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Hurricane SEYMOUR Forecast Discussion Number 15

2016-10-26 22:39:37| National Hurricane Center (East Pacific)

Issued at 200 PM PDT WED OCT 26 2016 000 WTPZ45 KNHC 262039 TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 200 PM PDT WED OCT 26 2016 Seymour has begun to quickly weaken. The hurricane's central dense overcast has taken on a distorted appearance, and the deep convection associated with the cyclone's circulation has become noticeably asymmetric. The eye has also become cloud-filled and has cooled rather dramatically in the last few hours. The recent deterioration of the cloud pattern is associated with a considerable increase of southwesterly shear over the cyclone. Dvorak T-numbers have decreased to T5.5/102 kt from both satellite agencies, and these estimates averaged with Dvorak CI-numbers, are used to lower the initial intensity to 110 kt. The shear, currently analyzed in the 15 to 20 kt range, is forecast to increase tremendously over Seymour during the next 24 to 36 hours as a strong mid- to upper-level tropospheric trough approaches the cyclone from the west. Since Seymour is a small tropical cyclone and will be traversing cooler waters by that time, the weakening trend should become increasingly more rapid. Global models show the vortex decoupling as a result of the strong deep-layer shear in 30 to 36 hours, and the official forecast shows Seymour degenerating into a remnant low by 48 hours. The NHC track forecast represents an update of the previous one and is near the various model consensus aids. Seymour continues to gain a greater northward component of motion, and the initial motion estimate is 300/11. The cyclone is forecast to turn even more poleward and slow down during the next 24 to 36 hours as it rounds the western periphery of a mid-level high located west of the Baja California peninsula. While the vortex remains intact, the strong south-southwesterly flow associated with the upstream trough should cause Seymour to turn north-northeastward before shearing off and eventually dissipating. The NHC intensity forecast is not much different than the previous one and near the multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 26/2100Z 17.5N 121.2W 110 KT 125 MPH 12H 27/0600Z 18.8N 122.2W 85 KT 100 MPH 24H 27/1800Z 20.6N 123.0W 60 KT 70 MPH 36H 28/0600Z 22.3N 122.6W 40 KT 45 MPH 48H 28/1800Z 23.5N 121.7W 30 KT 35 MPH...POST-TROP/REMNT LOW 72H 29/1800Z...DISSIPATED $$ Forecaster Kimberlain

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Hurricane SEYMOUR Forecast Discussion Number 14

2016-10-26 16:39:11| National Hurricane Center (East Pacific)

Issued at 800 AM PDT WED OCT 26 2016 000 WTPZ45 KNHC 261439 TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 14 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 800 AM PDT WED OCT 26 2016 Seymour continues to maintain an impressive central dense overcast, consisting of very deep convection around a 15 n mi well-defined eye. However, the distribution of convection has become slightly asymmetric since the last advisory, with the greatest coverage to the north and east of the center. The convective asymmetry is likely indicative of some increase in southwesterly shear. A blend of the latest Dvorak T- and CI-numbers, including UW-CIMSS ADT values, is used to lower the initial intensity estimate of 120 kt. Seymour is living on borrowed time. A large mid- to upper-level trough seen in water vapor imagery upstream of the cyclone should cause a considerable increase in southwesterly shear within 24 hours. By that time, Seymour will already have reached much cooler waters. This should result in weakening, with the rate of filling only increasing with time. By 36 hours, the shear should become extremely strong and cause the vortex to shear apart, with the middle and upper portions of the circulation moving well to the north or northeast of the low-level center. The official forecast shows Seymour becoming a post-tropical remnant low in 48 hours in general agreement with global models that depict this decoupling as soon as 36 to 42 hours. The NHC intensity forecast shows about the same rate of weakening as the previous one and is close to the multi-model consensus. The initial motion estimate is 290/13. Seymour has begun to gain a little more latitude recently as it nears the western periphery of a mid-level ridge west of the Baja California peninsula. The cyclone's motion should slow and become northwesterly soon and then shift north-northwestward and northward in 24 to 36 hours as Seymour becomes embedded in the flow between the high to the northeast and the trough to the northwest. Once Seymour becomes a shallow system in 48 hours, the remnant low will slow further and turn north-northeastward until dissipation. The NHC track forecast is only slightly to the left of the previous one after 24 hours and a little faster during the remnant low phase. FORECAST POSITIONS AND MAX WINDS INIT 26/1500Z 16.9N 120.2W 120 KT 140 MPH 12H 27/0000Z 18.0N 121.5W 105 KT 120 MPH 24H 27/1200Z 19.7N 122.6W 80 KT 90 MPH 36H 28/0000Z 21.1N 122.9W 60 KT 70 MPH 48H 28/1200Z 22.2N 122.4W 35 KT 40 MPH...POST-TROP/REMNT LOW 72H 29/1200Z 23.7N 121.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 30/1200Z...DISSIPATED $$ Forecaster Kimberlain

