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Tropical Depression NINE Forecast Discussion Number 4
2015-09-17 10:36:51| National Hurricane Center (Atlantic)
Issued at 500 AM AST THU SEP 17 2015 000 WTNT44 KNHC 170836 TCDAT4 TROPICAL DEPRESSION NINE DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092015 500 AM AST THU SEP 17 2015 Tropical Depression Nine continues to have a sheared appearance with an area of strong convection located 75-100 n mi east of the low-level center. Satellite intensity estimates from TAFB and SAB have increased to 30 kt, and reports from NOAA buoy 41041 indicate that the center pressure has fallen below 1007 mb. Based on these data, the initial intensity is increased to 30 kt. The depression is located between a developing upper-level low to its northwest and a tropical disturbance to the southeast. The global models are in poor agreement on how the depression will interact with these systems, which makes for a low confidence intensity forecast. The UKMET and Canadian models merge the depression with the disturbance as the latter system intensifies. The NAVGEM shows the depression surviving for several days as the disturbance weakens. The GFS weakens both the depression and the disturbance while developing another low pressure area nearby. Finally, the ECMWF keeps the depression separate from the other disturbance and forecasts it to survive for five days. Even if the depression and the disturbance do not merge, the upper-level low is likely to cause 20-25 kt of shear over the depression for at least 3-4 days. The new intensity forecast follows the no merger scenario but calls for no additional strengthening due to the shear. The depression is expected to degenerate to a remnant low by 48 hours and dissipate completely after 96 hours. Assuming that the depression and the disturbance do not merge, a weak low- to mid-level ridge to the north of the cyclone should steer it generally northwestward through the forecast period. The new forecast track is similar to the previous track, and it lies to the south of the center of the guidance envelope between the consensus models and the BAM shallow. An alternative forecast scenario is that the depression could move erratically if it and the disturbance get closer enough to try to merge. FORECAST POSITIONS AND MAX WINDS INIT 17/0900Z 16.3N 45.3W 30 KT 35 MPH 12H 17/1800Z 17.1N 45.6W 30 KT 35 MPH 24H 18/0600Z 18.1N 46.3W 30 KT 35 MPH 36H 18/1800Z 19.1N 47.3W 25 KT 30 MPH 48H 19/0600Z 20.0N 48.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 20/0600Z 21.0N 51.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 21/0600Z 22.5N 53.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 22/0600Z...DISSIPATED $$ Forecaster Beven
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