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Tropical Storm FIONA Forecast Discussion Number 18
2016-08-21 10:51:14| National Hurricane Center (Atlantic)
Issued at 500 AM AST SUN AUG 21 2016 000 WTNT41 KNHC 210851 TCDAT1 TROPICAL STORM FIONA DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL AL062016 500 AM AST SUN AUG 21 2016 After the earlier convective bursting episode and resultant spin up of the low-level wind field, strong westerly shear of at least 30 kt has stripped away the convective cloud shield, leaving a fully exposed low-level circulation center that is easy to locate. Assuming some spin down of the circulation due to the loss of convection, the initial intensity has been lowered to 40 kt. Now that Fiona's circulation has become a little more shallow, the low-level center has turned more westward and the cyclone is now moving 295/14 kt. The global models are in good agreement on a mid-level ridge to the north and northeast of Fiona remaining in place for the next few days, which should keep the small cyclone moving in a general west-northwest direction through 72 hours. After that time, the western portion of the ridge is expected to weaken due to an approaching frontal system, which should allow Fiona or its remnant circulation to slow down and turn northwestward. The new track forecast is essentially just an update of the previous advisory track, and lies close to the TVCN consensus model forecast. During the next 24 hours or so, Fiona is forecast to traverse through a band of strong westerly to northwesterly vertical wind shear of 25-30 kt and dry mid-level air with humidity values near 40 percent. As a result, gradual weakening is expected due to the small cyclone likely not being able to sustain deep convection for long periods of time. The new intensity forecast holds onto the previous trend of Fiona weakening into a remnant low by 72 hours. However, a lot of uncertainty remains with this intensity forecast. Strong instability that will be present due to very warm SSTs of 29C-30C and a cool upper-level troposphere, which could produce periodic bursts of intense convection that could sustain the low-level vortex until environmental become more favorable for strengthening on days 4 and 5. Most of the global models continue to depict a weak low for at least the next 5 days, except for the ECMWF model, which weakens Fiona to an open trough by 48 hours. The NHC intensity remains a compromise of these extremes and shows dissipation by day 5. FORECAST POSITIONS AND MAX WINDS INIT 21/0900Z 22.2N 51.7W 40 KT 45 MPH 12H 21/1800Z 22.9N 53.7W 35 KT 40 MPH 24H 22/0600Z 23.8N 56.3W 35 KT 40 MPH 36H 22/1800Z 24.5N 58.6W 35 KT 40 MPH 48H 23/0600Z 25.3N 60.7W 30 KT 35 MPH 72H 24/0600Z 27.0N 64.2W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 25/0600Z 29.7N 66.3W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 26/0600Z...DISSIPATED $$ Forecaster Stewart
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