ZCZC MIATCDAT4 ALL TTAA00 KNHC DDHHMM TROPICAL STORM NICHOLAS DISCUSSION NUMBER 17 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 PM EDT FRI OCT 17 2003 AFTER ALMOST LOOKING LIKE A HURRICANE THIS MORNING...NICHOLAS HAS WEAKENED A LITTLE DUE TO SOUTHWESTERLY VERTICAL SHEAR. THE LOW-LEVEL CENTER BECAME EXPOSED FOR A TIME BEFORE BEING COVERED BY THE LATEST BURST OF CONVECTION. WHILE SATELLITE INTENSITY ESTIMATES REMAIN 65 KT...THIS IS BASED MAINLY ON CONSTRAINTS FROM THE EARLIER ESTIMATES. THE INITIAL INTENSITY IS REDUCED TO 55 KT. IT SHOULD BE NOTED THAT THE CIRRUS OUTFLOW HAS INCREASED ON THE SOUTHWEST SIDE OF THE CYCLONE...WHICH IS A BIT DECEIVING AS SOUTHWESTERLY FLOW IS UNDERCUTTING THE OUTFLOW AND CAUSING SHEAR. THE INITIAL MOTION IS 325/7. THERE IS LITTLE CHANGE IN EITHER THE SYNOPTIC PATTERN OF THE FORECAST REASONING FROM THE PREVIOUS PACKAGE. NICHOLAS IS CAUGHT IN A COMPLEX PATTERN OF SURROUNDING WEATHER SYSTEMS THAT COMBINE TO PRODUCE WEAK STEERING CURRENTS. THE TRACK GUIDANCE SHOWS GREATER SPREAD THAN 6 HR AGO. AMONGST THE DYNAMICAL GUIDANCE...THE UKMET AND GFS FORECAST A MOTION TOWARD THE NORTHWEST OR WEST-NORTHWEST...THE GFDL FORECASTS A TURN TO THE NORTH...THE NOGAPS CALLS FOR A NORTHEASTWARD MOTION FOLLOWED BY A NORTHWESTWARD TURN...AND THE GFDN CALLS FOR NORTHWARD TO NORTHEASTWARD MOTION. THE KEY FACTOR IN THE SPREAD SEEMS TO BE THE STRENGTH OF THE STORM...AS THE STRONGER THE STORM IS IN THE MODEL THE FARTHER EAST IT GOES. SINCE THE GFS AND UKMET APPEAR TOO WEAK WITH NICHOLAS...AND THE OTHER DYNAMICAL GUIDANCE APPEARS TOO STRONG...THE FORECAST TRACK WILL SPLIT THE DIFFERENCE. THE NEW FORECAST IS AN UPDATE OF THE PREVIOUS PACKAGE. AN UPPER-LEVEL RIDGE SOUTHEAST OF NICHOLAS HAS A WELL-DEFINED WIND MAXIMUM ALONG ITS NORTHERN PERIPHERY. THIS WIND BAND IS UNDERCUTTING THE CIRRUS OUTFLOW OF NICHOLAS. THE FUTURE INTENSITY DEPENDS ON HOW CLOSE THE STORM STAYS TO THE WIND MAXIMUM. DYNAMICAL MODELS FORECAST THE RIDGE TO MOVE WESTWARD...WHICH COULD FORCE THE WIND MAXIMUM NORTH OF THE CYCLONE AND CREATE A MORE FAVORABLE ENVIRONMENT DURING THE NEXT 48 HR...ESPECIALLY IF NICHOLAS MOVES SLOWER THAN FORECAST. CONVERSELY...IF THE STORM MOVES FASTER THAN FORECAST...IT WOULD REMAIN IN STRONGER SHEAR. GIVEN THE UNCERTAINTY IN HOW MUCH SHEAR NICHOLAS WILL FACE...AND THAT IT WILL BE DIFFICULT FOR NICHOLAS TO STRENGTHEN MORE IF THE SHEAR DOES NOT DECREASE...THE INTENSITY FORECAST IS CHANGED TO THAT OF A STEADY-STATE 55 KT FOR 72 HR. AFTER THAT...NICHOLAS WILL LIKELY ENTER STRONGER SHEAR NORTH OF 20N AND WEAKEN. FORECASTER BEVEN FORECAST POSITIONS AND MAX WINDS INITIAL 17/2100Z 16.4N 48.5W 55 KT 12HR VT 18/0600Z 17.1N 48.9W 55 KT 24HR VT 18/1800Z 17.9N 49.4W 55 KT 36HR VT 19/0600Z 18.6N 50.0W 55 KT 48HR VT 19/1800Z 19.2N 50.5W 55 KT 72HR VT 20/1800Z 20.5N 51.5W 55 KT 96HR VT 21/1800Z 21.5N 52.5W 50 KT 120HR VT 22/1800Z 23.0N 54.0W 45 KT NNNN
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Page last modified: Monday, 07-Feb-2005 16:50:00 UTC