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Tropical Storm Isaac Forecast Discussion Number 8

2018-09-09 16:33:11| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sun Sep 09 2018 266 WTNT44 KNHC 091433 TCDAT4 Tropical Storm Isaac Discussion Number 8 NWS National Hurricane Center Miami FL AL092018 1100 AM AST Sun Sep 09 2018 WindSat microwave data from several hours ago show that Isaac has developed a well-defined low-level ring in the 37-GHz channel, and the center is embedded in the middle of a small central dense overcast. TAFB and SAB Dvorak estimates have increased to T3.5 given Isaac's improved structure, so the initial intensity is raised to 55 kt. Isaac's future intensity is the most challenging part of the forecast. Since sea surface temperatures will be warm (between 27-28 degrees Celsius) and vertical shear will be almost non-existent (less than 10 kt) for the next 36-48 hours, the tiny storm will be prone to a possibly significant increase in intensity during the next couple of days. For that period, the NHC intensity forecast is above the guidance, and above the previous forecast, due to relatively high numbers from the various rapid intensification indices. However, in 2-3 days, the global models are showing northwesterly shear steadily increasing and potentially reaching a maximum of around 30 kt by Thursday. The big question is the timing of this shear and how much it will affect Isaac's intensity before the cyclone reaches the Lesser Antilles. The HWRF, which brings Isaac to major hurricane intensity despite the forecast shear, appears to be an outlier, and is therefore pulling up the ICON intensity consensus and HCCA. As a result, the NHC intensity forecast is below ICON on days 4 and 5. This new forecast shows a more peaked maximum intensity, with Isaac possibly increasing and then decreasing in strength at faster rates than shown here. While the microwave data indicate that Isaac's center is slightly south of previous estimates, the current motion estimate is 270/8 kt. A low- to mid-level ridge is expected to move westward in tandem with Isaac for much of the forecast period, causing the cyclone to accelerate toward the west, reaching a forward speed of around 15 kt by 36 hours, and then holding that speed through day 5. With the exception of the UKMET and HWRF models, the guidance envelope is tightly packed from north to south, and the speed differences noted over the past few days have decreased. The updated NHC track forecast is just a little south of the previous one to account for the adjusted initial position, and lies slightly south of the multi-model consensus in the direction of HCCA and the Florida State Superensemble. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 14.3N 39.1W 55 KT 65 MPH 12H 10/0000Z 14.4N 40.8W 65 KT 75 MPH 24H 10/1200Z 14.4N 43.3W 75 KT 85 MPH 36H 11/0000Z 14.4N 46.3W 85 KT 100 MPH 48H 11/1200Z 14.3N 49.1W 85 KT 100 MPH 72H 12/1200Z 14.2N 54.8W 80 KT 90 MPH 96H 13/1200Z 14.5N 60.5W 70 KT 80 MPH 120H 14/1200Z 15.0N 66.5W 60 KT 70 MPH $$ Forecaster Berg

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Tropical Storm Helene Forecast Discussion Number 8

2018-09-09 10:45:41| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Sep 09 2018 747 WTNT43 KNHC 090845 TCDAT3 Tropical Storm Helene Discussion Number 8 NWS National Hurricane Center Miami FL AL082018 500 AM AST Sun Sep 09 2018 Satellite imagery indicates that Helene continues to become better organized with an increase in banding overnight. A couple of recent microwave images show that the inner core continues to organize with a low-level ring evident and indication that a banding-type eye is forming. Dvorak satellite T-numbers have increased and support an initial intensity of 55 kt. Helene is forecast to remain over SSTs of 27-28C during the next couple of days. The upper-level wind pattern is also expected to be quite favorable, and these conditions should support steady intensification. Slightly cooler waters and increasing southwesterly shear from an upper-level trough to the northwest of Helene are likely to put an end to the intensification process by day 3. After that time, Helene is forecast to move closer to the aforementioned trough with southwesterly shear and drier mid-level air likely to result in weakening by day 4 and 5. The NHC intensity forecast is near the IVCN intensity consensus model, but it is a little more conservative than the FSSE and HCCA models. The tropical storm is moving westward or 270 degrees at 12 kt. A deep-layer ridge to the north of the storm should steer Helene westward to west-northwestward during the next couple of days. By mid-week, Helene is expected to approach a weakness in the ridge between 40-50W longitude and the cyclone is predicted to turn northwestward, then north-northwestward by the end of the period. The track guidance is in fairly good agreement on this scenario, but there is some spread later in the period as to exactly what longitude the northwesterly turn takes place. The NHC forecast is between the various dynamical model solutions and is close to the HFIP corrected consensus model. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 13.2N 24.0W 55 KT 65 MPH 12H 09/1800Z 13.4N 26.0W 65 KT 75 MPH 24H 10/0600Z 14.0N 28.9W 75 KT 85 MPH 36H 10/1800Z 14.7N 31.9W 85 KT 100 MPH 48H 11/0600Z 15.4N 34.5W 90 KT 105 MPH 72H 12/0600Z 17.4N 38.3W 80 KT 90 MPH 96H 13/0600Z 20.5N 41.0W 65 KT 75 MPH 120H 14/0600Z 25.0N 42.5W 50 KT 60 MPH $$ Forecaster Brown

