WDPN32 PGTW 040300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 13W (THIRTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.7N 147.0E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 353 NM EAST-NORTHEAST OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
AFTER A COUPLE OF DAYS OF DIURNAL CONVECTIVE CYCLING, THE WIND
FIELD ASSOCIATED WITH TROPICAL DEPRESSION (TD) 13W HAS FINALLY
REACHED THE THRESHOLD TO COMMENCE WARNINGS. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI) CONTINUES TO DEPICT A RATHER SLOPPY SYSTEM,
WITH DISORGANIZED CONVECTION FLARING ALONG CONVERGENCE BANDS TO THE
EAST AND NORTH OF THE LOW LEVEL CIRCULATION CENTER (LLCC), AND
STRUGGLING TO PERSIST FOR MORE THAN A FEW HOURS. HOWEVER, A 032317Z
ASCAT-B PASS REVEALED A 25-30 KNOT WIND FIELD TO THE EAST, WHICH
HAS EXPANDED SINCE 24 HOURS AGO AND BEGUN TO WRAP AROUND THE
NORTHERN SIDE OF THE LLCC. ANIMATED WATER VAPOR IMAGERY REVEALS A
LARGE UPPER-LEVEL LOW OR TUTT-CELL POSITIONED JUST TO THE SOUTHWEST
OF TD 13W, WHICH IS PROVIDING GOOD POLEWARD OUTFLOW FOR THE SYSTEM.
THE INITIAL POSITION WAS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
SCATTEROMETER PASS NOTED ABOVE. THE INITIAL INTENSITY WAS ASSESSED
WITH HIGH CONFIDENCE, WITH THE SCATTEROMETER PASS PROVIDING
EVIDENCE TO HEDGE ABOVE THE AGENCY DVORAK CURRENT INTENSITY
ESTIMATES NOTED BELOW. THE SYSTEM LIES IN A MARGINALLY FAVORABLE
ENVIRONMENT, WITH WARM SSTS AND GOOD POLEWARD OUTFLOW OFFSET BY
SOME MODERATE SOUTHERLY SHEAR AND SIGNIFICANT DRY AIR ENSCONSED IN
THE WESTERN HALF OF THE SYSTEM.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: THE SYSTEM IS TRACKING
NORTH-NORTHWESTWARD ALONG THE SOUTHWESTERN PERIPHERY OF A
DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.0 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: SIGNIFICANT MID-LEVEL DRY AIR IN THE WESTERN HALF
OF THE SYSTEM. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TD 13W WILL TRACK GENERALLY NORTHWARD FOR THE
FIRST 24 HOURS OF THE FORECAST, REACH THE RIDGE AXIS SHORTLY AFTER
TAU 24, THEN TURN NORTHEAST AHEAD OF AN APPROACHING MID-LEVEL
TROUGH. THE SYSTEM WILL PICK UP SPEED AFTER TAU 24 AS IT MOVES
UNDER THE INCREASINGLY TIGHT GRADIENT BETWEEN THE RETREATING STR TO
THE EAST AND THE DEEP TROUGH MOVING IN QUICKLY FROM THE WEST. TD
13W WILL INTENSIFY SLIGHTLY IN THE SHORT-TERM, UNDER OVERALL
FAVORABLE ENVIRONMENTAL CONDITIONS, THOUGH IT WILL CONTINUE TO
STRUGGLE WITH DRY AIR ENTRAINMENT. AFTER PEAKING AT 35 KNOTS, THE
SYSTEM WILL BEGIN TO WEAKEN BY TAU 36 AS SHEAR SHARPLY INCREASES
AND THE SYSTEM STARTS TO MOVE OVER COOLER WATERS. EXTRATROPICAL
TRANSITION (ETT) IS EXPECTED TO START AROUND TAU 48 AS THE SYSTEM
MOVES NORTH OF THE 26C ISOTHERM, STARTS TO EMBED IN THE LOW-LEVEL
BAROCLINIC ZONE AND BECOMES IN PHASE WITH BOTH A 500MB SHORTWAVE
TROUGH AND 200MB JET MAX. ETT IS FORECAST TO BE COMPLETE NO LATER
THAN TAU 72 AS THE SYSTEM RACES OFF INTO THE NORTH PACIFIC. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN STRONG
AGREEMENT, WITH MINIMAL CROSS-TRACK SPREAD THROUGH THE DURATION OF
THE FORECAST PERIOD. AS EXPECTED WITH SYSTEMS UNDERGOING ETT,
SIGNIFICANT ALONG-TRACK SPREAD IS PRESENT AFTER THE SYSTEM ROUNDS
THE RIDGE AXIS, REACHING OVER 600NM BY THE END OF THE FORECAST. THE
JTWC FORECAST TRACK LIES WEST AND NORTH OF THE CONSENSUS MEAN
THROUGH THE FORECAST AND IS MUCH FASTER THAN THE MEAN AFTER TAU 36.
INTENSITY GUIDANCE IS IN GENERALLY GOOD AGREEMENT, THOUGH THE CTCX
AND HWRF DROP THE VORTEX AFTER 48 HOURS. THE REMAINDER OF THE
GUIDANCE BRIEFLY INTENSIFIES THE SYSTEM UP TO 35-40 KNOTS FOLLOWED
BY A VERY GRADUAL DECLINE OR EVEN SLIGHT REINTENSIFICATION AS THE
SYSTEM UNDERGOES ETT. THE JTWC FORECAST TRACKS CLOSE TO THE
CONSENSUS MEAN FOR THE DURATION OF THE FORECAST PERIOD. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   INTENSITY 0 - 72 HR: MEDIUM//
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