO THE EAST. HOWEVER, UPWELLING OF COOLER WATER WILL BEGIN TO 
STALL INTENSIFICATION OR EVEN BEGIN TO WEAKEN THE SYSTEM. THERE IS 
ALSO THE POSSIBILITY THAT THE TROUGH MAY STRIP THE CIRCULATION OF SO 
MUCH CONVECTION THAT IT IS UNABLE TO REBOUND BEFORE UPWELLING BEGINS 
TO CUT OFF INTENSIFICATION ALL TOGETHER. THERE IS STILL SOME 
UNCERTAINTY IN THE TRACK OF TS 19W GIVEN THAT IT IS IN A WEAK AND 
ILL-DEFINED STEERING ENVIRONMENT. IF RIDGING BUILDS TO THE WEST OF 
THE SYSTEM, THEN STORM WILL GRADUALLY BEGIN TO PULL OUT TO THE 
SOUTHWEST, AS THE MAJORITY OF THE DYNAMIC AIDS INDICATE. ON THE 
OTHER HAND, IF THE RIDGE TO THE EAST REBUILDS FASTER (AND STRONGER) 
THAN THE RIDGE TO THE WEST THEN THAT WILL BECOME THE PREDOMINANT 
STEERING MECHANISM FOR PARMA. THERE IS ALSO THE EXPECTATION THAT IF 
THE STORM IS UNABLE TO REACQUIRE DEEP CONVECTION, THE TRACKERS WILL 
INITIALIZE A STRONGER SYSTEM WITH ERRONEOUS TRACK SOLUTIONS.
    C. THERE IS NO STRONG INDICATION, THAT BEYOND DAY 4, THE CYCLONE 
WILL BEGIN TO BE PICKED UP BY ANY RIDGING/FLOW SUFFICIENT ENOUGH TO 
GUIDE IT OUT OF THIS QUASISTATIONARY STATE. AS SUCH, THE FORECAST 
REMAINS SLOW, BUT NEAR THE MODEL CONSENSUS AND PREVIOUS FORECAST.  
//
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