000 WTPZ42 KNHC 310852 TCDEP2 TROPICAL STORM ROSA DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL EP172012 200 AM PDT WED OCT 31 2012 RECENT MICROWAVE IMAGES CONTINUE TO SHOW THAT THE LOW-LEVEL CENTER OF ROSA IS LOCATED ON THE WESTERN EDGE OF THE DEEP CONVECTIVE MASS. DVORAK ESTIMATES FROM TAFB AND SAB ARE STEADY FROM 6 HOURS AGO...AND A PARTIAL ASCAT PASS AT 0434 UTC REVEALED A BAND OF 35-40 KT WINDS WITHIN THE EASTERN SEMICIRCLE. BASED ON THESE DATA...THE INTENSITY IS HELD AT 40 KT. ROSA APPEARS TO HAVE 12 TO 24 HOURS BEFORE STRONG UPPER-LEVEL WESTERLY WINDS SAG SOUTHWARD CLOSER TO THE CYCLONE. THE INTENSITY MODELS INDICATE LITTLE TO NO STRENGTHENING DURING THAT PERIOD...BUT THE NHC FORECAST LEAVES OPEN THE POSSIBILITY THAT ROSA COULD GET JUST A BIT STRONGER LATER TODAY. EVEN THOUGH THE STORM WILL BE OVER WARM WATERS DURING THE FORECAST PERIOD...INCREASING VERTICAL SHEAR IN THE NEXT DAY OR TWO SHOULD CAUSE ROSA TO GRADUALLY WEAKEN AND DEGENERATE INTO A REMNANT LOW BY DAY 4. ROSA IS MOVING VERY SLOWLY WESTWARD...OR 275/4 KT. A MID-LEVEL ANTICYCLONE LOCATED NEAR 130W APPEARS TO BE IMPEDING THE WESTWARD MOTION OF ROSA...AND THIS FEATURE IS EXPECTED TO MOVE LITTLE DURING THE NEXT COUPLE OF DAYS. ON DAYS 3 THROUGH 5...THE ANTICYCLONE IS FORECAST TO MIGRATE NORTHWESTWARD WHILE A MID- TO UPPER-LEVEL CUTOFF LOW DEVELOPS JUST WEST OF THE BAJA PENINSULA. THIS EVOLVING PATTERN SHOULD CAUSE ROSA TO DRIFT WESTWARD OR BE NEARLY STATIONARY OVER THE NEXT 5 DAYS. THE GFS CONTINUES TO BE THE OUTLIER SOLUTION BY SHOWING A STRONGER ROSA GETTING PICKED UP BY THE CUTOFF LOW AND MOVING NORTHEASTWARD BY THE END OF THE FORECAST PERIOD. HOWEVER... A BULK OF THE GFS ENSEMBLE MEMBERS SUPPORT THE SLOW WESTWARD MOTION SHOWN BY THE OTHER DETERMINISTIC MODELS. PERHAPS IN CELEBRATION OF HALLOWEEN...THE TRACK GUIDANCE ENVELOPE IS IN THE SHAPE OF A SQUASHED SPIDER...WHICH IS NORMALLY INDICATIVE OF VERY LITTLE MOTION. AS WAS FORESHADOWED IN THE 8 PM PDT DISCUSSION...AN EASTWARD ADJUSTMENT TO THE PREVIOUS NHC TRACK FORECAST HAS BEEN MADE...AND THE NEW OFFICIAL FORECAST LIES BETWEEN THE ECMWF MODEL AND THE MULTI-MODEL CONSENSUS TVCE. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 14.5N 117.3W 40 KT 45 MPH 12H 31/1800Z 14.6N 117.8W 45 KT 50 MPH 24H 01/0600Z 14.7N 118.3W 40 KT 45 MPH 36H 01/1800Z 14.7N 118.7W 35 KT 40 MPH 48H 02/0600Z 14.7N 118.8W 35 KT 40 MPH 72H 03/0600Z 14.5N 119.5W 30 KT 35 MPH 96H 04/0600Z 14.5N 120.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 05/0600Z 15.0N 120.5W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ FORECASTER BERG