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Tropical Depression Paul Forecast Discussion Number 15

2018-09-12 04:32:06| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Tue Sep 11 2018 661 WTPZ43 KNHC 120232 TCDEP3 Tropical Depression Paul Discussion Number 15 NWS National Hurricane Center Miami FL EP182018 800 PM PDT Tue Sep 11 2018 Active deep convection associated with Paul appears to have ceased a little earlier today, shortly after 1900 UTC. At least one more advisory will be issued just in case Paul makes an unexpected comeback, but this seems unlikely due to the 24 deg C SSTs beneath the cyclone. With no convection to sustain it, Paul will likely gradually spin down over the next few days until dissipating entirely by the weekend. Paul is now moving west with an initial motion of 275/10 kt. Now that Paul's circulation has become very shallow, it should continue to be steered generally westward to west-southwestward by the low-level tradewind flow until dissipation occurs in a few days. FORECAST POSITIONS AND MAX WINDS INIT 12/0300Z 22.4N 126.5W 30 KT 35 MPH 12H 12/1200Z 22.6N 128.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 13/0000Z 22.7N 129.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 13/1200Z 22.8N 130.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 14/0000Z 22.7N 131.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 15/0000Z 22.3N 132.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 16/0000Z...DISSIPATED $$ Forecaster Zelinsky

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Hurricane Florence Forecast Discussion Number 50

2018-09-11 22:57:03| National Hurricane Center (Atlantic)

Issued at 500 PM AST Tue Sep 11 2018 545 WTNT41 KNHC 112056 TCDAT1 Hurricane Florence Discussion Number 50 NWS National Hurricane Center Miami FL AL062018 500 PM AST Tue Sep 11 2018 Microwave satellite data indicate that Florence completed a full eyewall replacement cycle (ERC) a few hours ago, and recent visible and infrared imagery suggests that the eye has contracted slightly. Outflow continues to expand in all quadrants, and the outflow jets to the northwest and east have become better defined. Satellite intensity estimates from TAFB, SAB, and UW-CIMSS ADT are all T6.0/115 kt, and recent NHC objective intensity estimates are T6.2/120 kt. Given that the eye has mostly cleared out and has also warmed to near 19 deg C, the initial intensity has been bumped upward to 120 kt, which could be conservative. All of the wind radii had to be expanded/increased based on a blend of the earlier reconnaissance data and a 1430 UTC ASCAT scatterometer pass. The initial motion estimate is now 300/15 kt. There remains no significant to the previous track forecast or reasoning. Overall, the global and regional models have done a good job capturing the evolving synoptic- scale flow pattern across CONUS, with an amplifying trough moving onshore the the northwestern U.S. coast, which is inducing downstream ridging across the northeastern U.S. and the northwestern Atlantic Ocean. Water vapor imagery indicates that the blocking high pressure northwest of Bermuda is continuing to build and shift slowly eastward. The 12Z GFS model made a significant shift to the west, the UKMET made a shift to the east, and the ECMWF track has remained basically unchanged through 72 hours. As a result the consensus models have made only minor track shifts to the west. What is noticeable is that all of the global and regional models are indicating that the steering currents will collapse by 72 h when Florence is approaching the southeast U.S. coast. The weak steering currents are expected to continue through the weekend, which makes the forecast track on days 3-5 quite uncertain. The latest NHC forecast track is very similar to the previous two advisory tracks, and lies the middle of the guidance envelope between the consensus models TVCA to the north and the HCCA and FSSE models to the south. During the next 24 hours or so, Florence is expected to remain in a very favorable environment consisting of low shear near 5 kt, an expanding upper-level outflow pattern, and above-average SSTs of 29.0-29.5 deg C, which should result in additional strengthening. By 48 h, the decreasing forward speed along with the large inner-core wind field should induce some upwelling and gradual weakening. Although the GFS- and ECMWF-based SHIPS intensity models are indicating an increase in the southwesterly shear to near 20 kt, this could be due to the SHIPS model capturing Florence's own strong outflow from the GFS and ECMWF model fields. Despite the weakening shown at 72 hours, Florence is still expected to remain a dangerous hurricane through landfall. After Florence moves inland, the slow forward speed of less than 5 kt should cause a rapid spin down and weakening of the hurricane's circulation. The new NHC intensity forecast is a little above the highest guidance based on the aforementioned very favorable synoptic outflow pattern, and to maintain continuity with the previous forecast. Key Messages: 1. A life-threatening storm surge is now highly likely along portions of the coastlines of South Carolina and North Carolina, and a Storm Surge Warning is in effect for a portion of this area. 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, catastrophic flash flooding and significant river flooding is likely over portions of the Carolinas and Mid-Atlantic states from late this week into early next week, 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 Warning has been issued for a part of this area. 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 11/2100Z 27.5N 67.1W 120 KT 140 MPH 12H 12/0600Z 28.7N 69.3W 130 KT 150 MPH 24H 12/1800Z 30.4N 72.1W 135 KT 155 MPH 36H 13/0600Z 32.1N 74.5W 130 KT 150 MPH 48H 13/1800Z 33.4N 76.2W 120 KT 140 MPH 72H 14/1800Z 34.5N 77.7W 100 KT 115 MPH...NEAR THE COAST 96H 15/1800Z 35.0N 78.8W 40 KT 45 MPH...INLAND 120H 16/1800Z 35.7N 81.7W 25 KT 30 MPH...INLAND $$ Forecaster Stewart

