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Tropical Storm Alvin Forecast Discussion Number 8

2019-06-27 16:51:05| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Thu Jun 27 2019 000 WTPZ41 KNHC 271450 TCDEP1 Tropical Storm Alvin Discussion Number 8 NWS National Hurricane Center Miami FL EP012019 900 AM MDT Thu Jun 27 2019 Recent microwave imagery indicates that Alvin's convective structure has improved over the past several hours, and a 1057 UTC SSMI overpass showed a closed ring of convection around the mid-level center of the small tropical storm. However, the infrared presentation of Alvin has not changed significantly since the last advisory and satellite intensity estimates are also unchanged. The initial intensity is therefore held at 50 kt, in deference to ASCAT data from late last night. The dynamical models (primarily the HWRF, HMON, and GFS) continue to indicate that Alvin could strengthen during the next 12 to 24 hours before it reaches much colder waters to the north. Once the tropical storm reaches those waters in a day or so, it should quickly weaken, likely becoming a remnant low around or just after 48 h. The NHC intensity forecast has not been changed substantially since the previous advisory and remains close to the intensity consensus aids. It is worth noting that the small size of Alvin could make it susceptible to short-term swings of intensity, up or down, and once weakening begins it could occur even faster than currently indicated. The official track forecast has been nudged slightly northward, but is otherwise unchanged. The initial motion estimate is 300/12 kt, and Alvin is expected to continue to move west-northwestward or westward at a similar forward speed for the next 24 to 36 hours. After that time, the cyclone should turn toward the west as it weakens and becomes steered by shallow easterly flow to the north before it dissipates entirely. The NHC forecast closely follows the HCCA and TVCN aids. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 15.8N 113.1W 50 KT 60 MPH 12H 28/0000Z 16.9N 114.9W 55 KT 65 MPH 24H 28/1200Z 18.5N 117.0W 50 KT 60 MPH 36H 29/0000Z 19.7N 119.0W 35 KT 40 MPH 48H 29/1200Z 20.5N 120.7W 30 KT 35 MPH 72H 30/1200Z...DISSIPATED $$ Forecaster Zelinsky

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Tropical Storm Alvin Forecast Discussion Number 7

2019-06-27 10:47:31| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Thu Jun 27 2019 000 WTPZ41 KNHC 270847 TCDEP1 Tropical Storm Alvin Discussion Number 7 NWS National Hurricane Center Miami FL EP012019 300 AM MDT Thu Jun 27 2019 Alvin's satellite presentation is much improved since this time yesterday, consisting of a small but distinct CDO with extremely cold overshooting cloud tops of about -90C near the well-defined center. Outflow is fairly symmetrical due to the low vertical wind shear conditions affecting the compact cyclone. A 0435Z ASCAT-A pass revealed that tropical-storm-force winds only extended 15-20 nmi away from the center in the northern semicircle along with one peak wind vector of 46 kt located in the northwestern quadrant near the overshooting cloud tops. Given Alvin's small size and likely undersampling by the scatterometer instrument, the intensity is raised to 50 kt, which is consistent with ADT and SATCON estimates. The initial position and motion vector of 300/11 kt are based on ASCAT-A/-C scatterometer wind data and passive microwave fixes. Although the new track has been shifted north of the previous advisory track, there otherwise are no significant changes to the previous forecast track reasoning. Alvin is expected to move steadily west-northwestward around the southwestern periphery of a deep-layer subtropical ridge that extends westward from northwestern Mexico. The latest track guidance has again shifted northward and the new NHC track has likewise been shifted northward, but not quite as far the consensus models out of the respect for the lower latitude ECMWF and FSSE models. Additional strengthening is expected during the next 12 to 24 hours while Alvin remains over SSTs greater than 26 deg C and within a low vertical wind shear regime. By 36 hours, however, the combination of cooling SSTs and a more stable airmass is forecast to induce steady to rapid weakening. Alvin is expected to degenerate into a remnant low by late Saturday, and dissipate by Monday. The NHC intensity forecast is similar to a blend of the consensus models IVCN. HCCA, and FSSE. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 15.1N 111.9W 50 KT 60 MPH 12H 27/1800Z 15.9N 113.5W 55 KT 65 MPH 24H 28/0600Z 17.3N 116.0W 55 KT 65 MPH 36H 28/1800Z 18.7N 118.1W 45 KT 50 MPH 48H 29/0600Z 19.6N 119.9W 35 KT 40 MPH 72H 30/0600Z 20.7N 122.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 01/0600Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Alvin Forecast Discussion Number 6

