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Tropical Storm LESTER Forecast Discussion Number 7

2016-08-26 10:51:42| National Hurricane Center (East Pacific)

Issued at 300 AM MDT FRI AUG 26 2016 000 WTPZ43 KNHC 260851 TCDEP3 TROPICAL STORM LESTER DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132016 300 AM MDT FRI AUG 26 2016 While Lester continues to produce a large area of cold cloud tops, recent microwave imagery suggests that the storm is being affected by vertical wind shear and dry air entrainment that is displacing the convection mostly to the east and northeast of the center. A recent ASCAT-B overpass indicated 45-50 kt winds about 40 n mi northeast of the center, and based on this the initial intensity remains 50 kt. The microwave data show that Lester has moved westward for the past several hours, with the initial motion now 280/6. The cyclone should turn west-northwestward later today as it moves toward a weakness in the subtropical ridge. After 12-24 hours, Lester should resume a westward track with an increase in forward speed as the ridge strengthens and builds westward. The track model guidance is in excellent agreement with this scenario, and the new forecast track is near the center of the guidance envelope. The new forecast is a little to the south of the previous forecast based on the current position and motion. The dynamical models forecast the shear to decrease over the next 24 hours and remain low for the rest of the forecast period, which should allow Lester to again intensify. The forecast track keeps the cyclone over relatively warm water for about 72 hours, so the intensity forecast calls for gradual strengthening during that time. Subsequently, the waters cool a little along the forecast track, and the intensity forecast thus shows some weakening. The new intensity forecast is similar to the previous forecast, except slightly weaker during the first 24 hours due to the shear. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 16.9N 114.5W 50 KT 60 MPH 12H 26/1800Z 17.1N 115.5W 55 KT 65 MPH 24H 27/0600Z 17.3N 116.9W 60 KT 70 MPH 36H 27/1800Z 17.4N 118.7W 70 KT 80 MPH 48H 28/0600Z 17.4N 120.8W 75 KT 85 MPH 72H 29/0600Z 17.5N 126.0W 80 KT 90 MPH 96H 30/0600Z 17.5N 131.5W 80 KT 90 MPH 120H 31/0600Z 17.5N 136.0W 75 KT 85 MPH $$ Forecaster Beven

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Tropical Storm LESTER Forecast Discussion Number 6

2016-08-26 04:45:17| National Hurricane Center (East Pacific)

Issued at 900 PM MDT THU AUG 25 2016 000 WTPZ43 KNHC 260245 TCDEP3 TROPICAL STORM LESTER DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132016 900 PM MDT THU AUG 25 2016 Satellite images reveal that the cloud pattern has not changed very much during the past several hours, and deep convection is not as strong as earlier today. Some arc clouds have been moving away from the cyclone, suggesting that Lester is not intensifying at this time. This is confirmed by the latest Dvorak estimates which still lead to an intensity of 50 kt. Nevertheless, Lester's upper-level outflow is well established and, with low shear and warm waters in its future path, strengthening is in order. Thus, Lester is forecast to become a hurricane in a day or so. By the end of the forecast period, cooler waters should induce gradual weakening. The NHC forecast is basically the same as the previous one. Lester has slowed down a little bit and is now moving toward the west or 270 degrees at 5 kt. The cyclone should resume a west- northwest track soon toward a current weakness of the ridge. However, in about 2 days, the ridge should amplify and build westward, and this pattern should steer Lester westward through the rest of the forecast period. Given that the steering currents are expected to be very well established, tracks models are in very good agreement. The NHC forecast follows the multi-model consensus and has not deviated very much from the previous official prediction. FORECAST POSITIONS AND MAX WINDS INIT 26/0300Z 17.0N 113.8W 50 KT 60 MPH 12H 26/1200Z 17.4N 114.6W 55 KT 65 MPH 24H 27/0000Z 17.8N 115.9W 65 KT 75 MPH 36H 27/1200Z 18.0N 117.5W 70 KT 80 MPH 48H 28/0000Z 18.0N 119.5W 75 KT 85 MPH 72H 29/0000Z 18.5N 124.0W 80 KT 90 MPH 96H 30/0000Z 18.5N 129.5W 80 KT 90 MPH 120H 31/0000Z 18.5N 134.5W 75 KT 85 MPH $$ Forecaster Avila

