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Tropical Storm Isaac Forecast Discussion Number 9

2018-09-09 22:32:48| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sun Sep 09 2018 355 WTNT44 KNHC 092032 TCDAT4 Tropical Storm Isaac Discussion Number 9 NWS National Hurricane Center Miami FL AL092018 500 PM AST Sun Sep 09 2018 Isaac is a tiny tropical cyclone, but even still, recent SSMIS data showed that it has developed a mid-level microwave eye feature. At the time of the pass (1800 UTC), the convection was a little thin on the southern side, but geostationary satellite images suggest that it has filled in since that time. Dvorak intensity estimates are T4.0 from TAFB and T3.5 from SAB, so the maximum winds are estimated to be 60 kt. The intensity forecast philosophy has not changed since this morning. Isaac is strengthening in an environment of low shear and over warm sea surface temperatures, and for the next 36-48 hours, the NHC intensity forecast is above the bulk of the intensity models. The main reason for this is that Isaac's tiny size could allow the intensity to increase quickly within the favorable environment. However, the cyclone's tiny size will likely also be to its detriment after 48 hours when northwesterly vertical shear is expected to develop and increase to 20-30 kt. The shear will probably easily decouple the small system, causing the intensity to decrease much faster than suggested by most of the intensity models. For that reason, the official NHC intensity forecast is below the intensity consensus on days 3 through 5. It should be noted that both the GFS and ECMWF models show a weakening cyclone moving into the eastern Caribbean Sea by days 4 and 5, with the GFS even making the system an open wave by the end of the forecast period. These global model solutions lend credence to the belief that it may be difficult for Isaac to maintain hurricane status while it approaches the Lesser Antilles. Isaac is accelerating toward the west with an initial motion of 275/10 kt. Ridging to the north is expected to keep the cyclone on a westward trajectory for the entire forecast period, with acceleration continuing for the next 36 hours. The UKMET remains the biggest outlier, showing Isaac turning northwestward and northward into the central Atlantic after 48 hours. That still appears to be unlikely at this time, and the NHC track forecast is still close to the center of the guidance envelope. Only a slight northward shift was made to the new forecast based on the latest model solutions. Key Messages: 1. Isaac is expected to become a hurricane tonight or on Monday while it moves westward across the tropical Atlantic Ocean. 2. While Isaac is forecast to begin weakening by Tuesday while it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands, and the uncertainty in the intensity forecast is higher than usual. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 14.5N 40.3W 60 KT 70 MPH 12H 10/0600Z 14.6N 42.2W 70 KT 80 MPH 24H 10/1800Z 14.8N 45.0W 80 KT 90 MPH 36H 11/0600Z 14.9N 48.0W 85 KT 100 MPH 48H 11/1800Z 14.9N 50.9W 85 KT 100 MPH 72H 12/1800Z 15.0N 56.5W 70 KT 80 MPH 96H 13/1800Z 15.5N 63.0W 60 KT 70 MPH 120H 14/1800Z 15.5N 69.5W 50 KT 60 MPH $$ Forecaster Berg

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Tropical Storm Paul Forecast Discussion Number 6

