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Tropical Storm Florence Forecast Discussion Number 39

2018-09-09 04:52:11| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sat Sep 08 2018 736 WTNT41 KNHC 090252 TCDAT1 Tropical Storm Florence Discussion Number 39 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Sat Sep 08 2018 Satellite images indicate that the cloud pattern has continued to gradually become better organized with an eye feature trying to form. An average of all of the Dvorak satellite intensity estimates tonight suggest that Florence is not a hurricane yet, but is close to being one. The initial intensity is kept at 60 kt in this advisory. The recent ASCAT data showed less winds than previously observed by the NOAA plane when the cloud pattern was less organized. There is a band of convection trying to wrap around a possible eye feature, and that is an indication that Florence has continued to recover from the hostile shear environment, which brought the hurricane from Category 4 to a tropical storm in a matter of a day or so. The presence of developing upper-level outflow is a good indication that the shear has decreased, and with the presence of a warm ocean ahead, strengthening is anticipated. As indicated by my predecessor, the official forecast continues to call for a period of rapid intensification in about 12-24 hours, and Florence is expected to reach major hurricane intensity between 36-48 hours with additional strengthening thereafter. Florence is forecast to be an intense hurricane on days 3 through 5 as it moves across the warm waters of the southwestern Atlantic Ocean between the Bahamas and Bermuda, and then as it heads toward the southeast United States coast. Florence is still trapped within very light steering currents, and is slowly moving toward the west or 270 degrees at 5 kt. All indications are that this is about to change, as a strong ridge of high pressure builds to the north of the hurricane. This forecast flow pattern predicted by the global models will force Florence on a general west-northwest to northwest track with an increase in forward speed. The NHC forecast is basically on top of the previous one, and is in the middle of the narrow guidance envelope mainly for the next 3 to 4 days. By the end of the forecast period, when the hurricane is approaching the U.S. coast, the guidance envelope is wider and becomes bounded by the northernmost GFS and the southernmost HCCA and the FSSE ensembles. One thing all models coincide at the longer range is with the collapsing of the steering currents, resulting in a significant reduction of the forward speed of the hurricane. Key Messages: 1. Florence is forecast to be a dangerous major hurricane near the southeast U.S. coast by late next week, and the risk of direct impacts continues to increase. However, given the uncertainty in track and intensity forecasts at those time ranges, it's too soon to determine the exact timing, location, and magnitude of those impacts. 2. Interests along the U.S. East Coast, particularly from north Florida through North Carolina, should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials. 3. Large swells affecting Bermuda and portions of the U.S. East Coast will continue into next week. These swells will result in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 24.6N 55.2W 60 KT 70 MPH 12H 09/1200Z 24.5N 55.9W 70 KT 80 MPH 24H 10/0000Z 24.8N 57.4W 80 KT 90 MPH 36H 10/1200Z 25.2N 59.3W 100 KT 115 MPH 48H 11/0000Z 25.8N 61.8W 110 KT 125 MPH 72H 12/0000Z 27.5N 67.5W 125 KT 145 MPH 96H 13/0000Z 30.0N 73.5W 125 KT 145 MPH 120H 14/0000Z 33.8N 77.8W 115 KT 130 MPH $$ Forecaster Avila

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Tropical Depression Eighteen-E Forecast Discussion Number 3

