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Tropical Storm Irwin Forecast Discussion Number 27

2017-07-29 04:32:52| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Fri Jul 28 2017 000 WTPZ45 KNHC 290232 TCDEP5 Tropical Storm Irwin Discussion Number 27 NWS National Hurricane Center Miami FL EP102017 800 PM PDT Fri Jul 28 2017 Microwave data indicate that inner-core convection has recently developed near Irwin's center; however, the convection is not all that deep, with cloud top temperatures no colder than about -65C. Even though Dvorak intensity estimates from TAFB and SAB are T3.5/55 kt, the UW-CIMSS ADT and SATCON estimates are significantly lower, so the initial intensity will remain 50 kt. Vertical shear does not appear that it will be too prohibitive for the next 24 hours, and in fact, some models like the HWRF and HCCA suggest that Irwin could gain a little strength during that time. However, Irwin should reach waters colder than 26C in about 36 hours, which will induce a marked weakening trend. The NHC intensity forecast closely follows a blend of HCCA and the ICON intensity consensus, and it now shows Irwin becoming a tropical depression by day 3. The global models no longer show Irwin being absorbed by Hilary, probably because Hilary will reach colder waters sooner and will not be energetic enough to be the dominant system. The guidance shows one of two options: Irwin absorbing Hilary or the two staying as separate system. Either way, these scenarios required keeping Irwin through day 4, but as a remnant low since ocean waters will be quite cold. Center fixes off of recent microwave images indicate that Irwin had been drifting southward or meandering for much of the day. As Hilary moves northwestward, Irwin is expected to get pulled slowly northward during the next 24-36 hours, and then accelerate north-northwestward on the east side of Hilary on days 2-4. There is a fair amount of spread among the track models, especially after 48 hours, but the NHC track forecast is close to the tight clustering consisting of the GFS, ECMWF, HCCA, TVCN. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 14.8N 124.7W 50 KT 60 MPH 12H 29/1200Z 15.0N 124.7W 55 KT 65 MPH 24H 30/0000Z 16.0N 124.8W 55 KT 65 MPH 36H 30/1200Z 17.7N 125.1W 50 KT 60 MPH 48H 31/0000Z 20.0N 125.9W 45 KT 50 MPH 72H 01/0000Z 24.9N 128.8W 30 KT 35 MPH 96H 02/0000Z 27.5N 131.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 03/0000Z...DISSIPATED $$ Forecaster Berg

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Tropical Storm Irwin Forecast Discussion Number 26

2017-07-28 22:32:52| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Fri Jul 28 2017 000 WTPZ45 KNHC 282032 TCDEP5 Tropical Storm Irwin Discussion Number 26 NWS National Hurricane Center Miami FL EP102017 200 PM PDT Fri Jul 28 2017 Although Irwin's structure on visible satellite images looks well organized, infrared data show that the convection is not very deep, and the cyclone lacks an inner core. Dvorak T-numbers have not changed much and still support an initial intensity of 50 kt. Some moderate shear will be affecting the cyclone during the next 24 to 36 hours, but it is not expected to be strong enough to disrupt the cloud pattern and result in weakening. On this basis, the NHC forecast calls for only small fluctuations in intensity during that period. Thereafter, the circulation is expected to reach cooler waters, and weakening should then begin. This process will continue until Irwin and Hilary merge in about 96 hours or sooner. Irwin continues to be embedded in light steering currents, and the cyclone has barely moved today. Little motion is anticipated during the next 12 hours, but by Saturday, Irwin should be steered northward and northwestward by the southerly flow in the wake of Hilary. Track guidance is in good agreement for the next 3 days, but beyond that time, models become uncertain in how the interaction of Hilary and Irwin will take place. The NHC forecast continues to call for the merging of the two weakening cyclones. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 14.9N 125.0W 50 KT 60 MPH 12H 29/0600Z 15.0N 125.0W 55 KT 65 MPH 24H 29/1800Z 15.8N 125.0W 55 KT 65 MPH 36H 30/0600Z 16.8N 125.3W 50 KT 60 MPH 48H 30/1800Z 18.7N 126.0W 40 KT 45 MPH 72H 31/1800Z 22.5N 128.0W 35 KT 40 MPH 96H 01/1800Z...DISSIPATED $$ Forecaster Avila

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Tropical Storm Hilary Forecast Discussion Number 29

