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Hurricane Sam Forecast Discussion Number 29
2021-09-29 22:51:15| National Hurricane Center (Atlantic)
Issued at 500 PM AST Wed Sep 29 2021 108 WTNT43 KNHC 292051 TCDAT3 Hurricane Sam Discussion Number 29 NWS National Hurricane Center Miami FL AL182021 500 PM AST Wed Sep 29 2021 Sam continues to exhibit a well-organized cloud pattern on satellite imagery, although there is some asymmetry of the coldest-topped convection around the center. The hurricane is also maintaining numerous banding features with well-defined upper-level anticyclonic outflow. Observations from an Air Force Hurricane Hunter plane showed peak 700 mb fight-level winds of 130 kt in the northeast quadrant which, using the standard reduction, corresponds to maximum surface winds of 117 kt. However, the maximum SFMR-observed surface winds were 103 kt. Based on these data, with more emphasis on the flight-level winds, the current intensity estimate is held at 115 kt. The aircraft also measured a minimum central pressure of 945 mb by dropsonde. Since the hurricane is forecast to remain in an environment of low vertical shear and to remain over warm waters through 48 hours, Sam could restrengthen a little more during the next day or so. Since the Hurricane Hunters reported a concentric eyewall, short-term strengthening may be curtailed until the eyewall replacement cycle completes. Later in the forecast period, gradual weakening is likely due to cooler SSTs and increasing shear. The official intensity forecast is in very good agreement with the latest corrected consensus prediction. Sam will likely be close to extratropical transition around day 5. The initial motion is still northwestward, or 310/8 kt. During the next few days, Sam is likely to move northwestward and then northward around the western periphery of a large subtropical high pressure system centered over the eastern Atlantic. A turn to the north-northeast and northeast is expected later in the forecast period as Sam begins to encounter the flow on the southern side of a large mid-tropospheric trough moving through Atlantic Canada. The track guidance models remain in excellent agreement through 72 hours, including the time that Sam passes by Bermuda. At days 4-5 there is some divergence of the model guidance, apparently due to how Sam may interact with the trough. Most notably the ECMWF is a lot slower than the other global models suggesting that the trough will at least partially bypass it in that case. Not much change has been made to the previous official forecast, however, and the NHC track remains very close to the TVCN dynamical model consensus. Key Messages: 1. Swells generated by Sam will impact the Lesser Antilles during the next several days. Swells are expected to reach Bermuda and the Bahamas in a day or so, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 20.2N 57.6W 115 KT 130 MPH 12H 30/0600Z 21.2N 58.9W 120 KT 140 MPH 24H 30/1800Z 23.1N 60.4W 120 KT 140 MPH 36H 01/0600Z 25.6N 61.4W 120 KT 140 MPH 48H 01/1800Z 28.3N 61.9W 115 KT 130 MPH 60H 02/0600Z 31.0N 61.3W 110 KT 125 MPH 72H 02/1800Z 33.6N 59.9W 100 KT 115 MPH 96H 03/1800Z 37.5N 55.0W 85 KT 100 MPH 120H 04/1800Z 42.0N 48.5W 70 KT 80 MPH $$ Forecaster Pasch
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