WDPN31 PGTW 180300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 23W (TWENTYTHREE)
WARNING NR 02//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
TROPICAL DEPRESSION 23W (TWENTYTHREE), LOCATED APPROXIMATELY 525
NM EAST OF SAIPAN, HAS TRACKED WEST-NORTHWESTWARD AT 15 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
DEPICTS PERSISTENT BUT STRUGGLING CONVECTION THAT IS BEING SHEARED
AWAY FROM A BROAD LOW LEVEL CIRCULATION CENTER. THE CENTER POSITION
IS FIXED WITH LOW CONFIDENCE BASED ON THE MULTISPECTRAL SATELLITE
LOOP AND A 172321Z AMSU 89GHZ MICROWAVE IMAGE, WHICH SHOWS LOW LEVEL
BANDING WRAPPING INTO DISORGANIZED DEEP CONVECTION. THE CURRENT
INTENSITY IS ASSESSED AT 30 KNOTS, SLIGHTLY BELOW THE PGTW DVORAK
CURRENT INTENSITY ESTIMATE OF T2.5 (35 KTS), IN AGREEMENT WITH THE
KNES ESTIMATES OF T2.0 (30 KTS), AND SLIGHTLY ABOVE THE RJTD
ESTIMATE OF T1.5 (25 KTS). UPPER-LEVEL ANALYSIS REVEALS A MARGINAL
ENVIRONMENT WITH NORTHERLY OUTFLOW FROM TY 22W TO THE NORTHWEST
IMPINGING ON CONVECTION, LEADING TO INCREASED VERTICAL WIND SHEAR,
NOW 25-30 KTS, WHICH IS DISRUPTING CONVECTIVE STRUCTURE. TD 23W
FEATURES ONLY MARGINAL EQUATORWARD DIFFLUENCE ALOFT AT THIS TIME.
SSTS ARE VERY WARM (28-30 CELSIUS), SUPPORTING THE FLARING
CONVECTION. TD 23W IS TRACKING WEST-NORTHWESTWARD ALONG THE SOUTHERN
PERIPHERY OF THE SUBTROPICAL RIDGE TO THE NORTH.
3. FORECAST REASONING.
A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY FROM THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
B. TD 23W IS FORECAST TO TRACK NORTHWESTWARD ALONG THE WESTERN
PERIPHERY OF THE STR RIDGE CURRENTLY TO ITS NORTH, PASSING WELL
NORTH OF THE MARIANAS ISLANDS BY TAU 72. OUTFLOW FROM TY 22W TO ITS
NORTHWEST WILL CONTINUE TO BRING NORTHERLY VWS INTO THE AREA AND
WILL SLOW THE INTENSIFICATION RATE OF TD 23W. DUE TO THE SUSTAINED
MODERATE TO HIGH SHEAR ENVIRONMENT, WIND RADII ARE FORECAST TO BE
EXPANSIVE AND SOMEWHAT ASYMMETRIC. THE HWRF MODEL TRACK REMAINS THE
SOUTHERN OUTLIER, ABOUT 100 NM SOUTH OF THE MULTI-MODEL CONSENSUS.
OTHERWISE, MODELS ARE IN GOOD AGREEMENT ON STORM MOTION, WITH 170 NM
OF SOUTHWEST-NORTHEAST SPREAD AT TAU 72. THE JTWC FORECAST TRACK IS
PLACED CLOSED TO THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE, AND
THE JTWC FORECAST INTENSITY ALSO CLOSELY FOLLOWS THE INTENSITY
CONSENSUS.
C. AFTER TAU 72, TD 23W IS FORECAST TO ACCELERATE TO THE
NORTHWEST. BY TAU 96, A LONGWAVE MIDLATITUDE TROUGH WILL DIG SOUTH
NEAR JAPAN, ERODING THE STR. MODEL SPREAD INCREASES IN LATER TAUS,
DEPENDING ON THEIR DEPICTION OF THE STRENGTH OF THIS TROUGH AND HOW
MUCH IT ERODES THE STEERING RIDGE, WITH 400 NM OF SOUTHWEST-
NORTHEAST SPREAD AT TAU 120. MODELS ALSO DIFFER IN HOW THEY DEPICT
THE ORIENTATION OF TD 23W WITH RESPECT TO TY 22W AT LATER TAUS, WITH
NAVGEM AND ECMWF MAINTAINING A STRAIGHT NORTHWESTWARD MOTION
FOR TY 22W, THEN KEEPING TD 23W MORE EASTWARD, WHILE GFS PULLS TD
23W BEHIND THE TRACK OF TY 22W. HOWEVER, ALL MODELS DO SHOW
NORTHWEST MOTION AND RELATIVELY CONSISTENT STORM MOTION. THE
INTENSITY CONSENSUS HAS DROPPED FROM THE LAST MODEL RUN, WITH TD 23W
NOW REACHING 95 KTS BY THE END OF THE FORECAST PERIOD.
INTENSIFICATION AT THE END OF THE FORECAST PERIOD WILL DEPEND ON THE
RELATIVE POSITION OF TY 22W, AND IF TD 23W CAN TAP INTO THE JET AND
IMPROVE ITS POLEWARD OUTFLOW AT THAT TIME. THE JTWC FORECAST TRACK
IS PLACED CLOSE TO THE MULTI-MODEL CONSENSUS WITH FAIR CONFIDENCE,
AND THE INTENSITY FORECAST ALSO CLOSELY FOLLOWS THE INTENSITY
CONSENSUS.//
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