267 
 WTPA43 PHFO 301453
 TCDCP3
  
 Hurricane Miriam Discussion Number  18
 NWS Central Pacific Hurricane Center Honolulu HI   EP152018
 500 AM HST Thu Aug 30 2018
  
 Satellite imagery and a recent 1326Z GPM microwave pass indicate 
 that Hurricane Miriam is making the long awaited turn toward the 
 northwest this morning. The satellite presentation continues to
 show a well defined system with good outflow in all quadrants. There
 is some drier air working its way into the circulation however as 
 evident in the warming cloud tops to the southeast of the low level 
 circulation center. This drier air affecting the core of Miriam is 
 confirmed by the recent GPM pass which showed a closed eyewall in 
 the 37 GHz channel, but a lack of deep convection and ice return in 
 the southwest quadrant in the 85 GHz channel where the drier air is 
 being entrained. Given this recent data, confidence in the initial 
 position as well as intensity of the cyclone have increased 
 significantly. The latest subjective Dvorak intensity estimates
 from PHFO and JTWC were 4.5 (77 knots) and 4.0 (65 knots) from SAB.
 The Advanced Dvorak Technique from UW-CIMSS appears anomalously low
 at 3.5 (55 knots). Given the current structure of Miriam with some 
 drier air wrapping into the southeast quadrant of the system, 77 
 knots appears too high for the initial intensity. As a result, the 
 initial intensity will be held at 70 knots for this advisory, which 
 correlates reasonably well with a blend of the satellite intensity 
 estimates from the various agencies. The initial motion for this 
 advisory will be set at 315/07 knots. 
 
 Miriam is being steered by a large subtropical ridge to the 
 northeast of the system and a deep mid-upper level trough to the 
 north-northwest. The cyclone is expected to make a turn toward the 
 north later today through Friday night as it remains a deep system 
 steered by the deep layer steering flow between these two features. 
 West-southwesterly shear is expected to result in a decoupling of 
 the system by early Saturday, and this should allow Miriam to
 become increasingly influenced by the low level trade wind flow and
 steered back toward the northwest. The official forecast for this
 advisory is very close to the previous advisory, although it was
 nudged slightly to the left initially as a result of the current
 motion, and slightly to the right toward the end of the forecast
 period to better align with the consensus guidance.
 
 The environment will be conducive for additional intensification of 
 Miriam over the next 6 to 18 hours or so, with shear values 
 remaining low, sea surface temperatures holding around 28C, while 
 the system moves over a region of higher Ocean Heat Content. The 
 latest satellite imagery shows a nice banding structure developing 
 to the south and east of the cyclone, so this is expected to lead
 to a reduction in the dry air entrainment into the system in the
 near term. As a result the official forecast calls for slight 
 intensification over the next 12 hours, then holds the intensity 
 steady through 24 hours, as Miriam begins to feel the effects of 
 strengthening west-southwesterly shear but remains over warm water. 
 Beyond 24 hours the intensity forecast will show fairly rapid 
 weakening as shear values increase into the 30 to 45 knot range, 
 while the system moves over marginal sea surface temperatures of
 26C or below. The intensity forecast is very closely aligned with
 that of the previous advisory and closely follows the HWRF with
 some weighting given to the consensus guidance as well.
  
  
 FORECAST POSITIONS AND MAX WINDS
  
 INIT  30/1500Z 15.3N 141.6W   70 KT  80 MPH
  12H  31/0000Z 16.3N 141.9W   80 KT  90 MPH
  24H  31/1200Z 17.9N 141.7W   80 KT  90 MPH
  36H  01/0000Z 19.5N 141.7W   65 KT  75 MPH
  48H  01/1200Z 21.2N 142.3W   50 KT  60 MPH
  72H  02/1200Z 24.2N 144.7W   35 KT  40 MPH
  96H  03/1200Z 26.5N 148.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 120H  04/1200Z 28.0N 152.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
  
 $$
 Forecaster Jelsema