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Hurricane SEYMOUR Forecast Discussion Number 13

2016-10-26 10:49:15| National Hurricane Center (East Pacific)

Issued at 200 AM PDT WED OCT 26 2016 000 WTPZ45 KNHC 260849 TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 200 AM PDT WED OCT 26 2016 Seymour remains an impressive hurricane in geostationary satellite pictures, with a very distinct eye surrounded by a symmetric CDO. Subjective and objective Dvorak current intensity numbers are unchanged from before and the initial intensity remains 130 kt for this advisory. Some slight strengthening is still possible this morning while Seymour remains over warm water and in a low shear environment, however, by this afternoon the hurricane will begin moving over slightly lower SSTs, which should start the weakening process. Seymour is forecast to cross the 26 degree Celsius isotherm tonight and move over colder waters and into an area of strong southwesterly wind shear thereafter. These conditions are expected to cause a very rapid decrease in intensity on Thursday, and Seymour is forecast to become a tropical storm within 48 hours and degenerate into a post-tropical remnant low on Friday or early Saturday. The initial motion estimate is 290/12 kt. A deepening mid- to upper-level trough seen in water vapor imagery near 140W is expected to erode the western portion of the ridge that has been steering Seymour westward during the past few days. As the ridge weakens, Seymour is forecast to turn northwestward, then northward ahead of the trough. After 48 hours, Seymour's forward motion should decrease as it becomes vertically shallow and is steered by the weaker low-level flow. The latest guidance envelope has shifted northward and eastward at 36 h and beyond, and the NHC track forecast has been adjusted accordingly. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 16.4N 119.0W 130 KT 150 MPH 12H 26/1800Z 17.2N 120.5W 120 KT 140 MPH 24H 27/0600Z 18.6N 121.9W 95 KT 110 MPH 36H 27/1800Z 20.1N 122.5W 75 KT 85 MPH 48H 28/0600Z 21.2N 122.5W 50 KT 60 MPH 72H 29/0600Z 22.0N 122.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 30/0600Z 22.5N 122.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 31/0600Z...DISSIPATED $$ Forecaster Brown

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Hurricane SEYMOUR Forecast Discussion Number 12

2016-10-26 04:53:09| National Hurricane Center (East Pacific)