2018-09-09 04:46:11| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Sat Sep 08 2018 180 WTPZ43 KNHC 090246 TCDEP3 Tropical Depression Eighteen-E Discussion Number 3 NWS National Hurricane Center Miami FL EP182018 800 PM PDT Sat Sep 08 2018 The depression has changed little during the past several hours. The system remains sheared with the center located near the eastern edge of the main area of deep convection. This asymmetric cloud pattern is due to about 15 kt of east-northeasterly shear. The satellite intensity estimates range between 30-35 kt, so the initial intensity is held at 30 kt for this advisory. The depression should have some opportunity to strengthen during the next 2 to 3 days while it remains over warm waters and in a relatively moist environment. However, the global models suggest that the shear may not let up much during that time period, so only slow strengthening is anticipated. Beyond a few days, the water temperatures beneath the system are forecast to fall below 26 deg C. These unfavorable oceanic conditions and a drier and more stable airmass should cause a weakening trend. The NHC intensity forecast is a little lower than the previous one, but still lies at the high end of the model guidance. The depression is still moving westward but at a slower pace than before. The models all show the system turning northwestward overnight and maintaining that motion during the next few days as it is steered by a weakening mid-level ridge. Once the system moves over cooler waters and becomes shallow, a turn to the west-northwest is forecast as the cyclone is steered by the low-level flow. The new NHC track is a little north of the previous one to come into better agreement with the latest consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 16.1N 117.2W 30 KT 35 MPH 12H 09/1200Z 17.0N 118.0W 35 KT 40 MPH 24H 10/0000Z 18.3N 119.1W 40 KT 45 MPH 36H 10/1200Z 19.6N 120.3W 45 KT 50 MPH 48H 11/0000Z 20.5N 122.0W 50 KT 60 MPH 72H 12/0000Z 21.7N 125.8W 55 KT 65 MPH 96H 13/0000Z 22.8N 129.5W 45 KT 50 MPH 120H 14/0000Z 23.7N 132.1W 35 KT 40 MPH $$ Forecaster Cangialosi

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Tropical Storm Helene Forecast Discussion Number 7

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

Issued at 1100 PM AST Sat Sep 08 2018 252 WTNT43 KNHC 090242 TCDAT3 Tropical Storm Helene Discussion Number 7 NWS National Hurricane Center Miami FL AL082018 1100 PM AST Sat Sep 08 2018 Helene continues this evening with a large area of convective bands that wrap about three-quarters around the center. A blend of the subjective Dvorak classifications from TAFB and SAB with the CIMSS' SATCON gives an intensity of 50 kt, unchanged from the previous advisory. The tropical storm is situated just south of the upper-level subtropical ridge, which is inducing weak southeasterly vertical shear. That along with warm 28C waters and a moist, unstable atmosphere should support at least steady intensification for the next two days. By day three, however, the shear will increase out of the southwest as Helene moves close to an upper-level low. Simultaneously, the SSTs will drop to near 26C and the mid-troposphere should dry. The new intensity forecast shows a peak of 90 kt at 48 h, with gradual to steady weakening thereafter. This is based upon the IVCN multi-model consensus (minus the much weaker HMON solution) with a higher peak intensity than the previous advisory but a slightly faster decay. Helene's initial position was aided by the availability of a 2222Z ASCAT pass, which showed it to be somewhat farther south than earlier estimated. The initial motion is analyzed to be 260/11, as it is steered by a deep-layer ridge to its north. The tropical cyclone should gradually turn toward the west-northwest by Monday, then toward the north-northwest by Wednesday. The new track forecast - based on the TVCN multimodel consensus - is slightly south of the previous advisory throughout the forecast period, mainly because of the southward initial position. The aforementioned scatterometer pass allowed for a more accurate assessment of the initial size, which is somewhat larger than analyzed earlier. The new wind radii prediction indicates a larger Helene than the previous advisory, and is based upon the RVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 13.2N 22.8W 50 KT 60 MPH 12H 09/1200Z 13.3N 24.7W 60 KT 70 MPH 24H 10/0000Z 13.8N 27.4W 70 KT 80 MPH 36H 10/1200Z 14.5N 30.2W 80 KT 90 MPH 48H 11/0000Z 15.3N 33.0W 90 KT 105 MPH 72H 12/0000Z 17.1N 37.4W 85 KT 100 MPH 96H 13/0000Z 19.5N 40.5W 65 KT 75 MPH 120H 14/0000Z 22.5N 42.5W 50 KT 60 MPH $$ Forecaster Landsea

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Tropical Storm Isaac Forecast Discussion Number 6

