Tropical Storm NEOGURI Advisory Fri Oct 18

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE)
WARNING NR 004//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 21W (TWENTYONE), LOCATED APPROXIMATELY
555 NM EAST-NORTHEAST OF MANILA, PHILIPPINES, HAS TRACKED
NORTHWESTWARD AT 07 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (AMSI) SHOWS A SMALL CIRRUS SHIELD WITH
LIMITED SPIRAL RAIN BANDS WRAPPING INTO THE LOW LEVEL CIRCULATION
CENTER (LLCC). A 170458Z AMSR2 36 GHZ MICROWAVE IMAGE SHOWED SHALLOW
RAIN BANDS WRAPPING TOWARD A NOTCH FEATURE. THERE IS LOW CONFIDENCE
IN THE INITIAL POSITION BASED ON A COMBINATION OF THE AMSI AND
MICROWAVE IMAGE. THE INITIAL INTENSITY ALIGNS WITH DVORAK ESTIMATES
OF T1.5-T2.0 (25-30 KTS) WITH GOOD CONFIDENCE. MODERATE (15-20 KT)
VERTICAL WIND SHEAR (VWS), FAIR POLEWARD OUTFLOW, AND WARM (28-29
CELSIUS) SEA SURFACE TEMPERATURE (SST) MAKE THE ENVIRONMENT
MARGINALLY FAVORABLE. TD 21W IS TRACKING NORTHWESTWARD TOWARD A BREAK
IN A SUBTROPICAL RIDGE (STR) TO THE NORTH.
3. FORECAST REASONING.
   A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM
THE PREVIOUS PROGNOSTIC REASONING MESSAGE. HOWEVER, THE FORECAST HAS
BEEN EXTENDED TO TAU 96 AS THE TRACK SPEED IS SLOWER THAN THE
PREVIOUS FORECAST.
   B. BY TAU 12, THE STR IS EXPECTED TO BUILD AND THE TRACK IS
EXPECTED TO BECOME WESTWARD WHERE IT SHOULD REMAIN THROUGH TAU 72.
THE MARGINAL ENVIRONMENT WILL CONTINUE AND ALLOW FOR SLIGHT
INTENSIFICATION. BY TAU 72, INCREASING VWS SHOULD CAUSE THE SYSTEM TO
WEAKEN. UNCERTAINTY IN THE MODEL GUIDANCE IS QUITE LARGE, LENDING TO
LOW CONFIDENCE IN THIS PORTION OF THE JTWC TRACK FORECAST.
   C. BEYOND TAU 72, TD 21W SHOULD CONTINUE WESTWARD AND DISSIPATE
(TAU 96) AS THE STR TO THE NORTH IS EXPECTED TO BUILD AND THE
ENVIRONMENT IS EXPECTED TO BE UNFAVORABLE OVERALL. HOWEVER, THIS
FORECAST IS HIGHLY UNCERTAIN AS SEVERAL MODELS MAINTAIN THE WEAKNESS
IN THE STR AND ALLOW A POLEWARD TURN. EVEN IF THE POLEWARD TURN WERE
TO OCCUR, TD 21W SHOULD DISSIPATE AROUND TAU 96 DUE TO HIGH VWS.
BASED ON THE HIGH UNCERTAINTY IN THE MODEL GUIDANCE, THERE IS LOW
CONFIDENCE IN THE EXTENDED JTWC TRACK FORECAST.//
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