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Post-Tropical Cyclone Sam Forecast Discussion Number 51
2021-10-05 10:38:47| National Hurricane Center (Atlantic)
Issued at 900 AM GMT Tue Oct 05 2021 780 WTNT43 KNHC 050838 TCDAT3 Post-Tropical Cyclone Sam Discussion Number 51 NWS National Hurricane Center Miami FL AL182021 900 AM GMT Tue Oct 05 2021 Sam's low-level circulation center has become exposed on the south side of an east-to-west oriented cloud band containing cloud tops that have warmed to around -50 deg C. The cyclone's overall cloud pattern in satellite imagery resembles that of an occluded extratropical low pressure system, and surface analyses from the NOAA Ocean Prediction Center indicate that Sam's inner-core is now interacting with a frontal system. Furthermore, the system is located over sea-surface temperatures of around 14 deg C. Although the system likely still has a warm-core thermal structure in the mid- and upper-levels of the troposphere based on earlier 04/2321Z AMSU-A/-B data, the low-level circulation now is comprised of an extensive field of cold-air stratocumulus clouds. Based on these data, Sam has made the transition to an extratropical cyclone over the far north Atlantic. The initial wind speed has been lowered to 70 kt based on a standard decay rate. The initial motion estimate is northward or 010/20 kt. Extratropical-Sam has been moving north-northeastward at 35 kt since the previous advisory, but the latest global and regional models are all in very good agreement that the system will slow down significantly during the next 6-12 hours as it interacts with an approaching baroclinic mid- to upper-level trough/low. By late Tuesday, the post-tropical cyclone should turn east-northeastward, and then make a counter-clockwise loop southwest of Iceland on Wednesday. By Thursday, the large cyclone is expected to move east-northeastward or eastward and pass very near the southern coast of Iceland on days 4 and 5 as a weakening extratropical low pressure system. The new NHC track forecast is very similar to the previous advisory track, and lies close to the tightly packed simple- and corrected-consensus track models. Extratropical-Sam is expected to maintain hurricane-force winds for the next 12-18 hours, followed by a slow weakening trend as the cyclone loses its baroclinic forcing. However, Post-Tropical-Sam is still expected to pass close to Iceland in the 96-120-h period as an expansive low pressure system producing gale-force winds over a very large area. This is the last advisory issued by the National Hurricane Center on Sam. Additional information on this system can be found in High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFAT1, WMO header FZNT01 KWBC, and online at ocean.weather.gov/shtml/NFDHSFAT1.php. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, portions of the Bahamas, and southeastern Newfoundland through midweek. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 51.0N 39.4W 70 KT 80 MPH...POST-TROP/EXTRATROP 12H 05/1800Z 52.1N 38.9W 65 KT 75 MPH...POST-TROP/EXTRATROP 24H 06/0600Z 52.8N 35.1W 60 KT 70 MPH...POST-TROP/EXTRATROP 36H 06/1800Z 54.3N 30.0W 55 KT 65 MPH...POST-TROP/EXTRATROP 48H 07/0600Z 57.9N 24.6W 50 KT 60 MPH...POST-TROP/EXTRATROP 60H 07/1800Z 61.8N 23.9W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 08/0600Z 62.7N 27.4W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 09/0600Z 61.7N 26.2W 40 KT 45 MPH...POST-TROP/EXTRATROP 120H 10/0600Z 63.6N 15.0W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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Hurricane Sam Forecast Discussion Number 48
2021-10-04 16:44:02| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Mon Oct 04 2021 000 WTNT43 KNHC 041443 TCDAT3 Hurricane Sam Discussion Number 48 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Mon Oct 04 2021 Sam remains a formidable high-latitude hurricane this morning. An earlier 1102 UTC GMI microwave overpass continued to depict concentric eyewalls with a fairly well-defined inner-eye that was open over the southwestern quadrant. Although there has been a generally warming of the cloud tops in infrared satellite imagery, the eye remains apparent and the system is still fairly symmetric. A blend of the subjective Dvorak T- and CI-numbers from both SAB and TAFB yield an intensity of 85 