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Hurricane Florence Forecast Discussion Number 29

2018-09-06 16:39:47| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Sep 06 2018 231 WTNT41 KNHC 061439 TCDAT1 Hurricane Florence Discussion Number 29 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Thu Sep 06 2018 Vertical shear has increased since yesterday, which has caused a degradation of Florence's structure and a decrease in its maximum winds. The cloud-filled eye has been eroded over the past hour or so, and the deep convection is no longer symmetric, with the low-level circulation peeking out from under the higher clouds. Dvorak CI numbers from TAFB and SAB have fallen to T5.0-5.5 (90-100 kt), while the objective numbers from UW-CIMSS support 70-90 kt. The initial intensity is set at 90 kt, near the middle of this wide range of estimates. The intensity forecast has been somewhat of a self-defeating prophecy due to the nuances of the environmental shear. Even though Florence was able to rapidly intensify yesterday in an area just south of a zone of strong shear, the hurricane's stronger-than- expected intensity caused it to move more poleward, into that stronger shear. Right now, shear analyses range anywhere from 25-30 kt, and the latest available guidance suggests that this level of shear should continue for another 12-24 hours. As a result, continued weakening is forecast over the next day or so. After 36 hours, Florence is likely to encounter an upper-level environment that is more conducive for reintensification. The NHC forecast is adjusted downward toward the newest consensus aids, especially during the first 48 hours, but it still shows Florence reaching major hurricane strength again by days 4 and 5. Florence's forward motion has slowed just a little to 9 kt toward the northwest (315 degrees). A mid-level ridge is building to the north, which is likely to cause the hurricane to turn westward by 36 hours, with that motion continuing through about day 3. After that time, there is still considerable uncertainty in the evolution of the steering pattern over the western Atlantic, especially on day 4. On one hand, the GFS and HWRF dig a strong shortwave trough over Atlantic Canada by Monday, creating a break in the ridge which would allow Florence to turn northwestward. On the other hand, the ECMWF and UKMET both have weaker troughs and maintain stronger ridging over the northwestern Atlantic, allowing Florence to maintain a westward or west-northwestward course. All the models show a mid-level high over the western Atlantic by Tuesday. Due to typical biases among these models in the part of the Atlantic, we prefer to be between the GFS and ECMWF solutions at this time, which places the official NHC track forecast close to the TVCN multi-model consensus and just north of HCCA. There is still considerable model ensemble spread for Florence's track beyond day 5. Given the large uncertainty at these time ranges, it is far too soon to speculate what, if any, impacts Florence may have on the U.S. East Coast next week. Regardless of Florence's eventual track, large swells emanating from the hurricane will reach Bermuda beginning on Friday and portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 06/1500Z 24.6N 48.6W 90 KT 105 MPH 12H 07/0000Z 25.2N 49.8W 80 KT 90 MPH 24H 07/1200Z 25.6N 51.3W 75 KT 85 MPH 36H 08/0000Z 25.6N 52.6W 75 KT 85 MPH 48H 08/1200Z 25.7N 54.0W 80 KT 90 MPH 72H 09/1200Z 26.3N 56.4W 90 KT 105 MPH 96H 10/1200Z 28.0N 59.5W 100 KT 115 MPH 120H 11/1200Z 29.5N 65.0W 105 KT 120 MPH $$ Forecaster Berg

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Hurricane Olivia Forecast Discussion Number 24

2018-09-06 16:37:16| Tropical Depression LIDIA

Issued at 800 AM PDT Thu Sep 06 2018

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Hurricane Olivia Forecast Discussion Number 23

2018-09-06 13:46:47| Tropical Depression LIDIA

Issued at 500 AM PDT Thu Sep 06 2018

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Hurricane Florence Forecast Discussion Number 28

2018-09-06 10:46:14| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Sep 06 2018 505 WTNT41 KNHC 060846 TCDAT1 Hurricane Florence Discussion Number 28 NWS National Hurricane Center Miami FL AL062018 500 AM AST Thu Sep 06 2018 Southwesterly shear continues to affect Florence. Since the last advisory, the cloud pattern has become more asymmetric, and cloud tops surrounding the ragged eye have warmed. AMSR imagery around 0430 UTC indicated that the hurricane's eye is tilted slightly southwest to northeast with height, but the low- to mid-level inner-core was mostly intact. Based on an average of Final-T and CI numbers from TAFB and SAB, the initial intensity has been lowered to 100 kt, and objective estimates are even lower. Given the wind shear and current appearance of Florence, additional weakening in the short-term seems likely, and all of the intensity guidance agrees. From 24-72 h, the spread increases with the statistical models generally showing continued gradual weakening for another day or two, while the dynamical models show little change or slight restrengthening. By day 5, all of the guidance calls for Florence to restrengthen. The guidance envelope and consensus is lower with this model cycle, but the NHC intensity forecast has only been adjusted slightly lower, mainly in the first 72 h of the forecast. The official forecast is now a little above the IVCN intensity consensus and HCCA at most forecast hours, generally favoring the stronger solution of the dynamical models. Florence has continued to track northwestward, and the initial motion is 315/10 kt. The cyclone is still forecast to gradually turn westward over the next 48 h, in response to a building mid-level ridge to its north. Beyond that time, a mid-latitude trough over the northwestern Atlantic could create enough of a weakness in the ridge to steer the hurricane farther north, closer to Bermuda, as shown by the latest GFS. However, the ECMWF and UKMET suggest that the ridge will not be significantly affected, and Florence will move more westward. The ensembles from the GFS and ECMWF do not clearly favor one solution over another, and in fact many members track Florence somewhere in-between. While little change was made to the previous forecast and the new official track forecast remains close to the various consensus aids, the model spread has increased and confidence in the forecast is low. There is still considerable model ensemble spread for Florence's track beyond day 5. Given the large uncertainty at these time ranges, it is far too soon to speculate what, if any, impacts Florence may have on the U.S. East Coast next week. Regardless of Florence's eventual track, large swells emanating from the hurricane will reach Bermuda beginning on Friday and portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 06/0900Z 24.1N 47.9W 100 KT 115 MPH 12H 06/1800Z 24.8N 49.1W 90 KT 105 MPH 24H 07/0600Z 25.4N 50.8W 90 KT 105 MPH 36H 07/1800Z 25.6N 52.2W 90 KT 105 MPH 48H 08/0600Z 25.6N 53.6W 90 KT 105 MPH 72H 09/0600Z 26.1N 56.1W 95 KT 110 MPH 96H 10/0600Z 27.2N 59.0W 105 KT 120 MPH 120H 11/0600Z 29.0N 63.5W 110 KT 125 MPH $$ Forecaster Zelinsky

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Hurricane Olivia Forecast Discussion Number 22

2018-09-06 10:44:43| Tropical Depression LIDIA

Issued at 200 AM PDT Thu Sep 06 2018

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