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Tropical Storm Florence Forecast Discussion Number 32

2018-09-07 10:50:26| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Sep 07 2018 805 WTNT41 KNHC 070850 TCDAT1 Tropical Storm Florence Discussion Number 32 NWS National Hurricane Center Miami FL AL062018 500 AM AST Fri Sep 07 2018 Florence's structure continues to be negatively affected by strong southwesterly shear. Cloud tops have generally warmed over the past 6 hours, and recent microwave data show that the low-level circulation center of Florence is displaced nearly 20 nmi to the southwest of the mid-level center. Satellite intensity estimates have decreased since last night, and now support an initial intensity of 55 kt. Based on GFS and ECMWF SHIPS diagnostic output, the southwesterly shear is near its peak now, and should gradually decrease over the next 24 to 36 h. All of the intensity guidance shows little change in intensity through that time. From 48 h onward, a low shear/warm SST environment should allow the tropical storm to re-strengthen. However, the extent and timing of the strengthening varies greatly from model to model, with the dynamical models generally showing more intensification, and sooner, than the statistical models. As has been the case for most of Florence's existence thus far, confidence in the intensity forecast, especially beyond 36 h, is low. The new official forecast is a little lower than the previous advisory for the first 48 h, but close to it after that, and lies between the more aggressive HCCA and less aggressive IVCN consensus aids. Nighttime Proxy-Vis and earlier microwave imagery indicate that Florence has turned westward, with an estimated initial motion of 275/6 kt. Most of the track guidance has shifted slightly toward the southwest, so the NHC track forecast has also been nudged in that direction. Over the next 3 days of the forecast, Florence should be steered generally westward, and then west-northwestward, by a mid-level ridge to its north. By days 4 and 5, a developing mid-latitude trough could create a weakness in this ridge and allow Florence to move more toward the northwest, but there is still considerable uncertainty in the global models and their ensembles regarding the strength of the ridge and if the aforementioned trough will have any notable impact on the track of Florence. The NHC forecast follows HCCA very closely, and is also near the middle of the large combined envelope of the GFS, UKMET, and ECMWF ensembles. Key Messages: 1. Regardless of Florence's eventual track, large swells will begin to affect Bermuda later today and portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. 2. There is still very large uncertainty in Florence's track beyond day 5, and it is too soon to determine what, if any, other impacts Florence could have on the U.S. East Coast next week. 3. Since we are near the peak of hurricane season, this is a good time for everyone who lives in a hurricane-prone area to ensure they have their hurricane plan in place. FORECAST POSITIONS AND MAX WINDS INIT 07/0900Z 25.1N 50.7W 55 KT 65 MPH 12H 07/1800Z 25.1N 51.8W 55 KT 65 MPH 24H 08/0600Z 25.0N 53.2W 55 KT 65 MPH 36H 08/1800Z 25.0N 54.3W 60 KT 70 MPH 48H 09/0600Z 25.1N 55.4W 70 KT 80 MPH 72H 10/0600Z 26.2N 58.3W 95 KT 110 MPH 96H 11/0600Z 27.5N 63.5W 110 KT 125 MPH 120H 12/0600Z 30.0N 70.0W 110 KT 125 MPH $$ Forecaster Zelinsky

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Tropical Storm Florence Forecast Discussion Number 31

2018-09-07 04:54:22| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Sep 06 2018 738 WTNT41 KNHC 070254 TCDAT1 Tropical Storm Florence Discussion Number 31 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Thu Sep 06 2018 Deep convection near the center of Florence has continued to decrease during the past several hours, and the overall cloud pattern remains elongated from southwest to northeast. The latest Dvorak estimates and a recent scatterometer pass suggest that Florence is no longer a hurricane, and the initial wind speed is set to 60 kt. Some further weakening is anticipated since the shear isn't forecast to change much in the short term. However, all of the global models show a significant reduction of shear during the next several days, especially later this weekend. In combination with waters warming to near 29C, these conditions should promote strengthening into a hurricane again in a day or two and a major hurricane by early next week. The intensity guidance is higher than the last cycle at long range, which is hard to argue with considering the large upper-level anticyclone forecast in the vicinity of Florence. The NHC intensity forecast follows the trend of the previous prediction, a bit elevated from the previous forecast at days 4 and 5, although still below the intensity guidance. A recent scatterometer pass indicates that Florence has slowed down and turned left, estimated at 290/6. A narrow ridge over the subtropical Atlantic Ocean should continue to turn the storm westward overnight, albeit traveling at a relatively slow pace due to the strength of the ridge. This ridge is forecast is strengthen at long range, steering Florence faster to the west-northwest over the western Atlantic Ocean. Except for the UKMET, most of the historically reliable guidance is in reasonable agreement on this scenario, which is a bit surprising given the recent trials and tribulations from the guidance suite. The models continue to shift westward at long range, however, owing to differences on the strength and position of that ridge, and the official NHC track forecast is moved in that direction. Key Messages: 1. Regardless of Florence's eventual track, large swells will begin to affect Bermuda on Friday and portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. 2. There is still very large uncertainty in Florence's track beyond day 5, and it is too soon to determine what, if any, other impacts Florence could have on the U.S. East Coast next week. 3. Since we are near the peak of hurricane season, this is a good time for everyone who lives in a hurricane-prone area to ensure they have their hurricane plan in place. FORECAST POSITIONS AND MAX WINDS INIT 07/0300Z 25.1N 49.8W 60 KT 70 MPH 12H 07/1200Z 25.2N 50.8W 55 KT 65 MPH 24H 08/0000Z 25.2N 52.2W 60 KT 70 MPH 36H 08/1200Z 25.2N 53.6W 65 KT 75 MPH 48H 09/0000Z 25.3N 54.7W 75 KT 85 MPH 72H 10/0000Z 26.1N 57.3W 90 KT 105 MPH 96H 11/0000Z 27.6N 62.2W 105 KT 120 MPH 120H 12/0000Z 30.0N 68.5W 110 KT 125 MPH $$ Forecaster Blake

