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Tropical Storm MADELINE Forecast Discussion Number 6

2016-08-28 04:39:26| National Hurricane Center (East Pacific)

Issued at 800 PM PDT SAT AUG 27 2016 000 WTPZ44 KNHC 280239 TCDEP4 TROPICAL STORM MADELINE DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL EP142016 800 PM PDT SAT AUG 27 2016 The cloud pattern has changed little in organization during the past several hours and convection is not as deep as earlier today. However, the pattern still consists of a cyclonically curved convective band around the center. Dvorak estimates continue to support an initial intensity of 50 kt. The cyclone is expected to be moving within an environment of light shear for the next 2 to 3 days and over warm waters. This should encourage some intensification. After that time however, the shear is expected to increase to above 20 kt, and the environment will be less humid, resulting in gradual weakening. The NHC forecast is very similar to its predecessor, and is very close to the intensity consensus IVCN. Satellite fixes indicate that Madeline is moving toward the west- northwest or 285 degrees at 7 kt. The cyclone is located on the southwestern edge of the subtropical ridge and is approaching a large mid-latitude trough to its northwest. This pattern should steer Madeline on a more west-northwest course during the next day or two. The trough is forecast to lift out quickly and be replaced by a strong ridge in about 3 days. This pattern should force Madeline to turn toward the west or even south of due west by the end of the forecast period. Track guidance has been very consistent with this solution, and the NHC forecast is in a narrow gap between the previous NHC prediction and the multi-model consensus. Given the small southward shift of the consensus at day 5, the NHC forecast position was adjusted southward accordingly. The confidence in the track foreast late in the period is lower than usual and 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. Given that Madeline has moved into the Central Pacific basin, future information on this system will be issued by the Central Pacific Hurricane Center. FORECAST POSITIONS AND MAX WINDS INIT 28/0300Z 15.7N 140.1W 50 KT 60 MPH 12H 28/1200Z 16.5N 141.0W 55 KT 65 MPH 24H 29/0000Z 17.4N 142.4W 65 KT 75 MPH 36H 29/1200Z 18.4N 143.8W 70 KT 80 MPH 48H 30/0000Z 19.2N 145.5W 70 KT 80 MPH 72H 31/0000Z 20.0N 149.5W 60 KT 70 MPH 96H 01/0000Z 19.5N 153.8W 55 KT 65 MPH 120H 02/0000Z 19.0N 158.5W 45 KT 50 MPH $$ Forecaster Avila

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Tropical Storm GASTON Forecast Discussion Number 21

2016-08-27 22:40:42| National Hurricane Center (Atlantic)

Issued at 500 PM AST SAT AUG 27 2016 000 WTNT42 KNHC 272040 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 21 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 PM AST SAT AUG 27 2016 The inner-core convection of Gaston has continued to increase with the CDO having expanded and become more symmetrical. Subjective satellite intensity estimates remain T4.0/65 kt and T3.5/55 kt from TAFB and SAB, respectively, and the latest UW-CIMSS ADT value is T3.9/63 kt. An average of these estimates supports maintaining the initial intensity at 60 kt, which could be conservative. Gaston's initial motion estimate is 320/08 kt. There is no significant change to the previous forecast track reasoning. Gaston is expected to continue moving northwestward and gradually decelerate during the next two days as the cyclone moves into a break in the Bermuda-Azores ridge. A shortwave trough and associated frontal system is forecast to approach Gaston by 72 hours and slowly lift out the tropical cyclone to the north and northeast. On days 4 and 5, Gaston is expected to get caught up in the mid-latitude westerly flow ahead of the aforementioned frontal system and accelerate east-northeastward. The new NHC track model guidance is in much better agreement now that the ECMWF model has made a significant eastward shift in its track forecast and is much closer to the previous advisory track. The new official forecast track was only shifted eastward slightly to come more in line with the consensus models TVCN and GFEX. However, the track shift also resulted in a significant decrease in the forward speed on days 3, 4, and 5. The global and regional models continue to indicate that Gaston will remain in a very favorable upper-level outflow pattern for at least the next 72 hours. In fact, the Navy-COAMPS, GFS, and ECMWF models forecast Gaston to become a major hurricane by 72-120 h, with the ECMWF forecasting a central pressure of 945 mb on day 5. In contrast, the HWRF and the statistical-dynamical models SHIPS/LGEM only bring Gaston up to 85-90 kt. Due to the possibility of occasional dry air intrusions, with mid-level humidity values in the 40-percent range, weakening the inner-core convection, the official intensity forecast remains unchanged and on the conservative side through 72 hours, which is slightly above the consensus model IVCN. On days 4 and 5, strong westerly shear of about 30 kt should induce weakening, which also argues against this system becoming a major hurricane at that time like the ECMWF model is predicting, and the forecast is a little lower than the consensus on those days. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 29.2N 54.0W 60 KT 70 MPH 12H 28/0600Z 30.0N 54.6W 70 KT 80 MPH 24H 28/1800Z 30.7N 55.1W 80 KT 90 MPH 36H 29/0600Z 31.0N 55.3W 85 KT 100 MPH 48H 29/1800Z 31.4N 54.9W 90 KT 105 MPH 72H 30/1800Z 32.5N 53.0W 85 KT 100 MPH 96H 31/1800Z 34.0N 50.0W 75 KT 85 MPH 120H 01/1800Z 37.0N 44.0W 65 KT 75 MPH $$ Forecaster Stewart

