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Tropical Storm GASTON Forecast Discussion Number 45

2016-09-02 22:36:05| National Hurricane Center (Atlantic)

Issued at 500 PM AST FRI SEP 02 2016 000 WTNT42 KNHC 022035 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 45 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 PM AST FRI SEP 02 2016 Gaston is practically devoid of deep convection, and it is assumed that a significant spindown has occurred today. Under this assumption, the advisory intensity is set at 45 kt which is in agreement with a Dvorak Current Intensity number from TAFB. Since the cyclone will be moving over the colder waters to the north of the Azores and through an environment of strong shear, the forecast calls for the system to degenerate into a remnant low in 24 hours, and this event may occur even sooner than that. Microwave and geostationary satellite fixes, along with observations from Flores in the Azores, indicate that the motion is around 070/15 kt. Gaston should curve gradually toward the northeast, ahead of a broad trough in the mid-latitude westerlies, until dissipation. The official track forecast is very close to the dynamical model consensus. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 39.6N 30.4W 45 KT 50 MPH 12H 03/0600Z 40.3N 28.1W 35 KT 40 MPH 24H 03/1800Z 42.1N 25.1W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 04/0600Z 44.3N 21.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 04/1800Z...DISSIPATED $$ Forecaster Pasch

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Tropical Storm HERMINE Forecast Discussion Number 21

2016-09-02 16:56:00| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT FRI SEP 02 2016 000 WTNT44 KNHC 021455 TCDAT4 TROPICAL STORM HERMINE DISCUSSION NUMBER 21 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 1100 AM EDT FRI SEP 02 2016 Hermine is moving over southeastern Georgia and has weakened steadily since landfall. Convective tops have warmed, but several land stations have shown sustained winds near tropical storm force near the center with some gusts close to 50 kt. Stronger winds are also found over the Atlantic coastal waters, with buoy 41008 reporting winds around 40 kt in the last hour. Based on this, the initial intensity is set to 45 kt for this advisory. The central pressure, based on surface data, is about 989 mb. While the center of Hermine will remain over land for the next 24 hours or so, I don't expect the winds over water to decrease much, so the NHC forecast shows no change during that time. Strengthening through a combination of diabatic and baroclinic processes is shown after the system moves offshore. The GFS, ECMWF, and UKMET all show Hermine interacting with a potent upper-level shortwave trough in 2-3 days, and the system could re-strengthen to near hurricane force at that time. Slow weakening is expected later in the period, but Hermine is expected to remain a dangerous cyclone through 5 days. The NHC intensity forecast is based largely on global model guidance, and is a bit higher than the previous one. The initial motion estimate is 040/16. Hermine should continue moving northeastward for the next 36 to 48 hours ahead of an approaching mid-latitude trough. After that time, the cyclone should turn northward and nearly stall out as it interacts with the upper-level shortwave off the mid-Atlantic coast. A slow northeastward motion is shown at days 4 and 5, but there is a fair bit of model spread late in the period and confidence in the details of the track forecast at those times remains low. Note that the NHC forecast shows Hermine as a post-tropical cyclone in about 48 hours, but model fields do not show it undergoing a full extratropical transition, as it instead appears to acquire a warm seclusion structure. There is a possibility that the system could regain some tropical characteristics in 4-5 days, but this remains uncertain. Based on the latest forecast, tropical storm watches and warnings have been expanded northward along the U.S. mid-Atlantic coast. KEY MESSAGE: 1. Hermine is expected to become a post-tropical cyclone while still producing hazardous winds and storm surge over land. NWS policy allows NHC to write advisories on and issue tropical storm watches and warnings for post-tropical cyclones, when the system continues to pose a significant threat to life and property. NHC and the NWS Eastern Region have decided that this option will be invoked for Hermine. After Hermine becomes a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products for as long as the system remains a significant threat to land. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 31.9N 82.0W 45 KT 50 MPH...INLAND 12H 03/0000Z 33.5N 79.7W 45 KT 50 MPH...INLAND 24H 03/1200Z 35.4N 76.3W 45 KT 50 MPH...INLAND 36H 04/0000Z 36.7N 73.9W 55 KT 65 MPH...OVER WATER 48H 04/1200Z 37.5N 72.6W 55 KT 65 MPH...POST-TROPICAL 72H 05/1200Z 38.1N 72.6W 65 KT 75 MPH...POST-TROPICAL 96H 06/1200Z 38.8N 71.9W 60 KT 70 MPH...POST-TROPICAL 120H 07/1200Z 39.3N 71.5W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Brennan

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Tropical Storm GASTON Forecast Discussion Number 44

2016-09-02 16:52:41| National Hurricane Center (Atlantic)

