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Tropical Storm Arthur Forecast Discussion Number 5

2020-05-17 22:32:15| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Sun May 17 2020 000 WTNT41 KNHC 172032 TCDAT1 Tropical Storm Arthur Discussion Number 5 NWS National Hurricane Center Miami FL AL012020 500 PM EDT Sun May 17 2020 Arthur's satellite presentation has not changed much since the previous advisory. There are still some fragmented convective bands over the eastern portions of the circulation, but convective activity remains limited over the western half of the storm. A late-arriving ASCAT overpass from around the time of the previous advisory revealed somewhat lower wind speeds than reported by this morning's reconnaissance aircraft. This could be the result of the convection becoming more fragmented after the aircraft sampled that portion of the storm or related to the low bias of the ASCAT instrument. Regardless, the initial intensity remains a possibly generous 40 kt for now. Another reconnaissance aircraft is scheduled to investigate Arthur this evening. Arthur is forecast to remain within a low wind shear environment and over marginally warm sea surface temperatures through early Monday. These conditions favor some strengthening, however the sprawling structure of the cyclone and nearby dry mid-level air are likely to temper any increase in wind speed. After 36-48 hours, baroclinic forcing is expected to help the post-tropical cyclone maintain its intensity. Later in the period, the frontal gradients decrease which should cause weakening. Recent satellite fixes show that the Arthur is still moving north-northeastward at about 8 kt. The tropical storm should begin to accelerate northeastward overnight as a mid-level trough moves into the eastern United States. By Tuesday night, the steering flow is expected to become westerly which should cause Arthur to turn eastward, then southeastward later in the forecast period. The lastest dynamical track guidance has come into a little better agreement through 36-48 hours with the GFS and ECMWF converging on the previous NHC track. As a result, little change was needed to the first couple of days of the earlier NHC track forecast. After that time, most of the guidance has trended toward a faster eastward and east-southeastward motion, and the NHC forecast has been adjusted accordingly in that direction as well. Key Messages: 1. A Tropical Storm Warning is in effect for a portion of the North Carolina coast. Tropical-storm-force winds and heavy rains are expected there on Monday. 2. Dangerous coastal surf conditions and rip currents are expected to spread northward along the southeast U.S. coast to the mid-Atlantic states during the next couple of days. See products from your local National Weather Service Forecast Office for more details. FORECAST POSITIONS AND MAX WINDS INIT 17/2100Z 31.5N 77.2W 40 KT 45 MPH 12H 18/0600Z 33.1N 76.4W 40 KT 45 MPH 24H 18/1800Z 35.3N 74.6W 45 KT 50 MPH 36H 19/0600Z 36.6N 71.9W 50 KT 60 MPH 48H 19/1800Z 36.8N 69.2W 50 KT 60 MPH...POST-TROP/EXTRATROP 60H 20/0600Z 36.3N 67.5W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 20/1800Z 35.6N 66.3W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 21/1800Z 34.0N 64.0W 35 KT 40 MPH...POST-TROP/EXTRATROP 120H 22/1800Z...DISSIPATED $$ Forecaster Brown

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Tropical Storm Arthur Forecast Discussion Number 4

