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Hurricane Gert Forecast Discussion Number 12

2017-08-15 22:35:03| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Tue Aug 15 2017 000 WTNT43 KNHC 152034 TCDAT3 Hurricane Gert Discussion Number 12 NWS National Hurricane Center Miami FL AL082017 500 PM EDT Tue Aug 15 2017 Gert has exhibited hints of an eye in visible satellite imagery during the day, but that feature has not appeared in infrared images. Still, the satellite presentation has improved since the last advisory, and the initial intensity is set at 70 kt based on a blend of final-T Dvorak numbers of T4.0 from TAFB and SAB and T4.7 from the UW-CIMSS ADT. Vertical shear is expected to be relatively low--less than 20 kt--for the next 24 hours or so, which should allow Gert to strengthen a little more over the warm waters south of the northern wall of the Gulf Stream. The NHC forecast has been increased slightly through 36 hours to account for the latest intensity guidance, although it is still not as high as what is shown by models such as HWRF and the HFIP Corrected Consensus. Southwesterly shear increases substantially after 48 hours, which should cause Gert to weaken, and the cyclone is also expected to become extratropical by 72 hours. There is a lot of uncertainty on the ultimate fate of the extratropical low, but most of the global models keep Gert as a separate entity at least through day 4. Gert's trajectory continues to veer as it rounds the western edge of the subtropical ridge, and the initial motion is 020/10 kt. Gert will soon become embedded in the mid-latitude westerlies, sandwiched between a mid-oceanic high and a large cut-off low over eastern Canada. This pattern will cause Gert to turn northeastward and accelerate over the northwestern Atlantic during the next 48-72 hours. The cyclone will then likely slow down and turn more northward by day 4 as it interacts with the aforementioned cut-off low. There is very little cross-track spread among the track models, but the latest suite of guidance has slowed down slightly from previous runs. As a result, the new NHC forecast is just a little slower than the previous one, near the various multi-model consensus aids. Swells from Gert should spread northward along the mid-Atlantic coast of the United States during the next few days, with the first impacts expected to occur tonight. These swells are likely to produce dangerous surf and rip current conditions. Please consult products from your local National Weather Service office. FORECAST POSITIONS AND MAX WINDS INIT 15/2100Z 33.7N 71.2W 70 KT 80 MPH 12H 16/0600Z 35.3N 69.4W 75 KT 85 MPH 24H 16/1800Z 37.4N 64.9W 80 KT 90 MPH 36H 17/0600Z 39.7N 58.4W 80 KT 90 MPH 48H 17/1800Z 42.9N 50.2W 70 KT 80 MPH 72H 18/1800Z 51.5N 37.1W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 19/1800Z 55.5N 33.0W 40 KT 45 MPH...POST-TROP/EXTRATROP 120H 20/1800Z...DISSIPATED $$ Forecaster Berg

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Hurricane Gert Forecast Discussion Number 11

2017-08-15 16:34:27| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Tue Aug 15 2017 000 WTNT43 KNHC 151434 TCDAT3 Hurricane Gert Discussion Number 11 NWS National Hurricane Center Miami FL AL082017 1100 AM EDT Tue Aug 15 2017 Northerly shear of about 20 kt was inhibiting Gert overnight. A pair of microwave passes around 0900 UTC showed that the mid-level center was displaced about 20 n mi south-southeast of the low-level center. Since the time of the earlier microwave passes, outflow in the northwest quadrant has become re-established, suggesting that the shear may be lessening. The initial intensity has been held at 65 kt based on the latest Dvorak classification from TAFB. Environmental diagnostics from SHIPS indicate that the shear should continue to decrease for about the next 24 h, which should allow for a brief window of intensification. The GFS, ECMWF, and UKMET all indicate that extratropical transition will begin sometime between 36 h and 48 h, while the hurricane crosses a strong SST gradient and the shear increases above 30 kt. Gert is forecast to become fully extratropical about a day later. The initial motion is 015/9 kt. Gert has finally begun to round the western edge of the subtropical ridge, and should begin accelerating northeastward later today. The track guidance is tightly clustered for the first 48 h of the forecast. Beyond two days, there is some along-track spread in the models. The GFS forecasts that Gert will move faster and remain separated from a large extratropical low for nearly 5 days, while the ECMWF and UKMET forecast Gert to move slower, causing it to merge with the extratropical low sooner. The official forecast splits the difference between these scenarios for now and indicates that Gert will be absorbed by the extratropical low shortly after 96 h. Swells from Gert are expected to spread northward along the mid-Atlantic coast of the United States during the next few days. These swells are likely to produce dangerous surf and rip current conditions. Please consult products from your local National Weather Service office. FORECAST POSITIONS AND MAX WINDS INIT 15/1500Z 32.8N 72.0W 65 KT 75 MPH 12H 16/0000Z 34.4N 70.7W 70 KT 80 MPH 24H 16/1200Z 36.7N 67.0W 75 KT 85 MPH 36H 17/0000Z 38.9N 61.2W 75 KT 85 MPH 48H 17/1200Z 41.3N 53.3W 70 KT 80 MPH 72H 18/1200Z 48.8N 38.3W 60 KT 70 MPH...POST-TROP/EXTRATROP 96H 19/1200Z 54.0N 34.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 20/1200Z...DISSIPATED $$ Forecaster Zelinsky