Issued at 1100 AM AST FRI SEP 02 2016 000 WTNT42 KNHC 021452 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 44 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 AM AST FRI SEP 02 2016 Data from a recent ASCAT overpass indicate that the storm is a little stronger than previously estimated. The advisory intensity is set to 60 kt based on the scatterometer winds. Vertical shear over the system is currently around 20 kt and is forecast to increase to near 30 kt within the next day or so, and the cyclone will be moving over progressively cooler waters. Weakening is forecast in agreement with the IVCN intensity model consensus. Gaston should weaken to a remnant low before becoming absorbed by a frontal system in 48 hours or less. Based on the scatterometer data and conventional geostationary satellite fixes, the motion is east-northeastward or 070/16. Gaston should continue on an east-northeastward track, following the flow on the south side of the mid-latitude westerlies, until dissipation. The official track forecast is close to the multi-model consensus TVCN. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 39.1N 32.0W 60 KT 70 MPH 12H 03/0000Z 39.5N 29.8W 45 KT 50 MPH 24H 03/1200Z 40.7N 27.0W 35 KT 40 MPH 36H 04/0000Z 42.5N 24.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 48H 04/1200Z...DISSIPATED $$ Forecaster Pasch

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Tropical Storm GASTON Forecast Discussion Number 43

2016-09-02 10:47:00| National Hurricane Center (Atlantic)

Issued at 500 AM AST FRI SEP 02 2016 000 WTNT42 KNHC 020846 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 43 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 AM AST FRI SEP 02 2016 Conventional satellite imagery and two earlier microwave images show the center of circulation decoupled well to the west of the remaining deep convection. The initial intensity is decreased to 55 kt for this advisory and is based on a blend of the subjective and objective satellite intensity estimates. Continued weakening is expected as the cyclone traverses cooler oceanic sea surface temperatures and remains in a strong vertical shear environment. Gaston is forecast to degenerate into a remnant low in 36 hours, with dissipation in about 48 hours. The NHC intensity forecast is similar to the previous one, and is close to the IVCN intensity consensus model. Because Gaston's low-level circulation has decoupled from the mid-level flow due to the persistent strong shear, the cyclone's forward speed has decreased while moving eastward, or 080/13 kt. This general motion is expected to continue during the next couple of days. At the 36-hour period, Gaston should turn northeastward in response to a frontal system approaching from the northwest and open up into a trough of low pressure in a couple of days. The NHC track foreast remains near the middle of the tightly clustered model guidance and is similar to the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 38.5N 34.5W 55 KT 65 MPH 12H 02/1800Z 38.9N 31.8W 50 KT 60 MPH 24H 03/0600Z 39.7N 29.0W 40 KT 45 MPH 36H 03/1800Z 41.3N 26.3W 30 KT 35 MPH...POST-TROP/REMNT LOW 48H 04/0600Z...DISSIPATED $$ Forecaster Roberts/Stewart

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Tropical Storm HERMINE Forecast Discussion Number 20

2016-09-02 10:43:29| National Hurricane Center (Atlantic)

Issued at 500 AM EDT FRI SEP 02 2016 000 WTNT44 KNHC 020843 TCDAT4 TROPICAL STORM HERMINE DISCUSSION NUMBER 20 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 500 AM EDT FRI SEP 02 2016 Hermine made landfall near St. Marks, Florida, around 0530 UTC with 70-kt winds. Since that time radar data and surface observations indicate that the center has continued to move inland. The radar presentation has deteriorated, and it is estimated that Hermine has weakened to tropical storm status with 60-kt winds. Additional weakening is anticipated while the cyclone moves farther inland, but it could reinvigorate as an extratropical cyclone when it moves over water and interacts with a baroclinic trough in about 48 hours. The best estimate of the initial motion 025/12 kt. Hermine is embedded within the southwesterly flow ahead of a mid-latitude trough, and this flow pattern should steer the cyclone toward the northeast with an increase in forward speed during the next day or two. Track models are in very good agreement during this period. After that time, when Hermine begins to interact with a baroclinic trough, the cyclone should slow down significantly and begin to meander. On days 3-5, the track guidance becomes rather divergent, decreasing confidence in the forecast. The NHC prediction keeps an extratropical cyclone moving little or drifting northward a few hundred miles off the northeastern U.S coast during that time. There is a possibility that the system could regain some tropical characteristics in 4-5 days, but this remains uncertain as indicated in the previous NHC forecast. KEY MESSAGE: 1. Hermine is expected to become a post-tropical cyclone while still producing hazardous winds and storm surge over land. NWS policy allows NHC to write advisories on and issue tropical storm watches and warnings for post-tropical cyclones, when the system continues to pose a significant threat to life and property. NHC and the NWS Eastern Region have decided that this option will be invoked for Hermine. After Hermine becomes a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products for as long as the system remains a significant threat to land. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 30.8N 83.6W 60 KT 70 MPH 12H 02/1800Z 32.5N 81.4W 45 KT 50 MPH...INLAND 24H 03/0600Z 34.5N 77.9W 40 KT 45 MPH...INLAND 36H 03/1800Z 36.0N 74.5W 40 KT 45 MPH...OVER WATER 48H 04/0600Z 37.0N 72.5W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 05/0600Z 38.0N 72.0W 55 KT 65 MPH...POST-TROP/EXTRATROP 96H 06/0600Z 38.5N 72.0W 55 KT 65 MPH...POST-TROP/EXTRATROP 120H 07/0600Z 39.0N 72.0W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Avila

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