Tropical Storm NANMADOL Advisory Do, 15.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 16W (NANMADOL) WARNING
NR 010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 23.3N 138.8E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 164 NM WEST-SOUTHWEST OF IWO TO
   MOVEMENT PAST 6 HOURS: WESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A LARGE
ENVELOPE OF DEEP CONVECTION WRAPPING AROUND THE LOW-LEVEL
CIRCULATION CENTER (LLCC). A 142129Z SSMIS 91GHZ MICROWAVE IMAGE
REVEALS WELL-DEFINED FEEDER BANDS WRAPPING AROUND THE SOUTHERN AND
EASTERN PERIPHERY OF THE CIRCULATION, WITH THE DEEPEST INNER CORE
CONVECTION WRAPPING AROUND THE NORTHERN SEMICIRCLE. THE INITIAL
POSITION IS PLACED WITH MEDIUM CONFIDENCE USING MSI AND FORWARD
EXTRAPOLATION OF THE LLCC IN THE SSMIS PASS. THE INITIAL INTENSITY
IS ASSESSED AT 55 KNOTS WITH MEDIUM CONFIDENCE BASED ON AGENCY
DVORAK ESTIMATES AND A SATCON OF 54 KNOTS.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE NORTH
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T4.0 - 65 KTS
   RJTD: T3.5 - 55 KTS
   KNES: T4.0 - 65 KTS
   CIMSS SATCON: 54 KTS AT 142130Z
   CIMSS ADT: 63 KTS AT 142340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: INCREASE IN PEAK INTENSITY FORECAST
TO 115 KNOTS, SUGGESTING THE LIKELIHOOD OF RAPID INTENSIFICATION.
FORECAST DISCUSSION: TS 16W (NANMADOL) IS FORECAST TO TRACK
WEST-NORTHWESTWARD ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL
RIDGE (STR). THE CURRENT ENVIRONMENT IS FAVORABLE FOR CONTINUED
INTENSIFICATION, WITH LOW (0-5 KTS) VERTICAL WIND SHEAR (VWS), WARM
(29-30C) SEA SURFACE TEMPERATURES (SSTS), AND MODERATE OUTFLOW IN
ALL QUADRANTS EXCEPT FOR SOME RESTRICTION TO THE WEST. SEA SURFACE
TEMPERATURES ARE FORECAST TO WARM ALONG THE FORECAST TRACK, WITH AN
OVERALL IMPROVEMENT IN OUTFLOW, SO RAPID INTENSIFICATION IS
POSSIBLE OVER THE NEXT TWO DAYS. A PEAK INTENSITY OF 115 KNOTS IS
FORECAST TO BE REACHED BY TAU 48, BUT THAT MAY STILL BE A BIT
CONSERVATIVE DEPENDING ON WHEN THE RAPID INTENSIFICATION PHASE
BEGINS. AS THE SYSTEM APPROACHES THE AXIS OF THE STR, FORWARD
MOTION IS FORECAST TO SLOW, WITH THE POSSIBILITY OF A PERIOD OF
QUASISTATIONARY MOTION BEFORE RECURVING TOWARD THE NORTHEAST OVER
KYUSHU AND HONSHU. BEYOND TAU 72, STEADY WEAKENING IS FORECAST DUE
TO LAND INTERACTION AND AN INCREASE IN VERTICAL WIND SHEAR AS 16W
COMPLETES THE RECURVE AND ACCELERATES TO THE NORTHEAST INTO THE
MID-LATITUDE WESTERLIES. THERE IS SIGNIFICANT UNCERTAINTY WITH
RESPECT TO THE TIMING AND SPEED OF THE RECURVE, WITH THE SHIFT IN
STEERING FLOW POSSIBLY UNFOLDING GRADUALLY, SO THE TRACK FORECAST
CONFIDENCE IS MEDIUM THROUGH TAU 72 AND LOW BEYOND. THE INTENSITY
FORECAST CONFIDENCE IS MEDIUM THROUGH TAU 72 AND LOW BEYOND, GIVEN
THE UNCERTAINTY WITH THE START OF THE POTENTIAL RAPID
INTENSIFICATION PHASE AND THE TRACK UNCERTAINTY BEYOND TAU 72.
MODEL DISCUSSION: NUMERICAL MODELS ARE IN TIGHT AGREEMENT THROUGH
TAU 48, WITH ONLY A 60 NM SPREAD IN SOLUTIONS AMONG CONSENSUS
TRACKERS. THEREAFTER, ALTHOUGH MOST MODELS SUGGEST A RECURVE
SCENARIO OVER MAINLAND JAPAN, AN UNLIKELY BUT NOTABLE ALTERNATE
SCENARIO HAS EMERGED. THE GFS SUGGESTS A MUCH SLOWER TURN WITH SOME
QUASISTATIONARY MOTION BETWEEN TAU 48 AND 72, LIKELY DUE TO
ENCOUNTERING A SURGE OF OFFSHORE WINDS ACROSS THE KOREAN PENINSULA,
WHICH MAY TEMPORARILY MODULATE THE DEEP-LAYER STEERING FLOW. THE
NAVGEM PAINTS AN EVEN MORE EXTREME SCENARIO, WITH A
COUNTERCLOCKWISE LOOP JUST NORTH OF OKINAWA. THE VAST MAJORITY OF
GFS AND ECMWF ENSEMBLE MEMBERS ARE IN LINE WITH THE CONSENSUS
TRACKERS IN A STEADY RECURVE SCENARIO, BUT THERE ARE A FEW MEMBERS
THAT HINT AT THE OUTLIER NAVGEM SCENARIO. AT THIS TIME, THE
OFFICIAL JTWC FORECAST FOLLOWS THE CONSENSUS WITH THE RECURVE OVER
KYUSHU AND HONSHU, BUT WITH A SLOWER TURN BETWEEN TAU 48 AND 72 TO
ACCOUNT FOR THE UNCERTAINTY ASSOCIATED WITH THE DEEP-LAYER STEERING
FLOW AT THAT TIME.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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