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Tropical Storm MADELINE Forecast Discussion Number 4

2016-08-27 16:37:50| National Hurricane Center (East Pacific)

Issued at 800 AM PDT SAT AUG 27 2016 000 WTPZ44 KNHC 271437 TCDEP4 TROPICAL STORM MADELINE DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL EP142016 800 AM PDT SAT AUG 27 2016 A GPM overpass from 1146 UTC indicated that the low-level center of Madeline was located northeast of previous estimates, near the northeastern edge of the convection. This is consistent with the moderate northeasterly shear analyzed over the cyclone by the SHIPS model output based on the GFS and ECMWF fields. The initial intensity is held at 45 kt for this advisory in agreement with the latest Dvorak estimates from TAFB and SAB, and this value is also close to the latest UW-CIMSS SATCON analysis. Given the relocation of the center, the best estimate of the initial motion is an uncertain 310/08. Madeline is expected to continue moving generally northwestward for the next 72 hours around the southwesterly periphery of a mid-level ridge centered near 125W. After that time, a ridge centered north of the Hawaiian Islands becomes the dominant steering mechanism, and should impart a westward motion through the remainder of the forecast period. While the overall track forecast philosophy has not changed, the more northerly initial position has resulted in a northward shift of the guidance envelope and the NHC forecast by 30 to 50 n mi through the forecast period. The new NHC track is near the middle of the guidance envelope and close to the latest multi-model consensus. The SHIPS model shows the shear decreasing over Madeline during the next 12 hours or so, which should allow for some intensification through 72 hours while the cyclone is moving over SSTs above 27C. The NHC forecast continues to show Madeline reaching hurricane intensity in about 36 hours. At days 4 and 5, the SSTs along the track drop below 27C and the westerly shear increases, which should result in some weakening. The new NHC intensity forecast is a little lower than the previous one after 48 hours due to the more northerly track and is near or a little above the latest intensity consensus. It is too early to determine what impacts Madeline could have on the Hawaiian Islands late in the forecast period. It is important to remind users that the average day 4 and 5 track forecast errors for eastern Pacific tropical cyclones are around 145 and 170 miles, respectively. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 15.2N 138.5W 45 KT 50 MPH 12H 28/0000Z 15.9N 139.4W 50 KT 60 MPH 24H 28/1200Z 16.7N 140.7W 60 KT 70 MPH 36H 29/0000Z 17.7N 142.0W 70 KT 80 MPH 48H 29/1200Z 18.8N 143.6W 70 KT 80 MPH 72H 30/1200Z 20.5N 147.0W 70 KT 80 MPH 96H 31/1200Z 21.0N 151.2W 60 KT 70 MPH 120H 01/1200Z 21.0N 155.5W 50 KT 60 MPH $$ Forecaster Brennan

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Tropical Storm MADELINE Forecast Discussion Number 3

2016-08-27 11:03:51| National Hurricane Center (East Pacific)

Issued at 200 AM PDT SAT AUG 27 2016 000 WTPZ44 KNHC 270903 TCDEP4 TROPICAL STORM MADELINE DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL EP142016 200 AM PDT SAT AUG 27 2016 Madeline is gradually becoming better organized this morning, with increasing convective banding noted near the center. Satellite intensity estimates from TAFB and SAB have increased to 45 kt, and this intensity is supported by various objective estimates from CIMSS and CIRA. The initial motion is now 300/9. For the next few days, Madeline should move generally northwestward as it is steered by a segment of the subtropical ridge. After 72 hours, a turn toward the west is likely as Madeline encounters the westward-building subtropical ridge that is steering Hurricane Lester. The track guidance has shifted significantly to the north since the previous advisory, and near the end of the forecast period there is a fair amount of spread regarding whether Madeline will move near the Hawaiian Islands or north of them. The new forecast track is shifted northward, but it lies to the south of the center of the guidance envelope and the consensus models. Madeline will be moving over SSTs above 27C and in a light-to-moderate easterly shear environment for the next 2 to 3 days. This should allow for steady intensification, and the new intensity forecast follows the trend of the previous forecast in showing Madeline becoming a hurricane in 36 hours or so. After 72 hours, cooler sea surface temperatures and a drier air mass along the forecast track should result in a slow weakening. The new intensity forecast is a little stronger than the previous forecast, but it lies a little below the intensity consensus. It is too early to determine what, if any, impacts this cyclone could have on the Hawaiian Islands near the end of the forecast period. It is important to remind users that the average 5-day track forecast error for eastern Pacific tropical cyclones is about 170 miles. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 14.5N 138.4W 45 KT 50 MPH 12H 27/1800Z 15.1N 139.3W 50 KT 60 MPH 24H 28/0600Z 15.8N 140.5W 60 KT 70 MPH 36H 28/1800Z 16.7N 141.8W 70 KT 80 MPH 48H 29/0600Z 17.8N 143.4W 75 KT 85 MPH 72H 30/0600Z 19.5N 146.5W 75 KT 85 MPH 96H 31/0600Z 20.5N 150.5W 65 KT 75 MPH 120H 01/0600Z 20.5N 155.0W 60 KT 70 MPH $$ Forecaster Beven

