TROPICAL STORM HANNA DISCUSSION NUMBER 2 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 1100 AM EDT THU AUG 28 2008

SATELLITE IMAGERY INDICATES THAT THE CLOUD PATTERN ASSOCIATED WITH THE CYCLONE HAS IMPROVED DURING THE LAST 6 HOURS AND LATEST DVORAK INTENSITY ESTIMATES INDICATE WINDS ARE NOW NEAR 35 KT. BASED ON THIS INFORMATION...THE DEPRESSION IS UPGRADED TO TROPICAL STORM HANNA. VISIBLE IMAGERY SHOWS THAT HANNA'S LOW LEVEL CIRCULATION IS PARTIALLY EXPOSED ON THE WESTERN EDGE OF THE DEEP CONVECTION INDICATING WESTERLY SHEAR...ASSOCIATED WITH AN UPPER-LEVEL LOW TO THE WEST IS IMPACTING THE SYSTEM. SINCE THE OCEAN IS PLENTY WARM...THE INTENSITY FORECAST IS PRIMARILY A FUNCTION OF HOW HANNA INTERACTS WITH THE UPPER-LEVEL LOW. GLOBAL MODELS FORECAST THE UPPER-LEVEL LOW TO MOVE WEST-SOUTHWEST OVER THE NEXT SEVERAL DAYS PLACING HANNA IN A LOW VERTICAL SHEAR ENVIRONMENT WITH AMPLE DIVERGENT FLOW ALOFT. SUCH AN ENVIRONMENT WOULD FAVOR INTENSIFICATION SHOULD IT MATERIALIZE. HOWEVER...AS IS OFTEN THE CASE...THERE IS A FINE LINE BETWEEN A FAVORABLE AND UNFAVORABLE ENVIRONMENT FOR STRENGTHENING. THE OFFICIAL FORECAST ASSUMES THERE WILL BE SUFFICIENT SPACING BETWEEN THE UPPER-LEVEL LOW AND HANNA TO PROMOTE STRENGTHENING. BY DAY 5...UPPER-LEVEL OUTFLOW FROM GUSTAV COULD IMPART NORTHERLY SHEAR OVER HANNA POSSIBLY AFFECTING THE INTENSITY.

HANNA HAS BEEN MOVING TOWARD THE WEST-NORTHWEST...OR 300/10...DURING THE LAST 6 HOURS. HOWEVER...A MOTION TOWARD THE NORTHWEST IS EXPECTED OVER THE NEXT 2-3 DAYS AS HANNA MOVES AROUND THE SOUTHWESTERN PERIPHERY OF THE SUBTROPICAL RIDGE. THEREAFTER...THE TRACK BECOMES MUCH LESS CERTAIN AS IT REMAINS TO BE SEEN HOW HANNA WILL INTERACT WITH A MID-LATITUDE TROUGH FORECAST TO MOVE OFF THE U.S. EAST COAST. MOST OF THE DYNAMICAL MODELS SHOW HANNA BEING LEFT BEHIND IN AN AREA OF WEAK STEERING. HOWEVER...THE TRADITIONALLY GOOD PERFORMING GFS FORECASTS HANNA TO TURN NORTHEASTWARD AT DAYS 4 AND 5. THE OFFICIAL FORECAST ASSUMES THE FORMER OF THOSE SCENARIOS AND SHOWS A SLOW WESTWARD TRACK.