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Tropical Storm GASTON Forecast Discussion Number 13

2016-08-25 22:38:39| National Hurricane Center (Atlantic)

Issued at 500 PM AST THU AUG 25 2016 000 WTNT42 KNHC 252038 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 PM AST THU AUG 25 2016 Strong southwesterly shear continues to take a toll on Gaston. The cloud pattern has become more assymetric with all of the deep core convection located north and east of the center. This was confirmed by a recent SSMIS microwave overpass that showed significant southwest to northeast tilt between the low- and mid-level centers. Objective Dvorak T-numbers have decreased and subjective intensity estimates from TAFB and SAB support an initial wind speed of 55 kt. A large upper-level low near 23N 54W that is moving southwestward is expected to continue to produce an unfavorable upper-level environment over Gaston for another 12 to 18 hours. Some additional weakening is possible tonight, but the official NHC forecast shows little change in strength through Friday. After that time, Gaston should be moving around the northern portion of the upper-low and into a more favarable upper-level wind pattern. This combined with warm SSTs and a moist atmosphere should allow Gaston to re-strengthen over the weekend and once again become a hurricane. The updated NHC intensity forecast is slightly above the previous advisory after 36 hours, and is in good agreement with the latest SHIPS guidance. The initial motion remains northwestward at 15 kt. Gaston should move northwestward during the next day or so around a mid-level ridge over the east-central Atlantic. In 36 to 48 hours, a ridge is forecast to build to the north of Gaston, which should cause the cyclone to turn west-northwestward. Early next week, the ridge is forecast to weaken and Gaston is expected to turn northward, then recurve into the mid-latitude westerlies in 4 to 5 days. The track guidance is in good agreement during the first 48 hours, but there is some spread as to what longitude recurvature begins. The ECMWF shows a slower motion near the end of the forecast period and a track along the eastern side of the guidance envelope, while the GFS and GFS ensemble mean are along the western edge. The NHC track foreast remains near the middle of the guidance envelope and is in closest agreement with UKMET, FSSE, and multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 21.6N 45.5W 55 KT 65 MPH 12H 26/0600Z 23.4N 47.1W 55 KT 65 MPH 24H 26/1800Z 25.5N 49.6W 55 KT 65 MPH 36H 27/0600Z 27.0N 52.3W 60 KT 70 MPH 48H 27/1800Z 28.1N 54.7W 70 KT 80 MPH 72H 28/1800Z 30.2N 57.0W 85 KT 100 MPH 96H 29/1800Z 31.8N 57.7W 95 KT 110 MPH 120H 30/1800Z 34.0N 55.0W 95 KT 110 MPH $$ Forecaster Brown

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Tropical Storm LESTER Forecast Discussion Number 5

2016-08-25 22:38:11| National Hurricane Center (East Pacific)