kt, which is used as the initial intensity for this advisory. Now that Sam has moved north of the north wall of the Gulf Stream it will be moving over progressively colder waters and into much higher vertical wind shear conditions by this evening, which should cause some gradual weakening. However, the interaction of the cyclone with a strong upper-level trough is likely to lead to the rapid transition of Sam into a powerful extratropical cyclone later this evening. The post-tropical cyclone is expected to maintain hurricane-force winds until late Tuesday. Gradual weakening should occur after that time as the baroclinic forcing decreases, and the system is forecast to spin down slowly over the north Atlantic later this week. Sam continues to accelerate northeastward and is now moving at 045/28 kt. A faster northeastward motion is forecast through tonight as Sam moves within the strong southwesterly flow ahead of a mid-latitude trough. The post-tropical cyclone is forecast to slow down on Tuesday as it interacts with the trough/cut-off low. After that time, the system is forecast to turn east-northeastward, and then rotate cyclonically around the eastern flank of yet another cut-off low later in the week. The dynamical model guidance remains in generally good agreement on the overall track forecast scenario, albeit for some forward speed variations. The latest official forecast is similar to the previous advisory and lies near the TVCA multi-model consensus in deference to the aforementioned along-track differences. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, portions of the Bahamas, and southeastern Newfoundland for the next day or so. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 42.6N 45.7W 85 KT 100 MPH 12H 05/0000Z 46.5N 41.9W 75 KT 85 MPH 24H 05/1200Z 50.0N 40.5W 70 KT 80 MPH...POST-TROP/EXTRATROP 36H 06/0000Z 50.3N 38.3W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 06/1200Z 51.4N 33.2W 55 KT 65 MPH...POST-TROP/EXTRATROP 60H 07/0000Z 54.3N 27.7W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 07/1200Z 58.3N 24.8W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 08/1200Z 61.0N 29.7W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 09/1200Z 60.0N 29.8W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Brown
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Tropical Depression Victor Forecast Discussion Number 20
2021-10-04 10:56:24| National Hurricane Center (Atlantic)
Issued at 500 AM AST Mon Oct 04 2021 000 WTNT45 KNHC 040856 TCDAT5 Tropical Depression Victor Discussion Number 20 NWS National Hurricane Center Miami FL AL202021 500 AM AST Mon Oct 04 2021 Strong southerly vertical wind shear in excess of 25 kt along with entrainment of very dry mid-level air (less than 40 percent relative humidity) has continued to take its toll on Victor. Deep convection has been weakening during the past couple of hours and is now displaced more than 100 nmi north and northeast of the elongated low-level center. The shear is forecast by the global models to increase to more than 30 kt in the next 6-12 hours, while the surrounding environment becomes drier and more stable. As a result, the cyclone is forecast to degenerate into a remnant low later today and likely dissipate by Tuesday, if not sooner. Victor is moving west-northwestward or 300/12 kt, and this general motion is expected to continue through today based on the latest NHC model guidance. As a result, no significant changes were made to the previous track forecast. FORECAST POSITIONS AND MAX WINDS INIT 04/0900Z 18.1N 43.6W 25 KT 30 MPH 12H 04/1800Z 19.2N 45.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 05/0600Z...DISSIPATED $$ Forecaster Stewart
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Hurricane Sam Forecast Discussion Number 47
2021-10-04 10:42:50| National Hurricane Center (Atlantic)
