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Tropical Storm Sergio Forecast Discussion Number 8

2018-10-01 10:54:10| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Mon Oct 01 2018 428 WTPZ41 KNHC 010854 TCDEP1 Tropical Storm Sergio Discussion Number 8 NWS National Hurricane Center Miami FL EP212018 300 AM MDT Mon Oct 01 2018 Sergio's convective pattern in infrared satellite imagery has waxed and waned since the previous advisory. A previously large, solid curved band has become broken during the past couple of hours, but the comma head or CDO of the cloud band has increased in size while cloud tops have cooled to near -80C. Passive microwave imagery indicates that within the comma head feature an 18-20-nmi-wide closed eye has developed in the low-levels, while the mid- and upper-level eye remains open and ragged. Subjective intensity estimates remain T3.5/55 kt from TAFB and SAB. However, based on the well-defined, low-level eye, the intensity has been nudged upward to 60 kt for this advisory. The initial motion is now 270/12 kt, based primarily on microwave satellite fix positions. Sergio is forecast to move generally westward for the next 36 hours or so, under the influence of a strong, deep-layer ridge located to the north of the cyclone. By 48 hours, a mid- to upper-level trough moving southeastward along and offshore of the western U.S. is expected to gradually erode the western part of the ridge, allowing Sergio turn west-northwestward on day 2 and move northwestward on days 3-5. The new NHC track forecast is similar to the previous advisory track, and lies near a blend of the consensus models TCVA, TVCE, and HCCA. Both the statistical and dynamical intensity models have been too bullish on Sergio strengthening during the past 36 hours. Although the environment is quite moist with near 80 percent mid-level humidity values, a pronounced dry intrusion eroding the inner-core convection remains. By the time the dry air mixes out, modest northerly vertical wind shear is expected to hinder development in the 24-48 hour period. By 72 hours and beyond, the shear is forecast to decrease below 10 kt, but Sergio will be moving over cooler waters at that time. Given the mixed environmental signals, the official intensity is on the conservative side and remains well below the stronger HCCA and FSSE models, and closer to the simple consensus model IVCN. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 11.5N 108.6W 60 KT 70 MPH 12H 01/1800Z 11.4N 110.3W 70 KT 80 MPH 24H 02/0600Z 11.5N 112.5W 85 KT 100 MPH 36H 02/1800Z 12.0N 114.5W 95 KT 110 MPH 48H 03/0600Z 12.8N 116.1W 100 KT 115 MPH 72H 04/0600Z 14.9N 118.8W 100 KT 115 MPH 96H 05/0600Z 16.8N 121.2W 95 KT 110 MPH 120H 06/0600Z 17.9N 123.2W 90 KT 105 MPH $$ Forecaster Stewart

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Tropical Storm Leslie Forecast Discussion Number 20

2018-10-01 10:53:37| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Oct 01 2018 435 WTNT43 KNHC 010853 TCDAT3 Tropical Storm Leslie Discussion Number 20 NWS National Hurricane Center Miami FL AL132018 500 AM AST Mon Oct 01 2018 Leslie has become a little better organized overnight, with an increase in convective banding over the northern and northwestern portions of the circulation. ASCAT data around 0000 UTC revealed 40-45 kt winds over the northeastern quadrant of the cyclone and with the increase in organization since that time, the initial intensity has been raised to 50 kt. This is a little above the consensus Dvorak intensity estimates of T3.0 (45 kt) from TAFB and SAB. The shear that was affecting Leslie appears to have abated somewhat and with the system forecast to move southwestward toward slightly warmer waters, gradual strengthening is expected during the next couple of days. Later in the period, Leslie is forecast to move north-northeastward back over its previous track where cooler upwelled waters are likely to produce some weakening. The updated NHC intensity forecast shows a slightly higher peak intensity in 48 to 72 hours, but remains between the dynamical model guidance and the higher statistical models. Leslie continues to plod along with an initial motion estimate of 240/4 kt. The cyclone should move slowly southwestward to southward during the next couple of days as it remains within an area of weak steering currents. After that time, a shortwave trough to the northwest of Leslie is expected to allow the tropical cyclone to lift slowly north-northeastward by days 4 and 5. Although the dynamical model guidance is in agreement on this overall scenario, there is increasing cross-track (east-west) spread after 72 hours. The latest run of the ECMWF has shifted westward and brackets the western edge of the guidance envelope. The new NHC track forecast has been shifted in that direction, but it is not as far left as the various consensus aids out of respect of the previous track forecast. Regardless of the details of the forecast track, Leslie is still expected to meander over the central Atlantic through the remainder of this week. Large swells generated by Leslie when it was a strong extratropical low will continue to affect portions of the southeastern coast of the United States, Bermuda, the Bahamas, and most of the Greater and Lesser Antilles for another day or two. These swells could cause life-threatening surf and rip currents. Although the swells will diminish some by mid-week, they will likely remain hazardous for the same locations through the forecast period due to Leslie's slow motion. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 33.4N 53.9W 50 KT 60 MPH 12H 01/1800Z 33.1N 54.5W 55 KT 65 MPH 24H 02/0600Z 32.2N 55.3W 60 KT 70 MPH 36H 02/1800Z 31.2N 55.8W 65 KT 75 MPH 48H 03/0600Z 30.6N 56.0W 70 KT 80 MPH 72H 04/0600Z 31.7N 55.2W 70 KT 80 MPH 96H 05/0600Z 35.0N 54.3W 65 KT 75 MPH 120H 06/0600Z 37.4N 52.0W 60 KT 70 MPH $$ Forecaster Brown

