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Hurricane Isaac Forecast Discussion Number 11

2018-09-10 11:14:53| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Sep 10 2018 488 WTNT44 KNHC 100914 CCA TCDAT4 Hurricane Isaac Discussion Number 11...Corrected NWS National Hurricane Center Miami FL AL092018 500 AM AST Mon Sep 10 2018 Corrected intensity forecast in table below. Isaac has generally changed little during the past several hours. The small hurricane continues to have a central dense overcast feature with some fragmented bands to the south of the center. The Dvorak classifications remain 4.0/65 kt from both TAFB and SAB, and therefore, the initial intensity is held at 65 kt. Isaac is moving westward at 11 kt. The models remain in poor agreement this cycle as they are split into two camps. The UKMET, HWRF, and a few other models show Isaac recurving near or to the east of the Lesser Antilles toward a developing trough over the central Atlantic. Conversely, the GFS and ECMWF have been very consistent in showing a due westward motion across the Lesser Antilles and into the Caribbean as the cyclone moves on the south side of a strengthening mid-level ridge. The NHC track forecast continues to favor the usually more reliable GFS and ECMWF models, and this forecast is near a blend of those aids. It should be noted, however, that confidence in this track forecast is low given the model spread. The intensity forecast is also quite uncertain. One of the complex factors is Isaac's compact size as small hurricanes like this one are more prone to quick intensity changes, both up and down. In addition, the intensity guidance is in poor agreement as solutions range from steady weakening from this point onward to the HWRF that shows Isaac becoming a major hurricane. Given that the wind shear is expected to remain less than 10 kt for another day or two, strengthening is forecast during that time period. After that, however, an increase in shear first from a trough over the central Atlantic and later from the outflow from Florence should result in weakening when the system moves across the Lesser Antilles and into the Caribbean Sea. The NHC intensity forecast is not too different from the previous one and lies above the model consensus in the short term, but below it at the later forecast times. This forecast is in best agreement with the latest LGEM guidance. Key Messages: 1. Isaac is a small hurricane and uncertainty in the forecast is higher than normal. Although Isaac is forecast to begin weakening when it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands. 2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 10/0900Z 14.7N 42.7W 65 KT 75 MPH 12H 10/1800Z 14.8N 44.6W 70 KT 80 MPH 24H 11/0600Z 15.0N 47.4W 75 KT 85 MPH 36H 11/1800Z 15.0N 50.3W 80 KT 90 MPH 48H 12/0600Z 15.0N 53.0W 80 KT 90 MPH 72H 13/0600Z 15.3N 58.0W 70 KT 80 MPH 96H 14/0600Z 15.5N 64.0W 60 KT 70 MPH 120H 15/0600Z 15.6N 69.2W 50 KT 60 MPH $$ Forecaster Cangialosi

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Hurricane Florence Forecast Discussion Number 44