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Tropical Storm GASTON Forecast Discussion Number 19

2016-08-27 10:53:26| National Hurricane Center (Atlantic)

Issued at 500 AM AST SAT AUG 27 2016 000 WTNT42 KNHC 270853 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 19 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 AM AST SAT AUG 27 2016 Recent microwave data indicate that Gaston's center is now well embedded near the middle of the central dense overcast. The microwave images also show that a partial mid-level ring has formed, but it remains open to the north-northeast. Even though Gaston's structure appears to be improving, the maximum winds remain 55 kt based on Dvorak Current Intensity numbers from TAFB and SAB. Two outflow jets are emanating away to the east and southwest of Gaston, but the outflow remains restricted to the south and northwest. Still, the shear has decreased substantially from what it was a day or two ago, and it should remain generally low for the next 48 hours. In addition, sea surface temperatures ahead of Gaston are expected to be at least 28C for several more days. Therefore, strengthening is anticipated through day 3, followed by gradual weakening on days 4 and 5 due to increasing westerly shear. Although the intensity models all agree on this general scenario, there is a little more spread in the peak intensities than has been noted in previous advisories. On the high end, the Florida State Superensemble still shows Gaston approaching major hurricane intensity, while the LGEM model is at the other extreme being about 20 kt lower. The updated NHC intensity forecast is near the middle of the guidance and ends up just a little lower than the previous forecast. Gaston is moving northwestward, or 310 degrees at 13 kt, between a mid-tropospheric high to its northeast and a mid-/upper-level low to its southwest. The cyclone is expected to maintain a northwestward heading but steadily decelerate during the next 48 hours. After that time, Gaston should recurve sharply east-northeastward and accelerate through the end of the forecast period as it gets picked up by the mid-latitude westerlies. The track guidance has trended westward through 48 hours, delaying Gaston's recurvature just a bit, and it is then a little faster by days 4 and 5. The NHC track forecast has been adjusted accordingly and is close to the TVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 27.9N 52.0W 55 KT 65 MPH 12H 27/1800Z 28.8N 53.6W 60 KT 70 MPH 24H 28/0600Z 29.9N 55.0W 70 KT 80 MPH 36H 28/1800Z 30.6N 56.0W 80 KT 90 MPH 48H 29/0600Z 31.2N 56.8W 85 KT 100 MPH 72H 30/0600Z 32.5N 55.6W 90 KT 105 MPH 96H 31/0600Z 34.5N 50.5W 85 KT 100 MPH 120H 01/0600Z 37.0N 43.5W 75 KT 85 MPH $$ Forecaster Berg

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Hurricane LESTER Forecast Discussion Number 11

2016-08-27 10:50:21| National Hurricane Center (East Pacific)

