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Hurricane WILLA


Hurricane Willa Discussion Number   9
NWS National Hurricane Center Miami FL       EP242018
300 AM MDT Mon Oct 22 2018

Willa is an impressive hurricane with rapid to explosive deepening
having occurred over the past 48 hours, and yielding a 105-kt
intensity increase during that time. Satellite intensity estimates
range from subjective Dvorak T-numbers of T6.5/127 kt from TAFB and
SAB to objective T-numbers of T7.0/140 kt from UW-CIMSS ADT and NHC.
The most recent UW-CIMSS SATCON value around 0437Z was 127 kt. Based
on these data and the slightly more ragged appearance of the CDO
over the past couple of hours, the intensity is raised to 135 kt,
which is just below category 5 strength.

Willa is now moving northward and the initial motion estimate is
360/06 kt. Willa is forecast to continue moving northward today as
the hurricane rounds the western portion of a deep-layer ridge
located over central Mexico and which extends westward along 20N
latitude to near the Pacific coast of Mexico. On Tuesday, a turn
toward the north-northeast and northeast at a slightly faster
forward speed is expected as the hurricane gets caught up in
southwesterly flow ahead of an approaching mid-/upper-level trough.
The latest 00Z NHC model guidance is in decent agreement on this
general track scenario, but with noticeable differences in the
forward speed. Internal statistical analyses show that the ECMWF
and UKMET models have had a slow westward bias over the past 48
hours, which has significantly affected the consensus models. As a
result, the new NHC forecast is a little to the right of and
slightly faster the previous advisory track and the consensus
models, and lies close to GFS model which has verified the best
thus far. On the forecast track, Willa is expected to move over Las
Islas Marias on Tuesday and then move inland over southwestern and
west-central Mexico Tuesday night or early Wednesday. A 72-hour
position has been provided only for continuity purposes beyond the
48-hour period, and Willa will likely have dissipated by then.

Water temperatures ahead of the powerful hurricane are forecast to
increase from 28 deg C to near 29.5 deg C, and the depth of the warm
water is also expected to increase, which would tend to offset any
cold upwelling beneath Willa. The deep-layer shear is forecast to
increase to a little more than 10 kt during the next 36 hours, and
remain at that level during the duration of that time period. These
conditions would favor at least slight strengthening before the
internal dynamics of the hurricane in the form of an eyewall
replacement cycle (ERC) induces some weakening. With the eye
diameter already at 10-15 nmi, an ERC could begin at any time
during the next 12 hours. Shortly after 36 hours, the shear is
expected to increase sharply to more than 20 kt, which will cause
more significant weakening. However, Willa is still expected to be
near major hurricane strength when it makes landfall along the
west-central coast of Mexico. After landfall, the mountainous
terrain along with strong southwesterly wind shear of more than 30
kt will cause rapid weakening and dissipation of the cyclone inland
over north-central Mexico.

Key Messages:

1. Willa is forecast to be a dangerous hurricane when it reaches the
Islas Marias and the coast of southwestern Mexico by late Tuesday or
Wednesday morning, bringing a life-threatening storm surge and flash
flooding along with damaging hurricane-force winds. A Hurricane
Warning is in effect for these areas, and residents should rush
preparations to completion to protect life and property and follow
any advice given by local officials.


INIT  22/0900Z 18.2N 107.1W  135 KT 155 MPH
 12H  22/1800Z 19.2N 107.2W  135 KT 155 MPH
 24H  23/0600Z 20.4N 107.2W  125 KT 145 MPH
 36H  23/1800Z 21.8N 106.5W  115 KT 130 MPH
 48H  24/0600Z 23.8N 104.6W   50 KT  60 MPH...INLAND WNCTRL MEXICO
 72H  25/0600Z 27.0N  99.5W   20 KT  25 MPH...POST-TROP/INLAND

Forecaster Stewart