2018-09-09 22:31:47| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Sun Sep 09 2018 880 WTPZ43 KNHC 092031 TCDEP3 Tropical Storm Paul Discussion Number 6 NWS National Hurricane Center Miami FL EP182018 200 PM PDT Sun Sep 09 2018 ASCAT data from shortly before 1800 UTC indicated that Paul was slightly stronger than previously estimated, with several 35-40 kt vectors observed in the southeast quadrant. Based on this data, the initial intensity has been increased to 40 kt. In an effort to maintain some continuity, and since the ASCAT data nearly supports 45 kt, very slight strengthening is still forecast over the next 24 h, but little change in strength is realistically expected. Beginning in a day or two, a combination of dry, stable air, and decreasing SSTs should cause Paul to gradually weaken, eventually causing the cyclone to become a remnant low by the end of the forecast period. The new NHC intensity forecast is a little lower than the previous one, but is still near the high end of the intensity guidance. It is certainly possible Paul could weaken sooner than currently indicated. Paul has sped up slightly but is still moving northwestward, now at around 9 kt. Almost no change has been made to the official track forecast. Paul should continue moving northwestward for the next day or so, before gradually turning toward the west as the weakening cyclone becomes increasingly steered by low-level easterly flow to the north. The track guidance is in reasonably good agreement through about 72 h, at which point there are differences mainly related to how fast the cyclone will weaken. The NHC forecast remains close to HCCA and the other consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 18.5N 118.7W 40 KT 45 MPH 12H 10/0600Z 19.5N 119.6W 45 KT 50 MPH 24H 10/1800Z 20.5N 120.9W 45 KT 50 MPH 36H 11/0600Z 21.4N 122.5W 40 KT 45 MPH 48H 11/1800Z 21.9N 124.2W 40 KT 45 MPH 72H 12/1800Z 22.8N 127.4W 35 KT 40 MPH 96H 13/1800Z 23.5N 130.0W 30 KT 35 MPH 120H 14/1800Z 23.5N 131.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Zelinsky

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Hurricane Florence Forecast Discussion Number 41

2018-09-09 16:50:44| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sun Sep 09 2018 685 WTNT41 KNHC 091450 TCDAT1 Hurricane Florence Discussion Number 41 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Sun Sep 09 2018 Satellite images indicate that Florence is strengthening. Deep convection has intensified in the central dense overcast, with hints of a ragged eye in the latest GOES-16 visible channel. The NOAA Hurricane Hunters just flew through the eye, finding 70 kt winds at flight-level and 66 kt from the SFMR. This data confirms that Florence has become a hurricane again, and the initial wind speed is set to 65 kt. The aircraft also found that the minimum pressure has decreased to 984 mb. Overnight microwave data and the Hurricane Hunter vortex message show that a mostly complete eyewall has formed with Florence. In combination with low vertical wind shear and progressively warmer waters near 29C, this structure is a blueprint for rapid intensification. Almost all of the intensity guidance is showing at least one period of rapid strengthening during the next few days, which is rather rare. The NHC wind speed forecast is raised in the first couple of days following the guidance trend, then is very similar to the previous one. All indications are that Florence will be an extremely dangerous Category 4 hurricane while it moves over the western Atlantic toward the southeastern United States. Florence continues moving slowly westward, caught between a pair of mid-level ridges over the Atlantic Ocean. A very strong ridge is forecast to build over the northwestern Atlantic during the next few days, which should steer Florence west-northwestward at a much faster forward speed. By Wednesday, the hurricane should turn northwestward, and slow down somewhat due to another ridge forming over the Ohio Valley. It is interesting to note that while the ensemble means from the ECMWF and UKMET are west of the NHC forecast, the strongest members are on the right side of their ensemble envelope. Thus, the new NHC track forecast is very similar to the previous one, and continues to lie between the corrected consensus and consensus aids. The Ohio Valley ridge is concerning because Florence could stop moving pretty quickly around day 5, potentially leading to a serious heavy rain episode and inland flood hazard. Key Messages: 1. There is an increasing risk of two life-threatening impacts from Florence: storm surge at the coast and freshwater flooding from a prolonged heavy rainfall event inland. While it is too soon to determine the exact timing, location, and magnitude of these impacts, interests at the coast and inland from South Carolina into the mid-Atlantic region should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 24.4N 56.3W 65 KT 75 MPH 12H 10/0000Z 24.5N 57.4W 80 KT 90 MPH 24H 10/1200Z 24.9N 59.3W 95 KT 110 MPH 36H 11/0000Z 25.6N 61.7W 105 KT 120 MPH 48H 11/1200Z 26.4N 64.5W 120 KT 140 MPH 72H 12/1200Z 29.0N 70.8W 125 KT 145 MPH 96H 13/1200Z 32.2N 75.8W 120 KT 140 MPH 120H 14/1200Z 35.0N 78.5W 85 KT 100 MPH...INLAND $$ Forecaster Blake

