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Hurricane Miriam Discussion Number  25
NWS Central Pacific Hurricane Center Honolulu HI   EP152018
1100 PM HST Fri Aug 31 2018
 
Miriam's satellite presentation has deteriorated significantly over 
the past six hours, with 30 to 35 kt of vertical wind shear 
beginning to profoundly affect the core of this tropical cyclone. 
Deep convection lies only along the northern flank of the 
circulation and outflow has become almost non-existent except to
the north and northeast. Subjective Dvorak current intensity
estimates decreased to 4.5/77 kt from JTWC and SAB, while PHFO
remained at 5.0/90 kt. ADT from UW-CIMSS was 77 kt. Initial
intensity for this advisory is set at 75 kt, which may be high given
what appears to be the beginning of rapid system erosion. 

Initial motion is 355/08 kt, based mainly on a 0600 UTC fix 
influenced strongly by a 0409 UTC SSMIS pass. Miriam has likely 
begun its turn to the north northwest this evening as it is steered 
between an upper trough to the northwest and a deep ridge to the 
northeast. The track that Miriam actually takes will be determined 
by how fast the vertical wind shear decapitates it, with a weaker 
Miriam likely forced to turn northwestward faster as system
steering becomes influenced more by low level flow. The track
guidance envelope shifted a bit to the left, likely in anticipation
of more rapid Miriam weakening. The forecast track was shifted
slightly left as well, remaining within the rather tight envelope
between ECMWF and TVCE. 

Miriam will continue to weaken in the face of increasing shear and 
sub-26 degree C SSTs through the forecast period, with this system 
forecast to dissipate at 96 hours. We expect Miriam will weaken to
a tropical storm Saturday before becoming a post tropical remnant
low Monday. While some guidance, like SHIPS, wants to dissipate
Miriam within 48 hours, our intensity forecast follows HWRF between
ECMWF and FSSE. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  01/0900Z 20.9N 141.2W   75 KT  85 MPH
 12H  01/1800Z 22.2N 141.6W   65 KT  75 MPH
 24H  02/0600Z 24.1N 143.0W   50 KT  60 MPH
 36H  02/1800Z 25.9N 144.7W   40 KT  45 MPH
 48H  03/0600Z 27.6N 146.5W   35 KT  40 MPH
 72H  04/0600Z 30.7N 150.6W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  05/0600Z...DISSIPATED
 
$$
Forecaster Powell
 
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