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Tropical Storm Florence Forecast Discussion Number 40

2018-09-09 10:44:11| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Sep 09 2018 701 WTNT41 KNHC 090844 TCDAT1 Tropical Storm Florence Discussion Number 40 NWS National Hurricane Center Miami FL AL062018 500 AM AST Sun Sep 09 2018 Florence's cloud pattern has continued to gradually become better organized overnight, with an increase in convection near the center and a developing central dense overcast feature. However, the cloud tops are not particularly cold and the outer banding features remain fragmented likely due to some nearby dry mid-level air. Dvorak intensity estimates from TAFB and SAB range from 55 to 65 kt, so the initial intensity remains 60 kt, just shy of hurricane strength. The NOAA Hurricane Hunters are scheduled to investigate the storm later today, and that data should provide a better assessment of Florence's intensity. The upper-level outflow is becoming well established over the cyclone, and the global model guidance indicates that Florence will remain in a very favorable upper-level environment while the cyclone moves over the warm waters over the southwestern Atlantic. These conditions favor strengthening with the only apparent negative factor being nearby dry air, which will likely remain away from the inner core due to the low shear conditions. The NHC intensity forecast again calls for a period of rapid strengthening within the next 12-36 hours, and Florence is forecast to become a major hurricane on Monday with additional strengthening early in the week. This means that Florence is likely to be a very powerful hurricane as it moves over the western Atlantic toward the southeastern United States. The new NHC intensity forecast is near the various intensity consensus aids and is very similar to the previous official forecast. Florence is currently located between a couple of mid-level ridges and a slow westward motion is expected today. Another strong high pressure ridge is forecast to build to the north of Florence on Monday, which should cause the storm to begin moving west-northwestward to northwestward at an increasingly faster forward speed. The dynamical model guidance is tightly clustered for the first 2-3 days with increasing spread thereafter. The GFS remains along the right side of the guidance envelope with the HWRF and ECMWF bracketing the left edge. It should be noted that both the GFS and ECMWF ensemble means are a little to the left or west of their operational runs. As a result, the NHC track forecast lies to the left of the TVCA multi-model consensus, but is not as far to the west as the FSSE and HCCA corrected consensus models at day 5. The models are in agreement that Florence is likely to slow down near the end of the forecast period as a blocking high pressure ridge builds to the north of the hurricane. Key Messages: 1. Florence is forecast to be a dangerous major hurricane near the southeast U.S. coast by late this week, and the risk of direct impacts continues to increase. However, given the uncertainty in track and intensity forecasts at those time ranges, it's too soon to determine the exact timing, location, and magnitude of those impacts. 2. Interests along the U.S. East Coast, particularly from north Florida through North Carolina, should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials. 3. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week. These swells will result in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 24.5N 55.8W 60 KT 70 MPH 12H 09/1800Z 24.6N 56.7W 70 KT 80 MPH 24H 10/0600Z 24.8N 58.3W 85 KT 100 MPH 36H 10/1800Z 25.3N 60.4W 100 KT 115 MPH 48H 11/0600Z 26.1N 63.1W 110 KT 125 MPH 72H 12/0600Z 28.2N 69.1W 125 KT 145 MPH 96H 13/0600Z 31.3N 74.8W 120 KT 140 MPH 120H 14/0600Z 34.4N 77.9W 105 KT 120 MPH...INLAND $$ Forecaster Brown

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Tropical Storm Paul Forecast Discussion Number 4

