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Tropical Storm Nana Forecast Discussion Number 4

2020-09-02 04:37:15| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Tue Sep 01 2020 000 WTNT41 KNHC 020237 TCDAT1 Tropical Storm Nana Discussion Number 4 NWS National Hurricane Center Miami FL AL162020 1100 PM EDT Tue Sep 01 2020 Deep convection redeveloped over the center of Nana during the late afternoon and recent microwave imagery has revealed a small, well-defined low-level eye-like feature, and convective banding around the southwestern portion of the circulation. An Air Force Reserve hurricane hunter aircraft that has provided a couple of center fixes within the past hour or two has measured a peak 850-mb flight-level wind of 62 kt, and believable SFMR winds of around 50 kt, which supports an initial intensity of 50 kt. Data from aircraft indicated that the minimum pressure is around 999 mb. Nana is moving westward or 275/16 kt. A strong deep-layer ridge to the north of the tropical cyclone should steer it westward over the next day or so. After that time, the ridge is forecast to build west-southwestward and the dynamical models indicate that this will cause Nana to turn west-southwestward by Thursday. On this track, Nana is expected to approach the coast of Belize late Wednesday night or early Thursday, and move inland over Belize and Guatemala later that day. The track guidance is in good agreement on this scenario but has trended slightly southward, which seems reasonable given that Nana is vertically deep and is more likely to feel the deep-layered east-northeasterly flow. The new NHC track forecast is close to the FSU Superensemble and the HFIP corrected consensus model. The tropical storm is currently within an environment of light to moderate northeasterly shear. This shear does not seem to be strong enough to prevent strengthening during the next 12-24 hours, and the NHC intensity forecast shows a slightly faster rate of intensification during that time. This is in agreement with the latest SHIPS model output and the HCCA and HWRF models, and calls for Nana to become a hurricane within 24 hours. The shear is forecast to become more northerly just before landfall, and this is likely to slow the intensification process. Rapid weakening is expected after landfall, and the low-level circulation is expected to dissipate over Guatemala or southeastern Mexico by day 3. KEY MESSAGES: 1. Tropical Storm Nana is forecast to be a hurricane as it approaches the coast, and interests in Honduras, Guatemala, Belize, and Yucatan should closely monitor the progress of this storm. Strong winds, dangerous storm surge and very heavy rainfall causing flash flooding are becoming more likely from Nana. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 17.0N 80.9W 50 KT 60 MPH 12H 02/1200Z 17.1N 83.5W 60 KT 70 MPH 24H 03/0000Z 17.0N 86.3W 65 KT 75 MPH 36H 03/1200Z 16.8N 88.6W 65 KT 75 MPH...INLAND 48H 04/0000Z 16.5N 90.7W 35 KT 40 MPH...INLAND 60H 04/1200Z 16.2N 93.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 05/0000Z...DISSIPATED $$ Forecaster Brown

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Tropical Storm Omar Forecast Discussion Number 6

2020-09-02 04:35:16| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Tue Sep 01 2020 769 WTNT45 KNHC 020235 TCDAT5 Tropical Storm Omar Discussion Number 6 NWS National Hurricane Center Miami FL AL152020 1100 PM EDT Tue Sep 01 2020 Omar is battling strong northwesterly shear, as any burst of convection that tries to develop near the center is quickly pushed off well to the southeast of the cyclone's exposed low-level center. The initial intensity is being held at 35 kt based on a combination of the latest Dvorak CI values from UW-CIMSS, SAB and TAFB, as as well as data from a recent ASCAT overpass showing similar winds compared to earlier today. These intermittent bursts of convection should sustain Omar as a tropical storm at least through early Wednesday. However, the shear is expected to get even stronger by late Wednesday, which should cause a weakening trend to begin around that time. By Thursday night, if not sooner, any organized deep convection is expected to have diminished, and Omar is forecast to become a remnant low. An approaching frontal system should absorb what is left of the remnants a couple of days later. The latest NHC intensity forecast is the same as the previous one, and is in agreement with the various intensity aids. Omar continues to move east-northeast, now at 12 kt. The steering pattern is straightforward over the next few days, as the cyclone will be steered east-northeast then eastward around the northern periphery of a subtropical ridge. Later on in the forecast period, once Omar has become a remnant low, a turn to the northeast is expected ahead of the approaching frontal boundary. The latest NHC track forecast is little changed from the previous one, and lies in the middle of the tightly clustered track guidance. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 35.8N 70.0W 35 KT 40 MPH 12H 02/1200Z 36.4N 68.0W 35 KT 40 MPH 24H 03/0000Z 37.0N 65.4W 35 KT 40 MPH 36H 03/1200Z 37.2N 62.5W 30 KT 35 MPH 48H 04/0000Z 37.0N 60.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 04/1200Z 36.9N 58.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 05/0000Z 37.4N 56.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 06/0000Z 40.0N 52.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 07/0000Z...DISSIPATED $$ Forecaster Latto

