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ZCZC MIATCDAT5 ALL TTAA00 KNHC DDHHMM Hurricane Maria Discussion Number 23 NWS National Hurricane Center Miami FL AL152017 500 PM AST Thu Sep 21 2017 The last pass of today's Air Force reconnaissance flight found maximum flight-level winds of 117 kt in the northeastern eyewall, which supported increasing the intensity to 105 kt in the 2 PM intermediate advisory. The SFMR instrument continued to report higher winds, as was mentioned in the previous discussion. However, Maria has been moving over the Navidad and Silver Banks to the north of the Dominican Republic, where water depths are less than 30 meters deep in some places, and it is likely that shoaling effects inflated some of these numbers. Maria continues to move northwestward, or 315/8 kt, around the southwestern periphery of a mid-level high centered south of Bermuda. As Maria moves around this high, and toward an elongated trough extending from Tropical Storm Jose southwestward into the Gulf of Mexico, it is expected to turn north-northwestward within 24 hours and then north-northeastward by the end of the forecast period. This motion will take Maria's center very close to the Turks and Caicos Islands and the southeastern Bahamas during the next 24 hours, but the hurricane should then stay over the waters of the western Atlantic through day 5. The track models remain stable, and there is very little cross-track spread even at day 5. The guidance has slowed down a bit by the end of the forecast period due to Maria possibly interacting with a shortwave trough moving off the southeastern U.S. coast, and the new NHC forecast is therefore a little slower than the previous one at that time. SHIPS diagnostics indicate that southwesterly shear will be increasing over Maria during the next 12-24 hours, even though the hurricane will begin to move over an area of higher oceanic heat content. While it can't be ruled out that Maria might still strengthen a bit, the preponderance of the guidance suggests that the cyclone will only maintain its intensity for the next 12 hours and then begin a gradual decrease in strength on Friday. That trend should continue through the end of the forecast period, but Maria is expected to remain a hurricane through day 5. For now, the NHC forecast remains just above the intensity consensus, hedging toward the slower decay shown by the SHIPS model. KEY MESSAGES: 1. Flash flood warnings continue in portions of Puerto Rico due to persistent heavy rainfall from Maria's trailing rainbands. Catastrophic flooding is occurring on the island, especially in areas of mountainous terrain, and everyone in Puerto Rico should continue to follow advice from local officials to avoid these life-threatening flooding conditions. 2. A Hurricane Warning is in effect for the northern coast of the Dominican Republic, the Turks and Caicos Islands, and the southeastern Bahamas, where Maria is expected to bring dangerous wind, storm surge, and heavy rainfall. 3. Swells from Maria are expected to begin reaching the coast of the southeastern United States on Friday. These swells are likely to cause dangerous surf and life-threatening rip currents along the coast for the next several days, even with Maria forecast to remain well offshore over the western Atlantic Ocean. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 20.8N 69.8W 105 KT 120 MPH 12H 22/0600Z 21.6N 70.4W 105 KT 120 MPH 24H 22/1800Z 23.0N 71.2W 100 KT 115 MPH 36H 23/0600Z 24.5N 71.7W 100 KT 115 MPH 48H 23/1800Z 26.1N 72.1W 95 KT 110 MPH 72H 24/1800Z 28.9N 72.1W 95 KT 110 MPH 96H 25/1800Z 31.0N 71.0W 85 KT 100 MPH 120H 26/1800Z 33.0N 70.0W 75 KT 85 MPH $$ Forecaster Berg NNNN