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Hurricane Olivia Forecast Discussion Number 26

2018-09-07 04:32:22| Tropical Depression LIDIA

Issued at 800 PM PDT Thu Sep 06 2018

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Hurricane Olivia Forecast Discussion Number 25

2018-09-06 22:51:18| Tropical Depression LIDIA

Issued at 200 PM PDT Thu Sep 06 2018

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Hurricane Florence Forecast Discussion Number 30

2018-09-06 22:44:18| National Hurricane Center (Atlantic)

Issued at 500 PM AST Thu Sep 06 2018 111 WTNT41 KNHC 062044 TCDAT1 Hurricane Florence Discussion Number 30 NWS National Hurricane Center Miami FL AL062018 500 PM AST Thu Sep 06 2018 Vertical shear has taken its toll on Florence today as evidenced by a continued degradation of the overall cloud pattern. The circulation appears tilted, with the low-level center partially exposed to the southwest of the deep convection. Subjective and objective Dvorak Current Intensity (CI) numbers have fallen, and a blend of the various estimates supports an initial intensity of 70 kt. The intensity forecast is relatively straightforward in the short-term as shear is expected to remain strong, which should continue Florence's weakening trend, potentially taking the system below hurricane strength. By 24 hours, vertical shear is forecast to decrease, and the SSTs gradually warm along the forecast track. Assuming the overall circulation remains intact, Florence shouldn't have any problems restrengthening beginning in a day or so. In fact, guidance suggests that Florence could once again become a major hurricane in 4 or 5 days. The new NHC intensity forecast is essentially an update of the previous one, and is only adjusted to account for recent trends. Largely, the official forecast is close to the various consensus aids. Owing to the degraded structure and tilted nature of the system, Florence has wobbled a bit to the west, but the longer-term motion estimate is 305/09. Low- to mid-level ridging should cause Florence to turn toward the west-northwest and west between 12-48 hours while the cyclone recovers from the strong shear. Thereafter, Florence is anticipated to become a deep cyclone again, but an even stronger ridge should maintain the west-northwestward motion, at a faster speed, through day 5. The ridge is forecast to be sufficiently strong such that some track models show a motion just south of due west during the next 12-36 hours. Beyond day 3, the track forecast becomes increasingly uncertain due to differing evolutions of the steering pattern over the western Atlantic. While all of the global models show a progressive trough eroding the ridge, they differ in the strength of the trough and the ridge to the north of Florida. These differences result in a great deal of bifurcation in the track guidance, especially among global model ensemble members, at the end of the forecast period. In such situations, prudence suggests a reliance on continuity, and the the new official NHC track forecast is shifted only slightly south towards the TVCN multi-model consensus and HCCA. It is important to note that deterministic track models in these types of situations often display considerable run-to-run changes, and the uncertainty in this forecast remains larger than normal. Key Messages: 1. Regardless of Florence's eventual track, large swells will begin to affect Bermuda on Friday and portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. 2. There is still very large uncertainty in Florence's track beyond day 5, and it is too soon to determine what, if any, other impacts Florence could have on the U.S. East Coast next week. 3. Since we are near the peak of hurricane season, this is a good time for everyone who lives in a hurricane-prone area to ensure they have their hurricane plan in place. FORECAST POSITIONS AND MAX WINDS INIT 06/2100Z 25.0N 49.6W 70 KT 80 MPH 12H 07/0600Z 25.4N 50.7W 60 KT 70 MPH 24H 07/1800Z 25.5N 52.1W 65 KT 75 MPH 36H 08/0600Z 25.5N 53.4W 70 KT 80 MPH 48H 08/1800Z 25.6N 54.6W 75 KT 85 MPH 72H 09/1800Z 26.4N 57.1W 90 KT 105 MPH 96H 10/1800Z 28.0N 61.0W 100 KT 115 MPH 120H 11/1800Z 30.0N 67.0W 105 KT 120 MPH $$ Forecaster Berg/Rhome

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