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Tropical Storm HILDA Forecast Discussion Number 7

2015-08-07 16:35:28| National Hurricane Center (East Pacific)

Issued at 800 AM PDT FRI AUG 07 2015 000 WTPZ45 KNHC 071435 TCDEP5 TROPICAL STORM HILDA DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL EP102015 800 AM PDT FRI AUG 07 2015 Infrared satellite data, along with VIIRS moonlight visible imagery and a just received AMSR-2 overpass, show that Hilda is maintaining strong convection near the center. However, the system continues to have a sheared appearence, which is somewhat surprising since the available data show less than 10 kt of shear. The various satellite intensity estimates range between 45-55 kt, so the initial intensity remains 50 kt. The cirrus outflow is good to the west and poor to the east. The initial motion is 275/11. For the next 48-72 hours, Hilda should be steered westward to west-northwestward by the deep-layer subtropical ridge to its north. After that time, a mid- to upper-level low is expected to develop north of the Hawaiian Islands, which should cause Hilda to turn northwestward and north-northwestward. The track guidance again generally agrees on this scenario, but there remains a significant spread in how sharply Hilda will turn based on the strength of the cyclone after 72 hours. The GFS, GFDL, and GFDN, which depict a stronger cyclone, have sharper turns and end up on the east side of the guidance envelope. The ECMWF, HWRF, and GFS ensemble mean forecast a weaker Hilda, and they show a more westward motion. The UKMET and the various consensus models are between these extremes. The new forecast track, which is little changed from the previous track, follows this part of the guidance. The dynamical models are in good agreement that whatever shear is occurring should diminish in 24 hours or less. This should allow a faster rate of development, with Hilda forecast to become a hurricane in 24-36 hours and reach its peak intensity in 48-72 hours. After 72 hours, the cyclone is expected to encounter strong southwesterly vertical shear and decreasing sea surface temperatures, and this combination should cause significant weakening. The new intensity forecast has minor adjustments from the previous forecast and is in best overall agreement with the intensity consensus. FORECAST POSITIONS AND MAX WINDS INIT 07/1500Z 12.8N 136.7W 50 KT 60 MPH 12H 08/0000Z 13.0N 138.3W 55 KT 65 MPH 24H 08/1200Z 13.6N 140.5W 60 KT 70 MPH 36H 09/0000Z 14.3N 142.5W 70 KT 80 MPH 48H 09/1200Z 15.1N 144.5W 75 KT 85 MPH 72H 10/1200Z 16.5N 147.5W 75 KT 85 MPH 96H 11/1200Z 18.5N 149.5W 55 KT 65 MPH 120H 12/1200Z 20.5N 150.5W 40 KT 45 MPH $$ Forecaster Beven

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Tropical Storm HILDA Forecast Discussion Number 6

2015-08-07 10:38:00| National Hurricane Center (East Pacific)

Issued at 200 AM PDT FRI AUG 07 2015 000 WTPZ45 KNHC 070837 TCDEP5 TROPICAL STORM HILDA DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL EP102015 200 AM PDT FRI AUG 07 2015 A GPM Microwave Imager pass from 0431 UTC revealed that Hilda's center was located just under the eastern edge of the deep convection due to some easterly shear. The overall cloud pattern has not really improved since earlier advisories, and the initial intensity remains 50 kt. This is supported by Dvorak estimates of T3.5/55 kt from TAFB and T3.0/45 kt from SAB. Although Hilda's intensity has been steady for the past 12 hours, easterly shear is relaxing, and the cyclone should be able to strengthen soon. Hilda is forecast to be in a low-shear environment and over sea surface temperatures of 27-29C during the next three days or so, during which time intensification is expected. On days 4 and 5, weakening is anticipated as southwesterly shear of 25-35 kt begins to affect the cyclone. The NHC intensity forecast is near the upper bound of the guidance and shows a peak intensity occurring in about 48 hours. Hilda is moving due west, or 270/11 kt. The cyclone should turn west-northwestward by 36 hours as it approaches a weakness in the subtropical ridge. Then, a mid- to upper-level low is expected to develop north of the Hawaiian Islands in about three days, which should cause Hilda to turn northwestward and north-northwestward on days 4 and 5. The track guidance generally agrees on this scenario, but there is significant discrepancy on how sharply Hilda will turn after 72 hours. The GFS and GFDL, which depict a stronger cyclone, have sharper turns and end up on the east side of the guidance envelope. The ECMWF, on the other hand, does not intensify Hilda as much and is well to the west of the other track models. The updated NHC track forecast shows a sharper turn than in the previous advisory, but the turn is not nearly as sharp as suggested by the GFS, GFDL, and the various consensus models. FORECAST POSITIONS AND MAX WINDS INIT 07/0900Z 12.6N 135.6W 50 KT 60 MPH 12H 07/1800Z 12.8N 137.3W 60 KT 70 MPH 24H 08/0600Z 13.2N 139.3W 65 KT 75 MPH 36H 08/1800Z 13.9N 141.3W 70 KT 80 MPH 48H 09/0600Z 14.7N 143.3W 75 KT 85 MPH 72H 10/0600Z 16.1N 147.1W 70 KT 80 MPH 96H 11/0600Z 18.0N 149.0W 60 KT 70 MPH 120H 12/0600Z 20.5N 150.0W 45 KT 50 MPH $$ Forecaster Berg

