Tropical Storm Fernanda
500 AM HST Fri Jul 21 2017
Deep convection persisted to the northeast of Fernanda overnight,
but separated further from the low level circulation center (LLCC)
due to increasing southwesterly shear. The latest subjective Dvorak
intensity estimates from PHFO and SAB came in at 35 and 25 knots
respectively, with the Advanced Dvorak Technique (ADT) from
UW-CIMSS coming in with 33 knots. These values are likely still a
bit too low given the slow spin down time of these systems. Given
the increasing shear and resultant separation of the deep convection
from the LLCC however, the initial intensity has been lowered to 40
knots for this advisory package.
The initial motion for this advisory is 280/10 kt. The spread
between the reliable guidance remains large. The GFS continues to
show a stronger and deeper system tracking off toward the northwest
for the next several days around the southwestern periphery of a
strong sub-tropical ridge. The ECMWF and to a lesser extent the
HWRF and FSSE objective aids, show a more shallow system steered
along primarily by the low level trade wind flow. The GFEX and TVCN
consensus guidance appear to be influenced too far to the north due
to the outlier GFS solution. Given the unfavorable environmental
conditions, the forecast track is very close to the previous
official forecast, closely in line with the ECMWF solution, and to
the south of the consensus guidance. As a result, the forecast track
takes Fernanda off to the west-northwest through the forecast
period.
Fernanda remains in an unfavorable environment, with sea surface
temperatures around 25C, very dry air aloft as evident in water
vapor imagery, as well as strong southwesterly shear of around 35
knots. Strong shear of 25 to 35 kt, very dry air aloft, and
unfavorable sea surface temperatures hovering between 25 and 26C
are forecast to persist through the forecast period. As a result,
the forecast will continue to show slow and steady weakening, with
Fernanda expected to become a Tropical Depression in 12 to 24
hours, a remnant low in 24 to 36 hours, then dissipating after 72
hours.
FORECAST POSITIONS AND MAX WINDS
INIT 21/1500Z 18.6N 143.6W 40 KT 45 MPH
12H 22/0000Z 19.0N 145.0W 35 KT 40 MPH
24H 22/1200Z 19.5N 146.7W 30 KT 35 MPH
36H 23/0000Z 20.1N 148.6W 25 KT 30 MPH…POST-TROP/REMNT LOW
48H 23/1200Z 20.7N 150.7W 25 KT 30 MPH…POST-TROP/REMNT LOW
72H 24/1200Z 22.1N 155.9W 25 KT 30 MPH…POST-TROP/REMNT LOW
96H 25/1200Z…DISSIPATED