Tropical Storm NALGAE Advisory Sa, 29.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 26W (NALGAE) WARNING NR
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 13.6N 122.2E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 92 NM SOUTHEAST OF MANILA, PHILIPPINES
   MOVEMENT PAST 6 HOURS: WESTWARD AT 17 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL STORM 26W (NALGAE) MADE LANDFALL OVER CATANDUANES THIS
MORNING AT APPROXIMATELY 281700Z BASED ON ANIMATED RADAR IMAGERY.
SUBSEQUENT RADAR DATA AND ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) SUGGEST THE SYSTEM HAS MOVED WEST-SOUTHWEST AFTER THE 18Z HOUR,
AND THE CENTER NOW LIES IN THE VICINITY OF MARINDUQUE ISLAND.
ANIMATED RADAR SHOWS THAT THE LOW LEVEL CIRCULATION CENTER (LLCC)
HAS BECOME MORE RAGGED, WITH THE RUGGED TERRAIN DISRUPTING THE LOW
LEVEL INFLOW REGIME. DEEP CONVECTION CONTINUES TO FLARE UP, AROUND
THE LLCC, BUT REMAINS DISORGANIZED AT PRESENT. THE SYSTEM WAS
MOVING AT A GOOD CLIP UP TO THE 290000Z HOUR, AND THE LATEST RADAR
DATA SHOWS THAT THE SYSTEM HAS SLOWED DOWN CONSIDERABLY. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
MULTI-AGENCY FIX POSITIONS AS WELL AS THE PAGASA RADAR DATA. THE
INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
CONGRUENCE OF THE PGTW AND KNES INTENSITY ESTIMATES WITH THE ADT.
THE OVERALL ENVIRONMENT IS FAVORABLE, WITH LOW (10-15 KNOTS) VWS,
WARM SSTS IN THE SHALLOW SIBUYAN SEA, AND GOOD WESTWARD AND
EQUATORWARD OUTFLOW, WITH A DEVELOPING POLEWARD OUTFLOW AS WELL,
WITH A POINT SOURCE TO THE EAST. THE ONLY INHIBITING FACTOR IS THE
POSITIONING AMONGST THE COMPLEX TERRAIN OF THE CENTRAL PHILIPPINES,
WHICH IS DISRUPTING THE LOW-LEVEL FLOW INTO THE CORE OF THE SYSTEM.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE NORTHEAST.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T3.5 - 55 KTS
   RJTD: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   CIMSS ADT: 59 KTS AT 282340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: MODERATE EQUATORWARD AND WEAK POLEWARD OUTFLOW
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING. HOWEVER, THIS FORECAST WILL
INTRODUCE A POTENTIAL ALTERNATE FORECAST SCENARIO.
FORECAST DISCUSSION: HAVING MADE LANDFALL ABOUT SEVEN HOURS AGO, TS
26W HAS HELD TOGETHER SURPRISINGLY WELL, AND MAINTAINED A CURRENT
INTENSITY OF 55 KNOTS. AND WHILE THE OVERALL STEERING HAS NOT
CHANGED, THE SYSTEM HAS SLOWED DOWN AND TURNED MORE WEST-SOUTHWEST,
PRIMARILY DUE TO MESOSCALE TERRAIN INFLUENCES MORE THAN ANY
SIGNIFICANT CHANGE IN THE STEERING PATTERN. IT IS EXPECTED TO
RESUME A WEST-NORTHWEST TRACK IN THE NEXT FEW HOURS, AND EMERGE
BACK OVER THE OPEN WATERS OF SOUTH CHINA SEA TO THE SOUTHWEST OF
MANILA BY TAU 12. ALL OF THE AVAILABLE GUIDANCE SUPPORTS A GENERAL
WEST-NORTHWEST TRACK THROUGH TAU 36. THE SYSTEM REACHES A FORK IN
THE ROAD AT TAU 48, WITH A CHOICE BETWEEN A CURVE TO THE NORTH OR A
TRACK THAT CONTINUES TO THE WEST-NORTHWEST. THE CURRENTLY FAVORED
GUIDANCE SHOWS THE WESTERN SIDE OF THE STR ERODING AHEAD OF A
MODEST MID-LATITUDE TROUGH WHICH INDUCES A WEAKNESS IN THE RIDGE,
ENTICING TS 26W TO TURN NORTHWARD TOWARDS THE WEAKNESS AFTER TAU
48. THE SYSTEM IS EXPECTED TO WEAKEN A BIT AS IT MOVES OVER THE
RUGGED TERRAIN OF SOUTHERN LUZON, EMERGING INTO THE SCS AT AROUND
50 KNOTS. THE PRESENCE OF STRONG NORTHEAST SURGE FLOW PUSHING SOUTH
OUT OF THE TAIWAN STRAIT IS EXPECTED TO REMAIN RELATIVELY MOIST AND
THE ENHANCED FLOW INTO THE WESTERN SIDE OF TS 26W WILL SERVE TO
INCREASE THE VORTICITY OF THE SYSTEM. AT THE SAME TIME, THE
DEVELOPMENT OF AN UPPER-LEVEL POINT SOURCE ALOFT IS ANTICIPATED AND
THESE FACTORS WILL ENABLE A PERIOD OF RAPID INTENSIFICATION FROM
TAU 12 TO TAU 48, TO A PEAK OF 80 KNOTS. STEADILY COOLING SSTS,
INCREASING VWS AND DRY AIR ENTRAINMENT WILL LEAD TO STEADY
WEAKENING AFTER TAU 60, WITH THE SYSTEM FORECAST TO BE A WEAK
TROPICAL DEPRESSION OF ONLY 30 KNOTS BY TAU 120. AS THE SYSTEM
RAPIDLY WEAKENS, IT IS EXPECTED TO SLOW DOWN AND TURN TO THE WEST
AS THE LOW-LEVEL WIND FLOW TAKES OVER AS THE STEERING INFLUENCE.
