Tropical Storm SAUDEL Advisory Tue Oct 20

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 19W
(NINETEEN) WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 19W (NINETEEN), LOCATED
APPROXIMATELY 427 NM EAST OF MANILA, PHILIPPINES, HAS
TRACKED WEST-NORTHWESTWARD AT 13 KNOTS OVER THE PAST
SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) DEPICTS A BROAD SYSTEM, WITH WELL DEFINED LOW
LEVEL SPIRAL BANDS OF DEEP CONVECTION LYING OUTSIDE OF
A MOAT OF LITTLE TO NO CONVECTION SURROUNDING AN
OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC). THE
INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE BASED
ON ANALYSIS OF THE LOW LEVEL BANDS, WITH SUPPORT FROM
A 190456Z AMSR2 36GHZ AND 89GHZ COMPOSITE MICROWAVE
IMAGE. THE INITIAL INTENSITY OF 30 KNOTS IS ASSESSED
WITH HIGH CONFIDENCE BASED ON MULTI-AGENCY DVORAK
CURRENT INTENSITY ESTIMATES OF T2.0 (30 KNOTS) FROM
PGTW AND KNES) AND T1.5 (25 KNOTS) FROM RJTD, AS WELL
AS AN ADVANCED DVORAK TECHNIQUE ESTIMATE OF T1.8 (28
KNOTS), AND A SATELLITE CONSENSUS ESTIMATE OF 38 KNOTS.
TD 19W IS TRACKING ALONG THE SOUTHERN PERIPHERY OF A
SUBTROPICAL RIDGE EXTENDING SOUTHWESTWARD FROM A CENTER
NEAR 30N 155E TO FAR NORTHERN VIETNAM. THE SYSTEM IS
TRACKING THROUGH A GENERALLY FAVORABLE ENVIRONMENT WITH
WARM SSTS (29-30 DEG CELSIUS), LOW VWS (10-15 KNOTS) AND
MODERATE UPPER-LEVEL OUTFLOW ON THE DIVERGENT SOUTHWEST
QUADRANT OF AN UPPER LEVEL ANTICYCLONE.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TD 19W WILL CONTINUE TO TRACK GENERALLY WEST-
NORTHWESTWARD ALONG THE PERIPHERY OF THE STEERING STR
OVER THE NEXT 36 HOURS AS IT APPROACHES AND THEN CROSSES
CENTRAL LUZON. AFTER REEMERGING OVER WATER JUST AFTER TAU
36, THE SYSTEM IS FORECAST TO TAKE ON A SLIGHTLY MORE
NORTHWESTERLY COURSE FOR SHORT PERIOD TOWARDS A WEAKNESS
IN THE STEERING RIDGE, BEFORE AGAIN RESUMING A WESTWARD
TRACK BY TAU 96. TD 19W IS FORECAST TO STEADILY INTENSIFY
TO 45 KNOTS BEFORE MAKING LANDFALL ON THE CENTRAL LUZON
COASTLINE. THE SYSTEM WILL MAINTAIN INTENSITY AT 45 KNOTS,
OR EVEN SLIGHTLY WEAKEN BEFORE RESUMING INTENSIFICATION
AFTER TAU 48 OVER THE OPEN WATERS OF THE SOUTH CHINA SEA.
TD 19W IS FORECAST TO REACH A PEAK INTENSITY OF 60 KNOTS
BY TAU 72 AS MARGINAL UPPER LEVEL OUTFLOW IS OFFSET BY LOW
TO MODERATE (15-20 KNOTS) VWS AND WARM SSTS (29 DEG CELSIUS).
NUMERICAL TRACK GUIDANCE IS IN MODERATE AGREEMENT WITH 115
NM SPREAD AT TAU 12 INCREASING TO 255 NM BY TAU 72, WITH
NAVGEM REPRESENTING THE NORTHERN OUTLIER AND THE ECMWF
ENSEMBLE TRACKER BEING THE SOUTHERN OUTLIER. THE JTWC TRACK
FORECAST LIES ALONG THE SOUTHERN SIDE OF THE GUIDANCE
ENVELOPE THROUGH TAU 48. DUE TO THE INCREASING SPREAD
IN THE GUIDANCE ENVELOPE, THERE IS OVERALL LOW CONFIDENCE
IN THIS PORTION OF THE JTWC FORECAST TRACK. ADDITIONALLY,
THERE EXISTS AN ALTERNATE SCENARIO, IN WHICH THE WEAKNESS
IN THE STEERING RIDGE IS STRONGER AND LAST LONGER THAN
CURRENTLY FORECAST, LEADING TO A MORE NORTHWARD AND SLOWER
JOB IN THE TRACK BETWEEN TAUS 36 AND 72. THE NAVGEM AND
UKMET ENSEMBLE GENERALLY INDICATE THIS SCENARIO, SHOWING
A SHORT PERIOD OF MORE NORTHWARD MOVEMENT BEFORE RESUMING
A WESTWARD TRACK BY TAU 96. THE HWRF TRACKER REPRESENTS
THE OUTLIER IN THE FORECAST TRACK, TAKING THE SYSTEM
COMPLETELY INTO THE WEAKNESS IN THE RIDGE TOWARDS HONG KONG
THROUGH TAU 120. AT THIS TIME HOWEVER, THE WEAKNESS IN THE
RIDGE DOES NOT APPEAR TO BE STRONG OR LONG LASTING ENOUGH
TO LEAD TO A NORTHWARD TRACK FORECAST.
   C. IN THE EXTENDED FORECAST, TD 19W IS EXPECTED TO TRACK
WESTWARD AND THEN WEST-SOUTHWESTWARD AS THE PRIMARY STEERING
MECHANISM SHIFTS TO A DEVELOPING STR LOCATED THE NORTHWEST
OF THE SYSTEM. AS THE SYSTEM MOVES WEST AND COMES UNDER THE
INFLUENCE OF THE STR TO THE NORTHWEST, UPPER LEVEL OUTFLOW
BECOMES CONVERGENT OVER TOP OF THE SYSTEM WHILE VWS WILL
BEGIN TO INCREASE TO 20-30 KNOTS, OFFSETTING THE CONTINUED
WARM SSTS, AND LEADING TO A STEADY WEAKENING TREND. NUMERICAL
MODELS GENERALLY AGREE ON THE WESTWARD TRACK BUT WITH ACROSS
TRACK SPREAD INCREASING TO NEARLY 300 NM AT TAU 120, WITH
NAVGEM TAKING THE SYSTEM OVER HAINAN, WHILE GFS TAKES THE
SYSTEM INLAND OVER CENTRAL VIETNAM. AS DISCUSSED PREVIOUSLY,
THE HWRF MODEL IS THE NOTABLE OUTLIER, TAKING THE SYSTEM
NORTHWARD TOWARDS HONG KONG BY TAU 120 BUT THIS REPRESENTS
THE ALTERNATE SCENARIO AND ALL THE REMAINDER OF THE MODEL
GUIDANCE SHOWS A GENERALLY WESTWARD TRACK THROUGH TAU 120.
DUE TO POSSIBILITY OF THE ALTERNATE SCENARIO AND THE
INCREASED MODEL SPREAD EVEN WHEN DISCOUNTING THE HWRF
SOLUTION, THERE REMAINS LOW CONFIDENCE IN THIS PORTION
OF THE JTWC FORECAST TRACK.//
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