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Tropical Storm Nana Forecast Discussion Number 3

2020-09-01 22:55:42| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Tue Sep 01 2020 000 WTNT41 KNHC 012055 TCDAT1 Tropical Storm Nana Discussion Number 3 NWS National Hurricane Center Miami FL AL162020 500 PM EDT Tue Sep 01 2020 Although convection has waned somewhat since the previous advisory due to mid-level dry air entrainment, Nana still has enough deep convection over and surrounding the low-level center to keep the intensity at 45 kt. Upper-level outflow has continued to improve, with the associated anticyclone becoming more symmetrical. Another reconnaissance mission is scheduled for this evening. The initial motion estimate is 275/16 kt. There are no significant changes to the previous track forecast or reasoning. A pronounced deep-layer to the north of Nana is forecast to remain intact and even build westward over the next few days, keeping Nana moving in a general westward direction for the next 36 hours or so. Thereafter, the aforementioned ridge is expected to build west-southwestward across southern Mexico, driving the cyclone west-southwestward as well across northern Central America. The new NHC forecast track is similar to the previous advisory track, and lies close to a blend of the simple consensus models TVCN and GFEX, and the corrected- consensus models NOAA-HCCA and FSSE. The current northeasterly 15 kt of vertical wind shear and dry air intrusions should inhibit development for next 24 hours or so. However, the GFS and ECMWF models both forecast the shear to gradually decrease during the next 48 hours, becoming near 5 kt by 36 hours. The lower vertical shear, coupled with the already impressive outflow pattern, sea-surface temperatures of 29.5-30.5 deg C, and a moistening mid-level environment should allow for gradual strengthening for the next 24 hours, followed by more significant intensification thereafter, which will continue right up until landfall occurs. The NHC official intensity forecast is a blend of the simple consensus intensity model IVCN, and the corrected-consensus models NOAA-HCCA and FSSE. A Tropical Storm Warning and Tropical Storm Watch have been issued for the southern portion of the Yucatan Peninsula and the Caribbean Sea coast of Guatemala, respectively. KEY MESSAGES: 1. Tropical Storm Nana is now forecast to be a hurricane as it approaches the coast, and interests in Honduras, Guatemala, Belize, and Yucatan should closely monitor the progress of this storm. Strong winds, dangerous storm surge and very heavy rainfall causing flash flooding are becoming more likely from Nana. FORECAST POSITIONS AND MAX WINDS INIT 01/2100Z 16.8N 79.3W 45 KT 50 MPH 12H 02/0600Z 17.1N 81.5W 50 KT 60 MPH 24H 02/1800Z 17.3N 84.7W 55 KT 65 MPH 36H 03/0600Z 17.2N 87.3W 65 KT 75 MPH 48H 03/1800Z 16.9N 89.4W 45 KT 50 MPH...INLAND 60H 04/0600Z 16.5N 91.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 04/1800Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Omar Forecast Discussion Number 5

