Tropical Storm Isaac Advisory Number 21 NWS National Hurricane Center Miami FL AL092018 500 PM AST Wed Sep 12 2018 ...ISAAC EXPECTED TO BRING TROPICAL STORM CONDITIONS TO PORTIONS OF THE LESSER ANTILLES... SUMMARY OF 500 PM AST...2100 UTC...INFORMATION ---------------------------------------------- LOCATION...15.4N 56.6W ABOUT 295 MI...480 KM E OF MARTINIQUE ABOUT 325 MI...525 KM E OF GUADELOUPE MAXIMUM SUSTAINED WINDS...60 MPH...95 KM/H PRESENT MOVEMENT...W OR 275 DEGREES AT 20 MPH...31 KM/H MINIMUM CENTRAL PRESSURE...1003 MB...29.62 INCHES WATCHES AND WARNINGS -------------------- CHANGES WITH THIS ADVISORY: None. SUMMARY OF WATCHES AND WARNINGS IN EFFECT: A Tropical Storm Warning is in effect for... * Martinique * Dominica * Guadeloupe A Tropical Storm Watch is in effect for... * Antigua * Montserrat * St. Kitts and Nevis * Saba and St. Eustatius * St. Martin and St. Maarten A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area, in this case within 24 hours. A Tropical Storm Watch means that tropical storm conditions are possible within the watch area, in this case within 24 hours. Interests elsewhere in the Leeward Islands should monitor the progress of Isaac. For storm information specific to your area, please monitor products issued by your national meteorological service. DISCUSSION AND OUTLOOK ---------------------- At 500 PM AST (2100 UTC), the center of Tropical Storm Isaac was located near latitude 15.4 North, longitude 56.6 West. Isaac is moving toward the west near 20 mph (31 km/h), and this general motion with a decrease in forward is expected to continue through the weekend. On the forecast track, Isaac is forecast to move across the central Lesser Antilles and into the eastern Caribbean Sea on Thursday, and then move across the eastern and central Caribbean Sea through Saturday. Maximum sustained winds remain near 60 mph (95 km/h) with higher gusts. Some weakening is forecast during the next 48 hours. Tropical-storm-force winds extend outward up to 175 miles (280 km) from the center. The estimated minimum central pressure is 1003 mb (29.62 inches). HAZARDS AFFECTING LAND ---------------------- RAINFALL: Isaac is expected to produce total rainfall accumulations of 2 to 4 inches with isolated amounts up to 8 inches across Martinique, Dominica, and Guadeloupe. Rainfall of 1 to 2 inches with isolated amounts to 4 inches are forecast across Puerto Rico and the southern United States Virgin Islands, with up to an inch anticipated across the remaining Windward and Leeward Islands. This rainfall may cause life-threatening flash flooding. WIND: Tropical storm conditions are expected on Martinique, Dominica, and Guadeloupe early Thursday. Tropical storm conditions are possible within the tropical storm watch area on Thursday. STORM SURGE: Some coastal flooding is possible in areas of onshore winds. Near the coast, the surge will be accompanied by large waves. SURF: Swells generated by Isaac are affecting portions of the Lesser Antilles. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
Tropical Storm Isaac Discussion Number 21 NWS National Hurricane Center Miami FL AL092018 500 PM AST Wed Sep 12 2018 The satellite presentation of Isaac hasn't changed much in the past several hours, with the center exposed on the west side of a re-developing band of convection. On the last pass of the NOAA Hurricane Hunter aircraft, peak flight-level winds were 61 kt and SFMR values were about 50 kt. These data support an initial wind speed estimate of 50 kt. It is interesting to note that the surface center is estimated to be a tenth of a degree or two south of the well-defined swirl on satellite--a function of the fast forward speed and high shear. The shear is forecast to continue for the next day or two, and gradual weakening is anticipated, in line with the model guidance. Given the strong shear, it is possible that Isaac could degenerate into a wave near the Lesser Antilles. After that time, Isaac could escape the upper-level trough over the central Atlantic that has been shearing the cyclone, and find a more conducive environment. More of the model guidance is suggesting some reintensification of the cyclone in the western Caribbean Sea, with the biggest hold-out being the ECMWF. Continuity dictates that the forecast not change as much as the guidance, most of which is showing a hurricane at long range, so the compromise solution is no longer dissipating the cyclone and flat-lining the intensity at long range. It almost goes without saying that this is a low-confidence intensity forecast. Isaac has accelerated to near 17 kt over the past 12 hours, and the shorter-term motion is closer to 20 kt with the convection-less center. A mid-tropospheric high is forecast to persist for the next several days, which should keep the cyclone moving westward, albeit at a slightly slower pace. The model guidance has shifted a bit to the north on this cycle, but is mostly in response to the initial latitude. The NHC track forecast is moved northward as well but still lies on the south side of the consensus. At long range, it makes sense when forecasting a weaker system to stay on the left and faster side of the consensus, so that's where the new NHC prediction lies. Key Messages: 1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings are in effect for Martinique, Dominica, and Guadeloupe. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius. Interests on those islands should follow any advice given by their local officials. 2. Life-threatening flash flooding is possible with Isaac. The storm is expected to produce total rainfall accumulations of 2 to 4 inches with isolated amounts up 8 inches across Martinique, Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with isolated amounts to 4 inches are expected across Puerto Rico and the southern United States Virgin Islands. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 15.4N 56.6W 50 KT 60 MPH 12H 13/0600Z 15.6N 59.0W 50 KT 60 MPH 24H 13/1800Z 15.8N 61.8W 45 KT 50 MPH 36H 14/0600Z 16.0N 64.8W 45 KT 50 MPH 48H 14/1800Z 16.0N 67.6W 40 KT 45 MPH 72H 15/1800Z 16.0N 72.5W 40 KT 45 MPH 96H 16/1800Z 16.5N 76.5W 40 KT 45 MPH 120H 17/1800Z 17.5N 80.5W 40 KT 45 MPH