Issued at 300 PM MDT THU AUG 25 2016 000 WTPZ43 KNHC 252038 TCDEP3 TROPICAL STORM LESTER DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132016 300 PM MDT THU AUG 25 2016 The cloud pattern of Lester has become only slightly better organized than earlier today. Taking the mean of Dvorak estimates from TAFB and SAB leads to a current intensity estimate of 50 kt. An upper-level cyclonic shear axis just to the northwest of the storm is clearly evident in water vapor imagery, and this feature is somewhat impeding Lester's outflow to the north. Overall, however, the atmospheric and oceanic environment should be conducive for strengthening with low shear and a sufficiently warm ocean during the next several days. The NHC intensity forecast shows Lester becoming a hurricane within 24 hours, with additional intensification after that time frame. Late in the forecast period, marginal SSTs should halt the strengthening process. The official intensity forecast is fairly close to the multi-model consensus, IVCN. The storm is slowing down a bit and the initial motion is about 295/9. A slight weakness in the subtropical ridge to the north of Lester is expected to cause some further deceleration during the next day or two. Later in the forecast period, a stronger ridge should induce a westward motion at a faster forward speed. The official track forecast is similar to the previous one and about in the middle of the dynamical guidance suite. FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 16.9N 113.4W 50 KT 60 MPH 12H 26/0600Z 17.3N 114.3W 55 KT 65 MPH 24H 26/1800Z 17.7N 115.4W 65 KT 75 MPH 36H 27/0600Z 18.0N 117.0W 70 KT 80 MPH 48H 27/1800Z 18.0N 118.6W 75 KT 85 MPH 72H 28/1800Z 18.0N 123.0W 80 KT 90 MPH 96H 29/1800Z 18.0N 128.5W 80 KT 90 MPH 120H 30/1800Z 18.0N 133.5W 75 KT 85 MPH $$ Forecaster Pasch

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Tropical Storm GASTON Forecast Discussion Number 12

2016-08-25 16:40:45| National Hurricane Center (Atlantic)

Issued at 1100 AM AST THU AUG 25 2016 000 WTNT42 KNHC 251440 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 AM AST THU AUG 25 2016 The satellite presentation of Gaston has degraded some since the previous advisory, with deep convection bursting northeast of the center due to about 25 kt of southwesterly shear as analyzed by UW-CIMSS. An ASCAT-B pass from 1212 UTC was helpful in locating the center and showed peak winds of around 55 kt, which support an intensity of 60 kt for this advisory, a little above the latest Dvorak estimates of T3.5/55 kt. The strong shear is forecast to continue for the next 24 hours until Gaston moves poleward of an upper-level low currently located to its west. Given the shear and SSTs around 27-28C, little change in intensity, or perhaps some slight weakening, is expected in the first 24 hours. After that time, the shear decreases and Gaston moves over SSTs of 29-30C, which should allow for strengthening through the rest of the forecast period. The NHC intensity forecast is a little higher than the previous one later in the period and is close to the SHIPS model and the IVCN intensity consensus. The initial motion estimate is 315/15. Gaston should continue moving quickly northwestward for the next 24 to 36 hours around a mid-level ridge centered to its northeast. Ridging then builds westward to the north of the tropical cyclone, leading to a decrease in forward speed and a turn toward the west-northwest in 48 hours. Late in the period, Gaston will turn poleward into a weakness along 60W and then begin to recurve into the mid-latitude westerlies. The track model guidance agrees on this general scenario, however, there are some differences in the timing and sharpness of recurvature. The GFS, GFS ensemble mean, and UKMET are on the left side of the guidance envelope with a broader turn, while the ECMWF, HWRF, and GFDL show a sharper turn on the right side of the guidance. Given the uncertainty, the NHC forecast remains near the previous one in the middle of the guidance envelope, and is close to the TVCN multi-model consensus. The initial and forecast 34-kt and 50-kt wind radii were adjusted based on data from the ASCAT pass. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 20.4N 44.4W 60 KT 70 MPH 12H 26/0000Z 22.1N 45.9W 55 KT 65 MPH 24H 26/1200Z 24.4N 48.2W 55 KT 65 MPH 36H 27/0000Z 26.0N 50.9W 60 KT 70 MPH 48H 27/1200Z 27.1N 53.1W 65 KT 75 MPH 72H 28/1200Z 29.4N 56.6W 80 KT 90 MPH 96H 29/1200Z 31.0N 58.0W 90 KT 105 MPH 120H 30/1200Z 33.0N 57.0W 90 KT 105 MPH $$ Forecaster Brennan

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