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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
 
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