2017-07-28 16:38:51| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Fri Jul 28 2017 000 WTPZ44 KNHC 281438 TCDEP4 Tropical Storm Hilary Discussion Number 29 NWS National Hurricane Center Miami FL EP092017 800 AM PDT Fri Jul 28 2017 Cloud tops near the center of Hilary continue to warm this morning, and a consensus of objective and subjective Dvorak intensity estimates suggests a decreased initial intensity of 55 kt. An AMSR pass at 0913 UTC showed a band of deep convection spiraling into the center. There is also a pronounced dry slot, indicative of the drier environment that the cyclone is moving into. Hilary is crossing the 26 C SST isotherm this morning, so the window for any reintensification is closing quickly, despite the insistence of the HWRF and GFS which both show a hurricane in 24 hours. Instead, the official forecast continues to follow the statistical models, which depict steady weakening until Hilary becomes a remnant low. The intensity forecast is a little lower than indicated in the previous advisory based on the lower initial intensity, but no significant changes have been made. The low-level center of the tropical storm was briefly visible this morning, and the initial motion is a fairly confident 305/7. There has been a significant shift in the guidance toward the north for this advisory, especially the GFS and the GFS-based hurricane models. The 06Z GFS in particular has shifted nearly 500 nm to the northeast at day 5 and no longer shows any significant interaction between Irwin and Hilary. The 00Z ECMWF also shifted a little north, but not nearly to the same extent. Interaction between the two cyclones, and the eventual absorption of Irwin by Hilary, is still expected for now, as shown by the UKMET and ECMWF. Based on the changes in the guidance, the official forecast has been shifted north, but still lies well south of the multi-model consensus aids. It does, however, fall near the middle of the ECMWF and GFS ensemble tracks. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 19.2N 118.4W 55 KT 65 MPH 12H 29/0000Z 19.9N 119.5W 50 KT 60 MPH 24H 29/1200Z 20.9N 121.3W 45 KT 50 MPH 36H 30/0000Z 21.8N 123.2W 40 KT 45 MPH 48H 30/1200Z 22.8N 125.4W 35 KT 40 MPH 72H 31/1200Z 24.0N 129.4W 25 KT 30 MPH 96H 01/1200Z 24.8N 132.6W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 02/1200Z 26.5N 135.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Zelinsky

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Tropical Storm Irwin Forecast Discussion Number 25

2017-07-28 16:37:49| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Fri Jul 28 2017 000 WTPZ45 KNHC 281437 TCDEP5 Tropical Storm Irwin Discussion Number 25 NWS National Hurricane Center Miami FL EP102017 800 AM PDT Fri Jul 28 2017 Irwin's structure has not changed significantly during the past several hours, and deep convection near the center has been blossoming intermittently. Dvorak T-numbers have also been fluctuating, and the best estimate of the initial intensity is 50 kt. SHIPS model data indicate some shear over the cyclone but not strong enough to disrupt the cloud pattern. On this basis, the NHC forecast calls for only small fluctuations in intensity during the next 24 to 36 hours. Thereafter, the circulation is expected to reach cooler waters and weakening should begin. This process will continue until Irwin and Hilary merge in about 96 hours or sooner. Irwin is embedded in light steering currents, and the cyclone has barely moved during the past few hours. Little motion is anticipated today or early Saturday, but after that time, Irwin should be steered northward and northwestward by the southerly flow in the wake of Hilary. Track guidance is in good agreement that little motion will occur during the next day or so. There is a big difference in both direction and speed among models beyond 3 days with the northernmost GFS and the southernmost ECMWF marking the edges of the broad guidance envelope. However, by then, Irwin and Hilary should have already merged. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 14.8N 125.3W 50 KT 60 MPH 12H 29/0000Z 14.8N 125.3W 55 KT 65 MPH 24H 29/1200Z 15.2N 125.4W 55 KT 65 MPH 36H 30/0000Z 16.2N 125.3W 50 KT 60 MPH 48H 30/1200Z 18.0N 125.5W 45 KT 50 MPH 72H 31/1200Z 21.5N 128.0W 35 KT 40 MPH 96H 01/1200Z...Merge with Hilary $$ Forecaster Avila

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Tropical Storm Hilary Forecast Discussion Number 28

2017-07-28 10:40:23| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Fri Jul 28 2017 000 WTPZ44 KNHC 280840 TCDEP4 Tropical Storm Hilary Discussion Number 28 NWS National Hurricane Center Miami FL EP092017 200 AM PDT Fri Jul 28 2017 The structure of Hilary has changed little over the past several hours, as a small area of convection persists near the center mainly in the southeastern quadrant. Recent satellite intensity estimates range from 45-70 kt, and based on these, the initial intensity remains a possibly generous 60 kt. Hilary should cross the 26C isotherm in 12-18 h, and given the current level of convective organization, significant re- intensification seems unlikely during that time. After that, the cyclone is forecast to move over steadily decreasing sea surface temperatures and into a drier air mass, and this combination should result in steady weakening. The cyclone is now forecast to weaken to a depression in about 72 h over 24C sea surface temperatures and degenerate to a remnant low shortly thereafter. The new intensity forecast shows a faster weakening than the previous forecast after 12 h, and it is in best overall agreement with the SHIPS and LGEM models. It is possible that interaction with Tropical Storm Irwin could lead to a faster decay than currently forecast. The initial motion is 300/7, a little to the right of that in the previous advisory. A mid-level ridge to the north of Hilary is expected to steer the cyclone generally west-northwestward for 48 h or so. Around 72 h, a westward turn appears likely as the cyclone interacts with, and eventually absorbs Irwin. After this event, Hilary or its remnants should resume a west-northwestward motion as the weakening system is steered by the low-level flow. The new forecast track is nudged north of the previous track, but it lies south of the consensus models, which have shifted northward since the last advisory. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 18.7N 117.9W 60 KT 70 MPH 12H 28/1800Z 19.3N 119.0W 60 KT 70 MPH 24H 29/0600Z 20.3N 120.6W 55 KT 65 MPH 36H 29/1800Z 21.1N 122.6W 45 KT 50 MPH 48H 30/0600Z 21.9N 124.9W 40 KT 45 MPH 72H 31/0600Z 22.5N 129.0W 30 KT 35 MPH 96H 01/0600Z 23.5N 132.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 02/0600Z 25.0N 135.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Beven

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