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Tropical Storm Helene Forecast Discussion Number 9

2018-09-09 16:42:16| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sun Sep 09 2018 214 WTNT43 KNHC 091442 TCDAT3 Tropical Storm Helene Discussion Number 9 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Sun Sep 09 2018 Helene continues to exhibit a fairly well-organized cloud pattern, but does not yet have a well-defined inner core. The current intensity estimate, 55 kt, is based on a blend of subjective and objective Dvorak estimates and is also close to the latest SATCON value. The tropical cyclone is expected to remain over warm waters with low to moderate vertical wind shear for the next couple of days. Therefore, strengthening is likely through the early part of this week. Later in the period, marginal SSTs and increasing south-southwesterly shear should induce weakening. The official wind speed forecast is on the high side of the numerical intensity guidance. Helene continues to move westward, or about 270/11 kt. The system should move westward to west-northwestward to the south of a mid-level ridge over the eastern Atlantic over the next 48 hours or so. Thereafter, a mid-level trough is expected to create a pronounced weakness in the ridge along 40W-45W longitude. This steering pattern is likely to cause Helene to turn northwestward to north-northwestward during the latter part of the forecast period. The official track forecast is very similar to the previous NHC prediction and lies roughly in the middle of the numerical guidance suite. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 13.2N 25.0W 55 KT 65 MPH 12H 10/0000Z 13.7N 27.1W 65 KT 75 MPH 24H 10/1200Z 14.3N 30.0W 75 KT 85 MPH 36H 11/0000Z 14.9N 32.8W 85 KT 100 MPH 48H 11/1200Z 15.7N 35.2W 90 KT 105 MPH 72H 12/1200Z 17.8N 38.6W 80 KT 90 MPH 96H 13/1200Z 21.0N 41.0W 65 KT 75 MPH 120H 14/1200Z 26.0N 43.0W 55 KT 65 MPH $$ Forecaster Pasch

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Tropical Storm Paul Forecast Discussion Number 5

2018-09-09 16:42:16| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Sun Sep 09 2018 213 WTPZ43 KNHC 091442 TCDEP3 Tropical Storm Paul Discussion Number 5 NWS National Hurricane Center Miami FL EP182018 800 AM PDT Sun Sep 09 2018 Paul has not strengthened since the last advisory. Satellite imagery shows that the disorganized low-level center is located on the northeastern edge of the main convective mass, while the cyclone is being influenced by fairly strong easterly shear. The advisory intensity is held at 35 kt in agreement with a Dvorak estimate from TAFB. Although the shear is forecast to relax during the next couple of days, Paul will then be moving over marginal SSTs and into a drier air mass. Therefore only modest strengthening is predicted during the next day or so, followed by a leveling off of the intensity. The official forecast is a little below the previous one, but above most of the latest numerical guidance. Paul is moving slowly northwestward, or 320/7 kt. The cyclone is expected to continue northwestward on the western side of a weak mid-level ridge over the next day or so. Later in the forecast period, a ridge to the north of the system should cause Paul to turn toward the west-northwest. The official track forecast is similar to the previous NHC prediction and close to the dynamical model consensus. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 17.9N 118.1W 35 KT 40 MPH 12H 10/0000Z 18.9N 118.9W 40 KT 45 MPH 24H 10/1200Z 20.1N 120.1W 45 KT 50 MPH 36H 11/0000Z 21.1N 121.7W 45 KT 50 MPH 48H 11/1200Z 21.8N 123.7W 45 KT 50 MPH 72H 12/1200Z 22.8N 127.2W 45 KT 50 MPH 96H 13/1200Z 23.7N 130.0W 35 KT 40 MPH 120H 14/1200Z 24.5N 131.5W 30 KT 35 MPH $$ Forecaster Pasch

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