2019-06-27 04:32:29| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Wed Jun 26 2019 000 WTPZ41 KNHC 270232 TCDEP1 Tropical Storm Alvin Discussion Number 6 NWS National Hurricane Center Miami FL EP012019 900 PM MDT Wed Jun 26 2019 Alvin's presentation on satellite images consists of a well-defined CDO with very cold cloud tops near/over the center, with very limited banding features. Upper-level outflow is fairly distinct over the western semicircle of the system. The current intensity estimate is boosted slightly, to 45 kt, based on Dvorak CI numbers from both TAFB and SAB. Alvin is likely to strengthen some more during the next 12 to 24 hours since it should be moving over sufficiently warm waters, within low vertical shear, and in a modestly moist air mass for the next day or so. Beyond 24 hours, cooling SSTs and gradually increasing shear should cause a weakening trend to commence. All of the models call for rapid weakening in 2 to 3 days, and the cyclone should degenerate into a remnant low by Saturday. The official intensity forecast is a little below the latest model consensus. Recent center fixes indicate that there is no longer a southward component of motion, and the initial motion estimate is now westward or 270/11 kt. There have been no significant changes to the general track prediction philosophy. Over the forecast period, the cyclone is expected to move along the southwestern periphery of a mid-level ridge extending westward from northwestern Mexico. Most of the track guidance has shifted a little northward on this cycle, and the new official forecast is slightly north of the previous one, but a little south of the simple and corrected dynamical consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 27/0300Z 14.4N 111.3W 45 KT 50 MPH 12H 27/1200Z 15.0N 112.9W 50 KT 60 MPH 24H 28/0000Z 16.0N 115.1W 55 KT 65 MPH 36H 28/1200Z 17.0N 117.3W 50 KT 60 MPH 48H 29/0000Z 18.0N 119.2W 40 KT 45 MPH 72H 30/0000Z 19.3N 122.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 01/0000Z...DISSIPATED $$ Forecaster Pasch

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PHAC 2019-07-02 - HRCP Discussion Draft Summary

2019-06-26 22:33:04| PortlandOnline

PDF Document, 8,164kbCategory: July 2, 2019

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Tropical Storm Alvin Forecast Discussion Number 5

2019-06-26 22:31:53| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Wed Jun 26 2019 295 WTPZ41 KNHC 262031 TCDEP1 Tropical Storm Alvin Discussion Number 5 NWS National Hurricane Center Miami FL EP012019 300 PM MDT Wed Jun 26 2019 Alvin is a small tropical storm as indicated by a recent scatterometer pass that showed tropical storm force winds extending only about 30 n mi from the center in the northern semicircle. This scatterometer data as well as an average of satellite intensity estimates from TAFB and SAB support an initial intensity of 40 kt. Alvin has continued to move south of due west throughout the day, and the initial motion estimate remains 260/12 kt. Through 24 hours, little change was made to the NHC forecast track, as the cyclone is expected to move westward and then west-northwestward around the southwestern periphery of a subtropical ridge through dissipation. The latest forecast track is on the southern edge of the guidance envelope through 36 hours, and has been shifted a little northward closer to the consensus aids from 48 to 72 hours. Although dry air has been trying to entrain into the storm's circulation today, deep convection continues to develop over the center. Some additional strengthening is still expected through 24 hours as the cyclone moves over warm SSTs in a low-shear and sufficiently moist environment. After 36 hours, the forecast track takes Alvin over progressively cooler waters and into a drier and higher-shear environment. This should result in steady weakening, with the cyclone likely becoming a remnant low by 72 hours, and dissipating by 96 hours. The new NHC forecast is near or a little below the latest dynamical and consensus intensity guidance. FORECAST POSITIONS AND MAX WINDS INIT 26/2100Z 14.3N 110.3W 40 KT 45 MPH 12H 27/0600Z 14.3N 112.1W 45 KT 50 MPH 24H 27/1800Z 15.0N 114.3W 50 KT 60 MPH 36H 28/0600Z 16.0N 116.4W 50 KT 60 MPH 48H 28/1800Z 17.1N 118.5W 45 KT 50 MPH 72H 29/1800Z 18.3N 121.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 30/1800Z...DISSIPATED $$ Forecaster Latto/Brennan

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