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Hurricane SEYMOUR Forecast Discussion Number 10

2016-10-25 16:45:06| National Hurricane Center (East Pacific)

Issued at 800 AM PDT TUE OCT 25 2016 000 WTPZ45 KNHC 251444 TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 800 AM PDT TUE OCT 25 2016 Microwave satellite imagery suggests that Seymour underwent an eyewall replacement during the night, with an AMSR-2 overpass at 0917 UTC showing a larger eye than seen on a Windsat overpass at 0120 UTC. The hurricane is looking better organized than 6 hours ago in infrared satellite imagery, with the eye gradually becoming more apparent and greater symmetry in the cold cloud tops around the eye. The latest satellite intensity estimates include 115 kt from TAFB, 102, kt from SAB, and 108 kt from the CIMSS satellite consensus. Based on these and current trends, the initial intensity is increased to 110 kt. Seymour is expected to remain within an area of low wind shear and over SSTs of around 28C during the next 24 hours. These conditions favor additional intensification and the NHC wind speed forecast calls for a peak intensity at category 4 strength later today or early tonight. After that time, increasing southwesterly shear and cooler waters should cause steady to rapid weakening. The majority of the guidance shows 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 continues to lie near the upper end of the intensity guidance. The initial motion is 275/12. A mid- to upper-level ridge extending westward from Mexico is expected to steer Seymour westward to west-northwestward for another 24 hours or so. 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 between 72-96 hours as Seymour shears apart, and this is reflected in the track forecast. The new forecast track is a little to the south of the previous track during the first 24 hours, and then is similar to the previous track. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 15.6N 115.0W 110 KT 125 MPH 12H 26/0000Z 16.0N 117.0W 120 KT 140 MPH 24H 26/1200Z 16.8N 119.3W 115 KT 130 MPH 36H 27/0000Z 18.0N 121.0W 100 KT 115 MPH 48H 27/1200Z 19.5N 122.2W 85 KT 100 MPH 72H 28/1200Z 21.5N 122.5W 45 KT 50 MPH 96H 29/1200Z 22.0N 122.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 30/1200Z...DISSIPATED $$ Forecaster Beven

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Hurricane SEYMOUR Forecast Discussion Number 8

2016-10-25 04:37:01| National Hurricane Center (East Pacific)

Issued at 900 PM MDT MON OCT 24 2016 000 WTPZ45 KNHC 250236 TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 900 PM MDT MON OCT 24 2016 After rapidly intensifying during the past 30 hours, the rate of strengthening has slowed for the time being. The eye of Seymour has become cloud-filled on geostationary satellite images and microwave data indicate that the northwestern portion of the eyewall has eroded some. The observed arrested development this evening is likely due to the entrainment of dry air. However, there are already some signs that the hurricane is recovering from these effects with central convection increasing and becoming more circular during the past couple of hours. The initial intensity is set at 90 kt, slightly above the Dvorak estimates at 0000 UTC due to the improvement in organization of the system since that time. Since Seymour is expected to remain in low wind shear conditions, over warm water, and in an environment of good upper-level divergence during the next 24 hours, significant strengthening is anticipated. After that time, however, an increase in southwesterly shear should end the strengthening process. Seymour is forecast to rapidly weaken when it moves over cooler water and into hostile atmospheric conditions beginning in about 2 days. The NHC intensity forecast is a little higher than the multi-model consensus, and is in best agreement with the SHIPS and LGEM models. Seymour is moving westward, or 280 degrees, at 14 kt. The hurricane is expected to remain on the southwestern side of a mid-level high pressure system during the next 36 to 48 hours, which should allow Seymour to maintain a westward to west-northwestward motion. After that time, a large deep-layer low pressure system is expected to move eastward toward California and erode the ridge. This pattern change should cause Seymour to turn northwestward and then northward in 3 to 4 days. The track models are in relatively good agreement, and only small changes were made to the previous NHC forecast track. A pair of ASCAT passes from several hours ago indicate that the wind field of Seymour is very compact, with tropical-storm-force winds extending no more than 60 n mi from the center. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 15.5N 112.6W 90 KT 105 MPH 12H 25/1200Z 15.9N 114.6W 100 KT 115 MPH 24H 26/0000Z 16.4N 117.2W 110 KT 125 MPH 36H 26/1200Z 17.2N 119.5W 105 KT 120 MPH 48H 27/0000Z 18.4N 121.3W 90 KT 105 MPH 72H 28/0000Z 21.5N 122.8W 60 KT 70 MPH 96H 29/0000Z 22.5N 122.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 30/0000Z...DISSIPATED $$ Forecaster Cangialosi

