Tropical Storm Hanna: Forecast Discussion #2

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.

FORECAST POSITIONS AND MAX WINDS

INITIAL 28/1500Z 20.5N 59.2W 35 KT
12HR VT 29/0000Z 21.4N 60.4W 40 KT
24HR VT 29/1200Z 22.8N 61.9W 45 KT
36HR VT 30/0000Z 24.4N 63.7W 55 KT
48HR VT 30/1200Z 25.7N 65.6W 60 KT
72HR VT 31/1200Z 27.0N 68.5W 70 KT
96HR VT 01/1200Z 27.5N 70.0W 75 KT
120HR VT 02/1200Z 27.5N 71.5W 70 KT


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