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Tropical Storm FIONA Forecast Discussion Number 5
2016-08-18 04:44:45| National Hurricane Center (Atlantic)
Issued at 1100 PM AST WED AUG 17 2016 000 WTNT41 KNHC 180244 TCDAT1 TROPICAL STORM FIONA DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL062016 1100 PM AST WED AUG 17 2016 For a time late this afternoon, it seemed that Fiona might lose all its deep convection. However, a new burst developed around 2300 UTC and has persisted since that time. An 0026 UTC ASCAT-B pass revealed that the cyclone is still producing a small area of tropical-storm-force winds, and the intensity remains 35 kt. Fiona has a tough road ahead. Vertical shear right now is relatively low, but it is expected to begin increasing within the next 24 hours and then become belligerent out of the southwest by days 3 and 4. In addition, the cyclone is surrounded by dry air, and any negative impacts that the dry air is already having on the convection is likely to be compounded once the shear increases in a few days. With the exception of the GFDL, which makes Fiona a hurricane by the end of the forecast period, the rest of the intensity models respond to the unfavorable environment by showing relatively little strengthening. The NHC forecast follows this same thinking, and it shows Fiona only modestly strengthening during the next 36 hours, with some weakening probable by day 3. Based on the latest intensity guidance, the new NHC forecast has been adjusted downward from the previous forecast, but it still lies above the consensus at most forecast times. The recent ASCAT data helped to fix the center, and the initial motion estimate is now west-northwestward, or 300/14 kt. A break in the mid-tropospheric ridge to the north of Fiona is expected to to migrate westward during the next few days, but the cyclone's shallow nature is expected to help maintain a west-northwestward motion for much of the forecast period. The bulk of the track models are tightly clustered on such a heading. However, the GFDL (which makes Fiona hurricane) shows a sharp northward turn, while the HWRF (which essentially shows no intensification) takes a much more southern track. Since these two scenarios appear to cancel each other out, the NHC official forecast closely follows the consensus models, requiring a westward adjustment to the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 18/0300Z 15.5N 39.0W 35 KT 40 MPH 12H 18/1200Z 16.3N 40.4W 35 KT 40 MPH 24H 19/0000Z 17.1N 41.9W 40 KT 45 MPH 36H 19/1200Z 17.8N 43.5W 45 KT 50 MPH 48H 20/0000Z 18.6N 45.2W 45 KT 50 MPH 72H 21/0000Z 20.6N 49.1W 40 KT 45 MPH 96H 22/0000Z 22.5N 53.0W 40 KT 45 MPH 120H 23/0000Z 24.5N 57.0W 40 KT 45 MPH $$ Forecaster Berg
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