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Hurricane Sam Forecast Discussion Number 30
2021-09-30 04:56:52| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Sep 29 2021 000 WTNT43 KNHC 300256 TCDAT3 Hurricane Sam Discussion Number 30 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Wed Sep 29 2021 Sam is intensifying tonight after completing an eyewall replacement cycle. A recent NOAA Hurricane Hunter aircraft showed peak 700 mb flight-level winds of 138 kt, multiple SFMR surface winds of 120 kt or greater, plus falling central pressure to about 940 mb. This deepening trend is also seen on recent satellite imagery, with significant warming within the eye noted. These data support raising the initial intensity to 125 kt for this advisory. It is of note that the NOAA mission has been quite helpful for both the size and intensity analysis, with dropsondes and SFMR data showing that the hurricane has grown quite a bit since earlier today, mostly on the eastern side. Additionally, reflectivity data from the core confirmed the end of the eyewall replacement cycle. The hurricane could strengthen a little more overnight while it moves over a warm ocean eddy in fairly light shear. Afterward, gradually cooling SSTs, falling ocean heat content, and the inevitable future eyewall replacement cycles should cause a weakening trend. However, this will likely not be as steady as shown below, and significant deviations (upward or downward) from the forecast can be expected. Faster weakening is expected by the weekend due to cool SSTs and increasing shear. The new forecast is close to the intensity consensus, and is a little higher than the previous NHC prediction early on, primarily due to the initial wind speed. Sam has sped up tonight, now estimated at 315/10 kt. It sounds like a broken record, but there are no changes to the forecast during the first few days as model guidance is in excellent agreement on a gradually rightward-curving track, passing a couple of hundred n mi or more east of Bermuda. Afterward, guidance is actually in worse agreement over the weekend, with widely divergent solutions from an upper-level trough picking up Sam and turning it north toward Newfoundland or the trough leaving Sam behind, causing a slow eastward motion. The trend in the guidance is for a slower motion at long range, so I've elected to keep the day 4 and 5 points almost the same as the previous advisory, just a hair faster than the latest consensus aids. Key Messages: 1. Swells generated by Sam will impact the Lesser Antilles during the next several days. Swells are expected to reach Bermuda and the Bahamas in a day or so, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. A Tropical Storm Watch could be required for Bermuda on Thursday, and interests there should monitor the progress of Sam. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 20.6N 58.4W 125 KT 145 MPH 12H 30/1200Z 21.9N 59.6W 130 KT 150 MPH 24H 01/0000Z 24.1N 60.9W 125 KT 145 MPH 36H 01/1200Z 26.8N 61.8W 120 KT 140 MPH 48H 02/0000Z 29.6N 61.8W 115 KT 130 MPH 60H 02/1200Z 32.2N 60.7W 110 KT 125 MPH 72H 03/0000Z 34.3N 59.2W 100 KT 115 MPH 96H 04/0000Z 37.5N 54.5W 85 KT 100 MPH 120H 05/0000Z 42.0N 48.5W 70 KT 80 MPH $$ Forecaster Blake
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Tropical Storm Victor Forecast Discussion Number 3
2021-09-30 04:47:28| National Hurricane Center (Atlantic)
Issued at 200 AM CVT Thu Sep 30 2021 000 WTNT45 KNHC 300247 TCDAT5 Tropical Storm Victor Discussion Number 3 NWS National Hurricane Center Miami FL AL202021 200 AM CVT Thu Sep 30 2021 Victor has changed little in organization since the last advisory, with a large circulation and a complex of convective bands mainly in the northern semicircle. Satellite intensity estimates from TAFB and SAB remain at or near 35 kt, as do other objective intensity estimates. Thus, the initial intensity is still 35 kt, although the most recent ASCAT-A data suggests the possibility that this is a little generous. The center has moved or re-formed a bit to the west during the past several hours, although the longer-term motion still seems to be to the west-northwest at 290/11 kt. Other than the more westerly initial position, there is little change to the forecast philosophy or the forecast track since the last advisory. Victor is forecast to move west-northwestward for the next 36 h or so as it is steered by the flow on the south side of a deep-layer ridge. By 48 h, the global models forecast the western periphery of the ridge to get eroded by to a mid- to upper-level low over the subtropical central Atlantic. This evolution should cause the cyclone to turn northwestward at that time, followed by a northward turn by the end of the forecast period. There remains a fair amount of spread in the timing and location of the northward turn with the GFS and HWRF still on the right side of the guidance envelope and the ECMWF showing the slowest and westernmost solution. The new forecast track is in the center of the guidance envelope and lies very close to the various consensus models. For the next 36-48 h, Victor is expected to remain in an environment of warm sea surface temperatures and light vertical wind shear. This should allow steady strengthening, and rapid strengthening cannot be ruled out if the storm develops a better defined inner core. Based on a steady strengthening scenario, the new intensity forecast is identical to the previous one, taking Victor to hurricane intensity in 36 h with a peak intensity of 70 kt in 48 h. After that time, the aforementioned upper-level low is expected to cause significant shear over Victor while the cyclone moves into a drier air mass. This combination should cause the cyclone to weaken, and it is possible Victor could weaken faster than currently forecast after 60 hours. The new official intensity forecast lies near the upper edge of the intensity guidance. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 8.4N 26.7W 35 KT 40 MPH 12H 30/1200Z 8.9N 28.3W 45 KT 50 MPH 24H 01/0000Z 9.7N 30.2W 55 KT 65 MPH 36H 01/1200Z 10.6N 32.0W 65 KT 75 MPH 48H 02/0000Z 11.8N 33.5W 70 KT 80 MPH 60H 02/1200Z 13.4N 35.1W 70 KT 80 MPH 72H 03/0000Z 15.5N 36.7W 60 KT 70 MPH 96H 04/0000Z 20.5N 39.5W 45 KT 50 MPH 120H 05/0000Z 26.0N 41.0W 40 KT 45 MPH $$ Forecaster Beven
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Hurricane Sam Forecast Discussion Number 29
2021-09-29 22:51:15| National Hurricane Center (Atlantic)
Issued at 500 PM AST Wed Sep 29 2021 108 WTNT43 KNHC 292051 TCDAT3 Hurricane Sam Discussion Number 29 NWS National Hurricane Center Miami FL AL182021 500 PM AST Wed Sep 29 2021 Sam continues to exhibit a well-organized cloud pattern on satellite imagery, although there is some asymmetry of the coldest-topped convection around the center. The hurricane is also maintaining numerous banding features with well-defined upper-level anticyclonic outflow. Observations from an Air Force Hurricane Hunter plane showed peak 700 mb fight-level winds of 130 kt in the northeast quadrant which, using the standard reduction, corresponds to maximum surface winds of 117 kt. However, the maximum SFMR-observed surface winds were 103 kt. Based on these data, with more emphasis on the flight-level winds, the current intensity estimate is held at 115 kt. The aircraft also measured a minimum central pressure of 945 mb by dropsonde. Since the hurricane is forecast to remain in an environment of low vertical shear and to remain over warm waters through 48 hours, Sam could restrengthen a little more during the next day or so. Since the Hurricane Hunters reported a concentric eyewall, short-term strengthening may be curtailed until the eyewall replacement cycle completes. Later in the forecast period, gradual weakening is likely due to cooler SSTs and increasing shear. The official intensity forecast is in very good agreement with the latest corrected consensus prediction. Sam will likely be close to extratropical transition around day 5. The initial motion is still northwestward, or 310/8 kt. During the next few days, Sam is likely to move northwestward and then northward around the western periphery of a large subtropical high pressure system centered over the eastern Atlantic. A turn to the north-northeast and northeast is expected later in the forecast period as Sam begins to encounter the flow on the southern side of a large mid-tropospheric trough moving through Atlantic Canada. The track guidance models remain in excellent agreement through 72 hours, including the time that Sam passes by Bermuda. At days 4-5 there is some divergence of the model guidance, apparently due to how Sam may interact with the trough. Most notably the ECMWF is a lot slower than the other global models suggesting that the trough will at least partially bypass it in that case. Not much change has been made to the previous official forecast, however, and the NHC track remains very close to the TVCN dynamical model consensus. Key Messages: 1. Swells generated by Sam will impact the Lesser Antilles during the next several days. Swells are expected to reach Bermuda and the Bahamas in a day or so, and then spread to the United States east coast by this weekend. 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 29/2100Z 20.2N 57.6W 115 KT 130 MPH 12H 30/0600Z 21.2N 58.9W 120 KT 140 MPH 24H 30/1800Z 23.1N 60.4W 120 KT 140 MPH 36H 01/0600Z 25.6N 61.4W 120 KT 140 MPH 48H 01/1800Z 28.3N 61.9W 115 KT 130 MPH 60H 02/0600Z 31.0N 61.3W 110 KT 125 MPH 72H 02/1800Z 33.6N 59.9W 100 KT 115 MPH 96H 03/1800Z 37.5N 55.0W 85 KT 100 MPH 120H 04/1800Z 42.0N 48.5W 70 KT 80 MPH $$ Forecaster Pasch
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Tropical Storm Victor Forecast Discussion Number 2
2021-09-29 22:43:52| National Hurricane Center (Atlantic)