DUE TO A BIFURCATION IN THE MODEL GUIDANCE, THERE IS A POSSIBLE
ALTERNATE SCENARIO. IN THIS SCENARIO A WEAKER MID-LATITUDE TROUGH
RAPIDLY TRANSITS EAST IN LARGELY ZONAL FLOW ALOFT, THE STR HOLDS
STRONG AND DOES NOT ERODE AND TS 26W DOES NOT TURN NORTHWARD BUT
RATHER TRACKS OFF TO THE WEST, TOWARDS NORTHERN HAINAN.  A FURTHER
SOUTHWARD TRACK WOULD ALSO MEAN THE SYSTEM REMAINS IN A FAVORABLE
ENVIRONMENT FOR LONGER, OVER WARMER WATERS, AND WITH LESS
INTERACTION WITH THE NORTHEAST SURGE FLOW, RESULTING IN A
POTENTIALLY HIGHER PEAK INTENSITY.
MODEL DISCUSSION: AS MENTIONED ABOVE, TRACK GUIDANCE DISPLAYS A
BIFURCATION SCENARIO WITH TWO DISTINCT GROUPINGS. THE FIRST GROUP
MADE UP OF THE GFS, HWRF, UKMENT ENSEMBLE AND GALWEM ARE CONSISTENT
WITH THE JTWC FORECAST AND TURN THE SYSTEM NORTHWARD AFTER TAU 36.
THE SECOND GROUP COMPRISED OF ECMWF, NAVGEM AND THE GEFS AND ECENS
MEANS REPRESENT THE ALTERNATE SCENARIO, TRACKING THE SYSTEM ON A
FLATTER TRAJECTORY TOWARDS HAINAN. THE ENSEMBLE MEMBERS IN BOTH OF
GEFS AND ECENS SHOW A VERY WIDE SPREAD, THOUGH ONLY A COUPLE OF
MEMBERS SHOWS A RECURVING TRACK. THE JTWC FORECAST LIES ON THE
WESTERN EDGE OF THE FIRST GROUP OF MODELS, WEST OF THE CONSENSUS
MEAN AND ALL OF THE TRACKERS IN THIS GROUP. CONFIDENCE IS HIGH IN
THE FIRST 36 HOURS BUT RAPIDLY DETERIORATES TO LOW THEREAFTER DUE
TO THE VERY HIGH UNCERTAINTY AND THE BIFURCATION IN THE GUIDANCE.
INTENSITY GUIDANCE IS ALSO MIXED, WITH A WIDE ENVELOPE BETWEEN
50-110 KNOTS. THE RIPA AND SEVERAL OTHER RAPID INTENSIFICATION (RI)
AIDS HAVE BEEN TRIGGERED WITH THIS MODEL RUN, INDICATING A 40
PERCENT OR HIGHER PROBABILITY OF RI IN THE NEXT 24 TO 48 HOURS AND
THE COAMPS-TC ENSEMBLE AND GEFS ENSEMBLE INTENSITY PROBABILITIES
INDICATE 10-30 PERCENT CHANCE OF RI IN THE SAME TIME FRAME. THE
JTWC FORECAST CALLS FOR THE SYSTEM TO INTENSIFY AT A RATE
APPROACHING RI BETWEEN TAU 12 AND TAU 48, TO A PEAK OF 80 KNOTS AT
TAU 48, FOLLOWED BY STEADY WEAKENING THROUGH THE REMAINDER OF THE
FORECAST. CONFIDENCE IS MEDIUM THROUGH THE FORECAST PERIOD.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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