Tropical Depression Five - 4am Thursday Discussion

By: National Hurricane Center
By: National Hurricane Center

TROPICAL DEPRESSION FIVE DISCUSSION NUMBER 2
NWS NATIONAL HURRICANE CENTER MIAMI FL AL052013
500 AM AST THU AUG 15 2013

SATELLITE IMAGERY THIS MORNING SUGGESTS THAT TROPICAL DEPRESSION FIVE IS GETTING BETTER ORGANIZED. A CONVECTIVE CLUSTER WITH TOPS COLDER THAN -80C HAS FORMED NEAR AND TO THE WEST OF THE CENTER...AND THERE IS OUTER BANDING IN THE SOUTHEASTERN SEMICIRCLE. SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB ARE BELOW TROPICAL STORM STRENGTH...SO THE CYCLONE WILL REMAIN A 30-KT DEPRESSION FOR THIS ADVISORY. HOWEVER...THIS INTENSITY COULD BE A LITTLE CONSERVATIVE.

THE INITIAL MOTION IS 290/14. THE DEPRESSION IS CURRENTLY ON THE SOUTH SIDE OF A LOW/MID-LEVEL RIDGE OVER THE EASTERN ATLANTIC...AND THE TRACK GUIDANCE IS IN GOOD AGREEMENT THAT THIS WILL STEER THE CYCLONE WEST-NORTHWESTWARD FOR THE NEXT 2-3 DAYS. AFTER THAT...THE GUIDANCE DIVERGES. THE GFS...THE GFS ENSEMBLE MEAN...AND THE CANADIAN MODELS SHOW THE SYSTEM TURNING WESTWARD. THE ECMWF...THE UKMET...AND THE GFDL SHOW A WEST-NORTHWESTWARD TO NORTHWESTWARD MOTION...WITH THE CYCLONE MOVING TOWARD A WEAKNESS IN THE RIDGE OVER THE CENTRAL ATLANTIC. OVERALL...THE GUIDANCE ENVELOPE HAS SHOWN A NORTHWARD SHIFT SINCE THE LAST ADVISORY. SINCE THE CYCLONE IS EXPECTED TO WEAKEN AND BE STEERED BY THE LOW-LEVEL FLOW DURING THIS TIME...THE NEW FORECAST TRACK SHOWS A WESTWARD TURN IN BEST AGREEMENT WITH THE GFS. WHILE THE NEW FORECAST TRACK IS A LITTLE NORTH OF THE PREVIOUS TRACK AFTER 48 HR...IT IS WELL TO THE SOUTH OF THE ECMWF...THE UKMET...THE GFDL...AND THE CONSENSUS MODELS.

THE DEPRESSION IS CURRENTLY IN AN ENVIRONMENT OF WARM SEA SURFACE TEMPERATURES AND LIGHT VERTICAL WIND SHEAR. THIS SHOULD ALLOW SOME STRENGTHENING UNTIL THE SYSTEM REACHES COOLER WATERS IN 48 HR OR SO. WHILE THE CYCLONE IS FORECAST TO LEAVE THE COOLER WATERS AFTER THAT TIME...THE GFS...ECMWF...AND UKMET ARE IN GOOD AGREEMENT THAT IT SHOULD ENCOUNTER VERY DRY AIR AND WEAKEN...WITH THESE MODELS SHOWING THE SYSTEM DEGENERATING TO A TROPICAL WAVE BY 120 HR. THE NEW INTENSITY FORECAST DOES NOT YET FOLLOW THAT SCENARIO...AS THE SHIPS AND LGEM MODELS SHOW A STEADY-STATE OR STRENGTHENING SYSTEM. INSTEAD...THE FORECAST CALLS FOR A SLOW WEAKENING AS A COMPROMISE BETWEEN THESE EXTREMES.


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