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Hurricane Helene Forecast Discussion Number 15

2018-09-11 04:42:59| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Sep 10 2018 809 WTNT43 KNHC 110242 TCDAT3 Hurricane Helene Discussion Number 15 NWS National Hurricane Center Miami FL AL082018 1100 PM AST Mon Sep 10 2018 Helene remains a well organized hurricane with a fairly large 25 n mi diameter eye and ring of cold cloud tops surrounding that feature. There are a few dry slots beyond the inner core, however, which have prevented some of the Dvorak estimates from increasing this cycle. A blend of the latest subjective and objective satellite intensity estimates supports raising the initial intensity slightly to 95 kt. The hurricane has a limited amount of time to strengthen as the current environment of low wind shear and relatively warm SSTs will be changing soon. Helene is expected to cross the 26 deg C isotherm within 24 hours and it will enter an environment of increasing south-southwesterly shear just beyond that time period. These conditions should promote a steady weakening trend beginning late Tuesday. By the end of the forecast period, however, SSTs increase along the forecast track, and there could be some baroclinic enhancements to end the weakening trend and perhaps even cause some strengthening. This scenario is supported by the HWRF/HMON regional models and the GFS/ECMWF global models. The NHC intensity forecast is largely an update of the previous one, except it is slightly higher at the end of the period. The hurricane is gradually turning to the right, with the latest initial motion estimate being 295/13 kt. A large mid- to upper-level trough over the north Atlantic is expected to dig southward causing a significant weakness in the subtropical ridge. In response to this change in the steering pattern, Helene is forecast to gradually turn northward and then north-northeastward during the next several days. The track models are in fairly good agreement, and the NHC track forecast lies near the middle of the guidance envelope. This forecast is only a little to the right of the previous track. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 15.5N 32.4W 95 KT 110 MPH 12H 11/1200Z 16.2N 34.0W 95 KT 110 MPH 24H 12/0000Z 17.2N 35.7W 85 KT 100 MPH 36H 12/1200Z 18.6N 37.2W 75 KT 85 MPH 48H 13/0000Z 20.1N 38.0W 65 KT 75 MPH 72H 14/0000Z 24.6N 39.0W 60 KT 70 MPH 96H 15/0000Z 30.0N 38.0W 50 KT 60 MPH 120H 16/0000Z 36.2N 34.0W 55 KT 65 MPH $$ Forecaster Cangialosi

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Hurricane Florence Forecast Discussion Number 46

2018-09-10 22:55:56| National Hurricane Center (Atlantic)