2020-05-17 16:45:07| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Sun May 17 2020 000 WTNT41 KNHC 171445 TCDAT1 Tropical Storm Arthur Discussion Number 4 NWS National Hurricane Center Miami FL AL012020 1100 AM EDT Sun May 17 2020 Visible satellite imagery reveals a couple of convective bands over the eastern semicircle of the tropical storm, but convection is sparse over the western portion of the circulation. The latest reports from an Air Force reconnaissance aircraft show peak 925-mb flight-level winds of 48 kt, and believable SFMR winds of 35-40 kt. On this basis, the initial wind speed has been increased to 40 kt. The plane has reported a minimum pressure of 1002-1003 mb, which is down a few millibars from the previous flight. Arthur has a little more than 24 hours in which to gradually strengthen. The storm will be traversing the relatively warm waters of the Gulf stream, and the vertical shear is forecast to remain low through early Monday. After that time, increasing shear and cooler sea surface temperatures should put a halt to the tropical cyclone strengthening processes. Baroclinic forcing is likely to help the post-tropical cyclone maintain its strengthen through extratropical transition. Later in the forecast period, the global model guidance shows weakening as the frontal gradients decrease. The storm is moving north-northeastward or 015/8 kt. Arthur is expected to begin moving a little faster later today and tonight as a mid-level trough approaches the eastern United States. Later in the forecast period, the cyclone should turn eastward within the westerly steering flow. Although the dynamical models are in agreement on the overall scenario, there remains some spread as to how close the center of Arthur will track to the North Carolina Outer Bands. The GFS and HWRF remain along the western side of the guidance while the ECMWF and UKMET bracket the eastern side. The NHC track lies near the model consensus and little change was required to the previous track through 36-48 hours. After that time, the track guidance spread increases with the ECMWF showing a much faster east-southeastward motion than the GFS. The NHC forecast remains near the consensus after 48 hours, but there is less confidence in that portion of the track prediction. Key Messages: 1. A Tropical Storm Warning is in effect for a portion of the North Carolina coast. Tropical-storm-force winds and heavy rains are expected there on Monday. 2. Dangerous coastal surf conditions and rip currents are expected to spread northward along the southeast U.S. coast to the mid-Atlantic states during the next couple of days. See products from your local National Weather Service Forecast Office for more details. FORECAST POSITIONS AND MAX WINDS INIT 17/1500Z 30.5N 77.4W 40 KT 45 MPH 12H 18/0000Z 32.0N 76.8W 45 KT 50 MPH 24H 18/1200Z 34.3N 75.5W 50 KT 60 MPH 36H 19/0000Z 36.4N 73.2W 50 KT 60 MPH 48H 19/1200Z 37.3N 70.4W 50 KT 60 MPH...POST-TROP/EXTRATROP 60H 20/0000Z 37.1N 68.2W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 20/1200Z 36.5N 66.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 21/1200Z 35.5N 64.5W 35 KT 40 MPH...POST-TROP/EXTRATROP 120H 22/1200Z...DISSIPATED $$ Forecaster Brown

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Tropical Storm Arthur Forecast Discussion Number 3

2020-05-17 10:48:37| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Sun May 17 2020 000 WTNT41 KNHC 170848 TCDAT1 Tropical Storm Arthur Discussion Number 3 NWS National Hurricane Center Miami FL AL012020 500 AM EDT Sun May 17 2020 Arthur has not changed much in organization overnight. The storm features a large curved convective band over the eastern semicircle of the circulation, and the low-level center appears to be exposed to the west of that band. The intensity is held at 35 kt in agreement with a subjective Dvorak estimate from TAFB and ADT estimates from UW-CIMSS. An Air Force Hurricane Hunter aircraft is scheduled to investigate the cyclone around 12Z which should provide a good estimate of Arthur's intensity. Although Arthur will be crossing back over the Gulf Stream tonight and Monday, vertical shear is forecast to steadily increase during that time. With these counteracting influences on intensity change, only slight strengthening is anticipated during the next 24-36 hours. The official forecast is close to the model consensus and is similar to the previous one. In 2-3 days, some strengthening due to baroclinic processes is possible, since the global models depict extratropical transition around that time. Later in the forecast period, the dynamical guidance shows weakening as the frontal gradients decrease. The storm has been moving a little to the left of the previous estimates and the motion is now around 015/8 kt. An approaching mid-level trough over the eastern U.S. should cause the cyclone to accelerate northeastward during the next 48 hours or so. Later in the forecast period, Arthur or its post-tropical remnants should turn eastward within a general westerly flow field. There has been a bit of a westward shift in much of the track guidance for the next 1-2 days, which has necessitated a leftward adjustment of the official forecast track. Since the new NHC forecast is now closer to the North Carolina Outer Banks, the Tropical Storm Watch has been changed to a Tropical Storm Warning for that area. Key Messages: 1. A Tropical Storm Warning is now in effect for a portion of the North Carolina coast. Tropical-storm-force winds and heavy rains are expected there on Monday. 2. Dangerous coastal surf conditions and rip currents are expected to spread northward from Florida to the mid-Atlantic states during the next few days. See products from your local National Weather Service Forecast Office for more details. FORECAST POSITIONS AND MAX WINDS INIT 17/0900Z 30.0N 77.6W 35 KT 40 MPH 12H 17/1800Z 31.2N 77.2W 40 KT 45 MPH 24H 18/0600Z 33.2N 76.4W 45 KT 50 MPH 36H 18/1800Z 35.5N 74.7W 45 KT 50 MPH 48H 19/0600Z 36.9N 72.0W 50 KT 60 MPH 60H 19/1800Z 37.3N 69.4W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 20/0600Z 37.0N 67.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 21/0600Z 36.0N 66.0W 40 KT 45 MPH...POST-TROP/EXTRATROP 120H 22/0600Z...DISSIPATED $$ Forecaster Pasch

