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Tropical Storm FIONA Forecast Discussion Number 13
2016-08-20 04:47:09| National Hurricane Center (Atlantic)
Issued at 1100 PM AST FRI AUG 19 2016 000 WTNT41 KNHC 200246 TCDAT1 TROPICAL STORM FIONA DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL AL062016 1100 PM AST FRI AUG 19 2016 A 2346 UTC ASCAT-B overpass helped to locate the center, which is beneath the cirrus canopy but along the southern edge of the deep convection. The scatterometer pass suggested that the maximum winds could be as high as 40 kt, but the highest vectors had questionable directions and were on the edge of the pass. Therefore, the intensity remains 35 kt, which is supported by subjective and objective satellite intensity estimates. Southwesterly vertical shear has increased to about 20 kt and is forecast to go up further during the next 48 hours or so. Given the increasing shear, a relatively dry middle troposphere, and Fiona's small size, the cyclone is expected to gradually degrade during the next few days. The intensity and global models either maintain Fiona's intensity or show weakening through the forecast period, and the ECMWF even opens the system up into a trough by day 3 or 4. Given that Fiona will likely struggle to maintain deep convection as environmental conditions worsen, the NHC forecast now shows the cyclone degenerating into a remnant low by day 3. Although a remnant low is carried through day 5, dissipation is possible before the end of the forecast period. The initial motion remains west-northwestward but a little faster at 295/10 kt. The track models are in good agreement on Fiona accelerating a bit along this heading during the next 48 hours, and no significant changes to the NHC forecast was required during that time. After 48 hours, however, the track forecast is highly contingent on Fiona's status as a tropical cyclone, a remnant low, or a trough. The ECMWF, which dissipates the cyclone, has a track well to the south and west of the other models, while the GFS, GFS ensemble mean, and Florida State Superensemble still show a northward turn by day 5. Regardless, the entire guidance envelope has shifted westward significantly after 48 hours. The NHC official forecast has been moved in that direction, but it still lies east of the various consensus models. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 18.7N 45.5W 35 KT 40 MPH 12H 20/1200Z 19.4N 47.2W 35 KT 40 MPH 24H 21/0000Z 20.5N 49.5W 35 KT 40 MPH 36H 21/1200Z 21.6N 52.0W 30 KT 35 MPH 48H 22/0000Z 22.6N 54.7W 30 KT 35 MPH 72H 23/0000Z 24.6N 59.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 24/0000Z 26.5N 62.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 25/0000Z 29.0N 63.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Berg
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