Advisories

WDPN33 PGTW 300300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (BANG-LANG) WARNING 
NR 016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.3N 167.9E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 723 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 38 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 20W (BANG-LANG) AS A COMPACT SYSTEM FIGHTING THE EFFECTS
OF STRONG WESTERLY VERTICAL WIND SHEAR, EVIDENCED BY THE SHARP
WESTERN EDGE OF THE CENTRAL DENSE OVERCAST (CDO). ADDITIONALLY, THE
LOW-LEVEL CIRCULATION CENTER APPEARS TO BE NEARLY VISIBLE FROM
UNDER THE NORTHWESTERN PORTION OF THE CDO. ANIMATED BRIGHTNESS
TEMPERATURE REVEALS THE UPPER LEVEL CLOUD TOPS ASSOCIATED WITH THE
SYSTEM HAVE WARMED TO AN AVERAGE OF -40C. ENVIRONMENTAL ANALYSIS
REVEALS AN UNFAVORABLE ENVIRONMENT, CHARACTERIZED BY MODERATE TO HIGH 
(20-25 KTS) VERTICAL WIND SHEAR (VWS), MARGINAL (0-20 KJ PER CM2) 
OCEAN HEAT CONTENT (OHC), WEAK DRY AIR ENTRAINMENT, DECREASING 
OUTFLOW, AND TEMPERATE (26-27 C) SEA SURFACE TEMPERATURES (SST). THE 
INITIAL POSITION IS PLACED WITH LOW CONFIDENCE BASED ON THE LOW LEVEL 
CLOUD BANDING IN ANIMATED MSI. THE INITIAL INTENSITY OF 60 KTS IS 
ASSESSED WITH LOW CONFIDENCE BASED ON THE CONSISTENT WEAKENING TREND 
AND COLLAPSING DEEP CONVECTION.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: THE NORTHWESTERN FLANK OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE SOUTHEAST NEAR THE DATELINE

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T3.5 - 55 KTS
   RCTP: T3.5 - 55 KTS
   KNES: T4.0 - 65 KTS
   CIMSS ADT: 41 KTS AT 300030Z
   CIMSS AIDT: 41 KTS AT 300030Z
   CIMSS D-PRINT: 38 KTS AT 300030Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: WEAK EASTWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: LOW
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 20W WILL BEGIN TO TRACK NORTH-NORTHWESTWARD
IMMINENTLY, AS THE STEERING STR TO ITS EAST IS WEAKENED BY A PASSING 
UPPER-LEVEL TROUGH AND 20W CONTINUES TO INTERACT WITH TROPICAL STORM 
(TS) 21W TO ITS WEST. BETWEEN TAU 24-36, 20W IS EXPECTED TO CURVE TO 
THE NORTHEAST AS IT BEGINS SUBTROPICAL TRANSITION AND GETS PICKED UP 
BY THE UPPER-LEVEL TROUGH. SUBTROPICAL TRANSITION IS FORECAST TO BEGIN 
AROUND TAU 24 AND COMPLETE NO LATER THAN TAU 36. REGARDING INTENSITY, 
20W IS EXPECTED TO QUICKLY WEAKEN THROUGH TAU 12, FOLLOWED BY GRADUAL 
WEAKENING FROM TAU 12-36 AS IT BECOMES A SUBTROPICAL SYSTEM. THE 
ENVIRONMENT WILL GET INCREASINGLY HOSTILE AS 20W TRACKS POLEWARD, WITH 
VWS EXPECTED TO INCREASE FROM 25 KTS TO 55 KTS OVER THE COURSE OF THE
FORECAST PERIOD. CONSEQUENTLY, DRY AIR IS EXPECTED TO FURTHER ENTRAIN 
INTO THE CORE, ESPECIALLY AFTER TAU 24. 20W IS EXPECTED TO CROSS THE 
26C ISOTHERM BEFORE TAU 12, HASTENING WEAKENING THEREAFTER. 20W IS 
EXPECTED TO BE COMPLETELY EMBEDDED IN THE UPPER LEVEL JET BY ABOUT TAU 
36.

MODEL DISCUSSION: DETERMINISTIC MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT THROUGH THE FORECAST PERIOD, WITH THREE OUTLIERS --
NAVGEM, GALWEM, AND UKMET. DISREGARDING THOSE THREE, THE JTWC TRACK
FORECAST IS PLACED WITH HIGH CONFIDENCE. RELIABLE INTENSITY AIDS
ARE IN GENERAL AGREEMENT THROUGH TAU 36, WITH DISAGREEMENT IN RATE
OF WEAKENING. AS A RESULT, THE JTWC INTENSITY FORECAST IS HEDGED
CLOSE TO THE HAFS-A SOLUTION AND ASSESSED WITH OVERALL MEDIUM
CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   INTENSITY 00-72 HR: MEDIUM//
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