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Tropical Storm Miriam Forecast Discussion Number 9
2018-08-28 10:38:42| National Hurricane Center (East Pacific)
Issued at 200 AM PDT Tue Aug 28 2018 039 WTPZ45 KNHC 280838 TCDEP5 Tropical Storm Miriam Discussion Number 9 NWS National Hurricane Center Miami FL EP152018 200 AM PDT Tue Aug 28 2018 Miriam's cloud pattern has still not improved very much, with microwave data showing the low-level center still slightly displaced from the deep convection due to about 10 kt of northwesterly shear. Dvorak estimates from TAFB and SAB, as well as the UW-CIMSS SATCON, all remain near 55 kt, which will remain Miriam's initial intensity. Miriam is expected to remain over warm waters and in a relatively low-shear environment for the next 48-72 hours, which should allow the cyclone to strengthen modestly over the next few days. At 72 hours and beyond, Miriam will encounter significantly stronger shear ahead of an amplifying mid- to upper-level trough northeast of the Hawaiian Islands, and the cyclone will also move over sub-26C waters. These conditions should cause a rather quick weakening trend on days 4 and 5, with Miriam barely hanging on as a tropical storm by the end of the forecast period. The intensity guidance has decreased somewhat on this forecast cycle, probably because Miriam has less time before it reaches stronger shear. The updated NHC intensity forecast lies between the HCCA guidance and the IVCN intensity consensus, with the forecast peak intensity just slightly below that of the previous advisory. The initial motion is due westward, or 270/10 kt, with Miriam located along the southern periphery of the subtropical ridge. The amplifying trough northeast of Hawaii is breaking down the ridge and should cause Miriam to turn sharply toward the northwest and north between days 2 and 4. There are significant speed differences among the models by days 4 and 5, related to how deep Miriam is when it interacts with the deep-layer trough. The ECMWF, which maintains a deeper, stronger vortex, shoots Miriam quickly northward, ending up more than 600-700 nm north of where the GFS and HWRF models have the cyclone by day 5. Given where the consensus aids lie between those two extremes, the updated NHC track forecast has been slowed down a bit from the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 14.0N 133.5W 55 KT 65 MPH 12H 28/1800Z 14.0N 135.1W 60 KT 70 MPH 24H 29/0600Z 14.1N 137.1W 70 KT 80 MPH 36H 29/1800Z 14.4N 138.8W 75 KT 85 MPH 48H 30/0600Z 15.1N 140.1W 80 KT 90 MPH 72H 31/0600Z 17.9N 141.0W 80 KT 90 MPH 96H 01/0600Z 21.5N 141.5W 55 KT 65 MPH 120H 02/0600Z 26.0N 144.0W 35 KT 40 MPH $$ Forecaster Berg
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