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Tropical Depression SIX Forecast Discussion Number 3
2016-08-17 16:36:15| National Hurricane Center (Atlantic)
Issued at 1100 AM AST WED AUG 17 2016 000 WTNT41 KNHC 171436 TCDAT1 TROPICAL DEPRESSION SIX DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL062016 1100 AM AST WED AUG 17 2016 The depression's overall cloud pattern and low-level wind field have continued to improve, although cloud tops have warmed considerably near the center since the previous advisory. An 1139Z ASCAT-B overpass showed a well-defined low-level circulation center with a tight radius of maximum winds of only 10-15 nmi. Although there were a few vectors near tropical storm force, and satellite estimates at 1200Z were T2.5/35 kt from TAFB and SAB, the intensity is being maintained at 30 kt for this advisory due to the sharp decrease in deep convection near the center during the past few hours. The modest northeasterly vertical wind shear across the cyclone is forecast to weaken and veer to the southeast during the next 12 hours, and remain less than 10 kt for the next 36-48 hours, which typically favors strengthening. Sea-surface temperatures just below 27C are marginal for intensification and the small cyclone will be moving through dry mid-level air with humidity values dropping below 50 percent by 48 hours and beyond, conditions that are generally not conducive for significant strengthening. But given the tight inner-core wind field noted in recent ASCAT data, the cyclone is expected to be able to mix out any dry air intrusions and slowly strengthen for about the next 48 hours. By 72 hours and beyond, however, increasing southwesterly to westerly wind shear is expected to induce gradual weakening. The official intensity forecast is similar to the previous advisory and closely follows the intensity consensus model IVCN. The initial motion estimate is 300 /13 kt, based primarily on microwave and scatterometer fixes. The NHC model guidance is in good agreement on the cyclone moving west-northwestward to northwestward toward a weakness in the Bermuda-Azores ridge for the next 48 hours or so. After that, however, there is significant divergence in the models with the GFDL, GFS, and GFS-ensemble mean models taking a stronger and more vertically deep cyclone more toward the northwest, whereas the HWRF, UKMET, NAVGEM, and ECMWF models show a weaker and shallower cyclone turning more westward and moving along the southern periphery of the low-level ridge. As a result, the forecast track depends heavily on the strength and vertical structure of the cyclone. The new NHC forecast track is a little to the south or left of the previous advisory, but not nearly as far south as the ECMWF model due to expectations that the cyclone will not weaken nearly as much as that model is indicating. The forecast wind radii were decreased somewhat based on the smaller wind field depicted in recent ASCAT data. FORECAST POSITIONS AND MAX WINDS INIT 17/1500Z 14.0N 36.4W 30 KT 35 MPH 12H 18/0000Z 14.9N 37.7W 35 KT 40 MPH 24H 18/1200Z 16.0N 39.3W 40 KT 45 MPH 36H 19/0000Z 17.0N 40.7W 45 KT 50 MPH 48H 19/1200Z 17.9N 42.3W 50 KT 60 MPH 72H 20/1200Z 19.6N 45.9W 45 KT 50 MPH 96H 21/1200Z 21.8N 49.8W 40 KT 45 MPH 120H 22/1200Z 23.9N 53.4W 35 KT 40 MPH $$ Forecaster Stewart
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