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Hurricane Florence Forecast Discussion Number 27

2018-09-06 04:50:14| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Sep 05 2018 550 WTNT41 KNHC 060250 TCDAT1 Hurricane Florence Discussion Number 27 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Wed Sep 05 2018 It appears that southwesterly shear is finally affecting Florence. The cloud pattern has become asymmetric this evening, with the northeastern quadrant growing at the expense of the southwestern one, along with warming cloud tops noted across the central dense overcast. Satellite intensity estimates are a bit lower than before, so the initial wind speed is reduced to 110 kt. The current shear is forecast to persist by the global models during the next day or two until an upper-level low cuts off to south of Florence, which will likely reduce the shear. This, in combination with much warmer waters in the path of the hurricane, should set the stage for restrengthening. At long range, a rather large upper-level anticyclone is forecast to form near Florence, which would provide ample opportunity for the hurricane to regain its former strength and grow in size. Thus, the official forecast shows slow weakening in response to the initial shear, then levels off, with a significant increase in strength predicted by day 5. This solution is very close to the previous NHC advisory, but not quite as high as the HWRF and HMON models at long range. Florence has been moving more to the right during the past several hours, likely due to the vortex attempting to stay vertically aligned in the face of the southwesterly shear, but a longer-term motion is 310 degrees at 10 kt. A mid-level ridge is forecast to strengthen to the north and northwest of Florence, causing the hurricane to turn west- northwestward by Friday and even westward by the weekend. It appears that a mid-latitude trough over the northwestern Atlantic in a few days will likely miss Florence, with the only perceivable effects on the hurricane being a decrease in forward speed. The net result is that the model guidance continues to trend westward at long range, and the official forecast follows suit, lying just north of the model consensus in deference to the ECMWF deterministic model and ensemble mean. It should be noted that 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, resulting in life-threatening surf and rip currents on the island. Swells from Florence could also reach the U.S. East Coast by early next week. FORECAST POSITIONS AND MAX WINDS INIT 06/0300Z 23.4N 47.2W 110 KT 125 MPH 12H 06/1200Z 24.2N 48.4W 105 KT 120 MPH 24H 07/0000Z 25.0N 49.9W 100 KT 115 MPH 36H 07/1200Z 25.5N 51.6W 95 KT 110 MPH 48H 08/0000Z 25.7N 53.2W 95 KT 110 MPH 72H 09/0000Z 26.1N 55.7W 100 KT 115 MPH 96H 10/0000Z 27.2N 58.1W 105 KT 120 MPH 120H 11/0000Z 29.0N 62.0W 115 KT 130 MPH $$ Forecaster Blake

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Hurricane Olivia Forecast Discussion Number 21

2018-09-06 04:38:13| Tropical Depression LIDIA

Issued at 800 PM PDT Wed Sep 05 2018

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Hurricane Florence Forecast Discussion Number 26

2018-09-05 22:33:39| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Sep 05 2018 107 WTNT41 KNHC 052033 TCDAT1 Hurricane Florence Discussion Number 26 NWS National Hurricane Center Miami FL AL062018 500 PM AST Wed Sep 05 2018 Florence has continued to intensify during the day, with colder cloud tops completely encircling a clearer eye. Both subjective and objective Dvorak estimates have unanimously increased to T6.0/115 kt, and on this basis, the initial intensity is raised to 115 kt, making Florence a category 4 hurricane. Florence appears to be right along the southern edge of a zone of stronger vertical shear and continues to defy the intensity guidance, which has consistently been predicting weakening. That said, the last few satellite images indicate that the eye may be becoming slightly disrupted by the shear. Since the shear is anticipated to increase during the next day or so, gradual weakening is shown in the short-term official forecast, which lies along the upper end of the guidance through 48 hours. Beyond 48 hours, vertical shear is anticipated to weaken, which would allow Florence to intensify and potentially regain major hurricane status. However, Florence will need to thread the needle between areas of stronger shear for this to happen, and there is significant uncertainty in the intensity forecast. The new official intensity forecast is little changed from before except to adjust upward to account for the higher initial intensity. The initial motion estimate remains northwestward, or 305/11 kt. Strong upper-level ridging is forecast to strengthen north of Florence over the next few days, forcing the hurricane to turn toward the west-northwest through about 48 hours. Track guidance is good agreement during this period, and the official forecast is essentially an update to the previous one. Beyond day 2, a break forms in the ridge, which results in a weakening of the steering currents, a slowdown in Florence's forward speed, and a turn back toward the northwest. The new official forecast is shifted south a bit on days 3-5 to account for recent model trends and is closest to the TVCN consensus. It should be noted that there is considerable model ensemble spread and run-to-run variability 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, resulting in life-threatening surf and rip currents on the island. Swells from Florence could also reach the U.S. East Coast by early next week. FORECAST POSITIONS AND MAX WINDS INIT 05/2100Z 22.7N 46.6W 115 KT 130 MPH 12H 06/0600Z 23.5N 48.1W 110 KT 125 MPH 24H 06/1800Z 24.5N 49.7W 105 KT 120 MPH 36H 07/0600Z 25.2N 51.3W 100 KT 115 MPH 48H 07/1800Z 25.5N 52.9W 95 KT 110 MPH 72H 08/1800Z 26.0N 55.3W 100 KT 115 MPH 96H 09/1800Z 27.0N 57.4W 105 KT 120 MPH 120H 10/1800Z 29.0N 60.0W 105 KT 120 MPH $$ Forecaster Berg/Rhome

