013 
 WTNT41 KNHC 172042
 TCDAT1
 
 TROPICAL STORM FIONA DISCUSSION NUMBER   4
 NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
 500 PM AST WED AUG 17 2016
 
 During the past 6 hours, the tropical cyclone has made a transition
 from a large outer banding pattern to more of a CDO-type feature.
 Also, a 1635Z SSMI overpass confirmed the tight inner-core structure
 noted in the two earlier ASCAT passes. Dvorak satellite intensity
 estimates are T2.5/35 kt from TAFB and SAB, and a 1225Z ASCAT-A
 overpass indicated winds of near 35 kt at that time. Based on these
 data, the initial intensity is increased to 35 kt and the cyclone is
 upgraded to Tropical Storm Fiona.
 
 The initial motion estimate is 305/14 kt. The latest model guidance
 has come into much better agreement with the previous track
 forecast, with the GFS model shifting a little farther south and the
 ECMWF making a significant shift to the north due to that model not
 weakening Fiona as much now as compared to previous forecast cycles.
 These model changes have resulted in a northward shift in the
 consensus models TVCN and GFEX. Given the much better agreement in
 the NHC model guidance, the new track forecast was only shifted
 slightly southward of the previous advisory track after 72 hours in
 order to move closer to the consensus models.
 
 There are no significant changes to the previous intensity forecast.
 Fiona is a compact tropical cyclone with a radius of maximum winds
 of 15 nmi or less, which makes the storm susceptible to sharp
 fluctuations in intensity. The aforementioned SSMI satellite pass
 also indicated that the small inner-core region of Fiona now appears
 to be insulated from the very dry mid-level air that had earlier
 eroded the cyclone's convection. That, along with low shear
 conditions and marginal SSTs near 27C, should allow for some gradual
 strengthening for the next 36-48 hours, assuming that the cyclone
 can mix out any occasional intrusions of dry air. By 72 hours, the
 vertical shear is forecast to increase to around 20 kt from the
 southwest, which should act to arrest the intensification process
 and perhaps even induce some slight weakening. However, by 96 hours
 and beyond, the shear is forecast to steadily decrease when the
 cyclone will be moving over SSTs greater than 28C. These conditions
 could allow for the compact cyclone to maintain a steady intensity
 despite being embedded in mid-level humidity values of near 50
 percent. The official intensity forecast is similar to the previous
 advisory and closely follows the intensity consensus model IVCN
 through 48 hours, and then lies just below the Decay-SHIPS model at
 72-120 hours.
 
 FORECAST POSITIONS AND MAX WINDS
 
 INIT  17/2100Z 15.1N  37.8W   35 KT  40 MPH
  12H  18/0600Z 16.0N  39.0W   40 KT  45 MPH
  24H  18/1800Z 17.0N  40.6W   45 KT  50 MPH
  36H  19/0600Z 17.8N  42.1W   50 KT  60 MPH
  48H  19/1800Z 18.7N  43.8W   50 KT  60 MPH
  72H  20/1800Z 20.3N  47.4W   45 KT  50 MPH
  96H  21/1800Z 22.6N  51.3W   45 KT  50 MPH
 120H  22/1800Z 24.6N  55.0W   45 KT  50 MPH
 
 $$
 Forecaster Stewart