Tropical Storm TRAMI Advisory 9月26日

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 28W (TRAMI) WARNING NR 21//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 28W (TRAMI), LOCATED APPROXIMATELY 377 NM SOUTH OF
KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTHWARD AT 03 KNOTS
OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY
SHOWS THE CENTRAL CONVECTION HAS SIGNIFICANTLY WEAKENED AS EVIDENCED
BY RAPIDLY WARMING CLOUD TOPS. ADDITIONALLY, FEEDER BANDS HAVE BEGUN
TO UNRAVEL. HOWEVER, THE SYSTEM HAS MAINTAINED A WELL-DEFINED 32 NM
EYE WHERE THE INITIAL POSITION IS BASED ON WITH HIGH CONFIDENCE. THE
INITIAL INTENSITY OF 115 KNOTS IS BASED ON EQUIVALENT PGTW AND RJTD
DVORAK CURRENT INTENSITY ESTIMATES OF T6.0/115 KNOTS. THE UPPER
LEVEL ENVIRONMENT REMAINS GENERALLY FAVORABLE WITH LOW VERTICAL WIND
SHEAR (VWS) AND GOOD RADIAL OUTFLOW. HOWEVER, DRY AIR ASSOCIATED
WITH AN APPROACHING SHORT WAVE TROUGH OVER JAPAN IS NOW IMPACTING THE
NORTHWESTERN SIDE OF THE CYCLONE. TY 28W IS DRIFTING SLOWLY NORTHWARD
ALONG THE WESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) THAT RECEDED
TO THE EAST AND WEAKENED BY THE SHORTWAVE TROUGH.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TY 28W IS FORECAST TO CONTINUE ITS SLOW NORTHWARD DRIFT OR MAY
BECOME QUASI-STATIONARY DUE TO THE WEAK STEERING ENVIRONMENT. AFTER
TAU 24, THE STR WILL RE-BUILD AND DRIVE THE CYCLONE NORTHWESTWARD
THROUGH TAU 72. THE WEAKENING TREND - DUE TO THE SUBSIDING EFFECT OF
THE SHORT WAVE TROUGH - WILL CONTINUE IN THE NEAR TERM AND REDUCE THE
SYSTEM TO 95 KNOTS. HOWEVER, AFTER TAU 48, AS THE CYCLONE MOVES AWAY
FROM ITS COLD WAKE, THE OCEAN HEAT CONTENT IS EXPECTED TO INCREASE;
PLUS, POLEWARD OUTFLOW WILL SURGE, FUELING A SECONDARY
INTENSIFICATION PHASE. THE NUMERICAL MODEL GUIDANCE HAS COME INTO
BETTER AGREEMENT WITH JGSM AS THE SOLE OUTLIER TO THE LEFT OF THE
ENVELOPE. HOWEVER, THERE REMAINS SIGNIFICANT DIFFERENCES IN THE TRACK
SPEED AND TIMING OF THE NORTHWESTWARD TRACK. IN VIEW OF THESE, THERE
IS LOW CONFIDENCE IN THIS PORTION OF THE JTWC TRACK FORECAST.
   C. AFTER TAU 72, TY TRAMI IS FORECAST TO RECURVE AND ACCELERATE
NORTHEASTWARD AS IT ROUNDS THE STR AXIS AND BEGINS INTERACTING WITH
THE MID-LATITUDE WESTERLIES. BY TAU 120, TY 28W WILL REACH THE
SOUTHERN TIP OF KYUSHU, JAPAN. THE SECONDARY INTENSIFICATION PHASE
WILL CONTINUE AND BRING THE SYSTEM TO 110 KNOTS BY TAU 96. AFTERWARD,
INCREASING VERTICAL WIND SHEAR AND INTERACTION WITH THE JAPANESE
ISLANDS WILL GRADUALLY ERODE THE SYSTEM. BY TAU 120, AT 95 KNOTS, THE
CYCLONE WILL ALSO BEGIN EXTRA-TROPICAL TRANSITION. THE NUMERICAL
MODEL ENVELOPE IS AT ITS NARROWEST IN THE EXTENDED PORTION OF THE
FORECAST; HOWEVER, GIVEN THE INITIAL TRACK UNCERTAINTY, CONFIDENCE IN
THE JTWC LATTER TAU TRACK FORECAST REMAINS LOW.//
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