TC 22S(HALIMA) resilience despite shear and dry air//Invest 93W : MEDIUM//Invests 97P and 96S, 29/03utc, 06utc Update

JTWC IS ISSUING 12HOURLY WARNINGS ON TC 22S(HALIMA). 3HOURLY SATELLITE BULLETINS ARE ISSUED ON TC 22S AND INVEST 97P.

29/0730UTC.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE OVERALL DISSIPATION PHILOSOPHY HAS NOT CHANGED, HOWEVER THE FORECAST HAS BEEN LENGTHENED TO CARRY THE SYSTEM LATER INTO THE FORECAST PERIOD BEFORE DISSIPATION OCCURS. FORECAST DISCUSSION: TC 22S HAS OVER THE PAST TWELVE HOURS TURNED FIRST EAST AND NOW EAST-NORTHEAST, AS THE LOW TO MID-LEVEL RIDGE BUILDS IN TO THE SOUTH OF THE SYSTEM. OVERALL HOWEVER, THE STEERING PATTERN IS RELATIVELY WEAK AND THE OVERALL TRACK MOTION, WHILE GENERALLY TOWARDS THE NORTH, IS FRAUGHT WITH UNCERTAINTY. THE RIDGE EXTENSION TO THE SOUTH IS EXPECTED TO PUSH NORTHWARD OVER THE NEXT 12 TO 24 HOURS, ALLOWING FOR TC 22S TO MOVE NORTH WITH IT. HOWEVER, THE RIDGE WILL ERODE QUICKLY AHEAD OF AN APPROACHING TROUGH AND BY 36H, TC 22S WILL LIE IN A COL REGION BETWEEN THE TROUGH TO THE SOUTH, AND NEAR EQUATORIAL RIDGE CENTERS TO THE EAST AND WEST. THE NET EFFECT WILL BE A SLOW DRIFT GENERALLY NORTHWARD AT A VERY SLOW THREE KNOTS OR LESS. SHEAR HAS DECREASED OVER THE PAST TWELVE HOURS, DOWN TO MORE MODERATE LEVELS, WHICH HAS ALLOWED THE SYSTEM TO MAINTAIN ITS STRUCTURE AND INTENSITY, EVEN IN THE FACE OF THE SIGNIFICANT DRY AIR TO THE WEST. AS THE SYSTEM TRACKS NORTHWARD IT WILL MOVE UNDER THE UPPER-LEVEL RIDGE, WITH AN ACCOMPANYING REDUCTION OF THE OVERALL SHEAR. BUT THE CONTINUED INTRUSION OF VERY DRY AND STABLE AIR PROGRESSIVELY FURTHER INTO THE CORE OF THE SYSTEM AND THE REDUCED OUTFLOW WILL OFFSET THE REDUCED SHEAR AND LEAD TO A LONG AND SLOW DISSIPATION. THE SYSTEM IS EXPECTED TO DISSIPATE TO BELOW WARNING CRITERIA NO LATER 72H.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A COMPACT SYSTEM, WITH A CONVECTIVE STRUCTURE WHICH HAS BEEN RELATIVELY STABLE OVER THE PAST SIX TO TWELVE HOURS. A 282257Z SSMIS 37GHZ MICROWAVE IMAGE SHOWED A CYCLONICALLY CURVED REGION OF CURVED CONVECTION ALONG THE SOUTHERN PERIPHERY OF THE WELL DEFINED LOW LEVEL CIRCULATION CENTER (LLCC), AND LENT HIGH CONFIDENCE TO THE INITIAL POSITION. THE SYSTEM IS UNDER SOME WESTERLY SHEAR, WHICH IS IMPARTING SOME PRESSURE ON THE WESTERN SIDE OF THE CIRCULATION. ADDITIONALLY, DRY AND RELATIVELY STABLE AIR IS SOLIDLY IN PLACE ALONG THE WESTERN PERIPHERY AS INDICATED BY THE LARGE FIELD OF STABLE STRATOCUMULUS IN THE WESTERN HEMISPHERE OF THE CIRCULATION. EVEN IN THE FACE OF THE SHEAR AND DRY AIR, THE SYSTEM HAS BEEN ABLE TO MAINTAIN A CONSISTENT INTENSITY OF 45 KNOTS, BASED ON THE CONGLOMERATION OF AGENCY DVORAK CURRENT INTENSITY ESTIMATES AT T3.0 (45 KNOTS) AND THE SATCON ESTIMATE OF 46 KNOTS.