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Tropical Storm Isaac Forecast Discussion Number 17

2018-09-11 22:34:02| National Hurricane Center (Atlantic)

Issued at 500 PM AST Tue Sep 11 2018 014 WTNT44 KNHC 112033 TCDAT4 Tropical Storm Isaac Discussion Number 17 NWS National Hurricane Center Miami FL AL092018 500 PM AST Tue Sep 11 2018 Microwave and visible satellite data show that the center of Isaac is partially exposed on the western edge of the central dense overcast. Perhaps Isaac is a little weaker than earlier, but a blend of the latest subjective and objective estimates still gives an uncertain initial wind speed of about 60 kt. We should have a better idea of the maximum winds by tomorrow morning when a NOAA Hurricane Hunter enters the storm, or if we get lucky with a scatterometer pass this evening. Isaac is barely holding on in the face of increasing shear, which is much more evident in the 300-500 mb layer than a conventional analysis at 200 mb. However, convection could redevelop near the center while the storm moves over warming SSTs into a more moist and unstable environment east of the Lesser Antilles. These conflicting factors lead to a forecast of little change in the winds of Isaac until the cyclone moves into the eastern Caribbean. I should mention that the short-term trend on satellite makes me think the intensity forecast is too high over the next 36 hours. Yet, given our limited skill in forecasting intensity changes in marginal environments, plus the 5 or 10 kt of initial wind speed uncertainty, this is the course of least regret. We can see if this trend becomes more consistent before making substantial changes on the next advisory. Model guidance is very divergent in the long range for Isaac, with the regional hurricane and statistical models showing intensification, while the global models show continued weakening. The global models have done fairly well with this cyclone, so the forecast will lean more heavily on their weaker solutions. The latest initial motion estimate is 270/15, a little faster than before. Isaac is forecast to move westward at roughly the same speed for the next few days due to a mid-level ridge over the central Atlantic Ocean. Model guidance is tightly clustered on this cycle, with the UKMET now coming into good agreement with the rest of the guidance. The only change to this forecast is to accelerate Isaac roughly 1 kt over the course of the prediction, which over the course of a 5-day forecast results in a sizable westward shift. This forecast could still be too slow, since the GFS and ECMWF are a bit ahead of the new NHC track prediction. Key Messages: 1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles on Thursday, and hurricane and tropical storm watches have been issued for several islands. Interests in those areas should follow any advice given by their local officials, and warnings will likely be issued this evening. 2. Interests elsewhere in the Lesser Antilles should continue to monitor Isaac during the next couple of days. FORECAST POSITIONS AND MAX WINDS INIT 11/2100Z 14.6N 51.3W 60 KT 70 MPH 12H 12/0600Z 14.6N 53.4W 60 KT 70 MPH 24H 12/1800Z 14.8N 56.2W 60 KT 70 MPH 36H 13/0600Z 15.0N 59.2W 60 KT 70 MPH 48H 13/1800Z 15.3N 62.3W 55 KT 65 MPH 72H 14/1800Z 15.5N 68.5W 50 KT 60 MPH 96H 15/1800Z 15.5N 73.5W 45 KT 50 MPH 120H 16/1800Z 15.5N 77.5W 45 KT 50 MPH $$ Forecaster Blake

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Hurricane Helene Forecast Discussion Number 18

2018-09-11 22:32:35| National Hurricane Center (Atlantic)

Issued at 500 PM AST Tue Sep 11 2018 613 WTNT43 KNHC 112032 TCDAT3 Hurricane Helene Discussion Number 18 NWS National Hurricane Center Miami FL AL082018 500 PM AST Tue Sep 11 2018 Helene is feeling the effects of cooler waters, and the convection surrounding the distinct eye is weaker that earlier today. Dvorak numbers are decreasing, and only support an initial intensity of 90 kt. Both cooler waters and high shear will continue to impact the hurricane, and Helene will probably weaken to a tropical storm in about 36 hours or so. The NHC forecast calls for a decrease in the winds at the same rate as the intensity consensus aids. Helene is already on the southwestern side of the subtropical ridge, and it is now moving toward the northwest or 320 degrees at 10 kt. Global models continue to develop a large mid-level trough over eastern Atlantic, and the flow associated with this trough should steer Helene north-northwestward and northward during the next day to so. The cyclone should then recurve northeastward with an increase in forward speed and by Friday, a weakened Helene should be approaching the Azores. At the very long range, the cyclone will begin to lose tropical characteristics over the even colder waters of the north Atlantic. There has been little change in the track forecast since models have been very consistent with the current solution. FORECAST POSITIONS AND MAX WINDS INIT 11/2100Z 17.2N 34.9W 90 KT 105 MPH 12H 12/0600Z 18.3N 36.0W 80 KT 90 MPH 24H 12/1800Z 20.2N 37.2W 70 KT 80 MPH 36H 13/0600Z 22.3N 37.8W 60 KT 70 MPH 48H 13/1800Z 25.0N 38.0W 50 KT 60 MPH 72H 14/1800Z 31.5N 37.0W 45 KT 50 MPH 96H 15/1800Z 39.0N 32.5W 45 KT 50 MPH 120H 16/1800Z 43.5N 25.0W 45 KT 50 MPH $$ Forecaster Avila

