618 
 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