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Tropical Storm Irwin Forecast Discussion Number 11

2017-07-25 04:35:32| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Mon Jul 24 2017 000 WTPZ45 KNHC 250235 TCDEP5 Tropical Storm Irwin Discussion Number 11 NWS National Hurricane Center Miami FL EP102017 800 PM PDT Mon Jul 24 2017 Irwin has been gradually strengthening throughout the day. Satellite images show that deep convection has been persisting over the center and in fragmented curved bands, especially over the south and west portions of the circulation. Microwave images indicate that the system is vertically titled from south to north, likely due to southerly shear. The latest Dvorak CI numbers are 3.5/55 kt from TAFB and 4.0/65 kt from SAB. These estimates and higher ADT values from CIMSS at the University of Wisconsin support increasing the initial intensity to 60 kt. Irwin is crawling west-northwestward, with the initial motion estimated to be 285/3 kt. A slow westward to west-northwestward motion is forecast through early Tuesday as a weak mid-level ridge remains in place to the north of the storm. A west-southwestward motion is expected between 24 and 72 h as Hurricane Hilary approaches from the east. Beyond that time, the track forecast is very challenging as the models remain quite divergent with the spread being more than 700 n mi at day 5. The UKMET, ECMWF, and GFS global models show Irwin rotating around the east side of the circulation of Hilary and ultimately merging with that hurricane. The CMC model shows some binary interaction between the tropical cyclones, but keeps the systems separate, and the regional hurricane models HWRF and HMON show no interaction between the systems and continue to move Irwin westward. The NHC track forecast favors the global models and shows a slow northward motion on days 4 and 5. Needless to say, the long range track forecast is of low confidence. The environmental conditions should support some additional strengthening during the next day or so, and Irwin is expected to become a hurricane tonight or on Tuesday. Thereafter, an increase in vertical wind shear, associated with the outflow of Hilary, and slightly cooler waters along the forecast track should cause a gradual weakening trend. The NHC intensity forecast is similar to the previous one and in fair agreement with the consensus models. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 15.2N 118.2W 60 KT 70 MPH 12H 25/1200Z 15.3N 118.8W 65 KT 75 MPH 24H 26/0000Z 15.1N 119.9W 70 KT 80 MPH 36H 26/1200Z 14.8N 120.9W 65 KT 75 MPH 48H 27/0000Z 14.5N 121.7W 60 KT 70 MPH 72H 28/0000Z 14.2N 122.8W 55 KT 65 MPH 96H 29/0000Z 15.5N 123.0W 55 KT 65 MPH 120H 30/0000Z 18.0N 123.0W 50 KT 60 MPH $$ Forecaster Cangialosi

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Tropical Storm Greg Forecast Discussion Number 30

2017-07-24 22:38:54| Tropical Depression LIDIA

Issued at 200 PM PDT Mon Jul 24 2017

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Hurricane Hilary Forecast Discussion Number 14

2017-07-24 22:34:58| National Hurricane Center (East Pacific)

Issued at 400 PM CDT Mon Jul 24 2017 000 WTPZ44 KNHC 242034 TCDEP4 Hurricane Hilary Discussion Number 14 NWS National Hurricane Center Miami FL EP092017 400 PM CDT Mon Jul 24 2017 Hilary continues to display a small central core with very deep convective tops and lots of banding in the outer circulation. Although an eye is not apparent in visible imagery, WindSAT microwave data from earlier show that the hurricane has a distinct eye underneath the cloud canopy. Satellite estimates continue to increase, so the initial wind speed is set to 75 kt, close to the latest SAB fix. The hurricane has been strengthening quickly during the past 24 h, near the threshold of rapid intensification. With no significant changes to the favorable oceanic and atmospheric environments forecast, there is good reason to show a similar rate of strengthening for the first 24 hours of the forecast. After that time, models are indicating an increase in northerly wind shear, partially associated with an enhanced anticyclonic outflow from TS Irwin. This might limit the peak intensity, so the wind speed forecast has been leveled off on day 2. The latest model guidance has come in closer to the previous official forecast, so little change has been made in the new NHC intensity prediction. Some weakening is likely at longer range due to cooler SSTS, entrainment of drier air, and binary interaction with Irwin. The initial motion estimate is 300/7 kt. A ridge that extends into the eastern Pacific from the southwestern United States is forecast to strengthen slightly over the next couple of days, which should steer Hilary a little to the left and a bit faster during that time. The long-range forecast is quite challenging with the threat of binary interaction with Irwin, leading to widely divergent solutions by days 4-5. While the GFS keeps Hilary far separated from Irwin on the northeastern side of the guidance envelope, its ensemble mean is much more in the pack of the guidance, suggesting the latest GFS solution is not very likely. The UKMET and ECMWF continue to show rare TC binary interaction and merger scenarios which, although scientifically pleasing to see, lead to large errors potentially. Overall, the model trend toward more interaction with Irwin continues, which led to a westward shift of the latest NHC prediction at days 3-5. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 14.5N 104.9W 75 KT 85 MPH 12H 25/0600Z 14.9N 106.0W 85 KT 100 MPH 24H 25/1800Z 15.5N 107.8W 100 KT 115 MPH 36H 26/0600Z 16.1N 110.0W 105 KT 120 MPH 48H 26/1800Z 16.6N 112.0W 105 KT 120 MPH 72H 27/1800Z 17.7N 115.8W 95 KT 110 MPH 96H 28/1800Z 18.7N 119.3W 80 KT 90 MPH 120H 29/1800Z 19.5N 122.5W 70 KT 80 MPH $$ Forecaster Blake

