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Tropical Storm Miriam Forecast Discussion Number 11
2018-08-28 22:35:16| National Hurricane Center (East Pacific)
Issued at 200 PM PDT Tue Aug 28 2018 991 WTPZ45 KNHC 282035 TCDEP5 Tropical Storm Miriam Discussion Number 11 NWS National Hurricane Center Miami FL EP152018 200 PM PDT Tue Aug 28 2018 Northwesterly shear has taken a toll on Miriam, with visible satellite imagery depicting an exposed low-level center located northwest of the deep convection. Subjective Dvorak T-numbers have decreased to 3.0 by both SAB and TAFB, and a blend of the T- and CI-numbers yields an initial intensity of 50 kt for this advisory. Model guidance suggests that the northwesterly shear that is currently affecting the cyclone will continue for at least the next 24 hours, but it could diminish slightly by late Wednesday, allowing for some modest strengthening in 36 to 48 hours. Thereafter, a mid- to upper-level trough to the northwest of the cyclone is expected to bring about increasing southwesterly shear over the system, which is expected to cause steady weakening in 3 to 4 days. The cyclone is then expected to become a post-tropical cyclone by 120 hours as shear increases even further, and the system crosses cooler SSTs. The official intensity forecast has been lowered for much of the forecast period, reflecting both the lowering of the initial intensity and the expected continuation of the shear. Although not explicitly shown in the official forecast, there is still some chance that Miriam could reach hurricane status in a couple of days, before the shear increases. The initial motion continues to be due west, or 270/11 kt, as Miriam continues to be steered around the southern periphery of a subtropical ridge. The track forecast philosophy for the next few days remains unchanged as the aforementioned mid- to upper-level trough northeast of Hawaii weakens the western periphery of the ridge, and Miriam is forecast to turn northwestward then north-northwestward between the trough and ridge. There continues to be a large spread in the guidance by day 5, with the ECMWF and UKMET taking a more vertically coherent system northward, while the GFS, HWRF, and HMON turn Miriam westward as the system weakens and becomes a more shallow system. The latter scenario is beginning to seem more plausible and the NHC track forecast was adjusted southward and westward, but additional changes may be required if future forecasts shown a faster rate of demise. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 14.1N 136.4W 50 KT 60 MPH 12H 29/0600Z 14.2N 138.0W 50 KT 60 MPH 24H 29/1800Z 14.4N 139.8W 50 KT 60 MPH 36H 30/0600Z 14.9N 141.1W 55 KT 65 MPH 48H 30/1800Z 16.1N 141.7W 60 KT 70 MPH 72H 31/1800Z 19.3N 142.0W 55 KT 65 MPH 96H 01/1800Z 22.6N 143.1W 40 KT 45 MPH 120H 02/1800Z 26.0N 146.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown/Latto
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