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Tropical Storm Helene Forecast Discussion Number 29

2018-09-14 16:41:56| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Sep 14 2018 774 WTNT43 KNHC 141441 TCDAT3 Tropical Storm Helene Discussion Number 29 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Fri Sep 14 2018 Conventional satellite imagery and a 1147 UTC METOP-B AMSU microwave image show a deep convective curved band wrapping around the surface circulation in the west semicircle. Additionally, a recent 1148 UTC ASCAT-B scatterometer pass indicates that Helene is a little stronger than earlier estimated. The data showed a number of 60 kt winds in the northeast quadrant. Based on these data, Helene is still classified as a tropical cyclone, and the initial intensity is bumped up to 60 kt for this advisory. The large-scale models and the statistical intensity guidance indicate little change in strength during the next 36 hours or so. After that time, some gradual weakening is forecast as Helene completes extratropical transition while passing just to the north of the Azores Islands. The ECMWF, UKMET, and the CMC all show Helene absorbed by a larger baroclinic system just beyond the 72 hour period, as the cyclone moves over Ireland and the United Kingdom. The initial motion is estimated to be northward, or 010/20 kt, within the deep-layer southerly flow produced by a mid-tropospheric trough to the west. A north-northeast to northeast turn is expected over the weekend as the cyclone quickly moves within the mid-latitude westerlies. Only a slight adjustment to the left of the previous track forecast was made through 48 hours. The NHC official forecast is based primarily on the TVCN multi-model consensus. An orange wind warning is in effect for the Azores, equivalent to a tropical storm warning. Tropical Storm conditions with gusty winds and heavy rains are likely over the weekend as Helene passes the islands, regardless of whether the cyclone is a tropical storm or post-tropical. Helene is expected to be a post-tropical cyclone when it approaches Ireland and the United Kingdom in a few days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/. FORECAST POSITIONS AND MAX WINDS INIT 14/1500Z 32.9N 36.3W 60 KT 70 MPH 12H 15/0000Z 35.5N 35.5W 60 KT 70 MPH 24H 15/1200Z 38.4N 33.4W 55 KT 65 MPH 36H 16/0000Z 40.7N 30.0W 50 KT 60 MPH 48H 16/1200Z 42.7N 25.7W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 17/1200Z 47.4N 16.1W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 18/1200Z...ABSORBED BY FRONT $$ Forecaster Roberts

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Tropical Depression Isaac Forecast Discussion Number 28

2018-09-14 16:32:26| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Sep 14 2018 330 WTNT44 KNHC 141432 TCDAT4 Tropical Depression Isaac Discussion Number 28 NWS National Hurricane Center Miami FL AL092018 1100 AM AST Fri Sep 14 2018 Deep convection has been increasing near the center of Isaac, but visible imagery indicates that the circulation has become more elongated from northeast to southwest. It is possible that Isaac is in the process of losing a well-defined center, but we will know more about the wind structure in a few hours since the NOAA Hurricane Hunter aircraft should be in the area. The initial wind speed is held at 30 kt. Vertical shear has decreased near Isaac, with some outflow in the western semicircle noted and consolidation of the convection. Still, this area of the eastern Caribbean isn't known as the graveyard for nothing, and the system might just be too shallow and disorganized to take advantage of the changing environment. Almost all of the dynamical models, save the UKMET, show the cyclone moving westward to west-northwestward at a slower forward speed during the next few days and degenerating into an open wave in the central Caribbean Sea. This solution is also supported by fewer members of the GFS and ECMWF ensemble now maintaining a coherent system in the long range. Little change is made to the previous track and intensity forecasts, and we will see what the aircraft finds this afternoon and if the 1200 UTC model cycle maintains the dissipation trend. FORECAST POSITIONS AND MAX WINDS INIT 14/1500Z 14.9N 67.3W 30 KT 35 MPH 12H 15/0000Z 15.0N 69.0W 30 KT 35 MPH 24H 15/1200Z 15.1N 71.3W 30 KT 35 MPH 36H 16/0000Z 15.2N 73.3W 25 KT 30 MPH 48H 16/1200Z 15.6N 75.3W 25 KT 30 MPH 72H 17/1200Z 17.0N 79.5W 25 KT 30 MPH 96H 18/1200Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Helene Forecast Discussion Number 28

