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Potential Tropical Cyclone Six Forecast Discussion Number 5

2018-08-31 16:37:33| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Aug 31 2018 176 WTNT41 KNHC 311437 TCDAT1 Potential Tropical Cyclone Six Discussion Number 5 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Fri Aug 31 2018 The circulation is better defined this morning, but the associated convection is rather weak. In fact, Dvorak numbers have not changed overall, and do not support classifying the system as a tropical cyclone yet. It seems like both NHC and the global models predicted the development of a cyclone too soon. Nevertheless, the environmental conditions are favorable for both genesis and strengthening, and a tropical depression or a storm is expected to form later today or Saturday. The shear does not appear to be a problem at this time, but the cyclone should be moving over marginal SSTs, limiting the amount of intensification. The HWRF model changed its tune to a more reasonable one, and it now shows more modest strengthening. The NHC forecast is similar to the previous one, and continues to be very close to the intensity consensus. The disturbance is moving toward the west-northwest or 285 degrees at about 11 kt. The subtropical high should continue to steer the system toward the west-northwest with some increase in forward speed for the next 2 to 3 days. After that time, a weakness in the ridge will induce a more northwestward track over the open Atlantic Ocean. This is supported by the track guidance which continues to be in very good agreement for the next 3 days. Thereafter, the guidance envelope widens and becomes bounded by the westernmost ECMWF and the easternmost GFS models decreasing the confidence in the track forecast. FORECAST POSITIONS AND MAX WINDS INIT 31/1500Z 13.7N 22.7W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 01/0000Z 14.1N 24.5W 35 KT 40 MPH...TROPICAL STORM 24H 01/1200Z 14.8N 27.0W 40 KT 45 MPH 36H 02/0000Z 15.5N 29.5W 45 KT 50 MPH 48H 02/1200Z 16.2N 32.1W 50 KT 60 MPH 72H 03/1200Z 17.5N 37.5W 55 KT 65 MPH 96H 04/1200Z 19.5N 42.0W 60 KT 70 MPH 120H 05/1200Z 22.0N 45.5W 65 KT 75 MPH $$ Forecaster Avila

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Potential Tropical Cyclone Six Forecast Discussion Number 4

2018-08-31 10:36:59| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Aug 31 2018 322 WTNT41 KNHC 310836 TCDAT1 Potential Tropical Cyclone Six Discussion Number 4 NWS National Hurricane Center Miami FL AL062018 500 AM AST Fri Aug 31 2018 Even though the circulation is gradually becoming less elongated, there is still relatively little deep convection near the estimated center of circulation. Instead, most of the convection remains confined to the monsoon trough southwest of the center, where low-level convergence is strongest. Therefore, the system is not quite yet being designated as a tropical cyclone. Global model fields suggest that the low should detach from the monsoon trough within the next 12 hours, which would likely allow deep convection to form closer to the center. Assuming that happens, the system is also forecast to strengthen and become a tropical storm by this afternoon while it approaches the southern Cabo Verde Islands. Although vertical shear is expected to be relatively light for much of the forecast period, sea surface temperatures ahead of the system are marginal, and the low will likely move over a minimum of oceanic heat content in 48-72 hours. Adding on top of that a drier mid-level environment, only gradual strengthening is anticipated through day 5. A majority of the intensity models agree on this scenario, with the exception of the HWRF model, which brings the system close to major hurricane strength at the end of the forecast period. Since that solution is discounted at the moment, the NHC intensity forecast is below the HCCA guidance and closest to the intensity consensus. The low appears to have turned west-northwestward with an initial motion of 285/10 kt. A mid-level high centered west of the Canary Islands should continue to steer the disturbance west-northwestward at a faster clip for the next 4 days or so. By the end of the forecast period, a weakness in the ridge over the eastern Atlantic could cause the system to turn northwestward. The new NHC track forecast is a little slower than the previous one during the first 72 hours and then a little to the west on days 4 and 5 to better match the latest guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 13.6N 21.4W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 31/1800Z 14.2N 23.1W 35 KT 40 MPH...TROPICAL STORM 24H 01/0600Z 14.8N 25.6W 40 KT 45 MPH 36H 01/1800Z 15.5N 28.3W 45 KT 50 MPH 48H 02/0600Z 16.2N 30.9W 50 KT 60 MPH 72H 03/0600Z 17.4N 36.2W 55 KT 65 MPH 96H 04/0600Z 19.0N 41.0W 60 KT 70 MPH 120H 05/0600Z 21.5N 45.0W 65 KT 75 MPH $$ Forecaster Berg

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Hurricane Norman Forecast Discussion Number 13

