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Tropical Storm Walaka Discussion Number 5
NWS Central Pacific Hurricane Center Honolulu HI CP012018
1100 AM HST Sun Sep 30 2018
Deep convection around Walaka has assumed a good wrapping pattern
this morning, increasing position confidence with development of a
relative dry slot along the southeast quadrant of the circulation.
The northern portion of this slot, which is also quite warm, may
soon develop into an eye. Convective tops remain quite cold, in the
-80 to -90 degree C range. Subjective Dvorak current intensity
estimates ranged from 3.0/45 kt from JTWC to 4.0/65 kt from SAB.
PHFO provided 3.5/55 kt while ADT from UW-CIMSS was 58 kt. The
initial intensity is set at 55 kt based on a mix of these
estimates.
Walaka is tracking 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
the tropical cyclone northwestward, then northward on Tuesday. The
system should curve back north-northwestward on Thursday as it
begins to interact with the upper trough. Track guidance is very
tightly clustered through Friday, greatly increasing confidence in
the timing of the curve northward. Our forecast closely follows the
previous one, with a slight tap to the west beyond 36 hours needed
to stay within the very tight guidance envelope. FSSE and CTCI
mark the right and left envelope limits, with little difference
between them through day 3. With the official forecast track
bringing the center of Walaka very near Johnston Atoll on Tuesday, a
Hurricane Watch remains 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 days 4 and 5. Our intensity forecast
trends are close to those in the previous advisory, following LGEM
and IVRI as Walaka strengthens, then following FSSE and CTCI as it
weakens.
FORECAST POSITIONS AND MAX WINDS
INIT 30/2100Z 11.9N 165.2W 55 KT 65 MPH
12H 01/0600Z 12.2N 167.1W 75 KT 85 MPH
24H 01/1800Z 13.0N 168.8W 95 KT 110 MPH
36H 02/0600Z 14.3N 169.9W 115 KT 130 MPH
48H 02/1800Z 16.4N 170.5W 120 KT 140 MPH
72H 03/1800Z 21.8N 169.6W 120 KT 140 MPH
96H 04/1800Z 27.7N 168.5W 85 KT 100 MPH
120H 05/1800Z 31.5N 167.9W 65 KT 75 MPH
$$
Forecaster Powell
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