Tropical Storm MALAKAS Advisory Mon Sep 12

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN)
WARNING NR 04//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 18W (EIGHTEEN), LOCATED APPROXIMATELY 234 NM NORTH
OF FAIS, HAS TRACKED WEST-NORTHWESTWARD AT 13 KNOTS OVER THE PAST
SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY AND A 121158Z
MHS 89GHZ MICROWAVE IMAGE SHOWS FLARING CONVECTION NEAR THE LOW-
LEVEL CIRCULATION CENTER (LLCC), WITH THE DEEPEST CONVECTIVE CELL TO
THE SOUTHEAST. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE.
THE CURRENT INTENSITY REMAINS ASSESSED AT 40 KNOTS, USING A BLEND OF
DVORAK ESTIMATES RANGING FROM T1.5 (RJTD) TO T3.0 (PGTW). TS 18W
CONTINUES TO FEEL THE EFFECTS OF THE OUTFLOW OF STY 16W TO ITS
NORTHWEST AND IS SUFFERING THE EFFECTS OF MODERATE VERTICAL WIND
SHEAR (20 TO 25 KNOTS). THUS, THE UPPER-LEVEL OUTFLOW REMAINS
RESTRICTED IN THE NORTHWEST QUADRANT BUT IS GOOD ELSEWHERE. TS 18W
IS TRACKING ALONG THE SOUTHERN PERIPHERY OF THE DEEP-LAYERED
SUBTROPICAL RIDGE (STR) TO THE NORTH.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TS 18W IS FORECAST TO TRACK WEST-NORTHWESTWARD THEN
NORTHWESTWARD THROUGH TAU 72. THE STR IS FORECAST TO GRADUALLY ERODE
AS A SHORTWAVE TROUGH APPROACHES FROM THE WEST. DYNAMIC MODEL
GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 72, SO THERE IS FAIR
CONFIDENCE IN THIS PORTION OF THE FORECAST TRACK. AS STY 16W MOVES
FURTHER AWAY FROM TS 18W, THE UPPER-LEVEL ENVIRONMENT IS FORECAST TO
IMPROVE, WITH THE VERTICAL WIND SHEAR REDUCING AND A RIDGE BUILDING
OVER THE SYSTEM, WHICH WILL ALLOW FOR STEADY INTENSIFICATION TO
OCCUR, PARTICULARLY BEYOND TAU 24.
   C. AFTER TAU 72, TS 18W IS FORECAST TO TRACK POLEWARD TOWARD
THE VICINITY OF OKINAWA ALONG THE WESTERN PERIPHERY OF THE STR AND
INTO A WEAKNESS IN THE STR AXIS INDUCED BY A SHORTWAVE TROUGH.
HOWEVER, DYNAMIC MODEL GUIDANCE DIVERGES SIGNIFICANTLY AT THESE
EXTENDED TAUS, WITH DIFFERENCES IN THE TIMING AND DEGREE OF
CURVATURE TOWARD THE NORTH AND NORTHEAST. TS 18W SHOULD REACH PEAK
INTENSITY AROUND TAU 96, DURING THE POINT OF RECURVATURE. WEAKENING
IS FORECAST TO COMMENCE SHORTLY THEREAFTER DUE TO INTERACTION WITH
THE SHORTWAVE TROUGH. THERE IS LOW CONFIDENCE IN THE JTWC TRACK
FORECAST BEYOND TAU 72.//
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