2018-09-09 10:43:40| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Sun Sep 09 2018 441 WTPZ43 KNHC 090843 TCDEP3 Tropical Storm Paul Discussion Number 4 NWS National Hurricane Center Miami FL EP182018 200 AM PDT Sun Sep 09 2018 Satellite data indicate that depression has strengthened. The cyclone has a large curved band that wraps across the western half of the circulation, but banding features remain limited to the east of the center. A partial ASCAT pass from a few hours ago showed 30-35 kt winds in the southeastern quadrant. Based on that data and the 2.5/35 kt Dvorak classification from TAFB, the initial intensity is increased to 35 kt, making the system Tropical Storm Paul. Paul will likely strengthen some more during the next couple of days while it remains over warm waters and in a relatively moist environment. However, significant strengthening appears unlikely because of continued moderate east-northeasterly shear during the next day or so. The opportunity for intensification should end in a few days when the system moves over SSTs cooler than 26 deg C and into a drier and more stable air mass. The NHC intensity forecast is again nudged downward, but it still lies at the high end of the model guidance. Satellite fixes suggest that Paul has turned sharper to the northwest than expected. Smoothing through the fixes yields an initial motion estimate of 320/7. A continued northwestward motion is expected during the next day or two. After that time, Paul should turn to the west-northwest in response to a mid-level ridge to its north and then slow down by the end of the forecast period when it becomes shallow and is steered by the low-level flow. The NHC track has been adjusted northward to account for the change in the initial motion and position. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 17.1N 117.5W 35 KT 40 MPH 12H 09/1800Z 18.2N 118.2W 40 KT 45 MPH 24H 10/0600Z 19.4N 119.3W 45 KT 50 MPH 36H 10/1800Z 20.6N 120.8W 50 KT 60 MPH 48H 11/0600Z 21.4N 122.6W 50 KT 60 MPH 72H 12/0600Z 22.6N 126.5W 45 KT 50 MPH 96H 13/0600Z 23.5N 129.4W 40 KT 45 MPH 120H 14/0600Z 24.5N 131.3W 30 KT 35 MPH $$ Forecaster Cangialosi

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Tropical Storm Isaac Forecast Discussion Number 7

2018-09-09 10:40:41| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Sep 09 2018 335 WTNT44 KNHC 090840 TCDAT4 Tropical Storm Isaac Discussion Number 7 NWS National Hurricane Center Miami FL AL092018 500 AM AST Sun Sep 09 2018 The cloud pattern of Isaac has changed significantly during the past several hours. The storm no longer has a sheared appearance, and instead microwave images reveal a well-defined curved band that wraps a little more than halfway around the center. Despite the change in the storm's structure, the Dvorak classifications are unchanged at 3.0/45 kt. Based on that data, the initial intensity is held at 45 kt for this advisory. Now that the shear has lessened, Isaac should steadily strengthen during the next few days as the storm remains over warm waters and in a low wind shear environment. Beyond a few days, however, the SHIPS model shows a notable increase in shear, in part due to the outflow from Florence, which should end the strengthening trend and induce some weakening. There was no significant change in the intensity guidance this cycle, so the official forecast is largely an update of the previous one. This prediction lies fairly close to the IVCN and HCCA models. Isaac remains on the forecast track and the initial motion is the same as before, 270/7. A strengthening subtropical ridge to the north of the system should cause Isaac to move westward at an increasing forward speed during the next several days. Although there is some spread in the models, the usually more reliable GFS and ECMWF models have been consistent in showing a due westward motion through the forecast period. The NHC track forecast leans toward those models, and it is fairly similar to the previous prediction. Based on the current forecast, Isaac will be near the Lesser Antilles in 4 to 5 days and interests there should monitor the progress of this system. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 14.5N 38.1W 45 KT 50 MPH 12H 09/1800Z 14.6N 39.6W 55 KT 65 MPH 24H 10/0600Z 14.7N 41.9W 60 KT 70 MPH 36H 10/1800Z 14.7N 44.6W 70 KT 80 MPH 48H 11/0600Z 14.7N 47.5W 80 KT 90 MPH 72H 12/0600Z 14.7N 53.1W 85 KT 100 MPH 96H 13/0600Z 14.9N 58.9W 80 KT 90 MPH 120H 14/0600Z 15.3N 64.5W 70 KT 80 MPH $$ Forecaster Cangialosi

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