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Tropical Storm Isaac Forecast Discussion Number 16

2018-09-11 16:58:31| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Tue Sep 11 2018 079 WTNT44 KNHC 111458 TCDAT4 Tropical Storm Isaac Discussion Number 16 NWS National Hurricane Center Miami FL AL092018 1100 AM AST Tue Sep 11 2018 One-minute visible satellite data from GOES-16 indicate that the center of Isaac remains on the westward edge of the central dense overcast. The initial wind speed is kept at 60 kt, which is a compromise between the lower subjective estimates and higher ADTs and microwave estimates. This is a more uncertain estimate than normal due to the spread of the intensity fixes. The initial wind and 12 ft seas radii have been expanded out to the north based on TAFB estimates and microwave data. The intensity forecast continues to be tricky. Westerly shear should increase over the next couple of days, which would normally cause some weakening. However this effect could be tempered by increasing SSTs, mid-level moisture and instability in the path of the storm. Given our limited skill in forecasting intensity changes in marginal environments, plus the 5 or 10 kt of initial wind speed uncertainty, the official forecast will show no significant change in intensity for the next couple of days. Because of these uncertainties, a Hurricane Watch has been issued for many of the Leeward Islands, even though Isaac is not explicitly forecast to be a hurricane. Some weakening is indicated at long range after considering the global model trends in intensity, which have done fairly well for Isaac. The latest initial motion estimate is 270/14. Isaac is forecast to move westward at about the same speed for the next few days due to a mid-level ridge over the central Atlantic Ocean. Other than the UKMET model, there is not much spread in the guidance, except with the forward speed. Given the strength of the ridge, the cyclone isn't likely to slow down much, so the official forecast is basically along the previous forecast track but faster. The uncertainty increases at long range, and is somewhat dependent on the intensity of Isaac, but faster seems to be way to go given the weakening trend anticipated at that time. Key Messages: 1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles on Thursday, and hurricane and tropical storm watches have been issued for several islands. Interests in those areas should follow any advice given by their local officials. 2. Interests elsewhere in the Lesser Antilles should continue to monitor Isaac during the next couple of days, as additional watches could be needed for other islands. FORECAST POSITIONS AND MAX WINDS INIT 11/1500Z 14.6N 49.7W 60 KT 70 MPH 12H 12/0000Z 14.6N 51.8W 60 KT 70 MPH 24H 12/1200Z 14.7N 54.4W 65 KT 75 MPH 36H 13/0000Z 14.9N 57.1W 60 KT 70 MPH 48H 13/1200Z 15.1N 59.9W 60 KT 70 MPH 72H 14/1200Z 15.5N 65.5W 55 KT 65 MPH 96H 15/1200Z 15.5N 71.0W 50 KT 60 MPH 120H 16/1200Z 15.5N 75.0W 50 KT 60 MPH $$ Forecaster Blake

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Tropical Depression Paul Forecast Discussion Number 13

2018-09-11 16:44:03| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Tue Sep 11 2018 051 WTPZ43 KNHC 111443 TCDEP3 Tropical Depression Paul Discussion Number 13 NWS National Hurricane Center Miami FL EP182018 800 AM PDT Tue Sep 11 2018 The center of Paul remains to the east of a small area of deep convection; a consequence of persistently northeasterly shear. The initial wind speed is kept at 30 kt using a blend of the latest Dvorak estimates. Slow weakening is anticipated as Paul moves into a stable, drier air mass and over cooler sea surface temperatures. Most of the global models suggest deep convection will cease tomorrow, and remnant low status is indicated at that time. The initial motion is estimated to be west-northwestward, or 290/10 kt. A slow turn toward the west is forecast in 36 hours as the weakening cyclone becomes shallow and is steered by the low-level easterly tradewind flow. The southward model trend from the previous forecast continues this cycle, and the NHC forecast is adjusted in that direction. FORECAST POSITIONS AND MAX WINDS INIT 11/1500Z 22.3N 124.1W 30 KT 35 MPH 12H 12/0000Z 22.5N 125.8W 30 KT 35 MPH 24H 12/1200Z 22.8N 127.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 13/0000Z 23.1N 129.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 13/1200Z 23.2N 129.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 14/1200Z 23.0N 131.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 15/1200Z...DISSIPATED $$ Forecaster Blake

