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Tropical Depression Fernanda Discussion Number  41
NWS Central Pacific Hurricane Center Honolulu HI   EP062017
500 PM HST Fri Jul 21 2017
 
Fernanda's presentation continues to worsen in satellite imagery.
Deep convection remains within the northeast quadrant of Fernanda's
low level circulation center (LLCC), but the western half of the
circulation is beginning to open up as low clouds dissipate. 
Subjective Dvorak satellite intensity estimates range from 25 kt,
from SAB, to 30 kt, from PHFO. UW-CIMSS provided an ADT intensity of
30 kt. ASCAT missed this system, but cloud motion imagery shows no
more than 35 kt at low cloud level along the western edge of the
LLCC. Reduction to 30 kt at the surface seems reasonable and is
therefore the initial intensity for this round. Fernanda is now a
tropical depression. 
 
Initial motion is 285/07kt, representing a stronger northerly
component to motion over the past 6 to 12 hours. Fernanda is moving
along the southern flank of the subtropical ridge, gradually gaining
latitude as it approaches a weakness in the ridge west of 160W. The
spread in track guidance decreased with this cycle as GFS belatedly
shifted leftward. ECMWF also shifted leftward, but much less so,
allowing the total envelope to narrow noticeably. NAVGEM and GFS
are the left and right outliers, respectively, with ECMWF just
to the right of NAVGEM. The forecast track closely follows the
previous one through 36 hours, but shifts to the left at 48 hours to
follow the guidance envelope trend. This places the track close to
but to the right of ECMWF, which has served us nicely so far. This
track terminates with system dissipation after 48 hours, before it
reaches the longitude of the main Hawaiian Islands.
 
Fernanda remains in an unfavorable environment, with sea surface
temperatures around 25C and between 30 and 35 knots of southwesterly
shear. These conditions will persist along the forecast track. As a
result, the forecast calls for steady weakening. Fernanda is
expected to become a remnant low in 24 hours, then dissipate after
48 hours.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  22/0300Z 19.0N 144.8W   30 KT  35 MPH
 12H  22/1200Z 19.4N 146.2W   30 KT  35 MPH
 24H  23/0000Z 20.0N 148.2W   25 KT  30 MPH...POST-TROP/REMNT LOW
 36H  23/1200Z 20.6N 150.3W   20 KT  25 MPH...POST-TROP/REMNT LOW
 48H  24/0000Z 20.9N 152.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
 72H  25/0000Z...DISSIPATED
 
$$
Forecaster Powell
 
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