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Hurricane LESTER Forecast Discussion Number 13

2016-08-27 22:37:14| National Hurricane Center (East Pacific)

Issued at 200 PM PDT SAT AUG 27 2016 000 WTPZ43 KNHC 272036 TCDEP3 HURRICANE LESTER DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132016 200 PM PDT SAT AUG 27 2016 Lester's cloud pattern has increased some in organization during the last several hours. The cyclone's small central dense overcast has become more symmetric, with a well-defined 20 n mi wide eye evident in visible satellite imagery. A broken band is also wrapping around the southeastern semicircle of the Lester's circulation. Dvorak T-numbers are a consensus T5.0 from both satellite agencies. Based on these data, the initial intensity estimate is increased to 90 kt. Lester has been moving a little faster, and the initial motion estimate is 270/11. A strong subtropical ridge to the north of Lester should steer the cyclone on a nearly due-west course for at least the next few days. After about 72 hours, there are differences in the strength of the subtropical ridge north of Lester, which are somewhat dependent on the progression of a mid-latitude trough through the eastern Pacific. There also exists some potential for binary interaction with Madeline late in the forecast period, which could impart a more poleward motion to Lester. The GFS-based guidance continues to lie on the northern side of the guidance envelope, while the latest ECMWF solution remains on the southern edge. The new track forecast is nearly identical to the previous one through 48 hours but has been shifted northward in the direction of a consensus of the ECMWF, HWRF, GFS, and UKMET models after that time. A nearly uniform easterly flow over Lester throughout the troposphere is forecast for several days, resulting in a low-shear environment. SSTs should be warm enough next day or so for some intensification, but the environment around Lester will be drying. In about 36 to 48 hours, the hurricane should begin to traverse a region of lower SSTs between 130W and 140W. This will occur at a time when global models forecast the near-storm environment to become extremely dry. Slow weakening should commence around this time and throughout the remainder of the forecast period. This scenario is in line with the guidance, and the new intensity forecast is in best agreement with the multi-model consensus and LGEM. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 17.9N 119.6W 90 KT 105 MPH 12H 28/0600Z 18.0N 121.3W 95 KT 110 MPH 24H 28/1800Z 18.2N 123.9W 95 KT 110 MPH 36H 29/0600Z 18.3N 126.8W 90 KT 105 MPH 48H 29/1800Z 18.5N 129.6W 85 KT 100 MPH 72H 30/1800Z 18.8N 135.0W 80 KT 90 MPH 96H 31/1800Z 18.9N 139.8W 75 KT 85 MPH 120H 01/1800Z 19.5N 144.6W 70 KT 80 MPH $$ Forecaster Kimberlain

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Tropical Storm MADELINE Forecast Discussion Number 5

2016-08-27 22:33:58| National Hurricane Center (East Pacific)

