Hurricane Fernanda & Tropical Storm Greg Heading West in Pacific

Tropical Storm Greg will pass far South of Hawaii Thursday with tropical storm winds. We should get some higher surf but not much rain or wind. Hurricane Fernanda should be a post depression when it comes very close to Hawaii. Not sure yet of it’s impact. There is a 3rd disturbance in the Eastern Pacific expected to become a system within 48 hours.

500 AM HST Mon Aug 14 2023

Deep convection associated with Greg continues to develop near and 
west of the low-level circulation this morning, which has become 
partially exposed due to southeasterly shear estimated to be near 
10 to 15 kt. An ASCAT pass from several hours ago, however, 
revealed the system has a well-defined circulation, with 35 kt 
winds depicted north of the center. Based on the ASCAT data and 
subjective Dvorak intensity estimates ranging from T2.0 to T3.0 
between PHFO, SAB, AND JTWC, the initial intensity is held at 35 kt.

The initial motion for this advisory is 270/11 kt, as Greg 
continues to move south of the subtropical ridge. No significant 
change in this steering is expected over the next couple of days. 
Thereafter, a gradual turn to the west-southwest is forecast as the 
weakening system becomes steered by low- to mid-level flow. Similar 
to the previous advisory, the track guidance continues to shift 
slowly southward, with Greg passing far south of the Hawaiian 
Islands later this week. This forecast track reflects that and lies 
near a combination of the previous forecast, HCCA, and TVCE.  

Despite the tropical storm being located over very warm waters and 
within a relatively moist environment, the aforementioned 
southeasterly shear will continue to inhibit strengthening 
initially. Guidance shows this shear persisting over the next 12 to 
18 h before trending down. Therefore, the intensity is held steady 
for the first 12 h, then followed by some modest strengthening 
between 24 and 48 h. Beyond that, drier mid-level air along with 
increasing shear and slightly cooler waters should result in a 
gradual weakening trend, with Greg becoming post-tropical by day 5.


FORECAST POSITIONS AND MAX WINDS

INIT  14/1500Z 11.3N 140.9W   35 KT  40 MPH
 12H  15/0000Z 11.4N 142.6W   35 KT  40 MPH
 24H  15/1200Z 11.6N 144.9W   40 KT  45 MPH
 36H  16/0000Z 11.8N 147.3W   45 KT  50 MPH
 48H  16/1200Z 12.2N 149.7W   45 KT  50 MPH
 60H  17/0000Z 12.6N 152.2W   45 KT  50 MPH
 72H  17/1200Z 12.7N 154.7W   40 KT  45 MPH
 96H  18/1200Z 12.3N 159.8W   35 KT  40 MPH
120H  19/1200Z 11.7N 164.9W   30 KT  35 MPH...POST-TROP/REMNT LOW

500 AM HST Mon Aug 14 2023

Fernanda has continued to intensify this morning.  The 10 nmi- 
diameter eye is very well defined and is surrounded by a ring of
-70 deg C or colder convective cloud tops.  The hurricane's
upper-level outflow pattern remains quite symmetric.  Dvorak data
T-numbers for the system are now around 6.0 which supports an
intensity of 115 kt, i.e. category 4 intensity.  This is also in
good agreement with objective ADT estimates from UW-CIMSS.

A westward to west-northwestward track continues with an initial
motion estimate of about 280/7  kt.  There are no significant
changes to the track forecast reasoning from the previous
advisory.  A weak mid-tropospheric ridge to the north of
Fernanda should gradually strengthen over the next several days.
This should result in a continued west-northwestward or slightly
north of westward movement with some increase of forward speed over
the next several days.  Late in the forecast period, the weakening
cyclone should move mostly westward, following the low-level
environmental flow.  The official track forecast is close to both
the simple and corrected dynamical model consensus guidance.

The hurricane should traverse SSTs of 28-29 deg C today while
remaining in a low-vertical wind shear environment.  Therefore some
additional strengthening will probably occur today.  By early
tomorrow and thereafter, Fernanda should begin to encounter the
drier low- to mid-level air mass that is seen in the GOES-18
lower-level water vapor imagery not far to the west-northwest of the
tropical cyclone.  This, along with cooler ocean waters, should
result in steady weakening of the system over the next 2-5 days.
The official intensity forecast calls for Fernanda to degenerate
into a post-tropical cyclone at 96 h. However, based on the
latest simulated satellite imagery from the global models, a faster
rate of weakening is certainly possible.


FORECAST POSITIONS AND MAX WINDS

INIT  14/1500Z 15.8N 118.5W  115 KT 130 MPH
 12H  15/0000Z 16.3N 119.6W  125 KT 145 MPH
 24H  15/1200Z 17.0N 121.4W  105 KT 120 MPH
 36H  16/0000Z 17.4N 123.6W   90 KT 105 MPH
 48H  16/1200Z 17.6N 126.2W   75 KT  85 MPH
 60H  17/0000Z 17.8N 128.9W   60 KT  70 MPH
 72H  17/1200Z 18.0N 131.8W   45 KT  50 MPH
 96H  18/1200Z 18.3N 137.9W   35 KT  40 MPH...POST-TROPICAL
120H  19/1200Z 18.5N 144.0W   25 KT  30 MPH...POST-TROP/REMNT LOW