2018-09-10 11:06:51| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Sep 10 2018 672 WTNT41 KNHC 100906 CCA TCDAT1 Hurricane Florence Discussion Number 44...CORRECTED NWS National Hurricane Center Miami FL AL062018 500 AM AST Mon Sep 10 2018 Corrected to reflect that the 96 hour forecast point is inland. Florence is rapidly strengthening this morning. The satellite presentation has improved markedly overnight with a small 10-n-mi wide-eye becoming apparent in infrared satellite pictures. The upper-level outflow continues to expand over the northern and northwestern portions of the storm, but is somewhat restricted over the southeastern quadrant. Dvorak satellite classifications from TAFB and SAB supported an intensity of around 80 kt at 0600 UTC, but with the cooling of the cloud tops around the eye since that time, the initial intensity has been increased to 90 kt for this advisory. Satellite fixes indicate that Florence has turned west-northwestward (285 degrees), and is moving at a slightly faster forward speed of 8 kt. A high pressure ridge building to the north and northwest of Florence is expected to steer the hurricane west-northwestward to northwestward at a much faster forward speed over the southwest Atlantic during the next few days. After that time, a building ridge over the Ohio Valley is expected to cause a gradual reduction in the forward speed of the cyclone as it approaches the southeastern United States coastline. The latest run of the ECMWF has shifted southwestward, along with its ensemble suite, while there was little overall change in the GFS and its ensemble. On the other hand, the UKMET shifted northeastward and is now along the right side of the guidance envelope. With these changes to the guidance, the overall spread has increased this cycle, however, the corrected consensus aids (FSSE and HCCA) are not much different than before, and the NHC track again follows these models very closely. Users are cautioned to not focus on the exact forecast track as the average NHC errors at days 4 and 5 are about 140 and 180 n mi, respectively. Florence will be traversing very warm SSTs of around 29C and remain within a very favorable upper-level environment during the next couple of days. These conditions are expected to lead to significant strengthening during the next 12 to 24 hours, and Florence is forecast to be a very powerful major hurricane on its approach to the southeastern United States. The NHC intensity forecast is slightly above all of the intensity guidance during the first 24 hours, and is then a blend of the FSSE and HCCA models. The global model guidance also increases the size of Florence's wind field during the next few days, and this has been reflected in the NHC wind radii forecast. The NOAA G-IV jet is conducting another synoptic surveillance mission this morning in support of the 1200 UTC model cycle, and these flights will continue through Tuesday. A NOAA P-3 Hurricane Hunter aircraft is also scheduled to conduct a research mission into Florence this morning, with Air Force C-130 fix missions beginning late this afternoon. Additional upper-air data are being collected across portions of the central and eastern U.S. via special 0600 UTC and 1800 UTC radiosonde launches. Hopefully these data will help improve the track and intensity forecasts. Key Messages: 1. There is an increasing risk of life-threatening impacts from Florence: storm surge at the coast, freshwater flooding from a prolonged and exceptionally heavy rainfall event inland, and damaging hurricane-force winds. 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. 2. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week. These swells will result in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 10/0900Z 24.9N 58.9W 90 KT 105 MPH 12H 10/1800Z 25.4N 60.5W 110 KT 125 MPH 24H 11/0600Z 26.1N 63.1W 120 KT 140 MPH 36H 11/1800Z 27.3N 66.2W 125 KT 145 MPH 48H 12/0600Z 28.8N 69.3W 130 KT 150 MPH 72H 13/0600Z 32.2N 74.8W 125 KT 145 MPH 96H 14/0600Z 34.5N 78.1W 100 KT 115 MPH...INLAND 120H 15/0600Z 35.8N 79.6W 40 KT 45 MPH...INLAND $$ Forecaster Brown

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Hurricane Helene Forecast Discussion Number 12

2018-09-10 10:52:49| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Sep 10 2018 739 WTNT43 KNHC 100852 TCDAT3 Hurricane Helene Discussion Number 12 NWS National Hurricane Center Miami FL AL082018 500 AM AST Mon Sep 10 2018 Little change in the Helene's cloud pattern has been observed during the past 6 hours. The earlier mentioned ragged banding-type eye feature is still apparent in the enhanced infrared BD-curve imagery and a recent SSMI/S microwave overpass. The initial intensity is held at 75 kt, and is based on the subjective Dvorak intensity estimates. Helene is moving within a fairly ripe environment and over warm oceanic sea surface temperatures. These conducive conditions should promote additional strengthening over the next 36 hours. Afterward, increasing south-southwesterly shear, lower mid-tropospheric moisture, and cooler water are expected to induce weakening, however gradually, through day 5. The official forecast follows suit and is similar to the better performing IVCN intensity consensus model. Helene's initial motion is estimated to be west-northwestward, or 285/15 kt, and this general motion is forecast during the next 48 hours. Beyond that period, a break in the subtropical ridge to the northwest of Paul, created by a mid- to upper-level cut-off low digging southward, should induce a northward turn with a gradual increase in forward motion. The large-scale models all indicate this change in the synoptic steering pattern, although there is some disparity on how soon Helene will turn toward the north. The NHC track is quite close to the previous forecast and lies between the consensus guidance. FORECAST POSITIONS AND MAX WINDS INIT 10/0900Z 14.3N 28.9W 75 KT 85 MPH 12H 10/1800Z 14.8N 30.9W 80 KT 90 MPH 24H 11/0600Z 15.5N 33.4W 85 KT 100 MPH 36H 11/1800Z 16.3N 35.6W 85 KT 100 MPH 48H 12/0600Z 17.2N 37.2W 80 KT 90 MPH 72H 13/0600Z 20.3N 39.4W 65 KT 75 MPH 96H 14/0600Z 25.3N 40.8W 55 KT 65 MPH 120H 15/0600Z 31.2N 41.1W 50 KT 60 MPH $$ Forecaster Roberts

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Tropical Storm Paul Forecast Discussion Number 8

