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Tropical Depression Nine-E Forecast Discussion Number 1

2020-08-09 04:33:22| National Hurricane Center (East Pacific)

Issued at 1000 PM CDT Sat Aug 08 2020 000 WTPZ44 KNHC 090233 TCDEP4 Tropical Depression Nine-E Discussion Number 1 NWS National Hurricane Center Miami FL EP092020 1000 PM CDT Sat Aug 08 2020 Deep convection associated with the area of disturbed weather that we have been monitoring to the south of Mexico has persisted for more than 12 hours. Directional ambiguities from an earlier ASCAT-B scatterometer overpass suggest that the system has a closed circulation, and recent satellite imagery shows that convection is increasing near the estimated center while banding features are becoming better defined. The system has sufficient convective organization and circulation to be designated as a tropical depression, and advisories are being initiated. Based on a ship report and the scatterometer data, the current intensity is estimated to be near 30 kt. Upper-level outflow from the tropical cyclone is becoming better defined, and the system will be moving over warm waters and in a low-shear environment through early next week. With these conducive factors for strengthening, the cyclone is likely to become a tropical storm rather soon and a hurricane within a couple of days. The official forecast is similar to the model consensus but given the anticipated environment, this may be conservative. By days 3-5, cooler waters will likely lead to weakening. Although the center is not yet very well defined, my best estimate of initial motion is a fairly climatological 295/12 kt. The steering pattern looks quite straightforward for the next several days. A pronounced 500 mb ridge extending westward from the southwest United States over the Pacific should maintain a generally west-northwestward track for much of the forecast period. Late in the period, as the cyclone weakens, the shallower circulation should turn westward with the lower-level flow. The official track forecast is mainly a combination of the simple and corrected multi-model consensus solutions. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 14.7N 102.6W 30 KT 35 MPH 12H 09/1200Z 15.6N 104.5W 35 KT 40 MPH 24H 10/0000Z 16.7N 107.0W 45 KT 50 MPH 36H 10/1200Z 17.8N 109.5W 55 KT 65 MPH 48H 11/0000Z 18.8N 112.1W 65 KT 75 MPH 60H 11/1200Z 19.7N 114.7W 75 KT 85 MPH 72H 12/0000Z 20.5N 117.3W 70 KT 80 MPH 96H 13/0000Z 21.4N 122.7W 55 KT 65 MPH 120H 14/0000Z 21.0N 127.0W 40 KT 45 MPH $$ Forecaster Pasch

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Post-Tropical Cyclone Isaias Forecast Discussion Number 33

2020-08-05 10:38:17| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Wed Aug 05 2020 000 WTNT44 KNHC 050838 TCDAT4 Post-Tropical Cyclone Isaias Discussion Number 33 NWS National Hurricane Center Miami FL AL092020 500 AM EDT Wed Aug 05 2020 Surface observations indicate that Isaias continues to gradually weaken over southeastern Quebec. There have been a couple of observing sites with marine exposure along the St. Lawrence River that have reported winds of around 35 kt within the past few hours. Therefore, the initial intensity is set at that value for this advisory. The post-tropical cyclone should continue to weaken today, and the system is expected to be absorbed by a larger extratropical low over eastern Canada on Thursday. The cyclone has begun to decelerate and is now moving 015/24 kt. The post-tropical cyclone should continue on a general north-northeastward motion with an additional decrease in forward speed over the next 12-24 hours. The updated NHC track forecast is similar to previous advisory and is near the middle of the tightly clustered dynamical model guidance. This is the last NHC advisory on Isaias. Additional information on the post-tropical cyclone can be found in products issued by the Canadian Hurricane Center at: https://weather.gc.ca/hurricane/index_e.html FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 47.5N 71.8W 35 KT 40 MPH...POST-TROPICAL 12H 05/1800Z 50.7N 70.6W 30 KT 35 MPH...POST-TROP/EXTRATROP 24H 06/0600Z 53.3N 69.0W 25 KT 30 MPH...POST-TROP/EXTRATROP 36H 06/1800Z...DISSIPATED $$ Forecaster Brown

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Post-Tropical Cyclone Isaias Forecast Discussion Number 32