Issued at 200 AM PDT SAT AUG 27 2016 000 WTPZ43 KNHC 270850 TCDEP3 HURRICANE LESTER DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132016 200 AM PDT SAT AUG 27 2016 Satellite imagery shows that Lester is continuing to intensity, with an eye gradually becoming better defined inside a central dense overcast. Various subjective and objective satellite intensity estimates are near 75 kt, and that value is the initial intensity for this advisory. The hurricane has good cirrus outflow in the northwestern semicircle. The initial motion is now 280/9. Lester is south of a strong deep-layer subtropical ridge, and the dynamical models forecast this feature to build westward to the north of the hurricane through the forecast period. The forecast guidance remains tightly clustered around an almost due west motion with an increase in forward speed during the next several days. The new forecast track is an update of the previous track and lies near the center of the guidance envelope. Lester is expected to remain in a low-shear environment during the forecast period, so the main external factors controlling the intensity will be sea surface temperatures and possible dry air entrainment. During the first 48 hours or so, the hurricane will be over generally warm water. Based on current trends, this part of the intensity forecast has been raised significantly from the previous advisory and calls for a peak intensity of 95 kt. After 48 hours, a combination of slightly cooler water and dry air entrainment should cause a gradual weakening. Overall, the new intensity forecast lies near the upper edge of the guidance. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 17.9N 117.4W 75 KT 85 MPH 12H 27/1800Z 18.0N 118.8W 85 KT 100 MPH 24H 28/0600Z 18.1N 121.0W 90 KT 105 MPH 36H 28/1800Z 18.2N 123.5W 95 KT 110 MPH 48H 29/0600Z 18.4N 126.3W 95 KT 110 MPH 72H 30/0600Z 18.5N 132.0W 90 KT 105 MPH 96H 31/0600Z 18.5N 137.5W 85 KT 100 MPH 120H 01/0600Z 18.0N 142.5W 80 KT 90 MPH $$ Forecaster Beven

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Tropical Storm GASTON Forecast Discussion Number 18

2016-08-27 04:52:58| National Hurricane Center (Atlantic)

Issued at 1100 PM AST FRI AUG 26 2016 000 WTNT42 KNHC 270252 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 PM AST FRI AUG 26 2016 The structure of Gaston appears to be slowly improving, with less evidence of vertical shear than earlier today. Deep convection has been forming fairly close to the center in a more symmetric fashion, but still hasn't been able to persist for very long. Subjective estimates are a little higher than earlier, but still support an initial wind speed of 55 kt for this advisory. Gaston should move into a low-shear, warm-water environment during the next 2 or 3 days, which would usually support a great deal of strengthening. However, a key hindering factor remains the nearby low-level moisture, which is forecast to substantially decrease during the next few days. This should temper the expected intensification and reduce the chances for rapid strengthening. Gaston is likely to encounter increasing upper-level flow beyond 3 days, which probably will start a weakening trend in combination with gradually cooling SSTs. Model guidance is generally in good agreement on this scenario, with less spread in the intensity models than is typically seen. The new prediction is a blend of the previous forecast and the intensity consensus. Microwave data suggest that Gaston continues to move northwestward, now at about 13 kt, a bit slower than before. The cyclone is moving between a strong upper low to the southwest and a subtropical ridge to its northeast. The ridge is forecast to gradually weaken, resulting in Gaston decelerating over the next couple of days. Steering currents get quite light in about 3 days, and the cyclone is expected to turn northeastward or east-northeastward it moves around the northwest side of a distant ridge over the eastern Atlantic. Computer models are coming into better agreement on a sharper, slower recurvature, first suggested by the ECMWF yesterday. The latest NHC forecast is close to the previous one for the first few days, then is trended eastward to follow the model trend. FORECAST POSITIONS AND MAX WINDS INIT 27/0300Z 27.0N 50.8W 55 KT 65 MPH 12H 27/1200Z 28.0N 52.5W 60 KT 70 MPH 24H 28/0000Z 29.2N 54.1W 70 KT 80 MPH 36H 28/1200Z 30.2N 55.2W 80 KT 90 MPH 48H 29/0000Z 30.8N 55.7W 90 KT 105 MPH 72H 30/0000Z 31.8N 55.4W 95 KT 110 MPH 96H 31/0000Z 33.0N 52.0W 90 KT 105 MPH 120H 01/0000Z 35.0N 47.5W 80 KT 90 MPH $$ Forecaster Blake

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