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Hurricane Walaka Discussion Number  13
NWS Central Pacific Hurricane Center Honolulu HI   CP012018
1100 AM HST Tue Oct 02 2018
 
Morning visible and infrared satellite imagery trends showing Walaka
has degraded somewhat over the past several hours.  The eye has
become less distinct with a clouds apparently partially covering the
eye. Cloud tops have warmed in the eyewall and surrounding
convective ring, and Dvorak intensity estimates started to follow
this weakening trend which began overnight. Current intensities
range from 6.0 at SAB to 6.5 at PHFO and JTWC. Have conservatively
lowered the initial intensity to 130 kt for this forecast cycle as
Satcon estimates as of 1528 UTC were at 132 knots, with a very slow
weakening in the short term as Walaka appears to be completing an
eyewall replacement cycle based on earlier composite microwave data
overnight. A solid ASCAT pass from 0816 UTC was useful to make minor
adjustments to the wind radii, and an altimeter pass aided in
adjusting the 12 foot seas radii.

The initial motion was set at 355/09 knots, as Walaka continues to
be drawn northward by the deep low pressure system to the north.
This motion is expected to continue over the next day or two with
some acceleration as the influence of this low pressure over the
Walaka steering currents increases.  By Thursday and Friday,
Walaka slows as it interacts with the mid latitude trough which
remains cutoff from the mid latitude westerlies aloft.  By early
Saturday, another sharp upper trough picks the tropical cyclone up
and shifts it off to the northeast through the weekend, resulting in
accelerating northeastward motion. Overall, the track guidance
envelope remains fairly tightly clustered through Friday, with some
variations on how quickly Walaka gets picked up by the westerlies
over the weekend.  The official forecast stays very close to the
previous advisory and is only slightly nudged towards HCCA and ECMWF
beyond Friday.
 
Although experiencing a weakening trend since last night, the
environment surrounding Walaka remains somewhat conducive for
intensification during the next 12 to 18 hours. The tropical
cyclone will remain within a deep moist airmass with low vertical
wind shear, high ocean heat content and sea surface temperatures
near 29 or 30 degrees C. The forecast shows continued weakening in
the short term, due to current trends, then steady intensity
tonight through early Wednesday, as this period has the potential
for another round of intensity fluctuations within aforementioned 
conducive conditions.  Between 24 and 48 hours, vertical wind
shear increases dramatically due to interaction with the upper
level jet around the deep low to the north, resulting in steady
and rapid weakening of Walaka especially Thursday and Friday.
Weakening trend relaxes Saturday and Sunday as Walaka accelerates
to the northeast and transitions to an extratropical cyclone. The
official forecast follows the SHIPS and dynamical intensity
consensus through the 5 day forecast, except holds the intensity up
through the first 24 hours owing to potential fluctuations. 
 
The forecast track will take Walaka near Johnston Island later
today, with hurricane conditions expected this afternoon and this
evening. Therefore a Hurricane Warning remains in effect for this
location. The forecast track also takes the hurricane across the
Papahanaumokuakea Marine National Monument between French Frigate
Shoals and Maro Reef late Wednesday, therefore, a Hurricane
Warning has been issued for this area.  A Tropical Storm Warning
has been issued east of French Frigate Shoals to Nihoa as
tropical storm conditions are expected to reach that area late
Wednesday.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  02/2100Z 15.6N 170.1W  130 KT 150 MPH
 12H  03/0600Z 17.4N 169.9W  125 KT 145 MPH
 24H  03/1800Z 20.5N 169.1W  125 KT 145 MPH
 36H  04/0600Z 23.9N 167.8W  110 KT 125 MPH
 48H  04/1800Z 27.0N 167.3W   85 KT 100 MPH
 72H  05/1800Z 31.0N 167.2W   65 KT  75 MPH
 96H  06/1800Z 35.3N 162.7W   55 KT  65 MPH
120H  07/1800Z 44.6N 154.2W   45 KT  50 MPH...POST-TROP/EXTRATROP
 
$$
Forecaster Brenchley
 
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