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Tropical Depression IDA Forecast Discussion Number 25

2015-09-24 16:48:38| National Hurricane Center (Atlantic)

Issued at 1100 AM AST THU SEP 24 2015 000 WTNT45 KNHC 241448 TCDAT5 TROPICAL DEPRESSION IDA DISCUSSION NUMBER 25 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST THU SEP 24 2015 There has been no significant change in the satellite presentation of the cyclone. Ida is a sheared system and consists of a broad circulation with a cyclonically curved convective band around the eastern side. Based on a recent ASCAT pass the initial intenisty has been lowered to 30 kt. The hostile shear environment will likely continue today, but both the operational SHIPS and the experimental SHIPS based on ECMWF fields lower the shear allowing for some restrengthening. On this basis, the NHC forecast calls for little change in intensity during the next day or two, and some minor increase of the winds after that time. Ida has begun to move toward the north-northeast at 4 kt. The cyclone will become completely detached from the mid-level trough in about 12 to 24 hours, and will be slowly steered toward the north and north-northwest by the flow surrounding a developing subtropical ridge. After that time, a stronger ridge will force Ida to turn westward with some increase in forward speed. Track guidance is very consistent with this westward turn, however, by then, the status of Ida is highly uncertain. Ida could still be a tropical cyclone as indicated in the NHC forecast or it will be degenerating into a trough as indicated by the GFS. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 20.2N 44.9W 30 KT 35 MPH 12H 25/0000Z 20.8N 44.7W 30 KT 35 MPH 24H 25/1200Z 21.5N 45.0W 30 KT 35 MPH 36H 26/0000Z 22.5N 45.5W 30 KT 35 MPH 48H 26/1200Z 23.5N 46.5W 35 KT 40 MPH 72H 27/1200Z 24.5N 48.0W 40 KT 45 MPH 96H 28/1200Z 24.5N 50.0W 40 KT 45 MPH 120H 29/1200Z 24.0N 52.0W 40 KT 45 MPH $$ Forecaster Avila

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Tropical Storm IDA Forecast Discussion Number 24

2015-09-24 10:43:36| National Hurricane Center (Atlantic)

Issued at 500 AM AST THU SEP 24 2015 000 WTNT45 KNHC 240843 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 24 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST THU SEP 24 2015 Ida's low-level center is exposed to the west of a relatively small cluster of deep convection due to 25 kt of west-northwesterly shear. By themselves, satellite intensity estimates would no longer support Ida's status as a tropical storm, but the partial ASCAT passes from last evening had just enough wind that I'm hesitant to downgrade the system to a tropical depression just yet. Vertical shear is expected to decrease only slightly during the next couple of days, and along with abundant mid-level dry air, the environment will not be particularly favorable for strengthening. The GFDL, which brings Ida to hurricane strength in 4 days, remains an outlier and is discounted as a reasonable solution. Conversely, the ECMWF and GFS global models show Ida weakening as a result of the unfavorable environment, with the GFS even showing the cyclone degenerating to a trough by day 4 or 5. Because of these more believable scenarios, the NHC intensity forecast is a little lower than the previous one at the end of the forecast period, and overall is very close to the Florida State Superensemble. Ida had been moving east-southeastward during the past 12 hours, but the current motion estimate is 090/5 kt. Ida remains located within the base of a mid-tropospheric trough, but it should become detached from the trough during the next 24 hours and turn northward as low- to mid-level ridging develops over the eastern Atlantic. Now that the global models have been trending toward a weaker system by days 3 through 5, they show Ida becoming blocked by a surface high over the north Atlantic and moving westward at a faster forward speed by the end of the forecast period. The updated NHC track forecast is a little faster than and southwest of the previous forecast at days 4 and 5 to account for the model trends, but it is otherwise very similar before those times. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 19.6N 45.5W 35 KT 40 MPH 12H 24/1800Z 20.2N 45.4W 35 KT 40 MPH 24H 25/0600Z 20.8N 45.4W 35 KT 40 MPH 36H 25/1800Z 21.6N 45.8W 35 KT 40 MPH 48H 26/0600Z 22.5N 46.5W 35 KT 40 MPH 72H 27/0600Z 23.8N 48.2W 40 KT 45 MPH 96H 28/0600Z 23.5N 50.0W 40 KT 45 MPH 120H 29/0600Z 23.0N 52.5W 40 KT 45 MPH $$ Forecaster Berg

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Tropical Storm IDA Forecast Discussion Number 23

2015-09-24 04:31:39| National Hurricane Center (Atlantic)