Issued at 800 PM CVT Wed Sep 29 2021 000 WTNT45 KNHC 292043 TCDAT5 Tropical Storm Victor Discussion Number 2 NWS National Hurricane Center Miami FL AL202021 800 PM CVT Wed Sep 29 2021 The cloud pattern continues to exhibit a large circulation with numerous curved bands surrounding the center. The strongest thunderstorms remain on the system's west side. The latest Dvorak estimates from TAFB and SAB have both increased to 2.5/35 kt, and on that basis, the cyclone has been upgraded to a 35-kt tropical storm. Victor is still on a west-northwest course at about 11 kt. There has been little change to the track forecast reasoning. The cyclone is expected to continue west-northwestward during the next couple of days as it remains steered by the flow on the south side of a deep-layer ridge. By late Friday, however, the models show the western periphery of the ridge being eroded due to a mid- to upper-level low over the subtropical central Atlantic. As a result, the cyclone should turn northwestward by Friday night and then northward late this weekend or early next week. There remains a fair amount of spread in the timing and location of the northward turn with the GFS and HWRF still on the right side of the guidance envelope and the ECMWF showing the slowest and westernmost solution. Overall, the consensus models have not changed much and neither has the official track forecast. The storm is expected to remain in conducive environmental conditions for strengthening during the next couple of days. During that time period, Victor is forecast to move over 28-29 deg C waters while embedded within a moist airmass with very low wind shear (less than 10 kt). Rapid intensification sometimes occurs when conditions are this favorable, however, since the storm is broad and does not yet have an inner core, gradual strengthening seems more reasonable. The NHC forecast brings Victor to hurricane intensity in 36 hours. In a few days, however, the models show a pronounced increase in southwesterly shear and a notably drier atmosphere. These unfavorable conditions should end the opportunity for strengthening and promote a weakening trend. The NHC intensity forecast is similar to the previous one and remains near a blend of the HCCA, IVCN, and IVDR consensus models. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 8.3N 25.5W 35 KT 40 MPH 12H 30/0600Z 8.8N 26.9W 45 KT 50 MPH 24H 30/1800Z 9.5N 28.9W 55 KT 65 MPH 36H 01/0600Z 10.3N 30.8W 65 KT 75 MPH 48H 01/1800Z 11.4N 32.4W 70 KT 80 MPH 60H 02/0600Z 12.8N 34.1W 70 KT 80 MPH 72H 02/1800Z 14.7N 35.8W 60 KT 70 MPH 96H 03/1800Z 19.3N 38.9W 50 KT 60 MPH 120H 04/1800Z 24.9N 40.9W 40 KT 45 MPH $$ Forecaster Cangialosi
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Hurricane Sam Forecast Discussion Number 28
2021-09-29 16:42:08| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Wed Sep 29 2021 000 WTNT43 KNHC 291441 TCDAT3 Hurricane Sam Discussion Number 28 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Wed Sep 29 2021 Sam has a well-organized cloud pattern on satellite imagery, with a 90 n mi-wide Central Dense Overcast surrounded by numerous convective banding features. Upper-level outflow has now become better established to the southeast of the hurricane. A NOAA Hurricane Hunter mission from a couple of hours ago reported a 20 n mi-wide diameter circular eye. The current intensity is held at 115 kt for this advisory, which is a little higher than the maximum winds observed in the earlier aircraft mission. However, since the estimated minimum central pressure of 949 is a little lower than it was earlier this morning and Sam's intensity has been fluctuating, it is considered prudent not to lower the winds at this time. An Air Force Hurricane Hunter aircraft is scheduled to investigate the hurricane soon and will provide another intensity estimate. Sam continues its slow northwestward trek with the current motion remaining near 310/8 kt. Sam should move northwestward and then northward around the western portion of the subtropical ridge over the eastern Atlantic during the next few days, and turn toward the north-northeast and northeast later in the forecast period. The track guidance models are in very good agreement through 72 hours or so, with some increase in model spread thereafter. This increase in spread is largely due to how Sam will interact with a strong mid-latitude trough moving through Atlantic Canada. Some of the guidance suggests that this trough could partially bypass the hurricane around the end of the period and beyond, but this is not yet clear. The current NHC forecast track is not much different from the previous one and is in good agreement with the dynamical model consensus, TVCA. The hurricane will continue to move over warm waters and in an environment of low vertical shear and diffluent upper-level flow for the next 1-2 days. Thus, it would not be surprising to soon see some restrengthening. The official intensity forecast is essentially a blend of the latest Decay-SHIPS and LGEM predictions, and is very similar to the previous NHC forecast. Sam is likely to remain a major hurricane into this weekend. Key Messages: 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend. FORECAST POSITIONS AND MAX WINDS INIT 29/1500Z 19.4N 57.0W 115 KT 130 MPH 12H 30/0000Z 20.3N 58.1W 120 KT 140 MPH 24H 30/1200Z 21.8N 59.6W 120 KT 140 MPH 36H 01/0000Z 23.9N 61.0W 120 KT 140 MPH 48H 01/1200Z 26.6N 61.8W 115 KT 130 MPH 60H 02/0000Z 29.4N 61.8W 110 KT 125 MPH 72H 02/1200Z 32.0N 60.9W 105 KT 120 MPH 96H 03/1200Z 36.2N 57.3W 90 KT 105 MPH 120H 04/1200Z 39.5N 53.0W 75 KT 85 MPH $$ Forecaster Pasch
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