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Tropical Storm Arthur Forecast Discussion Number 2

2020-05-17 04:53:55| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Sat May 16 2020 127 WTNT41 KNHC 170253 TCDAT1 Tropical Storm Arthur Discussion Number 2 NWS National Hurricane Center Miami FL AL012020 1100 PM EDT Sat May 16 2020 An Air Force Reserve reconnaissance aircraft that has been investigating the tropical cyclone east of Florida has recently found maximum 925-mb flight-level winds of 45-46 kt to the southeast and east of the center, which equates to surface winds of 34-35 kt, along with uncontaminated SFMR surface wind speeds of 33-36 kt. Based on these data, the depression has been upgraded to Tropical Storm Arthur, the first named storm of the 2020 Atlantic hurricane season. The initial motion estimate remains north-northeastward or 030/11 kt. Arthur made a slight northeastward jog earlier this evening, but now appears to have returned to its previous base course. A motion toward the north-northeast is forecast to continue for the next 24 hours or so, keeping the cyclone well offshore the coasts of Florida and Georgia. A sharp shortwave trough currently moving across the southern Plains is forecast to dig east-southeast to southeastward over the next 48 hours, which will act to accelerate and eject Arthur more poleward. The more the shortwave trough digs and loses latitude, the more Arthur could get pulled closer to the North Carolina Outer Banks as per the GFS and HWRF scenarios. In contrast, the ECMWF and UKMET models show the shortwave losing amplitude quickly and lifting out, which acts to push Arthur farther away from the United States east coast. For now, the new NHC forecast track closely follows the various consensus models, which are about midway between the GFS-HWRF and ECMWF-UKMET solutions. However, the track was adjusted slightly to the right of the previous advisory track due mainly to the more eastward initial position. It should be noted that forecast track uncertainty is typically larger for weak systems like Arthur. Arthur has moved off of the warm waters of the Gulfstream current and currently is passing over a cold pool with SSTs near 24.5 deg C. These cooler waters should prevent any significant strengthening in the very near term. By 24 hours however, the cyclone is forecast to pass back over the warmer waters of the Gulfstream while moving into a very low vertical wind shear regime. These conditions, coupled with some cooler air aloft, should allow more vigorous convection to develop near the center, resulting in more strengthening as Arthur passes near the North Carolina coast. Extratropical transition should occur in about 48-60 hours over the much cooler waters of the North Atlantic. The NHC intensity forecast follows a blend of the consensus models HCCA and IVCN, and is similar to the previous intensity forecast. Key Messages: 1. A tropical storm watch is in effect for a portion of the North Carolina coast. Tropical-storm-force winds and heavy rains are possible there on Monday. 2. Dangerous coastal surf conditions and rip currents are expected to spread northward from Florida to the mid-Atlantic states during the next few days. See products from your local National Weather Service Forecast Office for more details. FORECAST POSITIONS AND MAX WINDS INIT 17/0300Z 29.4N 77.7W 35 KT 40 MPH 12H 17/1200Z 30.5N 77.2W 35 KT 40 MPH 24H 18/0000Z 32.2N 76.4W 40 KT 45 MPH 36H 18/1200Z 34.1N 75.2W 45 KT 50 MPH 48H 19/0000Z 36.2N 73.0W 45 KT 50 MPH 60H 19/1200Z 37.5N 70.6W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 20/0000Z 38.0N 69.1W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 21/0000Z 38.0N 67.6W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 22/0000Z...DISSIPATED $$ Forecaster Stewart

