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Hurricane Walaka Discussion Number 6
NWS Central Pacific Hurricane Center Honolulu HI CP012018
500 PM HST Sun Sep 30 2018
Walaka's well defined deep convective wrapping pattern has morphed
into a cloudy eye pattern in visible imagery this afternoon.
Satellite loop shows system outflow is good across all quadrants
and overall system symmetry and organization continue to improve.
Subjective Dvorak current intensity (CI) estimates range from
4.0/65 kt from JTWC and PHFO to 4.5/77 kt from SAB. ADT from
UW-CIMSS is 68 kt. Given the presence of a cloudy eye and higher
overall CI estimates from the analysis centers, we will set the
initial intensity at 65 kt for this advisory. Walaka is now a
hurricane.
The overall forecast philosophy for Walaka is unchanged. This
system continues moving slightly north of due west along the
southern flank of the subtropical ridge. Global models show a deep
upper trough will dig southward, eroding the western edge of the
subtropical ridge to the north of Walaka on Monday. This will steer
Walaka northwestward, then northward on Tuesday. Track guidance
remains very tightly clustered through Friday, greatly increasing
confidence in the timing of the curve northward. Our forecast
closely follows the previous one, with another tap to the west from
12 to 36 hours needed to stay within the very tight guidance
envelope. A small tap to the east at 72 hours was needed for the
same reason. With the official forecast track bringing the center of
Walaka very near Johnston Atoll on Tuesday, a Hurricane Warning is
now in effect there.
Walaka remains within ideal conditions for strengthening, with high
SSTs, low shear, high ocean heat content and plenty of deep
moisture. SHIPS shows vertical shear will remain below 20 kt
through 48 hours, ramping up to near 30 kt by day 3 and beyond. SSTs
stay above 28 degrees C until day 3 even as Walaka tracks northward.
Our forecast calls for rapid intensification through day 2, with
Walaka reaching major hurricane status on Monday. After leveling
off between days 2 and 3, increasing vertical shear and decreasing
SSTs should begin to take their toll on this system. Steady
weakening is forecast for day 3 and beyond. Our intensity forecast
trends are close to those in the previous advisory, following HWRF.
Some of the intensity guidance, like SHIPS, strengthens Walaka into
the 130 to 140 kt range by day 2, with the probability for rapid
intensification exceeding 90%. While we cap this system at 120 kt
for this advisory, subsequent advisory packages may begin to
reflect a stronger maximum intensity.
FORECAST POSITIONS AND MAX WINDS
INIT 01/0300Z 11.9N 166.4W 65 KT 75 MPH
12H 01/1200Z 12.3N 167.9W 85 KT 100 MPH
24H 02/0000Z 13.3N 169.4W 105 KT 120 MPH
36H 02/1200Z 14.8N 170.3W 115 KT 130 MPH
48H 03/0000Z 17.2N 170.3W 120 KT 140 MPH
72H 04/0000Z 22.7N 168.8W 110 KT 125 MPH
96H 05/0000Z 28.3N 168.3W 80 KT 90 MPH
120H 06/0000Z 32.1N 167.3W 60 KT 70 MPH
$$
Forecaster Powell
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