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Tropical Storm GUILLERMO Forecast Discussion Number 4
2015-07-30 22:32:05| National Hurricane Center (East Pacific)
Issued at 200 PM PDT THU JUL 30 2015 000 WTPZ44 KNHC 302032 TCDEP4 TROPICAL STORM GUILLERMO DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL EP092015 200 PM PDT THU JUL 30 2015 Guillermo is quickly becoming better organized, and both microwave and visible imagery suggest that the cyclone is developing a ring of inner core convection. Dvorak classifications at 1800 UTC were T3.0/45 kt from both TAFB and SAB, but given the quickly improving convective structure, the advisory intensity is set at 50 kt. This is close to the latest UW-CIMSS ADT estimate of T3.4/53 kt. The upper-level outflow of Guillermo continues to expand, and the storm is over very warm waters of around 29 degrees Celsius. Global model guidance indicates that Guillermo should remain in a light-shear environment for at least another 3 days, while also remaining over warm water and in a moisture-laden atmosphere. Therefore, continued strengthening is likely for the next 48 hours, and Guillermo could become a hurricane within the next 12-24 hours. Gradual weakening is forecast from day 3 through 5 due to stronger upper-level westerly winds which could affect the cyclone. The statistical-dynamical guidance continues to show only modest strengthening--barely taking Guillermo to hurricane status--but this scenario seems low given the seemingly favorable environment. The NHC official intensity forecast remains closer to the higher dynamical guidance and is largely unchanged from the previous forecast. The subtropical ridge is steering Guillermo west-northwestward, or 295/12 kt. The ridge is forecast to strengthen slightly, causing Guillermo to accelerate during the next 36 hours. After that time, a mid- to upper-level low well west of California is expected to produce a break in the ridge, which should cause the cyclone to slow down and turn northwestward by day 5. The latest track guidance ended up lying to the west of the previous official forecast track, and the updated NHC track has therefore been shifted a bit to the left, especially after 48 hours. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 9.8N 128.4W 50 KT 60 MPH 12H 31/0600Z 10.6N 130.6W 60 KT 70 MPH 24H 31/1800Z 11.5N 133.7W 70 KT 80 MPH 36H 01/0600Z 12.4N 136.8W 75 KT 85 MPH 48H 01/1800Z 13.2N 139.4W 80 KT 90 MPH 72H 02/1800Z 14.9N 143.3W 75 KT 85 MPH 96H 03/1800Z 16.5N 146.0W 65 KT 75 MPH 120H 04/1800Z 18.0N 148.5W 55 KT 65 MPH $$ Forecaster Berg
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