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| #803574 (Received by flhurricane at: 4:40 AM 29.Sep.2015) TCDAT1 TROPICAL STORM JOAQUIN DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL AL112015 500 AM EDT TUE SEP 29 2015 The low-level center of Joaquin remains exposed on the northwestern edge of the deep convection due to about 20 kt of shear. A pair of ASCAT passes around 01-02Z showed peak winds of around 35 kt, and that remains the intensity for this advisory, in agreement with the latest TAFB and SAB Dvorak estimates. The ASCAT data were also used to adjust the initial 34-kt wind radii. After moving quickly southwestward earlier, the center of Joaquin appears to be moving more slowly westward at about 4 kt. The track forecast remains highly uncertain, and if anything, the spread in the track model guidance is larger now beyond 48 hours due to uncertainty in the evolution of the synoptic-scale flow and the structure of Joaquin. In particular, the global models are having trouble with the evolution of a deep layer trough over the southeastern U.S. late in the forecast period, with added uncertainty about the structure of Joaquin and how it might interact with the trough. The latest runs of the UKMET and ECMWF bring Joaquin farther southwest early in the period and then show a northward and northeastward motion, respectively, ahead of the trough at days 4 and 5. The GFS shows a weaker Joaquin being absorbed into a frontal zone off the U.S. east coast in 3 to 4 days, while the HWRF and GFDL continue to show a faster northward and then northwestward track. In the first 48 hours, the new NHC track is a little to the south of the previous one accounting for the initial position and motion and a trend toward the multi-model consensus. At days 3 through 5, the official forecast is a little to the right of and slower than the previous one given the large spread in the track guidance. Needless to say, confidence in the details of the track forecast, especially beyond 48 hours, is extremely low. The SHIPS model output shows moderate to strong northerly shear continuing over the cyclone for the next 24 to 36 hours, which should only allow for slow strengthening. After that time, the shear decreases a bit and the cyclone could strengthen a little more while over warm waters. However, there is a lot of spread in the intensity guidance as well. As noted above, the latest run of the GFS dissipates Joaquin in 3 to 4 days. On the other hand, the ECMWF and UKMET show the cyclone deepening considerably in the short term, while the HWRF and GFDL show more strengthening later in the period. Given the uncertainty in Joaquin's structure and track, the new NHC forecast remains conservative, and is closest to the SHIPS model and below the intensity consensus. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 26.6N 70.6W 35 KT 40 MPH 12H 29/1800Z 26.6N 71.3W 40 KT 45 MPH 24H 30/0600Z 26.7N 72.2W 40 KT 45 MPH 36H 30/1800Z 26.8N 72.8W 45 KT 50 MPH 48H 01/0600Z 27.1N 73.2W 50 KT 60 MPH 72H 02/0600Z 28.5N 73.5W 55 KT 65 MPH 96H 03/0600Z 32.0N 73.0W 55 KT 65 MPH 120H 04/0600Z 36.5N 73.0W 55 KT 65 MPH $$ Forecaster Brennan |