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Tropical Storm HILDA Forecast Discussion Number 5

2015-08-07 04:42:29| National Hurricane Center (East Pacific)

Issued at 800 PM PDT THU AUG 06 2015 000 WTPZ45 KNHC 070242 TCDEP5 TROPICAL STORM HILDA DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL EP102015 800 PM PDT THU AUG 06 2015 The cloud pattern of Hilda has not changed much since the previous advisory. The cyclone has a couple of curved convective bands with a small symmetric central dense overcast. Recent microwave data also indicates that the inner core has not become any better organized during the afternoon. A blend of the subjective and objective Dvorak T-numbers support maintaining an initial intensity of 50 kt. Hilda is expected to remain in an environment of low vertical wind shear and over warm sea surface temperatures during the next couple of days. These conditions favor intensification and the NHC forecast brings Hilda to hurricane strength within the next 24 hours. The new official forecast shows a slightly higher peak intensity in about 48 hours, which is a blend of the latest statistical guidance. After that time, a drier and more stable air mass and increasing southwesterly shear around 96 h are expected to induce weakening. The NHC forecast is below the model guidance late in the forecast period, since the small tropical cyclone is likely to spin down more quickly as a result of the strong shear. The tropical storm is moving due west or 270 degrees at 12 kt. The forecast reasoning remains the same as before. Hilda is expected to turn west-northwestward in about 36 hours when it nears the western periphery of a subtropical ridge to its north. In about 96 hours, Hilda is forecast to turn northwestward due to a developing weakness in the subtropical ridge. The model guidance is in good agreement through day 3, but diverge somewhat thereafter. The ECMWF and UKMET show a faster forward motion and are along the western edge of the guidance envelope. Meanwhile, the GFS and HWRF are along the eastern edge. The updated NHC forecast is similar to the previous advisory and near the GFS ensemble mean late in the period. FORECAST POSITIONS AND MAX WINDS INIT 07/0300Z 12.7N 134.6W 50 KT 60 MPH 12H 07/1200Z 12.7N 136.3W 60 KT 70 MPH 24H 08/0000Z 13.0N 138.5W 70 KT 80 MPH 36H 08/1200Z 13.7N 140.5W 80 KT 90 MPH 48H 09/0000Z 14.5N 142.5W 80 KT 90 MPH 72H 10/0000Z 16.3N 146.1W 70 KT 80 MPH 96H 11/0000Z 17.8N 148.5W 60 KT 70 MPH 120H 12/0000Z 19.0N 150.0W 40 KT 45 MPH $$ Forecaster Brown

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Tropical Storm HILDA Forecast Discussion Number 4

2015-08-06 22:37:59| National Hurricane Center (East Pacific)