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Hurricane Helene Forecast Discussion Number 17

2018-09-11 16:42:01| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Tue Sep 11 2018 859 WTNT43 KNHC 111441 TCDAT3 Hurricane Helene Discussion Number 17 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Tue Sep 11 2018 It appears that Helene already peaked in intensity earlier this morning with 95 kt. The cloud pattern has not changed much, and still consists of a large eye surrounded by deep convection. However, these convective tops have warmed a little, and consequently Dvorak numbers have remained steady. From now on, Helene will begin to move over increasingly cooler waters, and become embedded within higher shear in about a day. These environmental conditions should result in weakening, and the NHC forecast calls for a decrease in the winds at the same rate as the intensity consensus aids. Helene has turned toward the northwest or 310 degrees at 10 kt. Models continue to erode the ridge to the north of the cyclone, and are also developing a large mid-level trough over eastern Atlantic. This forecast flow pattern will force Helene to turn more toward the north-northwest and north ahead of the trough for the next 2 to 3 days. After that time, the cyclone will recurve in the mid- latitude westerlies, and will begin to lose tropical characteristics at the very long ranges. Track models are in very good agreement with this solution. The NHC forecast is in the middle of the guidance envelope and in between HCCA and the multi-model consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 11/1500Z 16.5N 34.3W 95 KT 110 MPH 12H 12/0000Z 17.5N 35.5W 90 KT 105 MPH 24H 12/1200Z 18.9N 36.9W 80 KT 90 MPH 36H 13/0000Z 20.7N 37.8W 70 KT 80 MPH 48H 13/1200Z 22.5N 38.2W 60 KT 70 MPH 72H 14/1200Z 27.5N 38.0W 50 KT 60 MPH 96H 15/1200Z 35.0N 35.0W 50 KT 60 MPH 120H 16/1200Z 40.0N 30.0W 50 KT 60 MPH $$ Forecaster Avila

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Hurricane Florence Forecast Discussion Number 48

2018-09-11 10:56:27| National Hurricane Center (Atlantic)