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Tropical Depression One Forecast Discussion Number 1

2020-05-16 23:20:57| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Sat May 16 2020 000 WTNT41 KNHC 162120 CCA TCDAT1 Tropical Depression One Discussion Number 1...Corrected NWS National Hurricane Center Miami FL AL012020 500 PM EDT Sat May 16 2020 Corrected Key Message number 2 Satellite images, Doppler radar data, and surface observations indicate that the low pressure system located just east of the coast of central Florida has developed sufficient organization to now be classified as a tropical depression. The low-level center is well defined and deep convection has persisted near the center and in bands on the east side of the circulation for the past several hours. The cyclone is considered tropical instead of subtropical because it has central deep convection and a relatively small radius of maximum wind. The initial intensity is estimated to be 30 kt based on data collected earlier today by the Air Force Hurricane Hunters. Another aircraft is scheduled to investigate the system this evening. Based on satellite data and the earlier reconnaissance fixes, the initial motion of the system is north-northeastward at 11 kt. A continued north-northeastward motion at about the same forward speed is expected during the next 24 hours or so, keeping the cyclone offshore, but parallel to, the east coast of Florida. After that time, the model solutions begin to diverge and the details of the track forecast become more complicated. The important features for the future track of the depression appear to be a pair of mid- to upper-level troughs currently over the central U.S. The specific amplitude, timing, and location of those troughs will ultimately result in how close to the coast of North Carolina this system gets. The GFS and HWRF models are on the left side of the guidance and bring the system inland or along the coast, while the ECMWF and UKMET models show an offshore track. The NHC track forecast splits the difference of these solutions, and lies close to the various consensus models, which usually perform best. It should be noted that forecast uncertainty is typically larger for weak systems like this one. The system should gradually strengthen during the next couple of days as it remains over the Gulf Stream current and in relatively low wind shear conditions. However, the surrounding air mass is not particularly moist, so that and the marginally warm SSTs should limit significant intensification. The NHC intensity forecast calls for the system to become a tropical storm tonight or on Sunday with continued slow strengthening as it nears the North Carolina coast. Extratropical transition should occur in about 3 days, or sooner. The NHC intensity forecast generally lies roughly near the middle of the guidance envelope. Key Messages: 1. A tropical storm watch has been issued for a portion of the North Carolina coast. Tropical-storm-force winds and heavy rains are possible there on Monday. 2. Dangerous coastal surf conditions and rip currents are expected to spread northward from Florida to the mid-Atlantic states during the next few days. See products from your local National Weather Service forecast office for more details. FORECAST POSITIONS AND MAX WINDS INIT 16/2100Z 28.4N 78.6W 30 KT 35 MPH 12H 17/0600Z 29.7N 78.0W 35 KT 40 MPH 24H 17/1800Z 31.4N 77.4W 35 KT 40 MPH 36H 18/0600Z 33.0N 76.6W 40 KT 45 MPH 48H 18/1800Z 35.2N 74.8W 45 KT 50 MPH 60H 19/0600Z 37.1N 72.3W 50 KT 60 MPH 72H 19/1800Z 38.0N 70.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 20/1800Z 38.0N 68.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 21/1800Z...DISSIPATED $$ Forecaster Cangialosi

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