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Hurricane Florence Forecast Discussion Number 45

2018-09-10 16:50:54| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Mon Sep 10 2018 689 WTNT41 KNHC 101450 TCDAT1 Hurricane Florence Discussion Number 45 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Mon Sep 10 2018 Florence is quickly becoming a powerful hurricane. Satellite images show that the distinct eye has warmed in the center, with convection increasing in the eyewall during the past several hours. The initial wind speed is set to 100 kt, closest to the CIMSS-ADT value. A NOAA Hurricane Hunter will be in the area later this morning for a more accurate estimate. The hurricane is moving over progressively warmer waters over the next couple of days, with water temperatures peaking near 85F. In combination with the low vertical wind shear in the forecast during that time, Florence should continue to strengthen, and all models show it becoming a category 4 hurricane by tomorrow. The corrected-consensus guidance has done quite well with this intensification episode, and I don't see any reason to deviate much from them at this time. As Florence approaches the southeastern United States, there will likely be fluctuations in intensity from eyewall cycles, but even if this occurs, the hurricane's wind field is expected to grow with time, increasing the storm surge and inland wind threats. The bottom line is that there is increasing confidence that Florence will be a large and extremely dangerous hurricane, regardless of its exact intensity. During the last several hours, Florence has turned westward again, estimated at 11 kt. The steering currents are becoming well- defined as as a very strong ridge builds over the northwestern Atlantic Ocean, forcing Florence to move faster toward the west-northwest during the next couple of days. By late Wednesday, a turn toward the northwest is possible due to the orientation of the Atlantic ridge, along with a slight decrease in forward speed due to a new ridge building over the Great Lakes. The various models are shifting around at long range, but the model consensus has barely budged in the past few model cycles. Thus the new NHC forecast is close to the previous one, near the NOAA and FSSE consensus guidance. It is important not to focus on the exact forecast track as average NHC errors at days 4 and 5 are about 140 and 180 n mi, respectively, and dangerous hazards will extend well away from the center. 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 will likely be issued for some of these areas by Tuesday morning. 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 will likely be issued by Tuesday morning. 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 10/1500Z 25.0N 60.0W 100 KT 115 MPH 12H 11/0000Z 25.5N 61.9W 115 KT 130 MPH 24H 11/1200Z 26.4N 64.7W 125 KT 145 MPH 36H 12/0000Z 27.8N 67.9W 130 KT 150 MPH 48H 12/1200Z 29.5N 71.0W 130 KT 150 MPH 72H 13/1200Z 33.0N 76.3W 125 KT 145 MPH 96H 14/1200Z 35.0N 79.0W 75 KT 85 MPH...INLAND 120H 15/1200Z 36.0N 80.0W 30 KT 35 MPH...INLAND $$ Forecaster Blake

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Tropical Storm Paul Forecast Discussion Number 9

2018-09-10 16:37:55| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Mon Sep 10 2018 734 WTPZ43 KNHC 101437 TCDEP3 Tropical Storm Paul Discussion Number 9 NWS National Hurricane Center Miami FL EP182018 800 AM PDT Mon Sep 10 2018 Paul continues to have its low-level center displaced on the northeast side of the main area of deep convection. This convection is not well organized and there is little or no evidence of banding features. The current intensity estimate remains 35 kt, which is a blend of subjective Dvorak estimates from SAB and TAFB along with an ADT estimate from UW-CIMSS. Over the next few days, Paul will be moving into a drier air mass and over a cooler ocean. This should cause weakening, and the system if forecast to degenerate into a remnant low in 72 hours, if not sooner There is significant spread in the center fixes, so the initial motion is a rather uncertain 320/9 kt. A gradual turn to the west-northwest is forecast due to a mid-level ridge the north of Paul. Later in the period, the increasingly shallow cyclonic circulation should turn more to the west, following the low-level flow. The official forecast is similar to the previous NHC track an close to the correct model consensus, HCCA. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 20.8N 120.7W 35 KT 40 MPH 12H 11/0000Z 21.4N 121.9W 35 KT 40 MPH 24H 11/1200Z 21.9N 123.7W 35 KT 40 MPH 36H 12/0000Z 22.3N 125.4W 30 KT 35 MPH 48H 12/1200Z 22.7N 126.8W 30 KT 35 MPH 72H 13/1200Z 23.5N 129.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 14/1200Z 23.8N 130.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 15/1200Z 24.0N 132.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Pasch

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Hurricane Helene Forecast Discussion Number 13

2018-09-10 16:36:25| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Mon Sep 10 2018 274 WTNT43 KNHC 101436 TCDAT3 Hurricane Helene Discussion Number 13 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Mon Sep 10 2018 Helene's cloud pattern has become much better organized during the past few hours, with a clear 20 n mi wide eye seen in Meteosat infrared imagery, and a clear eye noted in a 1135 UTC AMSU overpass. A Dvorak classifications from TAFB and SAB were both T5.0/90 knots at 1200 UTC, and that is the initial intensity for this advisory. Helene will remain in an environment that should support some additional strengthening for the next 12 hours or so, with very low shear and SSTs of 27C or above. After that time, the SSTs cool quickly and the shear increases dramatically by 36 hours, which should induce steady weakening. Late in the period, the shear continues, and while SSTs increase the atmosphere dries out, and the intensity is held steady at day 5. The new NHC intensity forecast is above most of the guidance through 24 hours to account for the recent intensification of the storm, but then is close to or slightly above HCCA and IVCN. The initial motion estimate is 285/14. The subtropical ridge north of Helene will gradually weaken as an upper-level trough digs southward around 40W. This pattern will cause Helene to slow down and gradually recurve during the forecast period, with a faster northward motion expected at days 4 and 5 as Helene interacts with the aforementioned upper-level trough. While there is significant variability between the various global models in the exact evolution of the upper-level pattern, the track model guidance is in generally good agreement on this evolution, although the UKMET, its ensemble mean and the GEFS mean are notable outliers to the right. The new NHC track was not changed much from the previous one and lies near the middle of the guidance envelope and is close to HCCA. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 14.6N 30.0W 90 KT 105 MPH 12H 11/0000Z 15.2N 32.0W 100 KT 115 MPH 24H 11/1200Z 15.9N 34.3W 100 KT 115 MPH 36H 12/0000Z 16.8N 36.1W 90 KT 105 MPH 48H 12/1200Z 18.0N 37.7W 75 KT 85 MPH 72H 13/1200Z 21.5N 39.5W 60 KT 70 MPH 96H 14/1200Z 27.0N 40.5W 50 KT 60 MPH 120H 15/1200Z 32.5N 40.5W 50 KT 60 MPH $$ Forecaster Brennan

