Tropical Storm PHANFONE Advisory Tue Sep 30

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (PHANFONE) WARNING
NR 03//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (PHANFONE), LOCATED APPROXIMATELY 325 NM
EAST-SOUTHEAST OF SAIPAN, HAS TRACKED WEST-NORTHWESTWARD AT 12 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) DEPICTS A SMALL CENTRAL DENSE OVERCAST (CDO) FEATURE WITH
BROAD CONVECTIVE BANDING WRAPPING AROUND THE LOW-LEVEL CIRCULATION
CENTER (LLCC). THERE IS GOOD CONFIDENCE IN THE INITIAL POSITION,
WHICH IS LOCATED UNDER THE NORTHERN EDGE OF THIS CDO FEATURE BASED
ON THE EXPOSED LOW-LEVEL CLOUD LINES WRAPPING INTO THE CENTER. A
290355Z NOAA-19 89GHZ IMAGE DEPICTS FRAGMENTED DEEP CONVECTIVE
BANDING WRAPPING BROADLY INTO THE LLCC WITH THE BULK OF THE CORE
CONVECTION DISPLACED OVER THE SOUTHERN SEMI-CIRCLE OF THE LLCC DUE
TO WEAK NORTHEASTERLY VERTICAL WIND SHEAR. CONSEQUENTLY, THE DVORAK
INTENSITY ESTIMATES HAVE REMAINED FLAT AT 35 KNOTS AND THE INITIAL
INTENSITY REMAINS ASSESSED AT 35 KNOTS. OVERALL, UPPER-LEVEL
ANALYSIS INDICATES A FAVORABLE ENVIRONMENT WITH VIGOROUS POLEWARD
OUTFLOW INTO A TUTT CELL POSITIONED NEAR 20N 165E. TS 18W IS
CURRENTLY TRACKING ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL
RIDGE (STR).
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE, HOWEVER, THE PEAK INTENSITY HAS BEEN
INCREASED IN THE EXTENDED TAUS DUE TO HIGHLY FAVORABLE ENVIRONMENTAL
CONDITIONS AS THE SYSTEM TRACKS WEST OF THE NORTHERN MARIANAS.
   B. TS 18W IS FORECAST TO TRACK WEST-NORTHWESTWARD ALONG THE
SOUTHERN AND SOUTHWESTERN PERIPHERY OF THE STR THROUGH TAU 72.
DYNAMIC MODEL GUIDANCE IS IN EXCEPTIONALLY TIGHT AGREEMENT THROUGH
THIS PHASE WITH A 60NM SPREAD IN SOLUTIONS AT TAU 36 (OVER THE
NORTHERN MARIANAS) AND A 130 NM SPREAD IN SOLUTIONS AT TAU 72.
ENSEMBLE GUIDANCE ALSO SHOWS A TIGHT GROUPING OF SOLUTIONS AND
SUPPORTS HIGH CONFIDENCE. TS 18W IS FORECAST TO INTENSIFY SLOWLY
THROUGH TAU 24 DUE TO THE NORTHEASTERLY PRESSURE ON THE SYSTEM BUT
SHOULD RAPIDLY INTENSIFY AFTER PASSING OVER THE NORTHERN MARIANAS.
THE SYSTEM IS FORECAST TO PEAK AT 105 KNOTS BY TAU 72.
   C. IN THE EXTENDED PERIOD, MODEL UNCERTAINTY INCREASES DUE TO A
MORE COMPLEX STEERING ENVIRONMENT CREATED BY AN APPROACHING
MIDLATITUDE SHORTWAVE TROUGH. DYNAMIC MODELS AND ENSEMBLE SOLUTIONS
ARE SPLITTING INTO TWO DISTINCT CLUSTERS. THE FIRST, FAVORING A WEST-
NORTHWESTWARD TRACK THROUGH TAU 120. THE SECOND CLUSTER INDICATES A
SHARPER RE-CURVE SCENARIO NORTHWESTWARD TO NORTHWARD BY TAU 120. THE
JTWC FORECAST FAVORS THE RE-CURVE SCENARIO DUE TO THE EVOLUTION OF
THE DEEP, BROAD MIDLATITUDE TROUGHING EXPECTED TO DEEPEN OVER
EASTERN ASIA INTO WESTERN JAPAN BY TAU 96. TS 18W IS FORECAST TO
CONTINUE INTENSIFYING DUE TO ENHANCED POLEWARD OUTFLOW INTO THE
MIDLATITUDE WESTERLIES. PEAK INTENSITY HAS BEEN INCREASED TO 125
KNOTS AT TAU 120 WITH RAPID INTENSIFICATION A DISTINCT POSSIBILITY.
DUE TO SIGNIFICANT DIFFERENCES IN THE MODELS' DEPICTION OF THE
MIDLATITUDE TROUGH AND STR AS WELL AS THE LARGE MODEL SPREAD OF
320NM AT TAU 120, THERE IS OVERALL LOW CONFIDENCE IN THE JTWC
FORECAST TRACK.//
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