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Hurricane SEYMOUR Forecast Discussion Number 7

2016-10-24 23:05:03| National Hurricane Center (East Pacific)

Issued at 300 PM MDT MON OCT 24 2016 000 WTPZ45 KNHC 242104 CCA TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 7...CORRECTED NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 300 PM MDT MON OCT 24 2016 CORRECTED 120H STATUS Seymour is rapidly intensifying. A pinhole eye has formed within a small, nearly symmetric, central dense overcast (CDO) during the last several hours. In addition, a long curved band coils inward toward the center with a dry slot between it and the CDO. Dvorak classifications are T4.5/77 kt from TAFB and SAB, and the latest UW-CIMSS ADT value is T4.7/82 kt. A blend of these is used to raise the initial intensity estimate to 80 kt. Seymour has another 24 hours to strengthen under nearly ideal environmental conditions, consisting of low vertical wind shear, warm waters, strong upper-level divergence, and a sufficiently moist atmosphere. By 36 to 48 hours, an uptick in the southwesterly vertical shear associated with a mid- to upper-tropospheric trough approaching Seymour from the northwest should induce a weakening trend. With the waters cooling substantially and the shear becoming prohibitively high after that time, rapid weakening is likely. Seymour is now forecast to become a post-tropical remnant low by 96 hours, if not perhaps sooner, as depicted in global model guidance. The new intensity forecast is generally above the the guidance through 48 hours, closest to the FSU Superensemble and NOAA Corrected Consensus HCCA model output, and just above the multi- model consensus after that time. The initial motion estimate is 285/14 kt. Seymour should maintain a west-northwestward motion with some decrease in forward speed for the next 36 to 48 hours as it nears the western periphery of a mid-level ridge located to the south of Baja California. The trough upstream of Seymour should erode the ridge further in about 48 hours and cause the cyclone to turn northwestward and northward while it decelerates. The guidance has again shifted toward the left during the next few days, and the official forecast is adjusted in that direction but lies on the eastern side of the guidance envelope. The model guidance is now much slower after that time, around the time Seymour decouples and becomes a remnant low. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 15.3N 111.3W 85 KT 100 MPH 12H 25/0600Z 15.6N 113.3W 100 KT 115 MPH 24H 25/1800Z 16.0N 115.9W 110 KT 125 MPH 36H 26/0600Z 16.7N 118.3W 110 KT 125 MPH 48H 26/1800Z 17.6N 120.4W 100 KT 115 MPH 72H 27/1800Z 20.7N 122.4W 70 KT 80 MPH 96H 28/1800Z 22.3N 122.4W 35 KT 40 MPH...POST-TROP/REMNT LOW 120H 29/1800Z...DISSIPATED $$ Forecaster Kimberlain

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Hurricane SEYMOUR Forecast Discussion Number 6

2016-10-24 16:33:26| National Hurricane Center (East Pacific)

