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Tropical Depression ELEVEN Forecast Discussion Number 4
2015-09-28 22:45:57| National Hurricane Center (Atlantic)
Issued at 500 PM EDT MON SEP 28 2015 000 WTNT41 KNHC 282045 TCDAT1 TROPICAL DEPRESSION ELEVEN DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL112015 ISSUED BY THE NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 500 PM EDT MON SEP 28 2015 The low-level center has become well exposed due to continued northwesterly shear of 20 kt or more, with the mid-level center displaced well to the southeast of the low-level center. Flight-level and SFMR-observed wind data from an Air Force reconnaissance aircraft showed that the maximum winds remain near 30 kt. This is also consistent with the latest Dvorak estimates from SAB and TAFB. The center of the depression has been moving well to the left of the previous track, with the initial motion estimated to be 270/05 kt. In addition to the more westward initial motion, there have been significant changes in the model guidance, with the latest ECMWF forecast taking the cyclone much farther south and west than the GFS in 3-5 days. This appears to be the result of enhanced mid-level ridging to the north of the cyclone in the ECMWF at that time frame. This has necessitated a significant southwestward shift of the official forecast, although for continuity the new NHC track does not take the cyclone as far to the southwest as the ECMWF. Given the large spread now seen in the dynamical track guidance, confidence in the track forecast is rather low. The strong shear over the system is likely to continue for at least the next 12-24 hours and given the large displacement between the low- and mid-level centers, no intensification is anticipated in the short term. The dynamical guidance indicates some relaxation of the shear in a day or two, so the official forecast shows the cyclone strengthening into a tropical storm later on Tuesday. This is close to the intensity model consensus through 48 hours, and a little below it thereafter. Given the large uncertainty in the track forecast, there is also uncertainty as to what kind of environment the cyclone will encounter during the forecast period, which also leads to low confidence in the official intensity forecast. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 27.5N 70.2W 30 KT 35 MPH 12H 29/0600Z 27.7N 71.0W 30 KT 35 MPH 24H 29/1800Z 27.9N 72.0W 35 KT 40 MPH 36H 30/0600Z 28.3N 72.8W 40 KT 45 MPH 48H 30/1800Z 28.6N 73.4W 45 KT 50 MPH 72H 01/1800Z 30.0N 74.0W 45 KT 50 MPH 96H 02/1800Z 33.0N 74.0W 45 KT 50 MPH 120H 03/1800Z 38.5N 73.5W 45 KT 50 MPH $$ Forecaster Pasch/Hayes
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