2018-09-14 10:52:54| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Sep 14 2018 163 WTNT43 KNHC 140852 TCDAT3 Tropical Storm Helene Discussion Number 28 NWS National Hurricane Center Miami FL AL082018 500 AM AST Fri Sep 14 2018 Helene is looking increasingly like an extratropical cyclone, with a rain shield extending from its northwest quadrant and a line of clouds to the southeast, probably associated with a convergence zone, that looks like a developing a front. However, earlier ASCAT data suggested that the cyclone still had a compact wind field with maximum winds of around 55 kt on the west side, and the most recent available AMSU data still showed a distinct warm core. The extratropical transition (ET) process will likely be completed within the next 48 h, around the time that Helene will likely pass near or over the Azores. While this process occurs, little weakening is forecast by the various dynamical models, but gradual weakening should occur once ET is completed. By the end of the forecast period, nearly all of the global models depict the cyclone being absorbed into a larger mid-latitude low pressure system over the far northeast Atlantic. Helene continues to accelerate northward, and the initial motion is 010/20 kt. Almost no change has been made to the track forecast. Helene is still forecast to move quickly northward to north-northeastward between a mid-level ridge to its east and a sharp mid-latitude trough to its west. A northeastward turn should occur over the weekend as Helene reaches the northern extent of the ridge and becomes embedded within the mid-latitude westerlies. The official track forecast remains near the various consensus aids. The government of Portugal has issued an orange wind warning for the Azores, equivalent to a tropical storm warning. Tropical Storm conditions with gusty winds and heavy rains are likely over the weekend as Helene passes the islands, regardless of whether the cyclone is a tropical storm or post-tropical. Helene is expected to be a post-tropical cyclone when it approaches Ireland and the United Kingdom in a few days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 30.6N 36.0W 55 KT 65 MPH 12H 14/1800Z 33.4N 35.3W 55 KT 65 MPH 24H 15/0600Z 36.7N 33.8W 55 KT 65 MPH 36H 15/1800Z 39.3N 30.8W 55 KT 65 MPH 48H 16/0600Z 41.3N 26.9W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 17/0600Z 45.5N 17.7W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 18/0600Z 51.5N 8.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 19/0600Z...DISSIPATED $$ Forecaster Zelinsky

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Hurricane Florence Forecast Discussion Number 60

