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Tropical Storm Miriam Forecast Discussion Number 13
2018-08-29 10:53:20| National Hurricane Center (East Pacific)
Issued at 200 AM PDT Wed Aug 29 2018 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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