2020-08-05 04:46:49| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Tue Aug 04 2020 000 WTNT44 KNHC 050246 TCDAT4 Post-Tropical Cyclone Isaias Discussion Number 32 NWS National Hurricane Center Miami FL AL092020 1100 PM EDT Tue Aug 04 2020 Satellite and radar data, along with surface observations, indicate that Isaias has lost its tropical cyclone characteristics as it merges with a weak baroclinic zone over New England and southeastern Canada. Thus, it is being designated a post-tropical cyclone. The initial intensity is reduced to 40 kt based mainly on a recent observation at the Isle of Shoals, New Hampshire, and these winds are occurring over the Gulf of Maine well to the southeast of the center. The post-tropical cyclone is expected to weaken further, with winds dropping below 35 kt early tomorrow. After that, the system is forecast to be absorbed into a large baroclinic low over southeastern Canada between 36-48 h. The initial motion remains north-northeastward or 020/33 kt. A continued general north-northeastward motion with a decrease in forward speed is expected until the cyclone is absorbed by the larger low. The new NHC track forecast is similar to the previous forecast and is in the center of the tightly-clustered track guidance models. While Isaias is now post-tropical, the National Hurricane Center will continue to issue advisories until the Tropical Storm Warning can be discontinued along the coast of the United States. Key Messages: 1. Tropical storm force winds that could cause tree damage and power outages will continue over portions of New England for a few more hours. 2. As the rainfall exits the United States, scattered minor to moderate and isolated major river flooding is ongoing or forecast across portions of the Mid-Atlantic with most rivers falling below flood stage Wednesday. Quick-responding rivers in the Northeast will also be susceptible to minor and possible moderate river flooding. 3. The threat of tornadoes will continue over portions of Maine for a few more hours. FORECAST POSITIONS AND MAX WINDS INIT 05/0300Z 45.3N 72.5W 40 KT 45 MPH...POST-TROP/EXTRATROP 12H 05/1200Z 49.1N 71.1W 30 KT 35 MPH...POST-TROP/EXTRATROP 24H 06/0000Z 52.6N 70.0W 25 KT 30 MPH...POST-TROP/EXTRATROP 36H 06/1200Z 54.0N 68.2W 25 KT 30 MPH...POST-TROP/EXTRATROP 48H 07/0000Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Isaias Forecast Discussion Number 31

2020-08-04 22:57:11| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Tue Aug 04 2020 000 WTNT44 KNHC 042057 TCDAT4 Tropical Storm Isaias Discussion Number 31 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Tue Aug 04 2020 NOAA Doppler radar data from Islip, New York (KOKX), indicated 70-kt winds just off the surface south of Long Island earlier this afternoon, along with buoy reports from New York Harbor Inlet (44065) that supported lowering the intensity to 55 kt, so that value is used for this advisory. The motion remains north-northeastward or 020/35 kt. Isaias will continue moving north-northeastward through tonight, accompanied by a gradual decrease in forward speed. In 12 hours or, the cyclone is expected to interact with a larger extratropical over southeastern Canada and degenerate into a post-tropical cyclone near western Maine. By 24 hours, Isaias is expected to transition to an extratropical low, and dissipate or become absorbed by the aforementioned larger extratropical low. The new NHC track forecast is just an extension of the previous advisory, and lies very close to the tightly clustered consensus track models. Isaias will continue to gradually spin down and lose its tropical characteristics. The precipitation has decreased in size and also has become more stratiform in appearance, which are indications that the cyclone is gradually weakening. This downward trend should continue until Isais becomes an extratropical low in 12-24 hours. There also remains a risk of tornadoes across portions of New England this afternoon and evening Key Messages: 1. Tropical storm force winds that could cause tree damage and power outages will continue to spread across New England tonight. 2. Heavy rainfall near the path of Isaias, through the Hudson River Valley, is likely to result in flash flooding, particularly through urban areas and the surrounding terrain of the Catskills, Adirondack and Green Mountain Ranges through Tuesday night. Scattered minor to moderate river flooding is likely across portions of the Mid-Atlantic. Quick-responding rivers in the Northeast will also be susceptible to minor and possible moderate river flooding. 3. The threat of tornadoes will spread into New England this afternoon and evening. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 42.7N 74.2W 55 KT 65 MPH 12H 05/0600Z 46.3N 72.4W 35 KT 40 MPH...POST-TROP/INLAND 24H 05/1800Z 50.7N 71.1W 30 KT 35 MPH...POST-TROP/EXTRATROP 36H 06/0600Z 53.3N 70.0W 25 KT 30 MPH...POST-TROP/EXTRATROP 48H 06/1800Z 54.1N 67.8W 20 KT 25 MPH...POST-TROP/EXTRATROP 60H 07/0600Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Isaias Forecast Discussion Number 30