Issued at 500 PM AST Mon Sep 10 2018 366 WTNT41 KNHC 102055 TCDAT1 Hurricane Florence Discussion Number 46 NWS National Hurricane Center Miami FL AL062018 500 PM AST Mon Sep 10 2018 Unfortunately, the models were right. Florence has rapidly intensified into an extremely dangerous hurricane, with 30-second GOES-16 visible imagery showing well-defined eyewall mesovortices rotating inside of the eye. A NOAA Hurricane Hunter aircraft found peak SFMR winds of about 120 kt, with flight-level winds and dropsonde measurements also supporting that value for the initial wind speed estimate. Notably, the aircraft data also show the size of the hurricane-force winds has doubled in the past 12 hours. None of the guidance suggest that Florence has peaked in intensity, and this is supported by a continuation of a low-shear environment, and even warmer waters over the next 36 hours. Thus, the intensity forecast is raised from the previous one, bringing Florence close to category 5 strength tomorrow. Near landfall, the vertical wind shear could increase, along with the increasing likelihood of eyewall cycles. While the intensity forecast shows some weakening of the maximum winds near landfall, the wind field is expected to grow with time, which increases the storm surge and inland wind threats. The bottom line is that there is high confidence that Florence will be a large and extremely dangerous hurricane, regardless of its exact intensity. Florence has recently turned west-northwestward, still moving at 11 kt. The hurricane is expected to accelerate in that direction over the next day or two due to building mid-level ridge over the northwestern Atlantic Ocean. By late Wednesday, a turn toward the northwest is forecast due to the orientation of the Atlantic ridge, along with a decrease in forward speed due to a new ridge building over the Great Lakes. There is a new player in the forecast as well, with the disturbance over the northwestern Caribbean adding some uncertainty in the ridge strength over the southeastern United States. Perhaps it isn't surprising that the model spread has increased on this cycle, with a small eastward shift overall. The official forecast is nudged in the direction of the trend, but is west of the model consensus. 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. The NOAA G-IV jet will continue to conduct synoptic surveillance missions every 12 h through at least Wednesday. In addition, special 0600 UTC and 1800 UTC radiosonde launches have been expanded to additional upper-air stations across the U.S. are to collect extra data for the numerical models. 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 10/2100Z 25.4N 61.1W 120 KT 140 MPH 12H 11/0600Z 26.0N 63.2W 130 KT 150 MPH 24H 11/1800Z 27.0N 66.2W 135 KT 155 MPH 36H 12/0600Z 28.6N 69.3W 135 KT 155 MPH 48H 12/1800Z 30.4N 72.2W 130 KT 150 MPH 72H 13/1800Z 33.7N 77.0W 120 KT 140 MPH 96H 14/1800Z 35.6N 78.8W 50 KT 60 MPH...INLAND 120H 15/1800Z 36.5N 79.5W 25 KT 30 MPH...INLAND $$ Forecaster Blake

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Tropical Storm Paul Forecast Discussion Number 10

2018-09-10 22:34:24| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Mon Sep 10 2018 501 WTPZ43 KNHC 102034 TCDEP3 Tropical Storm Paul Discussion Number 10 NWS National Hurricane Center Miami FL EP182018 200 PM PDT Mon Sep 10 2018 Paul continues to have a rather ragged appearance in satellite imagery. The low-level center is completely exposed to the east of a limited area of deep convection. A pair of recent ASCAT passes showed maximum winds of just below tropical-storm-force, so the initial intensity has been held at 35 kt, assuming at least a little undersampling may have occurred due to limitations of the instrument. All of the intensity guidance continues to show weakening, and Paul is forecast to become a tropical depression later tonight, and a remnant low within the next few days. Given the cold SSTs and dry air ahead of Paul, it is certainly possible that Paul could become a remnant low or dissipate sooner than currently forecast. No major changes have been made to the previous official track forecast. Paul is still moving toward the northwest with an initial motion of 315/9 kt, but a turn toward the west-northwest should begin shortly as Paul is steered increasingly by low-level trade-wind flow. By the end of the forecast period, the remnant circulation of Paul should slow down and turn westward. The new official track forecast is basically a blend of the previous forecast and HCCA. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 21.3N 121.2W 35 KT 40 MPH 12H 11/0600Z 21.8N 122.7W 30 KT 35 MPH 24H 11/1800Z 22.2N 124.5W 30 KT 35 MPH 36H 12/0600Z 22.6N 126.1W 30 KT 35 MPH 48H 12/1800Z 22.9N 127.4W 30 KT 35 MPH 72H 13/1800Z 23.5N 129.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 14/1800Z 23.5N 131.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 15/1800Z 23.5N 132.5W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Zelinsky/Ramos

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Hurricane Helene Forecast Discussion Number 14

2018-09-10 22:33:26| National Hurricane Center (Atlantic)