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

Issued at 1100 PM AST Sat Sep 08 2018 251 WTNT44 KNHC 090242 TCDAT4 Tropical Storm Isaac Discussion Number 6 NWS National Hurricane Center Miami FL AL092018 1100 PM AST Sat Sep 08 2018 Isaac is strengthening this evening. Satellite images indicate that the deep convection has been increasing in intensity and coverage with better defined banding features. The center is not located in the center of the convection, however, due to some easterly shear. An ASCAT pass around 00Z showed maximum winds in the 40-45 kt range. These data are also in line with the latest Dvorak classifications of 3.0/45 kt from TAFB and SAB. Based on these estimates, the initial intensity is raised to 45 kt. Isaac will likely continue to strengthen during the next few days as the storm remains over warm waters and moves into an environment of decreasing wind shear. Beyond a few days, the SHIPS model shows a notable increase in shear, in part due to the outflow from Florence, which should end the strengthening trend and cause some weakening. With the exception of the HMON and COAMPS-TC models, the remainder of the intensity guidance is higher this cycle. The NHC intensity forecast is raised from the previous one, but it is a little lower than the HCCA and IVCN guidance. The storm is moving due westward at 7 kt. The track forecast seems fairly straightforward. A strengthening subtropical ridge to the north of the system should cause Isaac to move westward at an increasing forward speed during the next several days. This scenario is supported by the usually more reliable GFS and ECMWF models, and the NHC track forecast is near a blend of those aids. Based on the current forecast, Isaac will be near the Lesser Antilles in 4 to 5 days and interests there should monitor the progress of this system. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 14.4N 37.5W 45 KT 50 MPH 12H 09/1200Z 14.4N 38.7W 50 KT 60 MPH 24H 10/0000Z 14.5N 40.9W 60 KT 70 MPH 36H 10/1200Z 14.5N 43.5W 70 KT 80 MPH 48H 11/0000Z 14.5N 46.1W 75 KT 85 MPH 72H 12/0000Z 14.5N 51.5W 85 KT 100 MPH 96H 13/0000Z 14.6N 57.0W 80 KT 90 MPH 120H 14/0000Z 15.0N 62.9W 75 KT 85 MPH $$ Forecaster Cangialosi

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Tropical Storm Helene Forecast Discussion Number 6

2018-09-08 23:02:38| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 08 2018 912 WTNT43 KNHC 082102 TCDAT3 Tropical Storm Helene Discussion Number 6 NWS National Hurricane Center Miami FL AL082018 500 PM AST Sat Sep 08 2018 During the past few hours, Helene has developed a convective band that wraps about three-quarters of the way around the storm, and the intensity of the inner core convection has increased. In addition, microwave imagery shows an elliptical inner ring feature. Satellite intensity estimates at 18Z were in the 35-50 kt range, and given the recent increase in organization the initial intensity is increased to 50 kt. Helene should continue to strengthen through at least 72 h in a light vertical shear environment over sea surface temperatures near 27C. This part of the intensity forecast has been increased over the previous forecast and lies near the intensity consensus. However, there remain a couple of alternate forecast scenarios. The first is that Helene could rapidly intensify and become stronger than currently forecast, and the rapid intensification index of the SHIPS model has about a 35 percent chance of 25 kt of strengthening in the next 24 h. The second is that a large plume of African dust and associated dry air that has spread over the Cabo Verde Islands starts entraining into the cyclone and inhibits intensification. The former alternative is more likely than the latter at this time. After 72 h, Helene is expected to encounter increasing southwesterly shear associated with a mid- to upper-level trough over the central Atlantic, which should cause the system to weaken significantly. The initial motion is 275/11. There again is little change in the forecast guidance or the forecast track, with Helene expected to move westward to west-northwestward for the next 3 days or so on the south side of the subtropical ridge, then turn northwestward in response to the aforementioned trough. FORECAST POSITIONS AND MAX WINDS INIT 08/2100Z 13.6N 21.7W 50 KT 60 MPH 12H 09/0600Z 13.7N 23.5W 55 KT 65 MPH 24H 09/1800Z 14.1N 26.2W 65 KT 75 MPH 36H 10/0600Z 14.7N 29.1W 75 KT 85 MPH 48H 10/1800Z 15.5N 32.0W 85 KT 100 MPH 72H 11/1800Z 17.5N 37.0W 85 KT 100 MPH 96H 12/1800Z 19.5N 40.5W 65 KT 75 MPH 120H 13/1800Z 23.0N 42.5W 55 KT 65 MPH $$ Forecaster Beven

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