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Hurricane Miriam Discussion Number  22
NWS Central Pacific Hurricane Center Honolulu HI   EP152018
500 AM HST Fri Aug 31 2018
 
Miriam is beginning to feel the effects of increasing southwesterly 
shear, with the latest UW-CIMSS analysis depicting 30 knots of
shear over the system. Despite the increasing shear, the system
continues to hold a large area of deep convection with -65 to -80C
cloud tops over the low level circulation center. The latest
subjective Dvorak intensity estimates from PHFO, SAB, and JTWC all
came in at 4.5 (77 knots) with the Advanced Dvorak Technique from
UW-CIMSS slightly higher 4.6 (80 knots). The initial intensity for
this advisory will be held at 80 knots based on a blend of these
intensity estimates and recent satellite imagery suggesting the
intensity could be as high as T5.0 (90 knots). The motion is
somewhat uncertain given the deep convection over the center of
Miriam and a lack of timely microwave data. Using a combination of
interpolation of the movement when confidence in the center position
was high, along with current satellite trends, the motion is set at
005/09 knots.  

Hurricane Miriam is being steered by a large subtropical ridge to 
the east and northeast along with a deep mid-upper level trough to 
the north and northwest. A general northward motion is expected to 
continue today and tonight as the system remains fairly deep and 
follows the deep layer steering flow. Strong southwesterly shear is 
then expected to decouple the system by late tonight 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 nudged slightly to the left 
through 24 hours based on the estimated initial motion slightly to 
the east of due north. Beyond 24 hours the track forecast lies 
virtually on top of the forecast from the previous advisory. The 
track forecast closely follows the latest TVCE/HCCA consensus 
guidance through 48 hours, then is slightly to the left of the 
consensus guidance beyond 48 hours expecting a fairly shallow
system being steered by the trades and more in line with the GFS
solution. 

Miriam peaked at an intensity of 80 knots overnight, a high end 
category one hurricane, and it appears likely that this will be as 
strong as the system gets. Southwesterly shear is forecast to 
increase further today, with 35 to 50 knots of shear expected over 
the cyclone tonight through the end of the forecast period, while 
the system is passing over sea surface temperatures below 26C.
Rapid weakening should begin later today, with Miriam expected to
drop below hurricane strength late today or tonight, and become a
post-tropical remnant low Sunday or Sunday night. The intensity
forecast is closely aligned with the latest dynamical and consensus
guidance which were all in good agreement.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  31/1500Z 18.3N 141.2W   80 KT  90 MPH
 12H  01/0000Z 19.6N 141.2W   70 KT  80 MPH
 24H  01/1200Z 21.4N 141.7W   55 KT  65 MPH
 36H  02/0000Z 23.2N 142.8W   45 KT  50 MPH
 48H  02/1200Z 25.1N 144.4W   35 KT  40 MPH
 72H  03/1200Z 28.5N 148.4W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  04/1200Z...DISSIPATED
 
$$
Forecaster Jelsema
 
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