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Hurricane Florence Forecast Discussion Number 49

2018-09-11 16:59:00| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Tue Sep 11 2018 257 WTNT41 KNHC 111458 TCDAT1 Hurricane Florence Discussion Number 49 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Tue Sep 11 2018 Data from satellites and an Air Force Reserve reconnaissance aircraft mission indicate that Florence has completed an overnight eyewall replacement cycle (ERC). The recon data indicate that the eye has now expanded to a diameter of 30-32 n mi, and this was confirmed by an 1103Z SSMI/S microwave satellite image. The aircraft provided various intensity estimates with a peak SFMR surface wind of 113 kt noted in the northwest quadrant, a peak 700-mb flight-level wind of 143 kt in the northeast quadrant, and a central pressure of 950 mb. The 143-kt flight-level wind would normally correlate to an equivalent surface wind of about 129 kt. However, coincident SFMR surface winds were only 108 kt, indicating that the weak convection that region of the hurricane was not vigorous enough to bring down the strongest winds to the surface. The 950-mb central pressures corespondents to about 113 kt. Based on a blend of all these data, the initial intensity has been set to 115 kt. The initial motion estimate is now 295/14 kt based on the recent recon fix data. The broken record continues -- there is no significant to the previous track forecast or reasoning. Although the global and regional models continue to make minor shifts northward and southward, the consensus models have changed little. GOES-16 high-resolution water vapor imagery indicates that the amplifying large-scale flow pattern across CONUS is inducing a downstream ridge over the western Atlantic, with a high pressure cell centered northwest of Bermuda. This blocking ridge pattern is expected to keep Florence moving west-northwestward to northwest at around 15 kt for the next 48 hours or so. However, embedded within the large-scale flow is a weak shortwave trough over the central and southern Plains that is expected to eject out northeastward and weaken the ridging across the mid-Atlantic and southeastern U.S., causing Florence to slow down significantly in 72 hours as the powerful hurricane approaches the Carolinas. On days 4 and 5, an even slower motion or drift to the west and northwest is forecast, which will exacerbate the heavy rainfall threat. The new NHC forecast track is just an update of the previous one, and basically lies the middle of the guidance envelope between the consensus models TVCA to the north and HCCA and FSSE to the south. Water vapor imagery indicates that Florence has finally developed the much anticipated dual outflow pattern, with outflow jets noted in the northwestern and eastern quadrants. The latter outflow jet is flowing into an upper-level low, which is acting as an impressive mass sink near 25N/49W. These two outflow channels are producing significant deformation stretching across Florence's inner-core, which should aid in the re-strengthening process. Now that the eye has become stable with a diameter of about 30 n mi and since Florence is expected to remain in a low-shear environment of around 5 kt and over above-average SSTs of 29.0-29.5 deg C, slow but steady strengthening is expected for the next 24-36 hours. By 48 h and beyond, Florence's slow forward speed, coupled with the large eye and relatively shallow depth of the warm water should induce some upwelling beneath the cyclone that will initiate a slow weakening trend. By 72 hours the vertical wind shear is expected to increase to near 20 kt from the southwest, which will cause more significant weakening to occur. However, given the large overall wind field of Florence along with the large eye, only gradual weakening is expected. Once Florence moves inland, the slow forward speed of 3-5 kt will result in rapid spin down and weakening of the wind field. The new official intensity forecast is above of all of the intensity guidance based on the aforementioned very favorable synoptic outflow pattern, and to maintain continuity with the previous forecast. 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 is in effect for a portion of this area. 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, catastrophic flash flooding and significant river flooding is possible over portions of the Carolinas and Mid-Atlantic states from late this week into early next week, 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 is in effect for a part of this area. 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 11/1500Z 26.7N 65.3W 115 KT 130 MPH 12H 12/0000Z 27.7N 67.6W 120 KT 140 MPH 24H 12/1200Z 29.4N 70.6W 130 KT 150 MPH 36H 13/0000Z 31.1N 73.1W 130 KT 150 MPH 48H 13/1200Z 32.6N 75.2W 120 KT 140 MPH 72H 14/1200Z 34.2N 77.1W 105 KT 120 MPH 96H 15/1200Z 35.0N 78.1W 40 KT 45 MPH...INLAND 120H 16/1200Z 35.5N 79.5W 25 KT 30 MPH...INLAND $$ Forecaster Stewart

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