Tropical Storm TRAMI Advisory Fri Sep 21

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 28W (TWENTYEIGHT)
WARNING NR 02//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 28W (TWENTYEIGHT), LOCATED APPROXIMATELY
75 NM NORTHWEST OF ANDERSEN AFB, HAS TRACKED NORTHWESTWARD AT 13
KNOTS OVER THE PAST SIX HOURS. THE INITIAL POSITION IS PLACED WITH
FAIR CONFIDENCE BASED ON ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) WHICH DEPICTS A CONSOLIDATING CYCLONE WITH DEEP CONVECTION
OVER THE LOW LEVEL CIRCULATION CENTER (LLCC) AND BANDING FROM ALL
QUADRANTS, PARTICULARLY TO THE EAST, WRAPPING IN TO THE LLCC. THE
INITIAL INTENSITY IS CONSISTENT WITH THE PGTW DVORAK CURRENT
INTENSITY ESTIMATE OF T2.0 (30 KNOTS). LOW (5-15 KTS) VERTICAL WIND
SHEAR (VWS), EXCELLENT EQUATORWARD AND POLEWARD OUTFLOW, AND WARM
SEA SURFACE TEMPERATURES (28-29 CELSIUS) MAKE FOR A PRIME
ENVIRONMENT FOR INTENSIFICATION. TD 28W IS CURRENTLY TRACKING ALONG
THE PERIPHERY OF A SOUTHERN EXTENSION OF AN OTHERWISE EAST-WEST
ORIENTED SUBTROPICAL RIDGE (STR) TO THE NORTH.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY FROM THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TD 28W IS FORECASTED TO BEGIN A GRADUAL TURN TOWARD A WESTWARD
TRACK AS IT MOVES AWAY FROM THE SOUTHERN EXTENSION OF THE STR. AN
APPROACHING MID-LATITUDE TROUGH IS EXPECTED TO CREATE A WEAKNESS IN
THE STR AND, BY TAU 72, TD 28W IS EXPECTED TO BEGIN A POLEWARD TURN
AS IT ROUNDS THE STR AXIS. FOR THIS PERIOD, THE FORECAST INTENSITY
CLOSELY FOLLOWS THE HIGH END OF THE INTENSITY GUIDANCE, WHICH HAS
COME INTO BETTER AGREEMENT THIS CYCLE. THE JTWC FORECAST TRACK
FOLLOWS THE CONSENSUS TRACK WITH FAIR CONFIDENCE BASED ON 140 NM OF
SPREAD AMONG THE MODELS AT TAU 72.
   C. AFTER TAU 72, TD 28W IS EXPECTED TO CONTINUE ROUNDING THE STR
AXIS. AS IS OFTEN THE CASE, UNCERTAINTY IN THE MODEL GUIDANCE
INCREASES AS DIFFERENCES IN HOW THE MEMBERS RESOLVE THE INTERACTION
BETWEEN THE STR AND THE MID-LATITUDE TROUGH TRANSLATE INTO
DIFFERENCES IN DIRECTION OF MOTION AND ALONG TRACK SPEED. THE
INTENSITY IS FORECASTED TO CONTINUE TO GRADUALLY INCREASE, DESPITE A
DOWNWARD TREND IN THE MODEL GUIDANCE, BECAUSE THE ORIENTATION OF THE
MID-LATITUDE TROUGH IS EXPECTED TO AID POLEWARD OUTFLOW BEFORE
INCREASING VWS SHOULD CAUSE WEAKENING. BY TAU 120, SPREAD IN THE
MODEL TRACK GUIDANCE BECOMES QUITE LARGE, THEREFORE CONFIDENCE IS
LOW IN THE EXTENDED FORECAST TRACK.//
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