Issued at 1100 PM AST WED SEP 23 2015 000 WTNT45 KNHC 240231 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 23 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST WED SEP 23 2015 Ida is not a well-organized tropical storm. The center is on the western side of a small area of deep convection, with a few banding features noted in the southeastern quadrant of the cyclone. The initial intensity remains 35 kt since a recent partial ASCAT pass showed maximum winds of 30-35 kt. The storm continues to experience moderate-to-strong westerly shear. While the shear could relax somewhat over the next couple of days, very dry air is present in the mid- to upper-levels near and to the northwest of the cyclone, which could prevent Ida from fully taking advantage of the more favorable shear conditions for a few days. The latest NHC intensity forecast is close to the previous one and the intensity consensus. At longer range, some of the models are showing a resumption of the stronger shear, with only the GFDL forecasting significant intensification. Overall, the trend has been toward a less favorable environment at day 5, and the NHC intensity forecast is reduced at that time. Ida is moving eastward at about 4 kt while it remains embedded within the base of a mid- to upper-level trough. This trough is expected to lift out tomorrow with a ridge rebuilding over the central Atlantic. This pattern should cause the storm to move to the north by late Thursday, followed by a turn to the northwest on Friday around the ridge. After that time, another mid-latitude trough is expected to affect the cyclone. This feature, however, is not as deep as the last trough, and with the vertical representation of Ida being shallower in the model fields, much of the guidance is only show Ida stalling for a short amount of time rather than turning with the trough. Another ridge builds in by late weekend, which then forces Ida to move west-northwestward and westward by early next week. The guidance has shifted notably westward on this cycle in response to this pattern, and the NHC forecast is shifted that direction at long range, but is still pretty far east of the model consensus due to continuity concerns. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 19.9N 46.0W 35 KT 40 MPH 12H 24/1200Z 20.3N 45.5W 35 KT 40 MPH 24H 25/0000Z 21.0N 45.7W 35 KT 40 MPH 36H 25/1200Z 21.8N 46.2W 35 KT 40 MPH 48H 26/0000Z 22.7N 46.9W 35 KT 40 MPH 72H 27/0000Z 24.2N 48.4W 40 KT 45 MPH 96H 28/0000Z 24.5N 49.0W 45 KT 50 MPH 120H 29/0000Z 24.5N 50.5W 45 KT 50 MPH $$ Forecaster Blake

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Tropical Storm IDA Forecast Discussion Number 22

2015-09-23 22:33:04| National Hurricane Center (Atlantic)

Issued at 500 PM AST WED SEP 23 2015 000 WTNT45 KNHC 232032 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 22 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 PM AST WED SEP 23 2015 Ida's cloud pattern has not changed significantly during the day. The center of the cyclone is located to the west of a small area of deep convection, and the initial intensity is still estimated at 35 kt. Most of the global models indicate that a gradual relaxation of the shear should begin in a day or two, resulting in a little more favorable environment for Ida to re-strengthen. On this basis, the NHC forecast calls for a small increase in intensity beyond 48 hours. This is consistent with the previous forecast and the intensity consensus model. Ida has been drifting generally eastward embedded within the base of a mid- to upper-level trough. This trough is expected to lift out and keep the cyclone moving very slowly toward the northeast during the next 24 hours or so, while embedded within weak steering currents. After that time, Ida is forecast to turn to the north and north-northwest as a subtropical ridge gradually replaces the trough. The NHC forecast has changed very little from the previous one, and is very close to multi-model consensus. One interesting change is that the ECMWF and GFS models had been forecasting Ida to linger for a week or more over the Atlantic as a strong tropical cyclone. However, the most recent runs of both models now show a much weaker system. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 19.9N 46.6W 35 KT 40 MPH 12H 24/0600Z 19.9N 46.0W 35 KT 40 MPH 24H 24/1800Z 20.7N 45.8W 35 KT 40 MPH 36H 25/0600Z 21.5N 46.0W 35 KT 40 MPH 48H 25/1800Z 22.5N 46.7W 35 KT 40 MPH 72H 26/1800Z 24.0N 47.5W 40 KT 45 MPH 96H 27/1800Z 25.0N 48.0W 45 KT 50 MPH 120H 28/1800Z 25.5N 48.5W 50 KT 60 MPH $$ Forecaster Avila

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Tropical Storm IDA Forecast Discussion Number 21

2015-09-23 16:46:32| National Hurricane Center (Atlantic)

Issued at 1100 AM AST WED SEP 23 2015 000 WTNT45 KNHC 231446 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 21 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST WED SEP 23 2015 Ida's cloud pattern has deteriorated since yesterday, and the upper-level outflow is very disrupted due to shear. The center of the cyclone has been placed in the middle of a couple of low cloud swirls, and to the west of a small area of deep convection. Based on continuity, the initial intensity is kept at 35 kt, however, a recent partial ASCAT suggests that the winds could be lower. Both ECMWF and GFS SHIPS intensity models indicate that the shear has peaked, and a gradual relaxation should soon begin. However, it will take 36 to 48 hours for the shear to be low enough to favor re-strengthening. On this basis, only a small increase in intensity is forecast beyond that time. This is consistent with the previous forecast and the intensity consensus model. The cyclone, as anticipated, has become well embedded in the base of an upper-level trough and is drifting eastward or 090 degrees at 4 kt. Most of the global models lift the trough out and keep the cyclone moving very slowly within weak steering currents for the next day or two. After that time, the subtropical ridge is forecast to build over the Atlantic, and this flow pattern will force the cyclone to move toward the north and then to the north-northwest. For the next 3 days or so, the guidance is in good agreement showing a northward turn. The model spread increases after four days, but the general trend is to keep the cyclone moving slowly while is trapped south of the subtropical ridge. The NHC forecast is in the middle of the track envelope and is very similar to the previous one. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 20.0N 47.0W 35 KT 40 MPH 12H 24/0000Z 20.0N 46.4W 35 KT 40 MPH 24H 24/1200Z 20.5N 46.0W 35 KT 40 MPH 36H 25/0000Z 21.0N 46.0W 35 KT 40 MPH 48H 25/1200Z 22.0N 46.5W 35 KT 40 MPH 72H 26/1200Z 23.0N 47.5W 40 KT 45 MPH 96H 27/1200Z 25.0N 47.5W 45 KT 50 MPH 120H 28/1200Z 25.5N 48.0W 50 KT 60 MPH $$ Forecaster Avila

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