Issued at 200 PM PDT THU AUG 06 2015 000 WTPZ45 KNHC 062037 TCDEP5 TROPICAL STORM HILDA DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL EP102015 200 PM PDT THU AUG 06 2015 Hilda's cloud pattern has rapidly increased in organization during the past 12 hours. Conventional satellite imagery reveals a tiny tropical cyclone with a nearly symmetric dense overcast and a prominent band over the northern half of the circulation. A 1503 UTC Windsat pass showed a closed low-level ring of convection, suggesting that the inner core of the tropical cyclone is already well established. The initial intensity estimate is raised to 50 kt in best agreement with an earlier AMSU pass. The initial motion has been due west or 270/11. Nothing has changed regarding the forecast philosophy in the previous advisory. Hilda should continue moving westward and then turn west- northwestward in about 36 hours as it nears the western periphery of a subtropical ridge to its north. The cyclone should begin to respond to a weakness in the ridge located to the east of the Hawaiian Islands and turn northwestward at a significantly reduced forward speed by 96 hours. The official track forecast has been shifted significantly to the left during the first 48 hours but still lies on the right side of the guidance envelope. The track forecast after 48 hours is also left of the previous forecast but is closer to the multi-model consensus late in the forecast period. There are no obvious obstacles to additional intensification in the short term, except for somewhat drier and more stable air mass to the north and west of the cyclone. Given Hilda's small size, it is assumed that the entrainment of this air is not likely to be an inhibitor. The SHIPS model output indicates less conducive thermodynamic variables in 2 to 3 days which should halt any further intensification. Once the cyclone gains enough latitude in the central Pacific after 72 hours, increasing southwesterly shear associated with a mid- to upper-level trough near the longitude of Hawaii should result in weakening. In fact, the shear could be strong enough late in the period to cause a decoupling of the cyclone as depicted in global model fields. The new intensity forecast is boosted much higher in the short term to account for the current strengthening trend and is closest to the LGEM. The intensity forecast is near or below the multi-model consensus after 72 hours to emphasize the weakening expected at that time. FORECAST POSITIONS AND MAX WINDS INIT 06/2100Z 12.7N 133.4W 50 KT 60 MPH 12H 07/0600Z 12.6N 135.1W 55 KT 65 MPH 24H 07/1800Z 12.7N 137.4W 65 KT 75 MPH 36H 08/0600Z 13.1N 139.5W 70 KT 80 MPH 48H 08/1800Z 13.7N 141.6W 75 KT 85 MPH 72H 09/1800Z 15.5N 145.5W 70 KT 80 MPH 96H 10/1800Z 17.2N 147.9W 60 KT 70 MPH 120H 11/1800Z 18.5N 150.0W 45 KT 50 MPH $$ Forecaster Kimberlain

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Tropical Storm HILDA Forecast Discussion Number 3

2015-08-06 16:49:57| National Hurricane Center (East Pacific)

Issued at 800 AM PDT THU AUG 06 2015 000 WTPZ45 KNHC 061449 TCDEP5 TROPICAL STORM HILDA DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL EP102015 800 AM PDT THU AUG 06 2015 The cyclone's cloud pattern has significantly increased in organization since late yesterday. There is now a small central mass of deep convection, with a band on the northern side of the circulation. The center, according to an 1132 UTC SSM/I pass, is also directly underneath this central convection, and the same pass indicates increasing symmetry to the overall convective structure. Satellite classifications from 1200 UTC were T2.0/30 kt and T2.5/35 kt from SAB and TAFB, respectively. The initial intensity estimate is set at the higher end of these estimates (35 kt), based on the even greater organization seen in satellite imagery since that time. The initial motion is toward the west or 275/11. The general forecast scenario is for a subtropical ridge north of the cyclone to steer Hilda westward for the next couple of days with a slight bend toward the west-northwest after 36 hours. In about 72 hours, the cyclone should move around the western end of the ridge and encounter a break in this feature at the end of the forecast period. This pattern should cause Hilda to turn northwestward with a decrease in forward speed by 96 hours. The GFS-based guidance shows a stronger ridge and a west-southwestward to westward track during the next 2 to 3 days, while the ECMWF has a weaker ridge and a track farther north. The official forecast is closer to the ECMWF solution and near but faster than the previous one through 72 hours. Overall, the NHC forecast lies to the right of the multi-model consensus, which is dominated by the GFS guidance. After 72 hours, the official track is near the multi-model consensus but not as far west as the GFS/ECMWF model solutions. Large-scale dynamic and thermodynamic factors are forecast to be conducive for intensification through for the next 72 hours or so, except for a somewhat drier lower to middle troposphere along Hilda's path in 2 to 3 days. When the cyclone gains latitude late in the forecast period, it should begin to weaken due to increasing southwesterly shear associated with a mid- to upper-level trough expected to be near the longitude of Hawaii. The official intensity forecast is higher than the previous one through 72 hours and is higher than the multi-model consensus, but little change was made to the intensity forecast late in the period. FORECAST POSITIONS AND MAX WINDS INIT 06/1500Z 12.7N 132.3W 35 KT 40 MPH 12H 07/0000Z 12.9N 133.6W 45 KT 50 MPH 24H 07/1200Z 13.0N 135.8W 50 KT 60 MPH 36H 08/0000Z 13.5N 137.9W 55 KT 65 MPH 48H 08/1200Z 14.1N 139.9W 60 KT 70 MPH 72H 09/1200Z 15.4N 144.2W 65 KT 75 MPH 96H 10/1200Z 17.1N 146.8W 65 KT 75 MPH 120H 11/1200Z 18.8N 149.4W 55 KT 65 MPH $$ Forecaster Kimberlain

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