Issued at 500 PM AST Mon Sep 10 2018 089 WTNT43 KNHC 102033 TCDAT3 Hurricane Helene Discussion Number 14 NWS National Hurricane Center Miami FL AL082018 500 PM AST Mon Sep 10 2018 Helene's cloud pattern continues to be well organized with a large eye of about 20 nm in diameter, surrounded by a ring of deep convection. The cyclone's circulation is large with numerous cyclonically curved convective bands, and the outflow is fair in all quadrants. Dvorak classifications have not changed much, and support an initial intensity of 90 kt. Helene has the opportunity to strengthen some during the next 24 hours or so, while the shear is low and the ocean is still relatively warm. After that time, both shear and SSTs will become less favorable and gradual weakening should then begin. The NHC intensity forecast is similar to the previous one, and follows closely the intensity consensus aids. Helene is moving toward the west-northwest or 290 degrees at 15 kt. Soon, the hurricane will encounter a mid-level trough which the most reliable global models are forming in the eastern Atlantic. This new flow pattern will force Helene to turn to the northwest and north ahead of the developing trough. Track models are quite consistent with this solution and the the spread of the guidance envelope is small through the forecast period. This increases the confidence in the Helene's northward turn, followed by recurvature over the eastern Atlantic. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 14.9N 31.5W 90 KT 105 MPH 12H 11/0600Z 15.5N 33.3W 100 KT 115 MPH 24H 11/1800Z 16.4N 35.3W 100 KT 115 MPH 36H 12/0600Z 17.5N 37.0W 85 KT 100 MPH 48H 12/1800Z 19.0N 38.0W 75 KT 85 MPH 72H 13/1800Z 23.0N 39.5W 60 KT 70 MPH 96H 14/1800Z 28.5N 40.0W 50 KT 60 MPH 120H 15/1800Z 34.0N 37.0W 50 KT 60 MPH $$ Forecaster Avila

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Hurricane Isaac Forecast Discussion Number 13

2018-09-10 22:31:55| National Hurricane Center (Atlantic)

Issued at 500 PM AST Mon Sep 10 2018 407 WTNT44 KNHC 102031 TCDAT4 Hurricane Isaac Discussion Number 13 NWS National Hurricane Center Miami FL AL092018 500 PM AST Mon Sep 10 2018 Isaac has not become better organized today. The system continues to display a ragged CDO with limited banding features. Dvorak T-numbers from both TAFB and SAB continue to support an intensity of 65 kt. It is not clear why Isaac has failed to strengthen. Cirrus motions and microwave imagery suggest some shear over the tropical cyclone, which may have had some influence. Isaac should not experience a significant increase in shear until about 48 hours, so some strengthening is called for up to that time. Thereafter, the dynamical guidance shows a significant increase in shear, which should reverse the intensity trend. The HWRF model continues to be an outlier in predicting significant intensification over the Caribbean, whereas the other models do not. The official intensity forecast is above the model consensus over the first half of the period, and below the consensus during the latter part of the period, considering the HWRF could be making the consensus too high at days 3-5. Isaac continues moving westward or 270/12 kt. There continue to be a few differences among the track guidance, in particular the U.K. Met, which shows the system turning northwestward to northward well to the northeast of the Leeward Islands. The GFS takes the system into the Caribbean but shows a more west-northwestward motion. The official forecast stays close to the ECMWF and the corrected consensus predictions, which is very similar to the previous NHC track. Given the spread in the guidance, the confidence in the details of the track forecast beyond the first couple of days is larger than usual. Key Messages: 1. Isaac is a small hurricane and uncertainty in the intensity forecast is greater than normal. Although Isaac is expected to begin weakening when it approaches the Lesser Antilles, it is still likely 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. Watches will likely be required for portions of the Lesser Antilles tomorrow. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 14.4N 45.0W 65 KT 75 MPH 12H 11/0600Z 14.5N 47.1W 70 KT 80 MPH 24H 11/1800Z 14.6N 49.8W 75 KT 85 MPH 36H 12/0600Z 14.6N 52.3W 75 KT 85 MPH 48H 12/1800Z 14.8N 54.7W 75 KT 85 MPH 72H 13/1800Z 15.3N 60.2W 70 KT 80 MPH 96H 14/1800Z 15.5N 65.5W 60 KT 70 MPH 120H 15/1800Z 15.5N 70.5W 50 KT 60 MPH $$ Forecaster Pasch

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