04P(SETH) confounds high vertical wind shear and is the last 2021 Tropical Cyclone

JTWC IS ISSUING 6HOURLY WARNINGS AND 3HOURLY SATELLITE BULLETIN ON TC 04P.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: AS MENTIONED EARLIER, TC 04P (SETH) HAS RAPIDLY TRANSITIONED FROM A HYBRID SUBTROPICAL LOW TO A FULL-FLEDGED TROPICAL CYCLONE OVER THE PAST SIX TO TWELVE HOURS. THE FORECAST HOWEVER CALLS FOR A TRANSITION BACK TO A SUBTROPICAL CYCLONE WITHIN THE NEXT 48 HOURS. THE SYSTEM IS FORECAST TO SLOW DOWN AS IT TRACKS SOUTHEAST, THEN TURNS SOUTH BY 36H AS THE STEERING MECHANISM SHIFTS TO A STRONG SUBTROPICAL RIDGE (STR) TO THE SOUTHEAST. THE SYSTEM IS EXPECTED TO MAINTAIN ITS CURRENT INTENSITY THROUGH 24H, THEN WEAKEN STEADILY THROUGH THE REMAINDER OF THE FORECAST PERIOD UNDER INCREASING SHEAR, INCREASING MID-LEVEL DRYNESS AND DECREASING SSTS. TC 04P WILL MOVE UNDER A 500MB TROUGH BEGINNING AROUND 12H, THEN TRANSITION TO A SUBTROPICAL STORM NO LATER THAN 48H, AS IT BECOMES VERTICALLY STACKED UNDER A 500MB COLD-CORE LOW AND CROSSES INTO WATERS COOLER THAN 26C. WHILE THE FORECAST ONLY COVERS THE 48 HOUR PERIOD THROUGH SUBTROPICAL TRANSITION, THE REMNANT GALE-FORCE SUBTROPICAL LOW WILL LIKELY MEANDER SLOWLY WESTWARD THROUGH 72H WHILE CONTINUING TO STEADILY WEAKEN.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TC 04P (SETH) HAS RAPIDLY TRANSITIONED FROM A HYBRID-TYPE SYSTEM WITH PRIMARILY SUBTROPICAL CHARACTERISTICS, TO A WELL-DEFINED TROPICAL CYCLONE. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTED THE RAPID DEVELOPMENT OF A SYMMETRIC BALL OF DEEP CONVECTION NEAR THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC), WHICH DETACHED FROM THE COMMA-SHAPED BAND OF CONVECTIVE ACTIVITY BETWEEN THE 0600Z AND 1000Z HOURS. A 310910Z GPM COLOR 89GHZ COMPOSITE MICROWAVE IMAGE INDICATED THE LLCC HAD TUCKED UNDER THE AREA OF SOMEWHAT DISORGANIZED CONVECTION. AT THAT TIME, NORTHWESTERLY SHEAR WAS BEARING DOWN ON THE SYSTEM AND ELONGATING THE CONVECTIVE AREA, BUT SUBSEQUENT IMAGERY SUGGESTS THE SHEAR HAS RELAXED, AND ALLOWED THE CONVECTIVE AREA TO CONSOLIDATE. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON A 311146Z ASCAT-C BULLSEYE PASS. THE BULLSEYE PASS ALSO PROVIDED HIGH CONFIDENCE TO THE INITIAL INTENSITY, WHICH IS SET SLIGHTLY HIGHER THAN THE AGENCY DVORAK INTENSITY ESTIMATES. SURFACE WIND OBSERVATIONS FROM FREDERICK REEF WERE AS HIGH AS 63 KNOTS (10-MIN) AT 0900Z AS THE SYSTEM PASSED THE STATION, BUT THE HEIGHT OF THE OBSERVATION WAS 30-METERS, WHILE THE SURFACE PRESSURE READING OF 989MB WOULD SUGGEST A 45-50 KNOT INTENSITY. ENVIRONMENTAL CONDITIONS HAVE IMPROVED OVER THE PAST SIX TO TWELVE HOURS, BUT REMAIN ONLY MARGINALLY FAVORABLE. SSTS ARE WARM AND POLEWARD OUTFLOW IS GOOD, AND FOR THE TIME BEING SHEAR IS IS RELATIVELY LOW IN THE IMMEDIATE VICINITY OF THE SYSTEM. BUT DRY AIR IS FIRMLY ENTRENCHED TO THE WEST AND NORTH AND IS OFFSETTING THE OTHERWISE FAVORABLE CONDITIONS.

OUTFLOW: STRONG POLEWARD

MODEL DISCUSSION: TRACK GUIDANCE IS IN POOR AGREEMENT, WITH SIGNIFICANT SPREAD AFTER 36H. THERE ARE TWO DISTINCT GROUPINGS IN THE TRACK GUIDANCE, WITH THE NORTHERN GROUP OF UKMET ENSEMBLE, GALWEM, ECMWF AND THE ECMWF ENSEMBLE TURNING THE CIRCULATION SHARPLY WEST TOWARDS THE COASTLINE AFTER 36H. THE SOUTHERN GROUPING CONSISTING OF GFS, GFS ENSEMBLE AND HWRF TRACK THE SYSTEM STEADILY SOUTHWARD PARALLEL TO THE COAST THROUGH THE FORECAST PERIOD AND BEYOND. THE JTWC FORECAST LIES ON THE EAST SIDE OF THE ENVELOPE THROUGH 36H, THEN TURNS WEST ON THE SOUTHERN SIDE OF THE NORTHERN GROUPING THROUGH THE END OF THE FORECAST PERIOD. CONSIDERING THAT THE MAJORITY OF THE MODEL SPREAD OCCURS AFTER 36H, THERE IS MEDIUM CONFIDENCE IN THE JTWC FORECAST. INTENSITY GUIDANCE IS SIMILARLY MIXED, WITH THE SHIPS GUIDANCE BEING THE MOST AGGRESSIVE, PEAKING BETWEEN 65 AND 75 KNOTS, WHILE THE HWRF IS THE LEAST AGGRESSIVE, PEAKING AT THE ANALYSIS OF 50 KNOTS AND DECREASING STEADILY THEREAFTER. THE JTWC FORECAST MOST CLOSELY FOLLOWS THE HWRF SOLUTION, WITH MEDIUM CONFIDENCE.

ECMWF ENSEMBLE.