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Tropical Storm Rosa Forecast Discussion Number 25

2018-10-01 10:39:10| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Mon Oct 01 2018 431 WTPZ45 KNHC 010839 TCDEP5 Tropical Storm Rosa Discussion Number 25 NWS National Hurricane Center Miami FL EP202018 200 AM PDT Mon Oct 01 2018 Convection has been steadily decreasing due to the combination of southwesterly shear of about 30 kt and cool SSTs of less than 24 deg C. As a result, only a small patch of thunderstorms remains in the northeastern quadrant of Rosa's circulation. ASCAT passes around 0414Z and 0516Z only indicated wind speeds of 41-42 kt in the eastern semicircle. Assuming that some undersampling is occurring, the intensity has only been decreased to 45 kt for this advisory. Additional weakening is expected due to a further increase in the vertical wind shear and decreasing ocean temperatures beneath the cyclone. The official intensity forecast calls for Rosa to be near 35 kt at the time of landfall later today, but it is possible that more weakening could occur than currently indicated. Regardless, strong tropical-storm-force wind gusts will be widespread across the higher elevations of the northern Baja California peninsula. Rosa is expected to become remnant low before the cyclone reaches Arizona due to the rugged terrain of Baja and northwestern Mexico, and the continued strong wind shear. Rosa is moving northeastward now, or 035/10 kt. A northeastward to north-northeastward motion is expected until and after landfall, with some acceleration beginning later tonight. The latest NHC model guidance remains tightly clustered, and the official forecast track lies close to the previous track and an average of the TVCE, HCCA, and FSSE multi-model consensus aids. Key Messages: 1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office. 2. Tropical storm conditions are expected over portions of the central and northern Baja California peninsula later today, especially over higher elevations. These conditions could spread into the northern Gulf of California later tonight. Interests in those locations should monitor the progress of Rosa. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 26.7N 117.0W 45 KT 50 MPH 12H 01/1800Z 28.0N 116.1W 35 KT 40 MPH 24H 02/0600Z 30.1N 114.8W 30 KT 35 MPH...INLAND NRN BAJA CALIF 36H 02/1800Z 32.9N 113.5W 20 KT 25 MPH...POST-TROP/INLAND 48H 03/0600Z 35.9N 112.2W 15 KT 15 MPH...POST-TROP/INLAND 72H 04/0600Z...DISSIPATED INLAND $$ Forecaster Stewart

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Tropical Storm Leslie Forecast Discussion Number 19