Issued at 200 PM PDT SAT AUG 27 2016 000 WTPZ44 KNHC 272033 TCDEP4 TROPICAL STORM MADELINE DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL EP142016 200 PM PDT SAT AUG 27 2016 GOES-West visible imagery shows that the center of Madeline is located on the northeastern edge of a ragged area of central convection, with a large convective band wrapping around the western semicircle of the circulation. The initial intensity estimate is 50 kt, which is a blend of the latest subjective and objective Dvorak classifications. The SHIPS model output shows the vertical shear decreasing below 10 kt by tonight, while Madeline is moving over SSTs above 27C. This environment should allow for some additional strengthening during the next 24 to 36 hours. After that time, the tropical cyclone will be moving over cooler SSTs and into a drier environment, with mid-level relative humidity values falling below 50 percent. This should result in gradual weakening, followed by steadier weakening later in the period as the vertical wind shear increases above 20 kt. The new NHC intensity forecast is a little higher than the previous one in the short range given the observed strengthening, and is similar afterward. This forecast is close to or a bit below the latest IVCN consensus. The initial motion estimate is 305/09. The track forecast reasoning remains unchanged, as Madeline should continue moving generally northwestward for the next 48 hours under the influence of mid-level ridge centered well to its northeast. Later in the period, a strengthening mid-level ridge north of the Hawaiian Island will become the dominant steering mechanism and result in a westward or slightly south of due westward track. Through 72 hours the new NHC forecast is largely and update of the previous near one and close to the TVCN consensus near the middle of the guidance envelope. At days 4 and 5 the global models have shown run-to-run inconsistency about the details of Madeline's track near the Hawaiian Islands. The latest runs of the ECMWF, GFS, and HWRF have shifted south, with a track south of the Big Island, while much of the rest of the guidance is farther north. The NHC track has been shifted southward a little at days 4 and 5, but lies a little north of the multi-model consensus and well north of the southerly set of models. Given the lack of consistency in the track guidance, the confidence in the track foreast late in the period is lower than usual and 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 27/2100Z 15.7N 139.3W 50 KT 60 MPH 12H 28/0600Z 16.4N 140.3W 60 KT 70 MPH 24H 28/1800Z 17.3N 141.5W 65 KT 75 MPH 36H 29/0600Z 18.3N 143.0W 70 KT 80 MPH 48H 29/1800Z 19.3N 144.5W 70 KT 80 MPH 72H 30/1800Z 20.4N 148.2W 65 KT 75 MPH 96H 31/1800Z 20.5N 152.5W 60 KT 70 MPH 120H 01/1800Z 20.0N 157.0W 50 KT 60 MPH $$ Forecaster Brennan

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Tropical Storm GASTON Forecast Discussion Number 20

2016-08-27 16:41:28| National Hurricane Center (Atlantic)

Issued at 1100 AM AST SAT AUG 27 2016 000 WTNT42 KNHC 271440 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 20 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 AM AST SAT AUG 27 2016 A 0954Z WindSAT microwave image indicated that Gaston had developed a 15-nmi-diameter low-level eye that was embedded in the center of the nearly circular central dense overcast. Subjective satellite intensity estimates are T4.0/65 kt and T3.5/55 kt from TAFB and SAB, respectively, and the latest UW-CIMSS ADT value is T3.7/59 kt. An average of these estimates supports increasing the initial intensity to 60 kt. As anticipated, Gaston has slowed down and the motion estimate is now 310/09 kt. The cyclone is forecast to maintain a northwestward motion and continue to decelerate during the next 48 hours as Gaston moves into a break in the subtropical ridge located to its north. A mid-level shortwave trough is forecast to dig southeastward from Canada and the northeastern United States by 72 hours, and force Gaston to make a sharp turn toward the north and northeast when the cyclone is located several hundred miles east of Bermuda. On days 4 and 5, Gaston is expected to get caught up in the deep-layer mid- latitude westerlies and accelerate east-northeastward over the North Atlantic. The new official forecast track is a little slower than the previous advisory track and has been shifted a little to the east, but not nearly as far east as the consensus model TVCN out of respect for the ECWMF model, which is close to the previous forecast track. Gaston has maintained two pronounced upper-level outflow jets to its east and southwest. These jets are flowing into large upper-level lows that are acting as significant mass sinks, a pattern that favors intensification. Although the mid-level environment is expected to be characterized by low humidity values of 40-45 percent during the next 72 hours, the cyclone will be moving over SSTs of 29C and remain in a low vertical wind shear regime. Those latter conditions, along with the small eye feature and the outflow jet pattern should allow Gaston to overcome the dry conditions and result in strengthening at a typical rate of about 20 kt per day. By days 4 and 5, the vertical shear is forecast to increase sharply and become westerly at more than 30 kt, which should induce a weakening trend. The NHC intensity forecast is similar to the previous advisory, and lies close to the consensus model IVCN through 36 hours, and a little above IVCN and close to the SHIPS/LGEM models after that. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 28.4N 53.2W 60 KT 70 MPH 12H 28/0000Z 29.3N 54.3W 65 KT 75 MPH 24H 28/1200Z 30.1N 55.3W 75 KT 85 MPH 36H 29/0000Z 30.7N 56.0W 85 KT 100 MPH 48H 29/1200Z 31.3N 56.3W 90 KT 105 MPH 72H 30/1200Z 32.6N 54.4W 90 KT 105 MPH 96H 31/1200Z 34.2N 49.6W 80 KT 90 MPH 120H 01/1200Z 36.9N 43.7W 70 KT 80 MPH $$ Forecaster Stewart

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