Tropical Storm Otis Discussion Number 28
NWS National Hurricane Center Miami FL EP152017
800 AM PDT Mon Sep 18 2017
Otis is weakening at a remarkable rate. Only 12 hours ago, the
cyclone had an eye, but now most of the deep convection has
dissipated entirely. The cloud pattern now only supports an
intensity of about 45 kt, but Dvorak intensity estimates are
constrained to higher values by the rules of the technique. Given
the small size of Otis, it seems possible that this is a rare case
where the intensity is dropping faster than the Dvorak
technique allows. The initial intensity has been lowered to 60 kt,
but it is possible that Otis is even weaker than this. Continued
rapid weakening is expected, and most of the dynamical models show
Otis becoming a remnant low within about 36 h. The GFS and ECMWF
maintain a weak surface low for another couple days after that, but
suggest it will dissipate entirely shortly after 96 h.
Otis has continued to move northward over the past 6 hours, but
it appears to be slowing down. As the circulation becomes shallower
over the next 12 to 24 hours, Otis will be increasingly steered by
the low-level northeasterly flow associated with a ridge over the
central Pacific. This should cause the cyclone to make a sharp turn
toward the southwest later today or early tomorrow morning. Little
change has been made to the track forecast, which is close to an
average of the HCCA and TVCX consensus aids.
Hurricane Otis Discussion Number 26
NWS National Hurricane Center Miami FL EP152017
800 PM PDT Sun Sep 17 2017
Otis's rapid intensification rate has leveled off over the
past few hours, with the small clear eye not quite as distinct as
earlier. Still, the cloud pattern of the hurricane remains well
organized. While the 0000 UTC Dvorak estimates had subjective
values near 90 kt, a 6-hourly data-T average was 105 kt. As a blend
of these data, the initial wind speed is set to 100 kt. Otis is
moving over marginal SSTs now, so any further intensification is
expected to be slight. By tomorrow night, a combination of lower
SSTs, drier air and an increase in southerly shear is forecast to
cause a significant weakening of Otis. Almost all of the guidance
show rapid weakening by early Tuesday, and this is the solution
provided in the new NHC wind speed prediction. Global models
suggest Otis will lose organized deep convection within about 3
days, so the cyclone is forecast to degenerate into a remnant low at
that time.
Otis is moving faster to the north, now at 7 kt. The hurricane
should take a sharp westward turn in about 24 hours as it runs
into a building ridge over the eastern Pacific, then turn
west-southwest or southwest under that ridge. Very little change
was made to the previous forecast, which lies between the model
consensus and the ECMWF model.
Hurricane Otis Discussion Number 25
NWS National Hurricane Center Miami FL EP152017
200 PM PDT Sun Sep 17 2017
Otis continues to surprise us. Satellite images indicate that the
eye has become more distinct during the past few hours, and the
deep convection is wrapping around that center. The cyclone is
currently undergoing rapid intensification, and the initial
intensity is increased significantly to 90 kt based on a blend of
the latest satellite estimates. This makes Otis a category 2
hurricane on the Saffir-Simpson Hurricane Wind Scale. Even though
Otis is a powerful cyclone, recent ASCAT data confirms that the
wind field of the system is extremely compact.
Some additional strengthening is likely during the next 12 hours
or so while the hurricane remains in an environment of low wind
shear and over 27 deg C waters. After that time, however, lower
SSTs, drier air, and an increase in wind shear should cause
rapid weakening. Even though Otis will be moving over warmer waters
by the end of the forecast period, the models continue to show Otis
decaying due to unfavorable atmospheric conditions. The cyclone is
still forecast to become a remnant low by day 4 based on the global
models, but this is a low confidence prediction at this time.
The hurricane continues to move slowly northward at 4 kt. A
slow north to north-northwest motion is expected during the next
24 to 36 hours while Otis moves on the west side of a weak mid-level
ridge. Thereafter, a faster west and west-southwest motion is
expected when the weakening cyclone becomes more influenced by the
low-level tradewind flow. This track forecast is not too different
from the previous one, and it lies close to the various consensus
aids.
Tropical Storm Otis Discussion Number 21
NWS National Hurricane Center Miami FL EP152017
200 PM PDT Sat Sep 16 2017
The system has been maintaining a small area of deep convection
near the low-level center for the past several hours. The latest
Dvorak classification from TAFB and ADT values from CIMSS at the
University of Wisconsin have increased, and now support raising the
initial intensity to 35 kt, making the system a tropical storm.
Otis is expected to remain over 27 deg C waters and in a low
wind shear environment for the next couple of days, and this should
allow the cyclone to at least maintain its strength. Beyond that
time, an increase in shear, drier air, and cooler waters along the
expected track should cause weakening, and Otis is now
forecast to become a remnant low by day 4. The NHC intensity
forecast is slightly higher than the previous one, based on the
higher initial intensity.
The tropical cyclone continues to drift westward in weak steering
currents. The models remain in agreement that the storm should
turn slowly northward during the next day or two when it moves
along the west side of a weak mid-level ridge. Once the system
loses convection, a turn back toward the west or west-southwest is
predicted. The NHC track forecast is a little to the west of the
previous one in the short term to account for the more westward
initial position.
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