Issued at 500 AM AST Mon Oct 04 2021 000 WTNT43 KNHC 040842 TCDAT3 Hurricane Sam Discussion Number 47 NWS National Hurricane Center Miami FL AL182021 500 AM AST Mon Oct 04 2021 A 0255Z GPM microwave pass indicated that Sam had continued to go through an eyewall replacement cycle, consisting of a 20-nmi-wide inner eye and an 85-nmi-wide outer eye. The inner eye was rapidly eroding on the northwest side compared to microwave data from just a few hours earlier. Additionally, late-arriving ASCAT surface wind data from 0000-0100Z indicated that the inner-core wind field had contracted or weakened, while the outer wind field (34-kt radii) had continued to expand in all quadrants. Satellite intensity estimates have decreased to around 80 kt. However, the advisory intensity has been maintained at 90 kt based on the 10-kt increase in forward speed, which should offset any weakening of the tangential winds. Gradual weakening is expected now that Sam has passed north of the warm Gulf Stream and is moving over sub-25-deg-C sea-surface temperatures (SST), with much colder SSTs lying ahead of the hurricane. Vertical wind shear is forecast to increase to more than 30 kt by 18 h, which should cause further weakening. By 24 h and beyond, interaction with a strong upper-level trough/low should cause Sam to transition into a powerful extratropical cyclone by early Tuesday to the east of Newfoundland. This baroclinic interaction is expected to cause extratropical-Sam to maintain hurricane-force winds until early Wednesday. Slow weakening is forecast thereafter as the very large cyclone slowly spins down as an occluded low pressure system. No significant changes were made to the previous intensity forecast. Sam has continued to accelerate and is now moving at 045/26 kt. A northeastward motion along with continued acceleration is forecast through today as Sam moves into stronger southwesterly flow ahead of an approaching mid-latitude trough. The large cyclone is expected to remain the primary surface low center as it merges with the trough in a couple days. A gradual turn toward the north and northwest is expected on days 4 and 5 when extratropical-Sam moves around another high-latitude trough. The new NHC forecast track is essentially just an update of the previous advisory track, and lies close to the tightly packed consensus track models. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, the eastern United States and Atlantic Canada for the next couple of days. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 04/0900Z 40.8N 48.7W 90 KT 105 MPH 12H 04/1800Z 43.7N 45.0W 80 KT 90 MPH 24H 05/0600Z 47.8N 41.5W 75 KT 85 MPH...POST-TROP/EXTRATROP 36H 05/1800Z 50.1N 39.3W 70 KT 80 MPH...POST-TROP/EXTRATROP 48H 06/0600Z 51.0N 35.8W 60 KT 70 MPH...POST-TROP/EXTRATROP 60H 06/1800Z 52.8N 31.2W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 07/0600Z 56.0N 28.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 08/0600Z 60.6N 29.3W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 09/0600Z 61.5N 30.0W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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Tropical Depression Victor Forecast Discussion Number 19
2021-10-04 04:35:45| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Oct 03 2021 000 WTNT45 KNHC 040235 TCDAT5 Tropical Depression Victor Discussion Number 19 NWS National Hurricane Center Miami FL AL202021 1100 PM AST Sun Oct 03 2021 In its battle against dry air and shear, Victor is far from its namesake. The depression continues to produce only minimal deep convection to the north. Furthermore, an ASCAT overpass near 2219 UTC casts doubt on whether Victor still has a closed and well-defined circulation, showing generally light winds south of the suspect center. However, no visible imagery is currently available to confirm the state of Victor's surface center, so advisories will continue for now. ASCAT data showed winds a little above 25 kt to the northeast of Victor's center, so the initial intensity is held at 30 kt for this advisory. Victor is moving northwestward at a slightly faster speed (305/13 kt). Based on the ASCAT data and recent ProxyVis imagery, the center of Victor was relocated about 40 n mi south of previous estimates, which has resulted in a southern shift in the forecast track. That said, Victor is not expected to persist for much longer. The hostile surrounding environment will likely cause Victor to become a remnant low or dissipate on Monday. Victor, or its remnants, should move generally northwestward or west-northwestward until dissipation occurs within a day or so. FORECAST POSITIONS AND MAX WINDS INIT 04/0300Z 17.8N 43.0W 30 KT 35 MPH 12H 04/1200Z 18.8N 44.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 05/0000Z...DISSIPATED $$ Forecaster D. Zelinsky
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