Issued at 900 AM MDT MON OCT 24 2016 000 WTPZ45 KNHC 241433 TCDEP5 HURRICANE SEYMOUR DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 900 AM MDT MON OCT 24 2016 Seymour's cloud pattern continues to increase in organization. The cyclone's small central dense overcast has become circular and increasingly symmetric since the last advisory, with plenty of cold-topped deep convection, particularly near the center. Dvorak satellite classifications were T3.5/55 kt and T4.0/65 kt at 1200 UTC, while UW-CIMSS ADT values were around T4.5. Since that time, the cloud pattern has continued to exhibit greater organization. Thus, the initial intensity is raised to 65 kt, toward the higher end of the subjective estimates. For the next 36 hours or so, Seymour should be embedded in a nearly ideal environment for intensification, characterized by low vertical wind shear, warm waters, strong upper-level divergence and a sufficiently moist atmosphere. SHIPS model output continues to indicate the likelihood of rapid intensification during the next 24 hours, and so does the official forecast. Around 48 hours, southwesterly shear associated with a vigorous trough northwest of Seymour should begin to increase and induce weakening. The shear should become prohibitively high in 3 to 4 days and result in rapid weakening, and then a decoupling of the cyclone toward the end of the forecast period. Remnant low status is predicted in 5 days, but it very likely could be sooner. The NHC intensity forecast is largely an update of the previous one and is slightly above the multi-model consensus, close to the FSU Superensemble and the NOAA corrected consensus model, HCCA. The initial motion is 290/13. Seymour is being steered west- northwestward on the south side of deep-layer ridge near the southern tip of the Baja California peninsula. When the cyclone reaches the western edge of this ridge in about 2 days, it should encounter the flow associated with the deep-layer trough upstream of Seymour. This should result in a decrease in forward speed while the cyclone's heading turns northwestward and then sharply northward. Once a completely shallow system, Seymour or its remnants should turn north-northeast and lose its identity well west of the Baja California peninsula. The new track forecast has been adjusted westward in response to a shift of the guidance in that direction, but not as far left as the multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 15.2N 109.8W 65 KT 75 MPH 12H 25/0000Z 15.8N 111.9W 75 KT 85 MPH 24H 25/1200Z 16.2N 114.5W 95 KT 110 MPH 36H 26/0000Z 16.6N 116.9W 105 KT 120 MPH 48H 26/1200Z 17.4N 119.0W 95 KT 110 MPH 72H 27/1200Z 20.5N 121.8W 75 KT 85 MPH 96H 28/1200Z 23.9N 121.5W 45 KT 50 MPH 120H 29/1200Z 26.2N 120.2W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Kimberlain

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Tropical Storm SEYMOUR Forecast Discussion Number 5

2016-10-24 10:53:57| National Hurricane Center (East Pacific)

Issued at 300 AM MDT MON OCT 24 2016 000 WTPZ45 KNHC 240853 TCDEP5 TROPICAL STORM SEYMOUR DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL EP202016 300 AM MDT MON OCT 24 2016 Seymour continues to strengthen this morning. Satellite data indicate that the banding features have continued to improve and a small central dense overcast feature has developed and become more symmetric. Although earlier microwave imagery revealed a mid-level eye feature, there has been no recent microwave data to diagnose the structure of the inner core. Subjective and objective Dvorak intensity estimates have increased to 55 and 65 kt, respectively, and as a result the initial intensity has been raised to 55 kt. Seymour continues to move west-northwestward at about 13 kt. There has been no change to the track forecast reasoning from the previous advisory. Seymour should continue moving west-northwestward to the south of a deep-layer ridge that extends westward from Mexico. In a couple of days, the cyclone will approach the western portion of the ridge and Seymour should respond by turning northwestward. After 72 hours, an approaching shortwave trough should steer Seymour northward, then northeastward. The model guidance is in good agreement through the first 3 days, but after that time there is more spread in the track models this cycle. The ECMWF and UKMET models have trended faster and take Seymour much farther north at 96 h and 120 h than the remainder of the guidance. Since Seymour is expected to rapidly weaken and become an increasingly shallow system, the NHC forecast leans toward the slower and more southern solutions late in the forecast period. Seymour is forecast to traverse very warm water and remain within an environment of low vertical wind shear and high mid-level moisture during the next day or two. These conditions should allow the cyclone to quickly strengthen during the next 24 to 36 hours and like the previous advisory, the new NHC intensity forecast calls for rapid strengthening during the next 24 hours. In fact, the SHIPS Rapid Intensification Index gives a 78 percent chance of a 30 kt or more increase in wind speed during the next 24 hours. The updated NHC intensity forecast now shows a peak intensity of 100 kt in 48 hours, which is slightly below the LGEM, FSSE, and NOAA corrected consensus (HCCA) models. After 72 hours, increasing southwesterly shear and cooler SSTs are expected to cause a rapid spin down of the tropical cyclone. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 14.9N 108.5W 55 KT 65 MPH 12H 24/1800Z 15.4N 110.5W 70 KT 80 MPH 24H 25/0600Z 16.0N 113.2W 85 KT 100 MPH 36H 25/1800Z 16.4N 115.6W 95 KT 110 MPH 48H 26/0600Z 17.0N 117.9W 100 KT 115 MPH 72H 27/0600Z 19.6N 121.1W 85 KT 100 MPH 96H 28/0600Z 23.0N 121.3W 55 KT 65 MPH 120H 29/0600Z 24.5N 120.0W 30 KT 35 MPH $$ Forecaster Brown

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