Issued at 800 PM PDT TUE OCT 25 2016 000 WTPZ45 KNHC 260252 TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 800 PM PDT TUE OCT 25 2016 Seymour has continued to rapidly intensify since the last advisory. The eye is about 10-15 nmi wide in infrared satellite imagery, but is less than 10 nmi wide in a recent 0005Z SSMI/S microwave image, indicating that the eye diameter has contracted since the previous advisory. The initial intensity is increased to 130 kt for this advisory based on satellite intensity estimates of T6.5/127 kt from both TAFB and SAB, and a T6.7/132 kt from UW-CIMSS ADT. The initial motion is now 285/13 kt. The global and regional models remain in excellent agreement on Seymour moving steadily west-northwestward around the southwestern periphery of a deep-layer ridge for the next 24 hours, followed by a turn toward the northwest in 36 hours as a strong shortwave trough significantly erodes the western portion of the ridge. By 48 hours, Seymour is expected to slow down appreciably and turn north-northwestward as the cyclone rapidly weakens and becomes more vertically shallow. After that time, the shallow post-tropical low is forecast to turn back toward the west-northwest under the influence of the low-level trade wind flow. The official forecast track is similar to the previous advisory, and lies close to blend of the TVCX and HCCA consensus models. Some additional slight strengthening is possible during the next 6-12 hours due to the small eye, low shear conditions, and warm SSTs. After that time, increasing southwesterly vertical wind shear, cooler waters, including significant cold upwelling by 24 hours and beyond, should cause steady to rapid weakening. Seymour is expected to drop below hurricane status shortly after 48 hours, degenerate into a post-tropical low by 72 hours, and dissipate by 120 hours. The new intensity forecast is similar to the previous forecast, and is close to the IVCN intensity consensus model. FORECAST POSITIONS AND MAX WINDS INIT 26/0300Z 16.1N 117.7W 130 KT 150 MPH 12H 26/1200Z 16.7N 119.4W 125 KT 145 MPH 24H 27/0000Z 17.9N 121.1W 110 KT 125 MPH 36H 27/1200Z 19.2N 122.2W 95 KT 110 MPH 48H 28/0000Z 20.2N 122.6W 70 KT 80 MPH 72H 29/0000Z 21.1N 122.9W 40 KT 45 MPH...POST-TROPICAL 96H 30/0000Z 21.7N 123.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 31/0000Z...DISSIPATED $$ Forecaster Stewart

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Hurricane SEYMOUR Forecast Discussion Number 11

2016-10-25 22:37:36| National Hurricane Center (East Pacific)

Issued at 200 PM PDT TUE OCT 25 2016 000 WTPZ45 KNHC 252037 TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 200 PM PDT TUE OCT 25 2016 Seymour has continued to intensify since the last advisory, with the 15 n mi wide eye continuing to become better defined inside the central dense overcast. Satellite intensity estimates from TAFB, SAB, and the CIMSS satellite consensus are all 115 kt, so that is the initial intensity for this advisory. The hurricane currently has good cirrus outflow in all directions except to the south. Seymour is expected to remain within an area of low wind shear and over SSTs of around 28C for the next 12-18 hours, and thus some additional strengthening is possible. After 18 hours, increasing southwesterly shear and cooler waters should cause steady to rapid weakening. The majority of the guidance continues to show Seymour weakening below hurricane strength in less than 72 hours, degenerating to a remnant low by 96 hours, and dissipating completely by 120 hours. The new intensity forecast is an update of the previous forecast and again lies near the upper end of the intensity guidance. The initial motion is still 275/12. A mid- to upper-level ridge extending westward from Mexico is expected to steer Seymour westward to west-northwestward for another 12-24 hours. Subsequently, a deep-layer trough over the northeastern Pacific is expected to cause a break in the ridge, with Seymour turning northwestward and northward into the break. Most of the track guidance suggests the forward motion should slow after 48 hours as Seymour shears apart, and it now suggests that the hurricane should turn more westward before dissipation as it is steered by low-level easterly flow. The new forecast track is similar to the previous track for the first 72 hours, then it is shifted a little westward at the 96 hour point. FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 15.7N 116.4W 115 KT 130 MPH 12H 26/0600Z 16.2N 118.2W 120 KT 140 MPH 24H 26/1800Z 17.2N 120.3W 110 KT 125 MPH 36H 27/0600Z 18.5N 121.7W 95 KT 110 MPH 48H 27/1800Z 19.9N 122.4W 80 KT 90 MPH 72H 28/1800Z 21.0N 122.5W 40 KT 45 MPH 96H 29/1800Z 21.5N 123.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 30/1800Z...DISSIPATED $$ Forecaster Beven

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