2018-09-14 10:42:55| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Fri Sep 14 2018 755 WTNT41 KNHC 140842 TCDAT1 Hurricane Florence Discussion Number 60 NWS National Hurricane Center Miami FL AL062018 500 AM EDT Fri Sep 14 2018 An Air Force Hurricane Hunter aircraft has been investigating Florence over the past few hours. Data from the aircraft indicate little change in the intensity with the central pressure holding fairly steady. The current intensity estimate is kept at 80 kt for this advisory. There were a couple of SFMR-observed surface winds that were a little higher than that value, however these observations were very near Cape Lookout North Carolina, where shoaling likely caused some inflated wind speeds. The current intensity estimate is also consistent with peak WSR-88D Doppler radar velocities. The center of Florence will be moving inland very soon, but is expected to slow down even more today and tonight. As a result, it will remain fairly close to the coast today, with much of the circulation still over water. Therefore only a gradual decrease in intensity is called for through tonight. Over the weekend, a faster rate of weakening is likely while the center moves at a faster pace and goes farther inland. The hurricane is turning westward as it continues a slow forward motion of about 285/5 kt. Florence is currently in a region of weak steering currents associated with a col between two mid-level anticyclones. Over the next few days, a high pressure area is forecast to build to the east-northeast of the tropical cyclone. As a result, the system should gradually turn northwestward and northward in 2-3 days. Later in the forecast period, Florence should turn northeastward as it approaches the mid-latitude westerlies. The official track forecast is similar to the previous one and about in the middle of the dynamical guidance suite. It cannot be emphasized enough that the most serious hazard associated with slow-moving Florence is extremely heavy rainfall, which will cause disastrous flooding that will be spreading inland through the weekend. Key Messages: 1. A life-threatening storm surge is already occurring along portions of the North Carolina coast and will continue through today and tonight. This surge is also likely along portions of the South Carolina coast. The greatest storm surge inundation is expected between Cape Fear and Cape Hatteras, including the Neuse and Pamlico Rivers and western Pamlico Sound. 2. Life-threatening, catastrophic flash flooding and prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians through early next week, as Florence is expected to slow down while it moves inland. 3. Damaging hurricane-force winds are occurring along portions of the North Carolina coast and are expected to spread to portions of the South Carolina coast later today. Strong winds could also spread inland into portions of the Carolinas over the next couple of days. 4. Large swells affecting Bermuda, portions of the U.S. East Coast, and the northwestern and central Bahamas will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 34.2N 77.4W 80 KT 90 MPH 12H 14/1800Z 34.1N 78.3W 75 KT 85 MPH...INLAND 24H 15/0600Z 33.9N 79.1W 65 KT 75 MPH...INLAND 36H 15/1800Z 33.8N 79.9W 45 KT 50 MPH...INLAND 48H 16/0600Z 34.1N 81.1W 35 KT 40 MPH...INLAND 72H 17/0600Z 36.6N 83.2W 25 KT 30 MPH...INLAND 96H 18/0600Z 40.5N 80.5W 25 KT 30 MPH...POST-TROP/INLAND 120H 19/0600Z 43.5N 72.0W 25 KT 30 MPH...POST-TROP/INLAND $$ Forecaster Pasch

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Tropical Storm Joyce Forecast Discussion Number 7

2018-09-14 10:33:56| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Sep 14 2018 373 WTNT45 KNHC 140833 TCDAT5 Tropical Storm Joyce Discussion Number 7 NWS National Hurricane Center Miami FL AL102018 500 AM AST Fri Sep 14 2018 There has been little change in the convective organization of Joyce overnight. The system continues to produce bands of convection over the northeastern portion of the circulation, but the center remains exposed due to shear. The initial intensity is held at 35 kt, in agreement with the earlier ASCAT data and recent TAFB satellite estimate. Joyce is still moving southwestward or 210 degrees at 7 kt. Joyce is being steered in that direction around the larger circulation of Helene to its east-southeast. Once Helene passes east-northeast of Joyce later today, Joyce should turn eastward, then begin to accelerate northeastward ahead of a mid-latitude trough over the weekend. The global models have trended toward a faster northeastward motion after 24 h, and the NHC track forecast has been adjusted in that direction as well. Southwesterly upper-level winds are forecast to increase over the cyclone within the next day or two, which should prevent Joyce from strengthening. The NHC forecast calls for little change in wind speed over the next 36 h, then shows weakening as the shear increases further. Both the GFS and ECMWF dissipate Joyce by day 4, but the system could lose its deep convection and become post-tropical before that time. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 32.1N 44.9W 35 KT 40 MPH 12H 14/1800Z 31.7N 44.7W 35 KT 40 MPH 24H 15/0600Z 31.8N 43.7W 35 KT 40 MPH 36H 15/1800Z 32.7N 42.5W 35 KT 40 MPH 48H 16/0600Z 34.0N 40.7W 30 KT 35 MPH 72H 17/0600Z 36.5N 34.5W 30 KT 35 MPH 96H 18/0600Z...DISSIPATED $$ Forecaster Brown

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