MODEL DISCUSSION: ENSEMBLE AND DETERMINISTIC TRACK GUIDANCE IS IN RELATIVELY GOOD AGREEMENT ON THIS RUN, SHOWING A SIGNIFICANT CONSOLIDATION OVER THE PREVIOUS RUN. WITH THE EXCEPTION OF NAVGEM, ALL THE AVAILABLE GUIDANCE CONCURS ON A GENERALLY NORTHWARD TRACK, WITH THE GFS, GFS ENSEMBLE AND ECMWF ON THE WESTERN EDGE OF THE ENVELOPE AND THE EGRR AND GALWEM TRACKERS ON THE EASTERN SIDE. THROUGH THE FORECAST DURATION, THE SPREAD STEADILY INCREASES TO 370 KM AT 72H. THE ECENS AND GEFS MEANS ARE IN CLOSE AGREEMENT, BUT THE ECENS IN PARTICULAR SHOWS A WIDE SPREAD OF ENSEMBLE MEMBERS, WITH A SIGNIFICANT NUMBER TRACKING THE SYSTEM FAR TO THE EAST. THE JTWC FORECAST LIES CLOSE TO THE CONSENSUS MEAN WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS MIXED, WITH THE HWRF AND COAMPS-TC BEING THE MOST BEARISH, INDICATING DISSIPATION WITHIN 24 HOURS, WHILE THE DECAY SHIPS SHOWS THE SYSTEM MAINTAINING INTENSITY TO 72H, THEN DISSIPATION BY 96H. INTERESTINGLY, THE ECENS MEMBERS TAKING THE SYSTEM TO THE EAST INDICATE A MUCH STRONGER SYSTEM, WHILE THOSE MEMBERS TO THE WEST INDICATE A WEAKER, SUB-TS LEVEL SYSTEM. THE JTWC FORECAST IS CLOSE TO THE CONSENSUS MEAN THROUGH 48H, THEN IS BELOW THE MEAN THROUGH THE REMAINDER OF THE FORECAST WITH LOW CONFIDENCE.

THE AREA OF CONVECTION (INVEST 93W) PREVIOUSLY LOCATED NEAR 7.6N 118.0E IS NOW LOCATED NEAR 9.9N 116.2E, APPROXIMATELY 1090KM SOUTHEAST OF DANANG, VIETNAM. THE POSITION FOR 93W WAS RELOCATED BASED ON A 290211Z ASCAT-C IMAGE, WHICH SHOWS A DEVELOPING, ELONGATED CIRCULATION ADJACENT TO A SWATH OF 20-25 KNOT EASTERLIES OVER THE NORTHEAST QUADRANT WITH WEAKER WINDS (5-15 KNOTS) ELSEWHERE. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS STRONG MID-LEVEL TURNING WITH LOW-LEVEL CLOUD LINES WRAPPING INTO THE EASTERN QUADRANT SUGGESTING A DEVELOPING LOW-LEVEL CIRCULATION CENTER (LLCC). A 282236Z SSMIS 91GHZ COLOR COMPOSITE MICROWAVE IMAGE REVEALS A WEAKLY-DEFINED LLCC, HOWEVER, FORMATIVE DEEP CONVECTIVE BANDS ARE EVIDENT OVER THE NORTHERN SEMICIRCLE. UPPER-LEVEL ANALYSIS INDICATES NEAR-RADIAL OUTFLOW WITH LOW VERTICAL WIND SHEAR. SST VALUES ARE CONDUCIVE FOR FURTHER DEVELOPMENT AT 29-30C. GFS REMAINS THE MOST AGGRESSIVE MODEL WITH A TROPICAL DEPRESSION DEVELOPING QUICKLY WITHIN THE NEXT TWO DAYS AS THE SYSTEM TRACKS NORTHWESTWARD TOWARD VIETNAM. ECMWF AND NAVGEM SHOW A SIMILAR FORECAST TRACK BUT WEAKER DEVELOPMENT. THE GFS SOLUTION APPEARS MORE LIKELY CONSIDERING THE FAVORABLE ENVIRONMENTAL CONDITIONS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 12 TO 18 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1007 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS UPGRADED TO MEDIUM.

THE POSITION FOR 93W WAS RELOCATED BASED ON A 290211Z ASCAT-C IMAGE, WHICH SHOWS A DEVELOPING, ELONGATED CIRCULATION ADJACENT TO A SWATH OF 20-25 KNOT EASTERLIES OVER THE NORTHEAST QUADRANT WITH WEAKER WINDS (5-15 KNOTS) ELSEWHERE.