2020-09-01 22:48:15| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Tue Sep 01 2020 000 WTNT45 KNHC 012048 TCDAT5 Tropical Storm Omar Discussion Number 5 NWS National Hurricane Center Miami FL AL152020 500 PM EDT Tue Sep 01 2020 Satellite images show that the system remains sheared with a bursting pattern on satellite, occasionally exposing the center, and a large area of curved bands in the southeastern quadrant of the circulation. Almost all of the subjective and objective Dvorak estimates, along with SATCON values, are between 35 to 40 kt, and the lower number is chosen as the initial wind speed since scatterometer data suggests 30 to 35 kt. This makes Omar the 15th named storm of the 2020 Atlantic hurricane season, and is the earliest 15th storm on record, besting the previous mark by about a week from Ophelia of 2005. Any chance for strengthening should end by tomorrow afternoon due to greatly increasing shear, and weakening is likely to commence by then. The persistence of the shear should cause the cyclone to decay into a remnant low in about 48 hours, if not sooner. No significant changes were made to the previous forecast, which is near the model consensus. The initial motion remains east-northeast or 065/13 kt. The cyclone is being steered by the northern side of the subtropical ridge, which is forecast to cause a similar motion through tomorrow and an eastward turn late week due to the orientation of the ridge. The only notable change to the forecast is a slow down at long range in most of the guidance, probably due to a shallow system no longer feeling the stronger deep-layer winds, so the NHC track prediction follows suit. The remnant low should dissipate in 4-5 days. FORECAST POSITIONS AND MAX WINDS INIT 01/2100Z 35.3N 71.5W 35 KT 40 MPH 12H 02/0600Z 36.1N 69.4W 35 KT 40 MPH 24H 02/1800Z 36.7N 66.7W 35 KT 40 MPH 36H 03/0600Z 37.2N 63.9W 30 KT 35 MPH 48H 03/1800Z 37.2N 61.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 04/0600Z 37.0N 59.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 04/1800Z 37.0N 57.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 05/1800Z 39.5N 53.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 06/1800Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Nana Forecast Discussion Number 2

2020-09-01 18:56:37| National Hurricane Center (Atlantic)

Issued at 100 PM EDT Tue Sep 01 2020 000 WTNT41 KNHC 011656 TCDAT1 Tropical Storm Nana Special Discussion Number 2 NWS National Hurricane Center Miami FL AL162020 100 PM EDT Tue Sep 01 2020 An Air Force Reserve reconnaissance aircraft investigating the vigorous disturbance south of Jamaica this morning has found that the system has a well-defined, closed circulation center. The aircraft also measured 60 kt at 925 mb in the northeastern quadrant and reliable SFMR surface winds of 45 kt. Based on these data, the system has been upgraded to Tropical Storm Nana, the 14th named tropical storm of the 2020 Atlantic hurricane season. The initial motion estimate is an uncertain 280/14 kt, No significant changes were made to the previous track forecast. A strong deep-layer to the north of the system is expected to build slowly westward over the next few days, keeping the disturbance moving in a general west-northwestward to westward direction through 48 hours, followed by a turn toward the west-southwest shortly thereafter when the system will be inland over Central America. The NHC official forecast track lies close to a blend of the simple consensus models TVCN and GFEX, and the corrected-consensus model HCCA. Nana has developed a well-defined upper-level anticyclone, with outflow gradually improving in all quadrants. Modest northeasterly to easterly vertical wind shear is expected to gradually subside to less than 10 kt by 48 hours, which should allow for at least some slow but steady strengthening, given that sea-surface temperatures (SST) will be 29.5-30.5 deg C along the track. Some dry air intrusions might hinder development for the next 24 hours or so, but the global and regional models still indicate that the atmosphere will moisten thereafter, and continue to moisten right up until landfall. Based on the higher initial intensity and the mostly favorable environmental conditions expected after 24 hours, the peak forecast intensity has been increased to 65 kt at 48 hours, just prior to landfall. Due to the Nana's expected proximity to Belize in 48 hours, a Tropical Storm Watch has been issued for that country. Additional watches and possible warnings will likely be required later today for Guatemala and the southern portion of the Yucatan Peninsula. KEY MESSAGES: 1. Tropical Storm Nana is now forecast to be a hurricane as it approaches the coast, and interests in Honduras, Guatemala, Belize and Yucatan should closely monitor the progress of this storm. Strong winds, dangerous storm surge and very heavy rainfall causing flash flooding are becoming more likely from Nana. FORECAST POSITIONS AND MAX WINDS INIT 01/1700Z 16.6N 77.9W 45 KT 50 MPH 12H 02/0000Z 16.6N 79.5W 50 KT 60 MPH 24H 02/1200Z 16.7N 82.3W 55 KT 65 MPH 36H 03/0000Z 16.8N 85.1W 60 KT 70 MPH 48H 03/1200Z 16.8N 87.4W 65 KT 75 MPH 60H 04/0000Z 16.5N 89.6W 30 KT 35 MPH...INLAND 72H 04/1200Z 15.7N 91.7W 15 KT 15 MPH...POST-TROP/REMNT LOW 96H 05/1200Z...DISSIPATED $$ Forecaster Stewart

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