2020-08-04 16:59:12| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Tue Aug 04 2020 000 WTNT44 KNHC 041459 TCDAT4 Tropical Storm Isaias Discussion Number 30 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Tue Aug 04 2020 Doppler radar data from Dover, Delaware (KDOX), and Ft. Dix, New Jersey (KDIX) this morning continue to indicate 70-80 kt winds between 4000-7000 ft over the ocean just offshore the Delmarva peninsula and the southeastern coast of New Jersey, which would normally correspond to 65-70 kt surface winds. However, these velocity values have been collocated with only 15-25 dBZ reflectivity echoes, so the typical 80-85 percent reduction factors likely don't apply. Sustained observed surface winds of 50-55 kt have been reported, so the initial intensity will be held at 60 kt, but that is only for winds over the ocean and near the coast. Isaias is moving north-northeastward or 020/30 kt. The cyclone will continue to accelerate north-northeastward today and tonight within strong southwesterly flow ahead of a deep-layer trough and associated cold front approaching the mid-Atlantic and New England areas from the west. By 24 h, the Isaias is expected to slow down and degenerate into a post-tropical cyclone over Maine before it is absorbed by a larger extratropical low located over southeastern Canada. The new NHC track forecast is just an extension of the previous advisory, and lies very close to the tightly clustered consensus track models. As Isaias moves north-northeastward through the Mid-Atlantic coast, interaction with a strong upper-level jet maximum is forecast to maintain the tropical storm's intensity longer than what typically would be expected for inland decaying tropical cyclone. The global models continue to indicate that Isaias is likely to produce widespread tropical-storm conditions, with hurricane-force wind gusts possible along the mid-Atlantic coast through this afternoon. As a result, the gust factor at 12 h remains above the standard 20-percent value in the Forecast/Advisory Product (TCMAT4). In addition to the wind threat, Isaias is expected to produce heavy rainfall along and just west of the I-95 corridor today, and the Weather Prediction Center has placed a portion of this area in a high risk for life-threatening flash flooding. There also remains a significant risk of tornadoes across the mid-Atlantic coast and southeastern New York this afternoon, possibly spreading into southern New England tonight. Key Messages: 1. Widespread sustained tropical storm force winds and gusts to hurricane force are expected along the mid-Atlantic coast, including portions of the Chesapeake Bay region, through this afternoon, which could cause tree damage and power outages. Tropical storm conditions are expected to spread across New England tonight. 2. Heavy rainfall along the East Coast, near the path of Isaias, will result in flash flooding, some of which may be significant in the mid-Atlantic and Northeast through tonight. Potentially life-threatening urban flooding remains possible in Philadelphia and elsewhere along and just west of the I-95 corridor today. Scattered minor to moderate river flooding is likely across portions of the and the Mid-Atlantic. Quick-responding rivers in Northeast will also be susceptible to minor river flooding. 3. Numerous tornadoes have already occurred over portions of the mid-Atlantic coast this morning. The threat of tornadoes will continue along the mid-Atlantic coast spread into New England this afternoon. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 39.1N 76.1W 60 KT 70 MPH...INLAND 12H 05/0000Z 43.2N 73.8W 50 KT 60 MPH...INLAND 24H 05/1200Z 48.2N 71.7W 35 KT 40 MPH...POST-TROP/INLAND 36H 06/0000Z 52.0N 71.1W 30 KT 35 MPH...POST-TROP/EXTRATROP 48H 06/1200Z 53.6N 69.5W 20 KT 25 MPH...POST-TROP/EXTRATROP 60H 07/0000Z...DISSIPATED INLAND $$ Forecaster Stewart

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