Issued at 500 AM AST Tue Sep 11 2018 510 WTNT41 KNHC 110856 TCDAT1 Hurricane Florence Discussion Number 48 NWS National Hurricane Center Miami FL AL062018 500 AM AST Tue Sep 11 2018 Recent satellite imagery shows that the eye of Florence has become cloud filled and an earlier 0441 UTC microwave overpass revealed a double eyewall structure. These observations suggest that an eyewall replacement cycle is likely underway. Subjective and objective Dvorak current intensity numbers have not changed so the initial intensity will remain 120 kt for this advisory. An Air Force Reserve Hurricane Hunter mission is en route to the storm and should provide a better assessment of Florence's structure and intensity this morning. NOAA buoy 41049 located about 80 nmi north of the eye, has reported tropical-storm-force winds during the last several hours and seas as high as 23 ft. Florence's upper-level environment is predicted to remain quite favorable while the storm traverses sea surface temperatures of around 29C over the next 48 hours. Additional strengthening is forecast during this time, but some fluctuations in intensity are likely due to eyewall replacement cycles. The updated NHC intensity forecast once again calls for additional intensification and brings Florence to near category 5 strength within the next 24 to 36 hours. After 48 hours, a slight increase in southwesterly shear could result in some weakening, but Florence is expected to remain an extremely dangerous hurricane when it approaches the U.S. coastline. Florence has accelerated as anticipated and is now moving west-northwestward or 290 degrees at 13 kt. The track forecast reasoning has not changed much. A mid-level ridge to the northeast of Bermuda is expected steer Florence quickly west-northwestward to northwestward toward the southeast United States coast over the next 2 to 3 days. By 72 hours, a high pressure ridge building over the Upper-Midwest and Great Lakes regions is forecast to cause a significant reduction in Florence's forward speed and the hurricane is predicted to meander over the eastern portions of North or South Carolina at days 4 and 5. The ECMWF has trended slower this cycle at days 4 and 5, and as a result the NHC forecast shows slightly less motion at those time periods. The spread in the guidance increases by 72 hours, with the GFS and its ensemble mean along the right side of the guidance, while the ECMWF remains along the left edge. It should be noted that there are still a number of ECMWF members that are even farther left. The NHC track forecast has been nudged to the left and is close to the TVCN consensus aid. Given the amount of uncertainty by day 3, it is important not to focus on the exact forecast track as average NHC errors at days 3, 4, and 5 are about 100, 140 and 180 n mi, respectively, and dangerous hazards will extend well away from the center. Storm Surge and Hurricane watches have been issued for a portion of the coast of South and North Carolina. Additional watches may be required later today. Key Messages: 1. A life-threatening storm surge is likely along portions of the coastlines of South Carolina, North Carolina, and Virginia, and a Storm Surge Watch has been issued for a portion of this area. All interests from South Carolina into the mid-Atlantic region should ensure they have their hurricane plan in place and follow any advice given by local officials. 2. Life-threatening freshwater flooding is likely from a prolonged and exceptionally heavy rainfall event, which may extend inland over the Carolinas and Mid Atlantic for hundreds of miles as Florence is expected to slow down as it approaches the coast and moves inland. 3. Damaging hurricane-force winds are likely along portions of the coasts of South Carolina and North Carolina, and a Hurricane Watch has been issued for a part of this area. Damaging winds could also spread well inland into portions of the Carolinas and Virginia. 4. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 26.4N 64.1W 120 KT 140 MPH 12H 11/1800Z 27.2N 66.4W 125 KT 145 MPH 24H 12/0600Z 28.7N 69.4W 130 KT 150 MPH 36H 12/1800Z 30.5N 72.2W 130 KT 150 MPH 48H 13/0600Z 32.2N 74.5W 125 KT 145 MPH 72H 14/0600Z 34.3N 77.1W 115 KT 130 MPH 96H 15/0600Z 35.2N 78.0W 45 KT 50 MPH...INLAND 120H 16/0600Z 36.0N 79.0W 25 KT 30 MPH...INLAND $$ Forecaster Brown

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Tropical Depression Paul Forecast Discussion Number 12

2018-09-11 10:35:57| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Tue Sep 11 2018 524 WTPZ43 KNHC 110835 TCDEP3 Tropical Depression Paul Discussion Number 12 NWS National Hurricane Center Miami FL EP182018 200 AM PDT Tue Sep 11 2018 Paul's surface circulation continues to be decoupled well to the east of a rather shapeless convective mass that persists only as a result of intermittent bursts of deep convection. The cyclone's intensity is held at depression strength and is a compromise of the TAFB and SAB subjective Dvorak estimates. Within the next 12 hours, Paul will be moving into a stable, drier air mass and over cooler sea surface temperatures. This should support further weakening, and the cyclone is expected to degenerate into a remnant low in 48 hours, which is indicated in the large-scale models and the statistical intensity guidance. The initial motion is estimated to be west-northwestward, or 295/10 kt, within the steering current produced by a mid-level ridge extending westward over the eastern Pacific from the Baja California peninsula. A slow turn westward is forecast in 36 hours as the weakening cyclone becomes vertically more shallow and is steered by the low-level easterly tradewind flow. The official forecast is again adjusted to the south of the previous package to conform more with the TVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 21.9N 123.1W 30 KT 35 MPH 12H 11/1800Z 22.2N 124.7W 30 KT 35 MPH 24H 12/0600Z 22.5N 126.4W 30 KT 35 MPH 36H 12/1800Z 22.8N 128.0W 30 KT 35 MPH 48H 13/0600Z 23.0N 129.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 14/0600Z 23.2N 130.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 15/0600Z 23.2N 132.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 16/0600Z...DISSIPATED $$ Forecaster Roberts

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