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Tropical Storm Irwin Forecast Discussion Number 22

2017-07-27 22:39:29| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Thu Jul 27 2017 000 WTPZ45 KNHC 272039 TCDEP5 Tropical Storm Irwin Discussion Number 22 NWS National Hurricane Center Miami FL EP102017 200 PM PDT Thu Jul 27 2017 There has been no change in the structure of Irwin. The cyclone continues to be sheared with the center intermittently located in and out of the convection. Dvorak numbers remain unchanged, consequently, the initial intensity is kept at 50 kt in this advisory. The environment is neither hostile nor favorable for a significant change in strength for the next 48 hours, so only small fluctuations in intensity are anticipated during that period. Thereafter, steady weakening should begin as Irwin's circulation encounters cooler waters. By day four, Irwin should have merged with Hilary. This continues to be the solution provided by the ECMWF and GFS global models, and it is indicated in the NHC forecast. Steering currents are very weak and Irwin is basically meandering westward. No significant motion is forecast until Hilary passes to the north in about 2 days. By then, Irwin is expected to begin moving northward a little bit faster steered by the southerly flow in the wake of Hilary. Guidance is consistent with the very slow motion during the next day or so, and most of the models clearly capture the faster northward motion around Hilary later in the period. The NHC forecast, primarily beyond 2 days, follows the multi-model consensus, and is in between the faster ECMWF and the slower GFS. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 14.9N 124.3W 50 KT 60 MPH 12H 28/0600Z 14.8N 124.4W 50 KT 60 MPH 24H 28/1800Z 14.7N 124.7W 55 KT 65 MPH 36H 29/0600Z 14.6N 125.0W 55 KT 65 MPH 48H 29/1800Z 15.0N 125.0W 55 KT 65 MPH 72H 30/1800Z 19.0N 125.5W 45 KT 50 MPH 96H 31/1800Z...Merged with Hilary $$ Forecaster Avila

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Hurricane HILARY Forecast Discussion Number 26

2017-07-27 22:36:45| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Thu Jul 27 2017 000 WTPZ44 KNHC 272036 TCDEP4 Hurricane Hilary Discussion Number 26 NWS National Hurricane Center Miami FL EP092017 200 PM PDT Thu Jul 27 2017 Since the last advisory, the structure of Hilary has changed quite a bit. The primary feature is now a long band, extending almost 3/4 of the way around the circulation. Cloud tops have warmed during this time and a pronounced dry slot is visible over the eastern half of the circulation. Subjective and objective Dvorak intensity estimates have decreased, so the intensity has been decreased accordingly, to 65 kt. The intensity guidance remains divided, with the HWRF and COAMPS-TC models insisting on reintensification, and SHIPS and LGEM showing steady state or weakening. Simulated IR imagery from the HWRF forecast shows the development of a nearly clear eye in only 12 hours. Given the current structure of the cyclone, this seems unlikely. Therefore, the new official intensity forecast favors the statistical guidance and is a little above a blend of SHIPS and LGEM through 36 h. After that time, decreasing SSTs along the forecast track should result in steady weakening, ultimately causing the cyclone to become a remnant low. Since it is the stronger system, Hilary is still expected to dominate the interaction with Irwin, and is now forecast to absorb Irwin in about 96 h, as depicted by the ECMWF. 1-minute visible imagery from GOES-16 was very helpful in determining the initial position of of Hilary at 1800 UTC, and the low-level circulation has at times been visible through the higher cirrus cover. The initial motion is a somewhat more certain 290/8 kt. The extent to which Irwin will affect the track of Hilary still varies between the global models, with the GFS showing a stronger Hilary moving farther north, and the UKMET and ECMWF showing a weaker Hilary pulled farther south. The official track forecast still lies between these solutions, but favors the southern solution since the intensity forecast calls for gradual weakening throughout the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 17.9N 116.6W 65 KT 75 MPH 12H 28/0600Z 18.3N 117.9W 65 KT 75 MPH 24H 28/1800Z 19.1N 119.5W 65 KT 75 MPH 36H 29/0600Z 19.8N 121.3W 60 KT 70 MPH 48H 29/1800Z 20.5N 123.5W 60 KT 70 MPH 72H 30/1800Z 21.4N 128.0W 45 KT 50 MPH 96H 31/1800Z 22.5N 131.7W 35 KT 40 MPH 120H 01/1800Z 23.7N 134.8W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Zelinsky

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Tropical Storm Irwin Forecast Discussion Number 21

2017-07-27 16:35:45| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Thu Jul 27 2017 000 WTPZ45 KNHC 271435 TCDEP5 Tropical Storm Irwin Discussion Number 21 NWS National Hurricane Center Miami FL EP102017 800 AM PDT Thu Jul 27 2017 Satellite images indicate that Irwin's cloud pattern has not changed very much since the last advisory. The low-level center appears to be located on the eastern edge of the deep convection due to the shear caused by Hilary's outflow. The Dvorak estimates still yield an initial intensity of 50 kt. The environment is neither hostile nor favorable for a significant change in strength for the next 48 hours. Thereafter, a steady weakening should begin as the circulation encounters cooler waters. By day four, a weaker Irwin should be very close to Hilary, and by day five, the two cyclones should have merged, with Hilary becoming the dominant feature. This is the solution provided by the ECMWF and GFS global models, and it reflected in the NHC forecast. Currently, Irwin is forecast to remain embedded within very light steering currents and only a small westward drift is anticipated during the next two days. After that time, Hilary should have already passed by the north of Irwin. The cyclone should then begin to move northward and northwestward around Hilary's circulation until the the merge occurrs. The NHC forecast is in the middle of the guidance envelope formed by the northernmost GFS and southernmost ECMWF. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 15.0N 124.2W 50 KT 60 MPH 12H 28/0000Z 15.0N 124.8W 50 KT 60 MPH 24H 28/1200Z 15.0N 125.0W 55 KT 65 MPH 36H 29/0000Z 15.0N 125.0W 55 KT 65 MPH 48H 29/1200Z 15.5N 125.0W 55 KT 65 MPH 72H 30/1200Z 19.0N 125.5W 45 KT 50 MPH 96H 31/1200Z 23.0N 129.5W 35 KT 40 MPH MERGING 120H 01/1200Z...MERGED WITH HILARY $$ Forecaster Avila

