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Hurricane Miriam Discussion Number  21
NWS Central Pacific Hurricane Center Honolulu HI   EP152018
1100 PM HST Thu Aug 30 2018
 
The satellite presentation of Miriam has changed little over the 
past 6 hours, but a slight improvement in the convective cloud
coverage over the low level circulation center appears to have
occurred during the past several hours. A large area of -65 to -80C
cloud tops remain over the difficult to locate low level circulation
center. Several microwave passes since 30/2330Z have been helpful in
confidently determining the center location of Miriam as well as
estimating the motion of the system this evening, with a 0346Z
Windsat and 0636Z MHS Metop-A pass particularly useful. Subjective
Dvorak fixes from PHFO, SAB, and JTWC all came in with intensity
estimates of 4.5 (77 knots) with the Advanced Dvorak Technique from
UW-CIMSS slightly lower at 4.3 (72 knots). Several images around
31/06Z showed the center sufficiently within the deeper colder cloud
tops to yield a T5.0 (90 knots). Based on the slight improvement in
the convective cloud coverage over Miriam, and several images
around the synoptic hour yielding higher intensity estimates, the
initial intensity for this advisory has been weighted slightly
higher than the intensity estimates indicated from the various
agencies. The initial intensity has been raised to 80 knots with
this advisory with a motion of 360/07 knots.

Miriam is being steered by a large subtropical ridge to the 
northeast and a mid-upper level trough to the north and northwest.
A general northward motion is expected to continue tonight through 
Friday night as the system remains deep and intact following the 
deep layer steering flow. Strong southwesterly shear is then 
expected to decouple the system by late Friday night or early 
Saturday, and this is expected to result in a turn toward the 
northwest with Miriam being steered by the low level trade wind
flow. The official forecast has been shifted slightly to the right
through 48 hours to better align with the HWRF, TVCE, HCCA and GFS
which indicate a fairly deep circulation. Beyond 48 hours the
official forecast track lies roughly on top of the previous forecast
as the system becomes increasingly shallow and steered by the low
level trade wind flow.

It appears that the window for Miriam to intensify has closed, with 
the latest UW-CIMSS vertical shear analysis showing 21 knots of 
southwesterly shear over the core of the system. The hurricane is 
expected to slowly weaken tonight and Friday as shear steadily 
increases. Rapid weakening is then forecast Friday night and
Saturday as Miriam moves into a region of strong southwesterly shear
of 35 to 50 knots and moves over unfavorable sea surface
temperatures below 26C. Miriam is forecast to become a remnant low
by late Sunday. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  31/0900Z 17.4N 141.5W   80 KT  90 MPH
 12H  31/1800Z 18.7N 141.3W   75 KT  85 MPH
 24H  01/0600Z 20.5N 141.5W   65 KT  75 MPH
 36H  01/1800Z 22.3N 142.3W   50 KT  60 MPH
 48H  02/0600Z 24.2N 143.6W   35 KT  40 MPH
 72H  03/0600Z 27.7N 147.3W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  04/0600Z 31.0N 152.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  05/0600Z...DISSIPATED
 
$$
Forecaster Jelsema
 
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