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 WTPA43 PHFO 310901
 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. 
 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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