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Tropical Depression JULIA Forecast Discussion Number 15

2016-09-17 16:49:15| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT SAT SEP 17 2016 000 WTNT41 KNHC 171449 TCDAT1 TROPICAL DEPRESSION JULIA DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL AL112016 1100 AM EDT SAT SEP 17 2016 Convection has increased somewhat over the eastern quadrant of Julia, both in a cluster about 150 n mi east of the center and in thin bands closer to the center. This is just enough to keep the cyclone a tropical depression for this advisory. In the short term, the vertical wind shear is not favorable to sustain convection, and the cyclone should decay to a remnant low in 12 hours or less. The dynamical models forecast the shear to decrease after 36 hours, but it is likely that the remnant low will not be strong enough to take advantage of the more favorable environment. Thus, the intensity forecast continues to call for dissipation by 72 hours. The depression is currently nearly stationary. A low- to mid-level ridge north of Julia should move eastward into the Atlantic during the forecast period as a mid- to upper-level trough moves into the eastern United States. The flow between these features should steer the cyclone or its remnants slowly northwestward and northward for the remainder of the system's life. The new forecast track is similar to, but a little east of, the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 17/1500Z 30.2N 76.2W 25 KT 30 MPH 12H 18/0000Z 30.4N 76.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 18/1200Z 31.0N 77.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 19/0000Z 31.5N 77.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 19/1200Z 32.2N 77.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 20/1200Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm KARL Forecast Discussion Number 12

2016-09-17 10:33:11| National Hurricane Center (Atlantic)

Issued at 500 AM AST SAT SEP 17 2016 000 WTNT42 KNHC 170833 TCDAT2 TROPICAL STORM KARL DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 500 AM AST SAT SEP 17 2016 Karl is maintaining the status quo with its center at least 120 n mi to the southwest of the deep convection. Since the ASCAT pass from last evening showed a sizable area of 35-40 kt winds, and the Dvorak estimates have been steady, the initial intensity remains 40 kt. Karl is located just to the south of an upper-level low, and the resultant westerly shear should gradually relax and change direction during the next few days once Karl moves away from this feature. Sea surface temperatures will be steadily increasing along Karl's forecast path, and mid-tropospheric moisture may begin to increase after 48 hours. Therefore, the NHC intensity forecast continues to call for little change in intensity during the next 48 hours, followed by steady strengthening from day 3 to day 5. This forecast closely follows an average of the SHIPS and LGEM models and still allows for the possibility of Karl becoming a hurricane by day 5. There is no doubt that the center of Karl has been losing latitude during the past 6-12 hours, and the initial motion estimate is 265/12 kt. A strengthening low- to mid-level ridge over the central Atlantic is expected to continue pushing Karl south of due west during the next 36 hours. After that time, Karl will begin to approach a weakness in the ridge located over the western Atlantic, and the tropical cyclone is forecast to move west-northwestward on days 3-5 well to the northeast of the Leeward Islands. Although there is some uncertainty on how far south Karl will get before it turns west-northwestward, the spread among the track models is actually quite small. The updated NHC track forecast essentially lies along the TVCN multi-model consensus line, which ends up being a little south and west of the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 17/0900Z 18.2N 38.2W 40 KT 45 MPH 12H 17/1800Z 18.1N 40.1W 35 KT 40 MPH 24H 18/0600Z 17.9N 42.8W 35 KT 40 MPH 36H 18/1800Z 17.6N 45.4W 40 KT 45 MPH 48H 19/0600Z 17.8N 47.9W 40 KT 45 MPH 72H 20/0600Z 19.4N 53.1W 50 KT 60 MPH 96H 21/0600Z 22.0N 58.5W 60 KT 70 MPH 120H 22/0600Z 24.5N 63.0W 70 KT 80 MPH $$ Forecaster Berg

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Tropical Storm KARL Forecast Discussion Number 11

2016-09-17 04:50:11| National Hurricane Center (Atlantic)

Issued at 1100 PM AST FRI SEP 16 2016 000 WTNT42 KNHC 170250 TCDAT2 TROPICAL STORM KARL DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 1100 PM AST FRI SEP 16 2016 The deep convection with Karl is limited to the northeastern quadrant because of the 20 kt westerly vertical shear currently afflicting the tropical storm. Despite this, a 0007 UTC ASCAT scatterometer pass indicated maximum winds of about 40 kt, which is the initial intensity. Karl should continue to struggle for the next day or so due to the hostile vertical shear. The shear should relax as Karl reaches the west side of an upper-level trough in about 36 to 48 hours. From that point in time onward, Karl should be experiencing more conducive conditions: gradually warming SSTs, a moist low to middle troposphere, and low vertical shear. The NHC intensity forecast is for Karl to weaken slightly within the next day and then steadily strengthen starting in about two days onward. This is quite similar to the previous advisory and is based upon the IVCN multi-model consensus. However, confidence in Karl actually becoming a hurricane in five days is low because of the spread in the guidance. Karl is moving toward the west at 13 kt. The Bermuda-Azores high will be building in north of the tropical storm, helping to propel Karl on a general westward track for the next couple of days. Beginning in about three days, Karl will reach the western periphery of the Bermuda-Azores high and bend toward the west-northwest. The reliable track models have substantial spread - especially during days one to three - with some solutions showing a west-southwesterly track, which is rarely seen in this part of the Atlantic basin. The NHC track forecast is slightly south of the previous advisory, and is based upon the TVCN multi-model consensus. The initial size of Karl is determined by the recent scatterometer pass, which showed a large area of tropical-storm-force winds in the northern semicircle, but none to the south. The NHC wind radii forecast is based upon the RVCN consensus technique. FORECAST POSITIONS AND MAX WINDS INIT 17/0300Z 18.5N 37.0W 40 KT 45 MPH 12H 17/1200Z 18.5N 38.8W 35 KT 40 MPH 24H 18/0000Z 18.5N 41.5W 35 KT 40 MPH 36H 18/1200Z 18.5N 44.1W 35 KT 40 MPH 48H 19/0000Z 18.5N 46.8W 40 KT 45 MPH 72H 20/0000Z 20.0N 52.0W 50 KT 60 MPH 96H 21/0000Z 22.0N 57.0W 60 KT 70 MPH 120H 22/0000Z 25.0N 61.5W 70 KT 80 MPH $$ Forecaster Landsea

