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| #366548 (Received by flhurricane at: 4:40 AM 06.Aug.2010) TCDAT4 TROPICAL STORM COLIN DISCUSSION NUMBER 10 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL042010 500 AM AST FRI AUG 06 2010 VERTICAL WIND SHEAR...ANALYZED AT 25 KT BY THE SHIPS MODEL...HAS AGAIN TAKEN A TOLL ON COLIN...WITH THE LOW-LEVEL CENTER NOW COMPLETELY EXPOSED TO THE WEST OF THE PULSING DEEP CONVECTION. AN ASCAT PASS AROUND 0118 UTC SHOWED SEVERAL 35-40 KT RETRIEVALS EAST OF THE CENTER...AND THE INITIAL INTENSITY IS LOWERED TO 40 KT FOR THIS ADVISORY. THE SHIPS MODEL SHOWS THE SHEAR DECREASING TO LESS THAN 10 KT IN 24 HOURS...WHICH SHOULD GIVE COLIN AN OPPORTUNITY TO RE-INTENSIFY. HOWEVER...THE SHEAR INCREASES RAPIDLY AGAIN BY 48 HOURS...WHICH SHOULD LIMIT INTENSIFICATION BEYOND THAT TIME. THE OFFICIAL INTENSITY FORECAST IS ADJUSTED DOWNWARD TOWARD THE ICON INTENSITY CONSENSUS THROUGH 48 HOURS AND IS SIMILAR TO THE PREVIOUS PACKAGE AFTER THAT. THE 12-HOUR INITIAL MOTION ESTIMATE IS 325/12...TO THE LEFT OF THE PREVIOUS TRACK FORECAST. COLIN IS SITUATED TO THE SOUTHEAST OF AN UPPER-LEVEL SHEAR AXIS ANCHORED BY ONE LOW CENTER NEAR 27N 75W AND ANOTHER NEAR BERMUDA. THE GLOBAL MODELS SUGGEST THAT THESE LOWS WILL MOVE AWAY AND ALLOW COLIN TO MOVE NORTHWARD AT A SLOWER SPEED INTO A BREAK IN THE SUBTROPICAL RIDGE OVER THE NEXT COUPLE OF DAYS. BEYOND THAT TIME...THE GLOBAL MODELS SHOW A LONGWAVE TROUGH ADVANCING EASTWARD ACROSS THE NORTHEASTERN U.S. AND EASTERN CANADA. HOWEVER...THE TIMING OF THE INTERACTION BETWEEN COLIN AND INDIVIDUAL SHORTWAVES IN THE TROUGH VARIES WIDELY IN THE GUIDANCE...AND SO DOES THE FORWARD SPEED AND TRACK OF THE CYCLONE. THE GFS...ECMWF...AND GFDL MOVE COLIN MORE SLOWLY AND DELAY INTERACTION WITH THE TROUGH UNTIL AROUND DAY 4...WHILE THE UKMET... NOGAPS...AND HWRF TAKE COLIN MORE QUICKLY NORTHWARD AND SHOW INTERACTION WITH THE TROUGH ABOUT A DAY EARLIER. GIVEN THAT THE 72-HOUR FORECAST POSITIONS FROM THE GFS AND NOGAPS ARE ABOUT 1000 MILES APART...THIS TRACK FORECAST HAS LOW CONFIDENCE... ESPECIALLY IN THE LATER PERIODS. THE OFFICIAL FORECAST HAS BEEN ADJUSTED TO THE LEFT OF THE PREVIOUS NHC TRACK THROUGH 48 HOURS DUE TO THE MORE WESTERLY INITIAL POSITION AND MOTION...AND LIES ALONG THE LEFT SIDE OF THE GUIDANCE ENVELOPE. BEYOND THAT TIME...THE BIGGEST CHANGE TO THE OFFICIAL FORECAST WAS TO SLIGHTLY DECREASE THE FORWARD SPEED DUE TO THE LARGE SPREAD IN THE GUIDANCE. ONE ALTERNATIVE SCENARIO IS THAT IF COLIN REMAINS A SHALLOW SYSTEM WITHOUT MUCH DEEP CONVECTION...IT COULD MOVE FARTHER TO THE LEFT AND MORE SLOWLY IN THE SHORT TERM ALONG A TRACK SIMILAR TO THE SHALLOW BAM. FORECAST POSITIONS AND MAX WINDS INITIAL 06/0900Z 26.8N 67.4W 40 KT 12HR VT 06/1800Z 27.9N 67.7W 40 KT 24HR VT 07/0600Z 29.8N 67.4W 45 KT 36HR VT 07/1800Z 31.5N 66.8W 50 KT 48HR VT 08/0600Z 33.6N 65.7W 55 KT 72HR VT 09/0600Z 38.5N 62.5W 60 KT 96HR VT 10/0600Z 44.0N 56.0W 55 KT 120HR VT 11/0600Z 50.5N 45.0W 50 KT...POST-TROP/EXTRATROP $$ FORECASTER BRENNAN |