2018-10-01 04:52:38| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sun Sep 30 2018 471 WTNT43 KNHC 010252 TCDAT3 Tropical Storm Leslie Discussion Number 19 NWS National Hurricane Center Miami FL AL132018 1100 PM AST Sun Sep 30 2018 Leslie has changed little in organization over the past few hours, with the convective burst previously in the southeastern quadrant now in the northeastern quadrant. Recent scatterometer data indicates that the current intensity is 45 kt, with those winds occurring in the northeastern semicircle. The scatterometer data also suggests that the radii of tropical-storm force winds has decreased a little. While Leslie continues to be affected by westerly shear, the global models show the upper-level pattern becoming more favorable over Leslie in about a day. This, combined with slightly warmer SSTs along the forecast track should allow Leslie to gradually strengthen during the next 2 to 3 days. After that time, the storm is expected to move northeastward back over its previous track and cooler waters partly caused by its own upwelling, and that should cause at least a little weakening. The new intensity forecast is unchanged from the previous forecast, and it lies above the forecast of the regional hurricane models and below the forecast of the statistical-dynamical models. The initial motion is 250/5. Leslie is forecast to remain in weak steering currents for the next 72 h, with a southwestward to southward drift expected. Thereafter, a shortwave trough moving southward to the west of Leslie should cause a motion toward the north-northeast or northeast and an increase in forward speed. There are no important changes to the guidance or to the forecast track, and the bottom line remains that Leslie is forecast to meander over the central Atlantic for the next several days. Large swells generated by Leslie when it was a strong extratropical low will continue to affect the east coast of the United States and most of the Greater and Lesser Antilles for another day or two. These swells could cause life-threatening surf and rip currents. Although the swells will diminish some by mid-week, they will likely remain hazardous for the same locations through the forecast period due to Leslie's slow motion. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 33.3N 53.7W 45 KT 50 MPH 12H 01/1200Z 33.0N 54.2W 50 KT 60 MPH 24H 02/0000Z 32.4N 54.9W 55 KT 65 MPH 36H 02/1200Z 31.5N 55.5W 60 KT 70 MPH 48H 03/0000Z 30.8N 55.6W 65 KT 75 MPH 72H 04/0000Z 31.0N 55.0W 65 KT 75 MPH 96H 05/0000Z 34.0N 54.0W 60 KT 70 MPH 120H 06/0000Z 37.0N 52.5W 60 KT 70 MPH $$ Forecaster Beven

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Tropical Storm Sergio Forecast Discussion Number 7

2018-10-01 04:37:05| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Sun Sep 30 2018 903 WTPZ41 KNHC 010237 TCDEP1 Tropical Storm Sergio Discussion Number 7 NWS National Hurricane Center Miami FL EP212018 900 PM MDT Sun Sep 30 2018 Overall, Sergio has changed little in organization over the past several hours. The convective pattern is comprised of a ragged band wrapping about three-quarters of the way around the center, with the strongest convection to the north of the center. Recent microwave data show that the mid-level circulation center seen earlier has lost some definition. However, the low-level circulation center seems to be becoming better defined. The initial intensity remains 55 kt based on unchanged satellite intensity estimates. The initial motion remains 260/11. For the next 36 h or so, Sergio should move westward or just south of due westward around the southern periphery of a large deep-layer subtropical ridge extending from Mexico westward across the eastern Pacific. After that time, a mid- to upper-level trough moving southeastward along and offshore of the western U.S. will erode the western part of the ridge. This development will allow Sergio turn west-northwestward to northwestward with a gradual decrease in forward speed. The new forecast track is near the various consensus models and is an update of the previous track. Conditions appear favorable for Sergio to rapidly intensify in the next 36 h if the cyclone can become well enough organized to take advantage of the environment. This part of the intensity forecast presumes this will happen and is unchanged from the previous forecast of quick strengthening. From 36-48 h, a burst of northeasterly shear is expected to affect Sergio, which should at least slow intensification. Based on this and the shear letting up at 72 h, the intensity forecast is tweaked to show the peak intensity after the shear decreases. After 72 h, decreasing sea surface temperatures along the forecast track should cause a gradual weakening. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 11.6N 107.3W 55 KT 65 MPH 12H 01/1200Z 11.4N 109.0W 65 KT 75 MPH 24H 02/0000Z 11.3N 111.3W 80 KT 90 MPH 36H 02/1200Z 11.6N 113.4W 95 KT 110 MPH 48H 03/0000Z 12.4N 115.2W 100 KT 115 MPH 72H 04/0000Z 14.5N 118.0W 105 KT 120 MPH 96H 05/0000Z 16.5N 120.5W 100 KT 115 MPH 120H 06/0000Z 18.0N 122.5W 90 KT 105 MPH $$ Forecaster Beven

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