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| #292847 (Received by flhurricane at: 4:49 PM 01.Sep.2009) TCDAT1 TROPICAL STORM ERIKA DISCUSSION NUMBER 1 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL062009 500 PM EDT TUE SEP 01 2009 AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT HAS BEEN INVESTIGATING THE LOW PRESSURE AREA EAST OF THE LEEWARD ISLANDS AND HAS FOUND A MINIMUM PRESSURE OF ABOUT 1007 MB AND A CLOSED...ALBEIT...BROAD CIRCULATION. PEAK FLIGHT-LEVEL WINDS WERE 52 KT WITH SFMR DATA OF ABOUT 45 KT. THUS...TROPICAL STORM ERIKA HAS FORMED WITH AN INITIAL INTENSITY OF 45 KT. SATELLITE IMAGES SHOW THAT ERIKA IS EXPERIENCING MODERATE SOUTHWESTERLY SHEAR WITH THE CENTER NEAR THE WESTERN EDGE OF THE DEEP CONVECTION. SOME STRENGTHENING OF THE STORM SEEMS POSSIBLE DURING THE NEXT DAY OR TWO WHILE THE SSTS ARE WARM AND SHEAR REMAINS MODERATE. HOWEVER...IN ABOUT TWO DAYS...GLOBAL MODELS SIGNIFICANTLY INCREASE THE SHEAR NEAR THE TROPICAL CYCLONE AS UPPER-LEVEL WESTERLIES FROM A MID-OCEANIC TROUGH OVER THE WESTERN ATLANTIC IMPINGE ON THE SYSTEM. MOST OF THE GLOBAL MODELS SHOW ERIKA WEAKENING AFTER TWO DAYS DUE TO THE SHEAR. THE TWO BIG OUTLIERS IN THE MODEL GUIDANCE ARE THE GFDL/HWRF...WHICH FORM AN ANTICYCLONE NEAR ERIKA AND MAKE IT A HURRICANE. THE NHC FORECAST WILL SHOW SLOW INTENSIFICATION OVER THE FIRST COUPLE OF DAYS...THEN WEAKENING THEREAFTER AS THE SHEAR TAKES OVER. THE NHC FORECAST WILL BE CONSERVATIVE AT THIS TIME...BUT IT IS OF NOTE THAT ALMOST ALL OF THE RELIABLE INTENSITY GUIDANCE SHOW ERIKA EVENTUALLY BECOMING A HURRICANE...DESPITE THE SHEAR. THE INITIAL MOTION ESTIMATE IS 300/8...THOUGH THE CENTER APPEARS TO HAVE DONE SOME REFORMATION SINCE YESTERDAY. MID-LEVEL RIDGING OVER THE WEST-CENTRAL ATLANTIC SHOULD CAUSE ERIKA TO MOVE ALONG THAT GENERAL COURSE FOR THE NEXT COUPLE OF DAYS. THEREAFTER... THE STORM COULD TURN MORE TO THE NORTHWEST AS IT NEARS THE SOUTHWESTERN PERIPHERY OF THE RIDGE. THIS FORECAST IS RATHER UNCERTAIN AS THE TRACK PROBABLY HAS A STRONG DEPENDENCE ON THE FUTURE INTENSITY OF ERIKA. A WEAKER SYSTEM WOULD LIKELY MOVE MORE TO THE WEST...SIMILAR TO THE UKMET OR GFS MODELS. HOWEVER...A STRONGER SYSTEM WOULD LIKELY FEEL THE STRONG UPPER-LEVEL WINDS AND MOVE MORE TO THE RIGHT...LIKE THE HWRF/GFDL. THE OFFICIAL FORECAST IS LEFT OF THE DYNAMICAL CONSENSUS DUE TO THE STRONG SHEAR FORECAST IN THE LONGER-TERM. BECAUSE OF THE PROXIMITY TO THE NORTHERN LEEWARD ISLANDS...TROPICAL STORM WATCHES HAVE BEEN ISSUED IN THAT AREA. FORECAST POSITIONS AND MAX WINDS INITIAL 01/2100Z 17.2N 57.3W 45 KT 12HR VT 02/0600Z 17.7N 58.2W 45 KT 24HR VT 02/1800Z 18.4N 60.3W 50 KT 36HR VT 03/0600Z 19.1N 62.0W 55 KT 48HR VT 03/1800Z 19.8N 63.8W 60 KT 72HR VT 04/1800Z 21.0N 66.5W 55 KT 96HR VT 05/1800Z 23.0N 69.5W 50 KT 120HR VT 06/1800Z 25.0N 72.0W 45 KT $$ FORECASTER BLAKE/FRANKLIN |
