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Tropical Storm Fernanda Forecast Discussion


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000
WTPZ41 KNHC 200846
TCDEP1

Tropical Storm Fernanda Discussion Number  34
NWS National Hurricane Center Miami FL       EP062017
200 AM PDT Thu Jul 20 2017

Fernanda is beginning to experience moderate southwesterly vertical
wind shear, which has resulted in a pronounced degradation of the
convective cloud pattern since the previous advisory. Recent AMSU
and SSMI/S microwave satellite data indicate that the shear has
tilted the vortex toward the north, resulting in the initial
position being located a little south of the satellite fix
positions and closer to the southwestern edge of the coldest cloud
tops. The initial intensity has been lowered to 60 kt based on a
blend of consensus T-numbers of T3.5/55 kt and CI-numbers of T4.0/65
kt from both TAFB and SAB.

Fernanda's motion is now 290/08 kt. The weakening cyclone is
expected to continue moving generally west-northwestward along the
southern periphery of a broad deep-layer subtropical ridge for about
the next 72 hours. By days 4 and 5, a faster westward motion is
forecast as the shallow vortex comes under the influence of the
brisk low-level easterly trade wind flow. The latest GFS model run
appears to keep Fernanda too strong and vertically deep after 48 h,
which has resulted in a track well north of any of the other model
guidance. As a result, the new NHC forecast has been shifted a
little south of the previous advisory track through 72 hours, and
is then close to the middle of the guidance envelope after that.

Fernanda should steadily succumb to the effects of cooler water,
drier air, and shear increasing to 30 kt throughout the forecast
period, resulting in weakening and degeneration into a remnant low
by around 48 hours. It is possible that the system could even open
up into a tropical wave by 96-120 hours when it is passing just
north of the Hawaiian Islands. The official intensity forecast is
similar to the previous advisory and closely follows the IVCN
consensus model.

FORECAST POSITIONS AND MAX WINDS

INIT  20/0900Z 18.2N 137.7W   60 KT  70 MPH
 12H  20/1800Z 18.5N 138.9W   50 KT  60 MPH
 24H  21/0600Z 19.0N 140.7W   40 KT  45 MPH
 36H  21/1800Z 19.5N 142.5W   35 KT  40 MPH
 48H  22/0600Z 20.1N 144.2W   30 KT  35 MPH...POST-TROP/REMNT LOW
 72H  23/0600Z 21.6N 147.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  24/0600Z 22.7N 152.2W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  25/0600Z 23.8N 158.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Stewart