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Hurricane Walaka Discussion Number  19
NWS Central Pacific Hurricane Center Honolulu HI   CP012018
1100 PM HST Wed Oct 03 2018

Walaka's appearance has continued to degrade this evening, with no
signs of an eye. Strong vertical wind shear is likely to blame for
this. Fortunately, a plethora of microwave images were available
from the FNMOC and NRL web sites earlier this evening.  These
passes, which were useful for determining the location of the
low-level circulation center (LLCC), indicated the eyewall
structure appeared to remain somewhat intact, although there has
been some degradation in the southeastern quadrant. As a result of
this continued weakening, the latest subjective Dvorak current
intensity estimates were 5.5/102 kt from JTWC and PHFO, and 4.5/77
kt from SAB. The latest UW-CIMSS ADT estimate was 5.8/110 kt.
The initial intensity for this advisory is 110 kt.
 
Walaka's initial motion for this advisory is 025/19 kt. The track
forecast guidance remains tightly clustered, but it has shifted
slightly to the left during the 12 to 36 hour time periods.
Therefore, we have had to make some minor adjustments in the current
track forecast compared with the previous advisory during the 12 to
48 hour time periods. This is primarily due to the way the models
are handling the much anticipated interaction with a deep layer low
centered near 30N 170W this evening. Since there is a great deal of
uncertainty about how warm core Walaka will respond to the close
encounter with this baroclinic feature, we are assuming it will
survive the event intact. So with this uncertainty in mind, we have
Walaka moving northward at a slower speed within the next 12 hours,
followed by a turn toward the north-northwest between 12 and 24
hours. After that, again assuming Walaka is a viable system, it is
expected to turn toward the north-northeast by the 36 hour time
period. An accelerating north-northeast motion is forecast during
the 48 to 72 hour time periods.
 
With continued strong vertical wind shear into Thursday and sea
surface temperatures (SST) dropping to around 27 degrees C later
tonight, Walaka will likely experience a more rapid weakening trend
through early Thursday morning. We continue to weaken it below major
hurricane status within 12 hours. By late Thursday, the guidance
continues to suggest the vertical wind shear may relax somewhat.
However, water temperatures and ocean heat content will become
limiting factors, since SST would likely cool to 24-25 degrees C.
As was mentioned above, we are still not certain how the interaction
with the cold core low will affect Walaka. Assuming it survives
intact, we continue to maintain it as a tropical storm between 36
and 48 hours. After that, extratropical transition will likely
occur, so that Walaka will likely be an extratropical low by day 3.
Note that the latest intensity forecast was adjusted slightly up
from the previous package during the 36 to 72 hour time periods.
This is more in line with the latest GFS and ECMWF output.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  04/0900Z 24.8N 166.7W  110 KT 125 MPH
 12H  04/1800Z 27.5N 166.5W   90 KT 105 MPH
 24H  05/0600Z 30.3N 167.2W   75 KT  85 MPH
 36H  05/1800Z 32.5N 166.6W   60 KT  70 MPH
 48H  06/0600Z 35.4N 164.2W   55 KT  65 MPH
 72H  07/0600Z 46.0N 156.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  08/0600Z...DISSIPATED
 
$$
Forecaster Houston
 
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