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Tropical Storm Miriam Forecast Discussion Number 6
2018-08-27 16:36:06| National Hurricane Center (East Pacific)
Issued at 800 AM PDT Mon Aug 27 2018 364 WTPZ45 KNHC 271436 TCDEP5 Tropical Storm Miriam Discussion Number 6 NWS National Hurricane Center Miami FL EP152018 800 AM PDT Mon Aug 27 2018 The convective structure of Miriam has become better organized this morning, with a continued increase in overall banding. An earlier AMSR2 microwave overpass also revealed a well-defined band over the southeastern and southern portions of the circulation but there was some evidence of northwesterly shear with the center near the northwestern edge of the primary convective band. The initial intensity has been increased to 55 kt, which is supported by a Dvorak Data T-number of 3.5 from TAFB, earlier AMSU data, and recent UW/CIMSS ADT estimates of T3.7 (59 kt). Satellite fixes indicate that Miriam is moving westward or 270 degrees at 12 kt. The cyclone should remain on a westward heading over the next 48 hours while it is steered by a large deep-layer ridge to the north. After that time, a large mid- to upper-level low to the northeast of the Hawaiian Islands is expected to produce a break in the ridge between 140W-150W. This should cause Miriam to turn northwestward, then north-northwestward between days 3 through 5. The track guidance is in good agreement on this scenario, but there are some differences in Miriam's forward speed after recurvature begins. The ECMWF and UKMET take Miriam much faster northward than the remainder of the dynamical models, and the NHC track forecast is once again close to the various consensus aids at 72 h and beyond to account for the speed differences. The global models suggest that there will be a gradual increase in northwesterly shear over Miriam during the next day or so, however, this is not expected to prevent the cyclone from becoming a hurricane within the next 24 hours. The moderate shear is forecast to relax by Tuesday night, which should allow for additional intensification. The intensity guidance is not quite as aggressive as before, and the NHC intensity forecast, which lies between the IVCN intensity consensus model and the HFIP corrected consensus, has been adjusted slightly downward. Increasingly southwesterly vertical wind shear and cooler waters are expected to cause Miriam to weaken late in the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 13.9N 130.5W 55 KT 65 MPH 12H 28/0000Z 13.9N 132.3W 65 KT 75 MPH 24H 28/1200Z 13.8N 134.3W 70 KT 80 MPH 36H 29/0000Z 13.9N 136.3W 75 KT 85 MPH 48H 29/1200Z 14.1N 138.1W 85 KT 100 MPH 72H 30/1200Z 15.5N 140.8W 85 KT 100 MPH 96H 31/1200Z 18.7N 141.8W 75 KT 85 MPH 120H 01/1200Z 23.3N 143.2W 55 KT 65 MPH $$ Forecaster Brown
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