000 WTPZ45 KNHC 240239 TCDEP5 TROPICAL STORM DARBY DISCUSSION NUMBER 5 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP052010 800 PM PDT WED JUN 23 2010 THE RAPID INTENSIFICATION THAT DARBY WAS UNDERGOING EARLIER SEEMS TO HAVE SLOWED OR STOPPED...AS THERE HAS BEEN NO RECENT HINT OF AN EYE IN VISIBLE OR INFRARED IMAGERY AND MUCH OF THE OUTER BANDING ASSOCIATED WITH DARBY HAS DISSIPATED. WHY THIS IS OCCURRING IS NOT OBVIOUS...AS THERE ARE NO ANALYSES THAT SHOW SIGNIFICANT VERTICAL WIND SHEAR AFFECTING THE STORM. IT IS POSSIBLE THAT EASTERLY UPPER-LEVEL WINDS ARE UNDERCUTTING THE WELL-DEFINED ANTICYCLONIC OUTFLOW. SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB ARE 55 KT...AND THAT IS THE INITIAL INTENSITY FOR THIS ADVISORY. THE INITIAL MOTION REMAINS 285/10. DARBY IS SOUTH OF A LOW/MID- LEVEL RIDGE OVER MEXICO. THE LARGE-SCALE MODELS FORECAST THE RIDGE TO WEAKEN AS A DISTURBANCE OVER THE NORTHWESTERN CARIBBEAN SEA MOVES TOWARDS THE YUCATAN PENINSULA AND THE GULF OF MEXICO. THIS SHOULD LEAVE DARBY IN AN AREA OF LIGHT STEERING CURRENTS BY 72-96 HR...WITH THE LOW-LEVEL FLOW TRYING TO MOVE THE CYCLONE EASTWARD AND THE MID-LEVEL FLOW TRYING TO MOVE IT WESTWARD. THERE IS A LARGE SPREAD IN THE DYNAMICAL MODELS AFTER 72 HR...WHICH IS ANOTHER SIGN THAT THE CYCLONE IS LIKELY TO MOVE SLOWLY. THE NEW FORECAST TRACK IS SIMILAR TO THE PREVIOUS TRACK...CALLING FOR A WEST- NORTHWESTWARD MOTION AND DECELERATION FOR 48 HR OR SO...FOLLOWED BY A NORTHWARD TURN AT A SLOW FORWARD SPEED. THE NEW TRACK IS IN BEST AGREEMENT WITH THE FLORIDA STATE SUPERENSEMBLE. DESPITE THE CURRENT TRENDS...THE INTENSITY GUIDANCE FORECASTS DARBY TO STRENGTHEN FURTHER...WITH THE SHIPS AND LGEM MODELS SHOWING A PEAK INTENSITY OF 80 KT IN ABOUT 48 HR AND THE SHIPS RAPID INTENSIFICATION INDEX SHOWING A 40 PERCENT CHANCE OF 30 KT STRENGTHENING DURING THE NEXT 24 HR. AFTER THAT...INCREASING EASTERLY WIND SHEAR IS LIKELY TO ARREST DEVELOPMENT AND BEGIN A WEAKENING TREND. THE NEW INTENSITY FORECAST FOLLOWS THE TREND OF THE PREVIOUS FORECAST BUT AT SOMEWHAT LOWER INTENSITIES BASED ON CURRENT TRENDS AND THE FORECAST SHEAR. THE NEW FORECAST IS STRONGER THAN ALL THE GUIDANCE AT 96 AND 120 HR. IT SHOULD BE NOTED THAT THE GFS...UKMET...NOGAPS...AND ECMWF MODELS SHOW DARBY SHEARING APART AFTER 72 HR...WITH THE LOW-LEVEL CENTER MOVING EASTWARD AND THE MID-LEVEL CENTER MOVING WESTWARD. SHOULD THIS HAPPEN THE CYCLONE WOULD WEAKEN MUCH FASTER THAN CURRENTLY FORECAST. FORECAST POSITIONS AND MAX WINDS INITIAL 24/0300Z 12.3N 97.0W 55 KT 12HR VT 24/1200Z 12.7N 98.3W 65 KT 24HR VT 25/0000Z 13.1N 99.8W 75 KT 36HR VT 25/1200Z 13.4N 100.9W 80 KT 48HR VT 26/0000Z 13.6N 101.7W 85 KT 72HR VT 27/0000Z 14.0N 102.5W 85 KT 96HR VT 28/0000Z 14.5N 102.5W 80 KT 120HR VT 29/0000Z 15.0N 102.5W 65 KT $$ FORECASTER BEVEN