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 921 
 WTPZ45 KNHC 290853
 TCDEP5
 
 Tropical Storm Miriam Discussion Number  13
 NWS National Hurricane Center Miami FL       EP152018
 200 AM PDT Wed Aug 29 2018
 
 At face value, Miriam seems to be getting a little better
 organized.  Its convective canopy has been expanding during the
 past few hours, and continuous lightning strokes have been detected
 within a well-defined mid-level circulation.  However, a 0637 UTC
 METOP-B microwave pass shows that Miriam remains a sheared cyclone,
 with the low-level center running out ahead of the deep convection.
 Still, with the expanding convection, Dvorak estimates from TAFB and
 SAB have risen to T3.5, and the initial intensity is set at 55 kt.
 
 Miriam is moving westward, or 275/11 kt, near the western end of
 the subtropical ridge.  A deep-layer low located northeast of the
 Hawaiian Islands is expected to cause Miriam to slow down and turn
 sharply northwestward and northward during the next 48 hours.  The
 models are in excellent agreement on this scenario.  After day 3,
 however, there are significant differences among the guidance, with
 the GFS taking a shallow cyclone almost due westward and the ECMWF
 continuing to accelerate a deeper Miriam northward on the east side
 of the deep-layer low.  With these models being the most acute
 outliers, the NHC forecast track thinking continues to lie close to
 the tight clustering of the Florida State Superensemble, HCCA, and
 TVCX consensus aids.
 
 There is also greater-than-normal uncertainty in the intensity
 forecast.  The SHIPS and LGEM models show northwesterly shear
 persisting over Miriam and only strengthen the cyclone slightly
 during the next 24-36 hours.  The HWRF and HCCA models are still
 showing more significant strengthening, making Miriam a hurricane
 over the next day or two.  I was tempted to explicitly show Miriam
 becoming a hurricane in the official forecast, but I decided to hold
 off since the cyclone just hasn't been able to display an improved
 structure as of yet.  Even if some strengthening does occur,
 vertical shear is expected to increase substantially from 48 hours
 onward, which will cause fast weakening and Miriam likely
 degenerating into a remnant low by day 5.
 
 FORECAST POSITIONS AND MAX WINDS
 
 INIT  29/0900Z 14.2N 138.4W   55 KT  65 MPH
  12H  29/1800Z 14.4N 139.8W   60 KT  70 MPH
  24H  30/0600Z 14.9N 141.1W   60 KT  70 MPH
  36H  30/1800Z 15.8N 141.9W   60 KT  70 MPH
  48H  31/0600Z 17.2N 142.0W   60 KT  70 MPH
  72H  01/0600Z 20.4N 142.5W   50 KT  60 MPH
  96H  02/0600Z 23.0N 144.5W   35 KT  40 MPH
 120H  03/0600Z 26.5N 148.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
 $$
 Forecaster Berg
 
 
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