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Hurricane Olivia Forecast Discussion Number 19

2018-09-05 16:42:39| Tropical Depression LIDIA

Issued at 800 AM PDT Wed Sep 05 2018

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Hurricane Florence Forecast Discussion Number 25

2018-09-05 16:40:08| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Wed Sep 05 2018 150 WTNT41 KNHC 051440 TCDAT1 Hurricane Florence Discussion Number 25 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Wed Sep 05 2018 Remarkably, Florence has continued to strengthen. The hurricane has a compact central dense overcast with cold cloud tops completely encircling a clear, well-defined eye. With the improved structure, subjective Dvorak estimates have increased to T5.5/102 kt from TAFB and T6.0/115 kt from SAB, while the latest objective UW-CIMSS ADT estimate is T5.8/110 kt. The initial intensity is therefore set at 110 kt as a blend of these numbers, with Florence having become a major hurricane earlier this morning. Given the estimated maximum winds, Florence has been rapidly intensifying since yesterday, an event that was not foreseen by any intensity models, nor forecasters. Diagnostics from the SHIPS model and UW-CIMSS shear analyses have been consistently showing southwesterly shear of 20-25 kt in the vicinity of Florence, but it is possible that those schemes are averaging over a larger area than might be reasonable given the hurricane's small size. Florence has apparently been able to find a small pocket of relatively low shear, and with waters becoming progressively warmer, the hurricane has strengthened significantly more than anticipated. This makes the intensity forecast incredibly uncertain. SHIPS actually shows the shear increasing over the next 24 hours, but global model fields suggest that the hurricane may still be able to continue within the protected pocket of lower shear for the next several days. As a result, the NHC official forecast keeps Florence's intensity well above the available guidance, which all show the hurricane weakening over the next day or two. The new NHC prediction follows this trend and also shows some weakening, but this is a low-confidence forecast. Either way, Florence is expected to remain a hurricane throughout the 5-day period. The track forecast also has its challenges. The initial motion estimate is northwestward, or 305/11 kt. It now appears that stronger upper-level ridging may take shape to the north of Florence over the next few days, forcing the hurricane to turn back toward the west-northwest from 36-72 hours. After 72 hours, a break in the ridge should allow Florence to turn back toward the northwest, but the bulk of the track models have trended westward since yesterday. In light of these trends, the NHC official track forecast has been shifted westward on days 4 and 5, but not quite as far as the various model consensus aids. It is worth stressing that there is still a significant amount of spread among the GFS and ECMWF ensemble members by the end of the forecast period, and just like the intensity forecast, the track forecast is of low confidence. FORECAST POSITIONS AND MAX WINDS INIT 05/1500Z 22.0N 45.7W 110 KT 125 MPH 12H 06/0000Z 22.7N 47.2W 110 KT 125 MPH 24H 06/1200Z 23.8N 49.0W 100 KT 115 MPH 36H 07/0000Z 24.7N 50.6W 95 KT 110 MPH 48H 07/1200Z 25.1N 52.1W 90 KT 105 MPH 72H 08/1200Z 25.7N 54.7W 90 KT 105 MPH 96H 09/1200Z 27.0N 57.0W 95 KT 110 MPH 120H 10/1200Z 28.5N 59.0W 95 KT 110 MPH $$ Forecaster Berg/Rhome

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