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Hurricane Isaac Forecast Discussion Number 12

2018-09-10 16:36:25| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Mon Sep 10 2018 273 WTNT44 KNHC 101436 TCDAT4 Hurricane Isaac Discussion Number 12 NWS National Hurricane Center Miami FL AL092018 1100 AM AST Mon Sep 10 2018 Isaac's cloud pattern is currently a bit ragged-looking, with an irregular CDO and limited convective banding. Dvorak intensity estimates from both TAFB and SAB are 65 kt, and that value will be used for the advisory. There is significant uncertainty in the intensity forecast for this system. Vertical wind shear over the hurricane is forecast to be weak to moderate during the next day or so, which should allow Isaac to strengthen somewhat. By 48 hours, however, the global models depict significantly stronger northwesterly shear, associated with an upper-level trough over the central Atlantic, affecting the tropical cyclone. Most of the guidance does not show Isaac strengthening over the Caribbean. The HWRF model is an outlier, strengthening Isaac into a major hurricane later in the period, and it is the main contributor to high consensus forecasts at days 3-5. The NHC intensity forecast is above the model consensus through 72 hours and below it by the end of the period. Isaac continues to move westward, or 275/12 kt. A subtropical ridge should be maintained to the north of the tropical cyclone for the next several days, so a continued westward track seems likely. The U. K. Met. Office model is an outlier and shows Isaac turning northwestward and northward to the northeast of the Leeward Islands. The official track forecast, like the previous one, follows the other reliable global models and is also close to the HFIP corrected consensus guidance. Key Messages: 1. Isaac is a small hurricane and uncertainty in the forecast is greater than normal. Although Isaac is forecast to begin weakening when it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands. 2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 14.7N 43.9W 65 KT 75 MPH 12H 11/0000Z 14.9N 45.9W 70 KT 80 MPH 24H 11/1200Z 15.0N 48.7W 75 KT 85 MPH 36H 12/0000Z 15.0N 51.4W 75 KT 85 MPH 48H 12/1200Z 15.1N 54.0W 75 KT 85 MPH 72H 13/1200Z 15.4N 59.4W 70 KT 80 MPH 96H 14/1200Z 15.5N 64.5W 60 KT 70 MPH 120H 15/1200Z 15.7N 70.0W 50 KT 60 MPH $$ Forecaster Pasch

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Hurricane Helene Forecast Discussion Number 12

2018-09-10 13:33:52| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Sep 10 2018 768 WTNT43 KNHC 101133 CCA TCDAT3 Hurricane Helene Discussion Number 12...Corrected NWS National Hurricane Center Miami FL AL082018 500 AM AST Mon Sep 10 2018 Corrected to change Paul to Isaac in third paragraph Little change in the Helene's cloud pattern has been observed during the past 6 hours. The earlier mentioned ragged banding-type eye feature is still apparent in the enhanced infrared BD-curve imagery and a recent SSMI/S microwave overpass. The initial intensity is held at 75 kt, and is based on the subjective Dvorak intensity estimates. Helene is moving within a fairly ripe environment and over warm oceanic sea surface temperatures. These conducive conditions should promote additional strengthening over the next 36 hours. Afterward, increasing south-southwesterly shear, lower mid-tropospheric moisture, and cooler water are expected to induce weakening, however gradually, through day 5. The official forecast follows suit and is similar to the better performing IVCN intensity consensus model. Helene's initial motion is estimated to be west-northwestward, or 285/15 kt, and this general motion is forecast during the next 48 hours. Beyond that period, a break in the subtropical ridge to the northwest of Isaac, created by a mid- to upper-level cut-off low digging southward, should induce a northward turn with a gradual increase in forward motion. The large-scale models all indicate this change in the synoptic steering pattern, although there is some disparity on how soon Helene will turn toward the north. The NHC track is quite close to the previous forecast and lies between the consensus guidance. FORECAST POSITIONS AND MAX WINDS INIT 10/0900Z 14.3N 28.9W 75 KT 85 MPH 12H 10/1800Z 14.8N 30.9W 80 KT 90 MPH 24H 11/0600Z 15.5N 33.4W 85 KT 100 MPH 36H 11/1800Z 16.3N 35.6W 85 KT 100 MPH 48H 12/0600Z 17.2N 37.2W 80 KT 90 MPH 72H 13/0600Z 20.3N 39.4W 65 KT 75 MPH 96H 14/0600Z 25.3N 40.8W 55 KT 65 MPH 120H 15/0600Z 31.2N 41.1W 50 KT 60 MPH $$ Forecaster Roberts

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