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Hurricane Gert Forecast Discussion Number 10

2017-08-15 10:33:32| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Tue Aug 15 2017 000 WTNT43 KNHC 150833 TCDAT3 Hurricane Gert Discussion Number 10 NWS National Hurricane Center Miami FL AL082017 500 AM EDT Tue Aug 15 2017 After a period of intensification several hours ago, recent satellite images show that the cloud pattern has deteriorated a little bit. The eye is no longer discernible, and the center is now located to the west of an area of very deep convection. Dvorak numbers still support an initial intensity of 65 kt. Gert has an opportunity to gather some strength in the next day or so after recurvature. The new NHC forecast is not as aggressive as the previous one, and it follows very closely the intensity consensus. Only the HWRF model significantly strengthens the cyclone. After two days, Gert will be completely embedded within the mid-latitude westerlies and should become a post-tropical cyclone. The best estimate of the initial motion is toward the north or 360 degrees at 10 kt. Gert is being steered northward by the flow around the western edge of the subtropical ridge. The cyclone should then turn to the northeast with an increase in forward speed later today as it becomes embedded within the mid-latitude flow. The NHC track forecast is similar to the previous one, and is in the middle of the tight guidance envelope. Swells from Gert are expected to spread northward along the mid-Atlantic coast of the United States during the next few days. These swells are likely to produce dangerous surf and rip current conditions. Please consult products from your local National Weather Service office. FORECAST POSITIONS AND MAX WINDS INIT 15/0900Z 31.8N 72.5W 65 KT 75 MPH 12H 15/1800Z 33.3N 71.5W 70 KT 80 MPH 24H 16/0600Z 35.5N 68.9W 75 KT 85 MPH 36H 16/1800Z 37.8N 64.0W 75 KT 85 MPH 48H 17/0600Z 40.0N 57.0W 70 KT 80 MPH 72H 18/0600Z 46.5N 41.0W 65 KT 75 MPH...POST-TROP/EXTRATROP 96H 19/0600Z 52.0N 36.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 20/0600Z...DISSIPATED $$ Forecaster Avila

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Hurricane Gert Forecast Discussion Number 9

