Tropical Storm ATSANI Advisory sáb, 31-10

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 23W (ATSANI)
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 23W (ATSANI), LOCATED APPROXIMATELY 147 NM
SOUTH-SOUTHWEST OF ANDERSEN AFB, GUAM HAS TRACKED NORTHWESTWARD AT
26 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY (MSI) DEPICTS A VERY DISORGANIZED SYSTEM WITH FLARING
CONVECTION WRAPPING INTO A BROAD AREA OF WEAK TURNING, WITH NO
DISTINCT LOW LEVEL CIRCULATION CENTER (LLCC) EVIDENT IN AVAILABLE
IMAGERY. THERE HAVE BEEN NO MICROWAVE OR SCATTEROMETER PASSES SINCE
THE 0000Z HOUR AND THERE ARE POSSIBLY MULTIPLE CIRCULATIONS EMBEDDED
IN THE BROADER CIRCULATION. THE INITIAL POSITION IS PLACED WITH LOW
CONFIDENCE BASED PRIMARILY ON SPIRAL BAND ANALYSIS OF THE FEW LOW-
LEVEL CLOUD BANDS EVIDENT IN ANIMATED MSI. THE INITIAL INTENSITY IS
ASSESSED AT 30 KTS BASED ON SUBJECTIVE DVORAK ESTIMATES OF T2.0 (30
KNOTS) FROM PGTW, AN ADVANCED DVORAK TECHNIQUE ESTIMATE OF 2.8 AND A
SATELLITE CONSENSUS ESTIMATE OF 39 KNOTS; HOWEVER, THE AUTOMATED
TECHNIQUES ARE ANALYZING A POSITION DISPLACED WELL TO THE SOUTHEAST
OF THE ASSESSED LLCC, AND ARE UNREPRESENTATIVE. ENVIRONMENTAL
ANALYSIS INDICATES THE SYSTEM LIES IN A REGION OF MODERATE TO STRONG
(20-25 KNOT) VWS FROM THE OUTFLOW FROM STY 22W, OFFSETTING ROBUST
EQUATORWARD OUTFLOW AND WARM (30 TO 31 DEGREE CELSIUS) SEA SURFACE
TEMPERATURES. TD 23 IS CURRENTLY TRACKING RAPIDLY NORTHWESTWARD
ALONG THE SOUTHWEST PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE
(STR) CENTERED TO THE NORTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TD 23W IS FORECAST TO CONTINUE TRACKING NORTHWESTWARD,
TRACKING ALONG THE SOUTHWESTERN EDGE OF THE STR TO THE NORTHEAST
THROUGH TAU 72. THE SYSTEM WILL CONTINUE TO SLOWLY AND STEADILY
INTENSIFY THROUGH TAU 48 IN SPITE OF MARGINAL ENVIRONMENTAL
CONDITIONS, WITH THE CONVERGENT EQUATORWARD OUTFLOW FROM STY 22W
LIMITING THE OUTFLOW AVAILABLE FOR TD 23W TO A SINGLE EQUATORWARD
CHANNEL. THIS CONVERGENCE, COMBINED WITH MODERATE VWS (15-20 KTS),
WILL SERVE TO OFFSET THE VERY WARM (30 DEG CELSIUS) WATERS. AFTER
TAU 48 THE SYSTEM IS FORECAST TO MOVE UNDER A DEVELOPING UPPER-LEVEL
ANTICYCLONE, WHICH WILL PROVIDE FOR MORE ROBUST RADIAL OUTFLOW AND
RESULT IN A STEEPER RATE OF INTENSIFICATION THROUGH TAU 72. DYNAMIC
MODEL GUIDANCE IS IN POOR AGREEMENT, WITH A DISTINCT BIFURCATION
FROM TAU 12. THE NAVGEM MODEL IS CURRENTLY THE ONLY CONSENSUS MEMBER
THE CONTINUES TO RECURVE THE SYSTEM NORTHEAST INTO THE WESTERLIES,
WHILE GFS, ECMWF AND THEIR ENSEMBLES, AS WELL AS THE AFUM MODEL, ALL
INDICATE A SLOWING OF THE FORWARD SPEED BEGINNING AT TAU 72, AND
THEN A TURN WESTWARD, LEADING TO A CROSS-TRACK SPREAD OF 400 NM AT
TAU 72. THE REMAINDER OF THE GUIDANCE PACKAGE LIES VERY CLOSE TO THE
MULTI-MODEL CONSENSUS AVERAGE. THE JTWC FORECAST TRACK HAS BEEN
PLACED ALONG THE SOUTHERN EDGE OF THE GUIDANCE ENVELOPE THROUGH TAU
72, AS A NOD TOWARDS THE GFS AND ECMWF SOLUTIONS WITH LOW
CONFIDENCE.
   C. BEYOND TAU 72, THE SYSTEM WILL SLOW SIGNIFICANTLY AS IT MOVES
INTO A WEAK STEERING ENVIRONMENT, DOMINATED BY A COL AREA WHICH
DEVELOPS DUE TO A WEAKNESS IN THE STR INDUCED BY THE APPROACH OF
DEEP MID-LATITUDE TROUGH TO THE NORTH. AS IT TRACKS NORTHWESTWARD IT
BEGINS TO TAP INTO DUAL-CHANNEL OUTFLOW, ALLOWING IT TO INTENSIFY TO
95 KNOTS AT TAU 120. THE STEERING MECHANISM SHIFTS BY TAU 96 TO THE
STR TO THE WEST OVER CHINA LEADING TO A MORE WESTWARD TRACK THROUGH
THE END OF THE FORECAST. NUMERICAL MODEL GUIDANCE IS HIGHLY
DIVERGENT, WITH THE AFOREMENTIONED BIFURCATION BECOMING INCREASINGLY
EVIDENT IN LONG RANGE FORECAST. THE CROSS-TRACK SPREAD BETWEEN THE
NAVGEM AND GFS OUTLIERS INCREASES TO NEARLY 900 NM BY TAU 120 WITH
THE REMAINDER OF THE CONSENSUS MEMBERS TIGHTLY PACKED AND FAIRLY
EVENLY DISTRIBUTED IN ROUGHLY THE MIDDLE OF THE BIFURCATION
ENVELOPE. THE JTWC TRACK IS PLACED ALONG THE SOUTHERN SIDE OF THE
TIGHTEST MODEL PACKING, AS A HEDGE TOWARDS THE GFS AND ECMWF
SOLUTIONS, WITH VERY LOW CONFIDENCE.//
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