2018-08-31 10:36:59| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Fri Aug 31 2018 323 WTPZ41 KNHC 310836 TCDEP1 Hurricane Norman Discussion Number 13 NWS National Hurricane Center Miami FL EP162018 200 AM PDT Fri Aug 31 2018 Norman appears to have weakened over the past several hours. Cloud tops around the eye have warmed, and the eye itself has become less distinct. Several recent microwave overpasses show little indication that an eyewall replacement cycle is underway, but it is possible that our ability to observe such a cycle is limited by the resolution of the available instruments. Satellite intensity estimates from all agencies have decreased and supported at most an intensity of 125 kt at 0600 UTC. Given the continued warming of cloud tops near the inner core since that time, the initial intensity has been lowered to 120 kt. Due to the lower initial intensity, the intensity forecast has also been lowered slightly, particularly for the first 24 h. Gradual weakening is expected through the forecast period, though larger short-term fluctuations in intensity due to eyewall replacement cycles are possible, if not likely, while Norman remains a hurricane. By the end of the forecast period, a faster weakening rate is anticipated as Norman is forecast to move over cooler SSTs and reach a drier environment. The NHC forecast is near the center of the intensity guidance envelope and near the intensity consensus. The hurricane has continued to move west-southwestward at 8 kt. Almost no change has been made to the track forecast, which remains close to the various consensus aids. The hurricane should turn westward, and then west-northwestward, as it is steered primarily by a deep-layer ridge to the north for the next 5 days. With the exception of the UKMET global model, which is an outlier to the south, the global and regional dynamical models are in good agreement on the future path of Norman, and confidence in the track forecast is fairly high. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 16.9N 120.2W 120 KT 140 MPH 12H 31/1800Z 16.6N 121.4W 110 KT 125 MPH 24H 01/0600Z 16.3N 123.1W 110 KT 125 MPH 36H 01/1800Z 16.3N 125.1W 105 KT 120 MPH 48H 02/0600Z 16.7N 127.4W 100 KT 115 MPH 72H 03/0600Z 18.4N 133.0W 85 KT 100 MPH 96H 04/0600Z 19.5N 139.0W 70 KT 80 MPH 120H 05/0600Z 20.5N 143.5W 55 KT 65 MPH $$ Forecaster Zelinsky

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Potential Tropical Cyclone Six Forecast Discussion Number 3

2018-08-31 04:40:31| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Aug 30 2018 870 WTNT41 KNHC 310240 TCDAT1 Potential Tropical Cyclone Six Discussion Number 3 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Thu Aug 30 2018 Satellite imagery indicates that the disturbance is gradually becoming better organized. However, the convection is not yet well enough organized to call the system a tropical cyclone, and recent scatterometer data show that the system still lacks a well-defined center of circulation. The scatterometer did indicate winds of 25-30 kt, so the initial intensity is nudged upward to 30 kt. The system should be in an environment of light to moderate easterly vertical shear for the next three to four days. Sea surface temperatures decrease along the forecast track during this time, though, reaching 26C by 72-96 h. The intensity guidance suggests that the system should at least slowly develop despite the falling SSTs, and based on this the new intensity forecast calls for it to become a tropical depression in about 12 h, a tropical storm in 12-24 h, and a hurricane near the 96-h point. While the system is expected to reach warmer SSTs near the end of the forecast period, it is also expected to encounter westerly shear at that time, which should limit additional intensification. The initial motion is a somewhat uncertain 280/11. The trade winds on the south side of the subtropical ridge should steer the system west-northwestward with an increase in forward speed during the next 3-4 days, passing near or over the southern Cabo Verde Islands Friday or Friday night. Near the end of the forecast period, a turn toward the northwest is forecast as the system approaches a weakness in the ridge. The new track forecast is again little changed from the previous forecast and lies near the various consensus models. It should be noted that some adjustments to the early parts of the forecast track may occur until the center becomes better defined. FORECAST POSITIONS AND MAX WINDS INIT 31/0300Z 13.2N 20.9W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 31/1200Z 13.7N 22.3W 30 KT 35 MPH...TROPICAL DEPRESSION 24H 01/0000Z 14.5N 24.9W 40 KT 45 MPH 36H 01/1200Z 15.3N 27.7W 45 KT 50 MPH 48H 02/0000Z 16.0N 30.4W 50 KT 60 MPH 72H 03/0000Z 17.5N 35.5W 60 KT 70 MPH 96H 04/0000Z 19.0N 40.0W 65 KT 75 MPH 120H 05/0000Z 21.5N 44.0W 65 KT 75 MPH $$ Forecaster Beven

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Hurricane Norman Forecast Discussion Number 12

2018-08-31 04:33:59| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Thu Aug 30 2018 999 WTPZ41 KNHC 310233 TCDEP1 Hurricane Norman Discussion Number 12 NWS National Hurricane Center Miami FL EP162018 800 PM PDT Thu Aug 30 2018 After the explosive intensification observed last night and earlier today, Norman's intensity has leveled off. The eye of the major hurricane remains quite circular and distinct with a solid eyewall surrounding the center. The eastern portion of the eyewall, however, has eroded a little during the past few hours. Based on the steady state appearance, the initial intensity is held at 130 kt, which lies at the upper end of the satellite intensity estimates. The environmental conditions surrounding Norman are expected to only gradually become less favorable with SSTs and mid-level humidity values slowly decreasing along Norman's track during the next few days. There is a possibility of eyewall replacement cycles in major hurricanes like Norman, which typically cause fluctuations in strength and are challenging to forecast. The intensity models all show a slow weakening trend, and the NHC forecast follows that theme. This forecast is slightly below the previous one, trending toward the latest consensus models. Norman is moving west-southwestward at 8 kt steered by a northeast-southwest oriented deep-layer ridge to the north of the tropical cyclone. This general motion is expected to continue for the next day or so. After that time, a change in the orientation of the ridge should cause Norman to turn to the west and then west-northwest with an increase in forward speed. The track models remain in relatively good agreement, and the NHC track forecast lies near the middle of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 31/0300Z 17.3N 119.6W 130 KT 150 MPH 12H 31/1200Z 17.0N 120.7W 125 KT 145 MPH 24H 01/0000Z 16.6N 122.3W 120 KT 140 MPH 36H 01/1200Z 16.2N 124.3W 110 KT 125 MPH 48H 02/0000Z 16.3N 126.3W 105 KT 120 MPH 72H 03/0000Z 17.8N 131.5W 95 KT 110 MPH 96H 04/0000Z 19.5N 137.0W 75 KT 85 MPH 120H 05/0000Z 20.5N 142.0W 60 KT 70 MPH $$ Forecaster Cangialosi

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