Tropical Storm MALAKAS Advisory Tue Sep 13

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN)
WARNING NR 03//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 18W (EIGHTEEN), LOCATED APPROXIMATELY 170 NM WEST
OF NAVSTA GUAM, HAS TRACKED WESTWARD AT 15 KNOTS OVER THE PAST SIX
HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY AND A 120549Z MHS
89GHZ MICROWAVE IMAGE SHOWS THE SYSTEM BECOMING BETTER ORGANIZED
WITH DEEP CONVECTION WRAPPING INTO THE LOW-LEVEL CIRCULATION CENTER
(LLCC) WITH IMPROVED BANDING. THE INITIAL POSITION IS PLACED WITH
FAIR CONFIDENCE. THE CURRENT INTENSITY IS ASSESSED AT 40 KNOTS,
GIVEN DVORAK ESTIMATES RANGING FROM T1.5 (RJTD) TO T3.0 (PGTW). TS
18W IS IN A REGION OF MODERATE NORTHWESTERLY VERTICAL WIND SHEAR (20
TO 25 KNOTS) ASSOCIATED WITH THE EASTERN PORTION OF THE UPPER-
TROPOSPHERIC OUTFLOW EMANATING FROM STY 16W TO THE NORTHWEST. UPPER-
LEVEL OUTFLOW IS SUBSEQUENTLY RESTRICTED IN THE NORTHWEST QUADRANT
OF 18W, BUT GOOD ELSEWHERE, WITH A ROBUST OUTFLOW CHANNEL TO THE
EAST. 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 CHINA. DYNAMIC MODEL GUIDANCE
IS IN GOOD AGREEMENT THROUGH TAU 72, SO THERE IS FAIR CONFIDENCE IN
THIS PORTION OF THE FORECAST TRACK. ATMOSPHERIC CONDITIONS ARE
FORECAST TO IMPROVE IN THE COMING DAYS, AS STY 16W MOVES FURTHER
AWAY TO THE WEST, ALLOWING FOR AN IMPROVEMENT IN THE POLEWARD
OUTFLOW. A PERIOD OF RAPID INTENSIFICATION IS POSSIBLE FROM TAU 24
TO 48, IF THE TUTT CELL TO THE NORTH OF TS 18W BECOMES ORIENTED TO
ALLOW FOR A POLEWARD OUTFLOW CHANNEL.
   C. AFTER TAU 72, TD 18W IS FORECAST TO TRACK POLEWARD TOWARD
THE VICINITY OF OKINAWA ALONG THE WESTERN PERIPHERY OF THE STR.
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 72 TO 96, WHEN INTERACTION WITH THE SHORTWAVE
TROUGH IS FORECAST TO OCCUR. THERE IS LOW CONFIDENCE IN THE JTWC
TRACK FORECAST BEYOND TAU 72.//
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