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Tropical Storm Miriam Forecast Discussion Number 12
2018-08-29 04:33:18| National Hurricane Center (East Pacific)
Issued at 800 PM PDT Tue Aug 28 2018 680 WTPZ45 KNHC 290233 TCDEP5 Tropical Storm Miriam Discussion Number 12 NWS National Hurricane Center Miami FL EP152018 800 PM PDT Tue Aug 28 2018 There's been little change in the overall cloud pattern of Miriam during the past several hours. An earlier Advanced Technology Microwave Sounder (ATMS) overpass and a 2053 UTC GCOM-W1 AMSR2 image indicate that the surface center is still sheared to the northwest of the deep convective banding feature encompassing the southern portion of the cyclone. Dvorak satellite intensity estimates from TAFB and SAB remain unchanged, and a 2213 UTC SATCON analysis showed 51 kt. Based on these data, the initial intensity is held at 50 kt. The intensity forecast is rather problematic this evening. Decay SHIPS guidance (both GFS and ECMWF background fields) shows the moderate northwesterly shear increasing after 48 hours, while indicating little change in strength up to that period. After that time, these statistical/dynamical guidance models quickly degenerate the cyclone into a remnant low in 4 days. The NOAA-HCCA, HWRF, and the IVCN consensus model, on the other hand, all show Miriam becoming a hurricane in 36-48 hours. For this advisory, I've elected to maintain continuity and show gradual strengthening to just below hurricane strength in 36 hours, with a weakening trend commencing in 3 days. All the deterministic models agree with Miriam becoming a remnant low in 5 days, and this is reflected in the NHC forecast. The initial motion is estimated to be westward, or 270/11 kt, within the easterly mid-level steering flow produced by a mid-tropospheric high to the north of Miriam. The cyclone is expected to turn toward the northwest in 36 hours, then rather abruptly north-northwestward in 2 days and continuing on through day 4, in response to a large mid- to upper level cutoff low digging southwestward toward the Hawaiian Islands. A large spread in the models still exists in the latter portion of the forecast, as mentioned in the previous advisory. The track forecast philosophy remains unchanged with a nudge more toward a blend of the TVCN consensus and the global models that show a gradual turn back toward the northwest due to a shallower vertical structure near the end of the period. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 14.1N 137.4W 50 KT 60 MPH 12H 29/1200Z 14.2N 138.9W 50 KT 60 MPH 24H 30/0000Z 14.5N 140.4W 55 KT 65 MPH 36H 30/1200Z 15.4N 141.4W 60 KT 70 MPH 48H 31/0000Z 16.7N 141.8W 60 KT 70 MPH 72H 01/0000Z 20.2N 142.0W 55 KT 65 MPH 96H 02/0000Z 23.4N 143.6W 40 KT 45 MPH 120H 03/0000Z 26.5N 146.7W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Roberts
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