2017-08-15 04:36:59| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Mon Aug 14 2017 000 WTNT43 KNHC 150236 TCDAT3 Hurricane Gert Discussion Number 9 NWS National Hurricane Center Miami FL AL082017 1100 PM EDT Mon Aug 14 2017 Satellite images show that Gert has developed an eye, although the convection is weak in the western eyewall. Still, microwave images show a healthy inner-core structure, with plenty of deep convection in the eastern eyewall. Satellite intensity estimates unanimously support making Gert a hurricane on this advisory, and the initial wind speed is set to 65 kt, close to the TAFB figure. Some light or moderate northerly shear is forecast to continue to affect Gert for the next day or so, which might limit the intensification rate. On Wednesday, however, the shear is forecast to drop off as Gert accelerates northeastward over warm waters with a large increase in upper-level divergence also noted. This is typically a favorable environment for intensification, and the official forecast is raised from the previous one, lying between the model consensus IVCN and the higher corrected consensus aids. Gert is expected to complete extratropical transition within 72 hours, then gradually weaken over the North Atlantic before it merges with a larger extratropical cyclone in 4 to 5 days. Gert continues to move northward at about 7 kt. The storm is expected to turn north-northeastward and begin to accelerate Tuesday ahead of a shortwave trough approaching the northeastern United States. Like the last cycle, the track guidance continues to be in excellent agreement, but is generally slower than before. The new forecast is on the fast side of the guidance, assuming that Gert is a strong hurricane that stays vertically coherent in the faster mid/upper-level flow. Swells from Gert are expected to spread northward along the mid-Atlantic coast of the United States during the next few days. These swells are likely to produce dangerous surf and rip current conditions. Please consult products from your local National Weather Service office. FORECAST POSITIONS AND MAX WINDS INIT 15/0300Z 31.2N 72.3W 65 KT 75 MPH 12H 15/1200Z 32.3N 72.0W 70 KT 80 MPH 24H 16/0000Z 34.3N 70.6W 75 KT 85 MPH 36H 16/1200Z 36.6N 66.9W 85 KT 100 MPH 48H 17/0000Z 39.3N 60.8W 90 KT 105 MPH 72H 18/0000Z 46.0N 46.0W 70 KT 80 MPH...POST-TROP/EXTRATROP 96H 19/0000Z 52.0N 37.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 20/0000Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Gert Forecast Discussion Number 8

2017-08-14 22:47:21| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Mon Aug 14 2017 000 WTNT43 KNHC 142047 TCDAT3 Tropical Storm Gert Discussion Number 8 NWS National Hurricane Center Miami FL AL082017 500 PM EDT Mon Aug 14 2017 The satellite presentation of Gert has continued to improve today. Convective banding has increased around the center, and a large banded eye has appeared in recent visible satellite pictures. Satellite intensity estimates from TAFB, SAB, and UW-CIMSS supported an intensity of around 55 kt at 18z, but due to the continued increase in organization, the initial wind speed is set at 60 kt for this advisory. Although there is some light northerly shear over the system, it should not prevent additional strengthening during the next day or so while Gert remains over SSTs of 28 to 29 deg C. Gert is expected to become a hurricane within 12 hours, and the updated NHC intensity forecast calls for a bit higher peak intensity than before. The NHC forecast is a little above the statistical guidance and calls for peak intensity a little sooner than most of the guidance. The HWRF continues to be very bullish in bringing the system to major hurricane status, but since it shows peak intensity well after the system is expected to encounter westerly shear and cooler SSTs, less weight has been placed on that scenario. Gert is expected to complete extratropical transition within 72 hours, then gradually weaken over the North Atlantic before it merges with a larger extratropical cyclone in 4 to 5 days. Gert has been moving more slowly than expected today while it has been rounding the western portion of the ridge. The storm is expected to turn north-northeastward and begin to accelerate Tuesday ahead of a shortwave trough approaching the northeastern United States. The track guidance continues to be in excellent agreement, but is generally slower than before. As a result, little cross-track change was required in the NHC track through 72 h, but the new forecast is slower than the previous one to account for the latest trend in the latest guidance. Swells from Gert are expected to spread northward along the mid-Atlantic coast of the United States during the next few days. These swells are likely to produce dangerous surf and rip current conditions. Please consult products from your local National Weather Service office. FORECAST POSITIONS AND MAX WINDS INIT 14/2100Z 30.6N 72.3W 60 KT 70 MPH 12H 15/0600Z 31.6N 72.3W 70 KT 80 MPH 24H 15/1800Z 33.3N 71.6W 75 KT 85 MPH 36H 16/0600Z 35.6N 69.0W 80 KT 90 MPH 48H 16/1800Z 38.0N 64.0W 80 KT 90 MPH 72H 17/1800Z 44.5N 48.0W 70 KT 80 MPH...POST-TROP/EXTRATROP 96H 18/1800Z 52.0N 39.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 19/1800Z...ABSORBED BY LARGER EXTRATROPICAL LOW $$ Forecaster Brown

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