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Tropical Depression JULIA Forecast Discussion Number 13

2016-09-17 04:49:41| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT FRI SEP 16 2016 000 WTNT41 KNHC 170249 TCDAT1 TROPICAL DEPRESSION JULIA DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL AL112016 1100 PM EDT FRI SEP 16 2016 Deja vu all over again. Julia lost all its deep convection tonight and now consists of a very tight swirl of low clouds. Assuming that the cyclone is spinning down, the initial intensity has been lowered to 30 kt. I would not be surprised if another round of convection develops near the center despite the strong westerly shear, but the cyclone will likely become decapitated again by strong upper-level westerlies. Given that the strong shear will likely persist for the next 2 days or so, weakening is indicated in the NHC forecast. By the time the shear is expected to decrease, it is probably too late for the cyclone to recover, and in a couple of days, if not sooner, Julia is expected to be a remnant low. There has been no significant motion during the past day or so, and basically the cyclone has been meandering while it is trapped in very light steering currents. This pattern is not expected to change, and Julia or its remnants will likely continue milling around for the next 2 to 3 days until dissipation. FORECAST POSITIONS AND MAX WINDS INIT 17/0300Z 30.2N 76.3W 30 KT 35 MPH 12H 17/1200Z 30.2N 76.5W 30 KT 35 MPH 24H 18/0000Z 30.5N 77.0W 25 KT 30 MPH 36H 18/1200Z 31.0N 77.5W 25 KT 30 MPH 48H 19/0000Z 31.5N 77.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 20/0000Z 32.5N 76.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 21/0000Z...DISSIPATED $$ Forecaster Avila

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Tropical Storm KARL Forecast Discussion Number 10

2016-09-16 22:46:07| National Hurricane Center (Atlantic)

Issued at 500 PM AST FRI SEP 16 2016 000 WTNT42 KNHC 162046 TCDAT2 TROPICAL STORM KARL DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 500 PM AST FRI SEP 16 2016 Convection is quite strong in the northeastward quadrant of Karl, although the center has recently become exposed due to moderate-to-strong westerly shear. There has been little change to the overall satellite presentation in the past several hours, and the initial wind speed is kept at 40 kt. A continuation of the shear is forecast to keep Karl from strengthening much over the next day or two. This shear is forecast to relax by all of the models by early next week, and some strengthening should then take place while Karl moves over warmer waters. The amount of intensification is a big question mark, with the GFS/ECMWF now showing only a weaker tropical storm, while the rest of the standard intensity guidance (HWRF, GFDL, SHIPS and LGEM models) have Karl as a hurricane by the end of the period. Because of the uncertainty, the intensity forecast is kept the same as the previous one, not too far from the model consensus. The initial motion estimate is about the same as before, 280/12. Model guidance has made a large northward shift today due to the forecasted interaction of a mid/upper-level trough with Karl over the next few days. Initially, the tropical cyclone was expected to be on the western side of the trough, which would have helped steer the cyclone more to the west-southwest over the next couple of days. However, almost all of the guidance now show the trough staying to the west of Karl, which would impart a west or west-northwest motion during that time. The lack of a west-southwest track has large implications down the line, with the cyclone gaining a lot more latitude since Karl is closer to a weakness in the subtropical ridge. Continuity dictates that the NHC forecast not be shifted as much as the guidance suggests for this package, since the guidance could still shift back to the south. A large northward adjustment is made to the NHC forecast at all times, but it remains south of the model consensus and the ECMWF solutions. FORECAST POSITIONS AND MAX WINDS INIT 16/2100Z 18.8N 35.6W 40 KT 45 MPH 12H 17/0600Z 19.0N 37.3W 40 KT 45 MPH 24H 17/1800Z 19.0N 39.7W 40 KT 45 MPH 36H 18/0600Z 19.0N 42.5W 40 KT 45 MPH 48H 18/1800Z 19.0N 45.3W 45 KT 50 MPH 72H 19/1800Z 20.0N 50.5W 50 KT 60 MPH 96H 20/1800Z 22.0N 55.5W 60 KT 70 MPH 120H 21/1800Z 25.0N 60.5W 70 KT 80 MPH $$ Forecaster Blake

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