2018-09-10 10:45:21| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Mon Sep 10 2018 630 WTPZ43 KNHC 100845 TCDEP3 Tropical Storm Paul Discussion Number 8 NWS National Hurricane Center Miami FL EP182018 200 AM PDT Mon Sep 10 2018 Paul's poorly defined surface circulation remains displaced well to the east of an intermittent burst of deep convection. A 0448 UTC ASCAT-A pass covering the western semicircle showed only 30 kt winds. Based on the scatterometer pass, and a blend of the satellite intensity estimates from TAFB and SAB, and an earlier 0228 UTC SATCON analysis of 37 kt, the initial intensity is lowered to 35 kt. The UW-CIMSS shear analysis indicates 15-20 kt of east-northeasterly shear impeding the eastern side of the cyclone. The Decay SHIPS intensity model shows the shear diminishing within the next 24 hours or so, however, Paul will be moving over cooler waters and into a more stable thermodynamic environment. Therefore, weakening is forecast in 36 hours, or less, and Paul is expected to degenerate into a remnant low in 3 days. The intensity forecast is similar to the previous advisory and is based on the NOAA-HCCA and IVCN consensus models. The exposed surface circulation continues to move a bit erratically this morning, but the 12 hour motion is an uncertain northwestward, or 320/8 kt. Paul should continue to move within the mid-level steering flow produced by a ridge to the north through day 3, then a turn westward is indicated as the vertically shallower cyclone moves within the low-level trades. The NHC track forecast is an update of the last one, and agrees with the various consensus guidance. FORECAST POSITIONS AND MAX WINDS INIT 10/0900Z 20.1N 119.9W 35 KT 40 MPH 12H 10/1800Z 20.8N 120.9W 35 KT 40 MPH 24H 11/0600Z 21.5N 122.5W 35 KT 40 MPH 36H 11/1800Z 22.1N 124.3W 30 KT 35 MPH 48H 12/0600Z 22.5N 125.9W 30 KT 35 MPH 72H 13/0600Z 23.4N 128.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 14/0600Z 23.6N 130.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 15/0600Z 23.6N 132.3W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Roberts

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Hurricane Isaac Forecast Discussion Number 10

2018-09-10 04:56:17| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sun Sep 09 2018 684 WTNT44 KNHC 100256 TCDAT4 Hurricane Isaac Discussion Number 10 NWS National Hurricane Center Miami FL AL092018 1100 PM AST Sun Sep 09 2018 Isaac's IR satellite presentation has generally improved since the last advisory. While there is no evidence of an eye in conventional satellite imagery, the most recent microwave imagery around 2100 UTC indicated the presence of a pinhole eye. Dvorak intensity estimates are T4.0 from both TAFB and SAB, and on this basis Isaac has been upgraded to a hurricane, the 5th of the 2018 Atlantic season and 3rd in the past few days. I must stress that all aspects of the forecast are highly uncertain at this time. The dynamical track model spread is nearly 1000 miles at day 5, while the intensity guidance spread is nearly 100 kt. Although the NHC track and intensity forecasts have not been changed very much, this should not be interpreted as increasing confidence in the forecast. With regard to track, there is a clear split in the model guidance regarding the speed of Isaac. The GFS and ECMWF models forecast that a mid-level ridge to the north will accelerate the hurricane westward for the next 36 h, with a westward motion continuing through day 5 as the ridge is reinforced in the wake of Florence. On the other hand, the UKMET and HWRF models show a weakness developing in the ridge which causes Isaac to move much slower, and eventually turn northward. The NHC forecast has been favoring the former solution, and continues to do so with this advisory since I see no reason to make a drastic change to the forecast at this point. Isaac is a very small hurricane, which makes the intensity forecast particularly tricky since small storms are susceptible to rapid changes of intensity, up and down. Since the wind shear is expected to remain low, and Isaac is forecast to remain over fairly warm SSTs for the next 48 h, the current forecast of further intensification seems reasonable, and the NHC forecast is still at the top end of the guidance envelope. From 72 h onward, there could be a sudden, and substantial increase in wind shear associated first with an upper-level trough to the north, and later with the outflow of Hurricane Florence. Assuming this shear affects Isaac as expected, the tiny hurricane would likely weaken quickly, perhaps even faster than currently shown in the forecast. By the end of the forecast period, the NHC forecast remains a fair amount below the intensity consensus, closer to the weakening solutions of the GFS and ECMWF. Key Messages: 1. Isaac is a small hurricane and uncertainty in the forecast is higher than normal. Although Isaac is forecast to begin weakening by Tuesday when it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands. 2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 10/0300Z 14.5N 41.6W 65 KT 75 MPH 12H 10/1200Z 14.7N 43.5W 75 KT 85 MPH 24H 11/0000Z 14.9N 46.3W 80 KT 90 MPH 36H 11/1200Z 15.0N 49.3W 85 KT 100 MPH 48H 12/0000Z 15.0N 52.0W 85 KT 100 MPH 72H 13/0000Z 15.2N 57.3W 75 KT 85 MPH 96H 14/0000Z 15.5N 63.5W 60 KT 70 MPH 120H 15/0000Z 15.5N 70.0W 50 KT 60 MPH $$ Forecaster Zelinsky

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