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Tropical Storm Irwin Forecast Discussion Number 20

2017-07-27 10:55:16| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Thu Jul 27 2017 000 WTPZ45 KNHC 270855 TCDEP5 Tropical Storm Irwin Discussion Number 20 NWS National Hurricane Center Miami FL EP102017 200 AM PDT Thu Jul 27 2017 Overall, Irwin's cloud pattern has change little during the past several hours. An earlier 0218 UTC SSMIS microwave pass shows a considerable structural vertical tilt toward the northwest due to the impinging southeasterly shear produced by Hilary to the east. Experimental GOES-16 channel 9 mid-level water vapor imagery indicates drier air intruding in the northeast quadrant of Irwin with subsequent convective erosion in that portion of the cloud canopy. The Dvorak satellite intensity estimates from SAB and TAFB support maintaining the initial intensity at 50 kt for this advisory. The dynamical intensity models indicate that the moderate southeasterly shear will back more toward the northeast during the next 12-24 hours as Hilary invades from the northeast. Beyond the 24 hour period, the intensity forecast becomes a bit problematic. The hurricane models, the HFIP Corrected Consensus, the Florida State Superensemble, and the GFS/ECMWF global models all show Irwin strengthening back into a hurricane under much improved diffluent flow aloft. The statistical-dynamical intensity models, on the other hand, show only slight strengthening. As a compromise for this forecast package, the official forecast shows modest strengthening, then gradual weakening after 48 hours. The initial motion is estimated to be west-southwestward, or 255/05 kt. As Hilary continues to moves closer and passes to the north of Irwin, a significant reduction in Irwin's forward speed, essentially drifting in a cyclonic fashion, is expected during the next 24-36 hours. After that time, the cyclone is forecast to accelerate northward and then northwestward up the eastern and northern portion of Hilary's peripheral circulation. The deterministic guidance remains in fair agreement with this synoptic pattern scenario, and the NHC forecast is similar to the previous one and is based on a blend of the TVCX and HCCA consensus models. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 14.7N 123.8W 50 KT 60 MPH 12H 27/1800Z 14.6N 124.4W 50 KT 60 MPH 24H 28/0600Z 14.5N 124.8W 55 KT 65 MPH 36H 28/1800Z 14.3N 124.7W 55 KT 65 MPH 48H 29/0600Z 14.7N 124.3W 55 KT 65 MPH 72H 30/0600Z 17.8N 124.4W 50 KT 60 MPH 96H 31/0600Z 22.0N 128.0W 40 KT 45 MPH 120H 01/0600Z 24.5N 133.5W 35 KT 40 MPH $$ Forecaster Roberts

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Hurricane Hilary Forecast Discussion Number 24

2017-07-27 10:54:17| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Thu Jul 27 2017 000 WTPZ44 KNHC 270854 TCDEP4 Hurricane Hilary Discussion Number 24 NWS National Hurricane Center Miami FL EP092017 200 AM PDT Thu Jul 27 2017 Hilary has lost organization due to the effects of moderate northerly vertical shear, with recent Windsat and GPM overpasses indicating that the center is now near the northwestern edge of the main convective area. Subjective satellite intensity estimates are 75-90 kt, while estimates based on microwave sounding data and the CIMSS ADT are 55-65 kt. Based on a blend of these, the initial intensity is reduced to a possibly generous 75 kt. The large-scale models forecast Hilary to move under a cyclonic shear axis between two upper-level high pressure areas during the next 24-48 h. This evolution may allow the current shear to decrease, but it is not an ideal environment for a tropical cyclone. Thus, the intensity forecast calls for little change in strength during this time, which is in good agreement with the intensity guidance. After 48 h, Hilary should move over cooler sea surface temperatures, and it is likely to significantly interact with Tropical Storm Irwin. The large-scale models differ on the details, but they agree that the combination of the interaction, dry air, and the presence of cool water should cause Hilary to quickly weaken as a result. Thus, the new intensity forecast now calls for the system to degenerate to a remnant low by the end of the forecast period. The initial motion is 290/10. For the first 48 h or so, Hilary should be steered generally west-northwestward by the subtropical ridge. After that, the track forecast becomes very uncertain due to the interaction with Irwin, where the latter cyclone is likely to move around the eastern and northern sides of Hilary before a possible merger. The Canadian model merges the two cyclones in about 72 h, while the GFS finishes the merger at about 120 h. On the other hand, the ECMWF delays the merger until after the forecast period, while the UKMET keeps the two systems separate. The new track forecast will not attempt to show the details of the interaction, but it will show more of a westward turn and changes in speed during the interaction than the previous forecast. Overall, the new track is close to the consensus models. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 17.3N 115.1W 75 KT 85 MPH 12H 27/1800Z 17.7N 116.5W 70 KT 80 MPH 24H 28/0600Z 18.4N 118.2W 70 KT 80 MPH 36H 28/1800Z 19.1N 119.9W 70 KT 80 MPH 48H 29/0600Z 19.8N 121.7W 70 KT 80 MPH 72H 30/0600Z 21.0N 126.0W 60 KT 70 MPH 96H 31/0600Z 21.5N 130.5W 45 KT 50 MPH 120H 01/0600Z 22.0N 133.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Beven

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