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Tropical Storm Irwin Forecast Discussion Number 10

2017-07-24 22:33:27| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Mon Jul 24 2017 000 WTPZ45 KNHC 242033 TCDEP5 Tropical Storm Irwin Discussion Number 10 NWS National Hurricane Center Miami FL EP102017 200 PM PDT Mon Jul 24 2017 Irwin has been displaying a dimple in visible satellite imagery, which appears to coincide with a mid-level eye that has been apparent in recent microwave imagery. However, the microwave data also suggest that Irwin's circulation is tilted from south to north with height, with the low-level center located south of the feature noted in visible imagery. Subjective Dvorak estimates have risen to T3.5/55 kt from TAFB and T4.0/65 kt from SAB, while the objective ADT is still down around T3.1/47 kt. Given the cyclone's tilted structure, I'm comfortable going down the middle and initializing the intensity at 55 kt. Vertical shear is currently low over Irwin and should remain so for the next 24 hours or so. This will likely allow Irwin to continue strengthening, reaching hurricane status overnight. Increased shear, possibly enhanced by Hilary's outflow, could begin to impinge on Irwin after 24 hours, which would likely limit the intensification trend. The new intensity guidance has come down a bit after 24 hours, and only gradual weakening is anticipated through the end of the forecast period as Irwin and Hilary interact. Irwin will also be approaching the 26 deg C SST isotherm in about 5 days, which will also foster some weakening. The updated NHC forecast is close to the HCCA guidance, and it's just slightly below the previous forecast from 36 hours and beyond. Irwin's is drifting westward with an initial motion of 280/3 kt. As Irwin and Hilary's circulations get closer together over the next few days, Irwin will respond by first moving slowly west-southwestward on days 2 and 3, and then turning northwestward and accelerating behind or around the southeast side of Hilary's circulation. The regional HWRF and HMON models continue to show a persistent westward track that ignores Hilary's existence, while the global models show varying degrees of interaction between the two cyclones. Because of the unrealistic scenarios shown by the regional models, the NHC track forecast continues to lie east of the multi-model consensus on days 4 and 5, and it's very close to HCCA. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 15.0N 117.8W 55 KT 65 MPH 12H 25/0600Z 15.2N 118.3W 65 KT 75 MPH 24H 25/1800Z 15.2N 119.2W 70 KT 80 MPH 36H 26/0600Z 15.1N 120.3W 65 KT 75 MPH 48H 26/1800Z 14.8N 121.3W 65 KT 75 MPH 72H 27/1800Z 14.2N 122.7W 60 KT 70 MPH 96H 28/1800Z 15.0N 123.0W 60 KT 70 MPH 120H 29/1800Z 17.5N 124.5W 55 KT 65 MPH $$ Forecaster Berg

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Tropical Storm IRWIN Forecast Discussion Number 9

2017-07-24 16:58:55| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Mon Jul 24 2017 000 WTPZ45 KNHC 241458 TCDEP5 Tropical Storm Irwin Discussion Number 9 NWS National Hurricane Center Miami FL EP102017 800 AM PDT Mon Jul 24 2017 Irwin's structure continues to improve, with the low-level center embedded beneath a CDO feature that has persisted for the past several hours. In addition, recent microwave data have revealed the formation of a mid-level eye. Dvorak satellite intensity estimates now range between T3.0/45 kt and T3.5/55 kt, so the initial intensity is raised accordingly to 50 kt. For once, the track forecast for a tropical cyclone is more challenging and more uncertain that its intensity forecast. Irwin is expected to have some binary interaction with Hurricane Hilary beginning in about 3 days, but the amount of interaction is still highly uncertain. The ECMWF, UKMET, and Canadian models all show a strong Fujiwhara interaction, with Irwin swinging to the east and northeast of Hilary by day 5. The GFS shows much less interaction, with Irwin staying well to the southwest of Hilary on day 5, and the regional HWRF and HMON models don't even seem to know about Hilary's existence to allow an interaction to occur. Needless to say, there is significant, higher-than-normal spread in the track guidance beginning in about 72 hours. The updated NHC track forecast continues to show a slow westward motion through day 3, followed by an acceleration toward the northwest and north-northwest on days 4 and 5. At the longer ranges, this forecast is closest to HCCA, but future adjustments are likely due to the significant spread among the models and their ensemble members. Since it appears that Irwin is developing a well-defined inner core, it is likely well on its way to becoming a hurricane. Vertical shear is forecast to be low for the next 48 hours or so, while sea surface temperatures are very warm between 28-29 deg Celsius. The NHC intensity forecast is generally a blend of HCCA and the ICON intensity consensus, bringing Irwin to hurricane status by 24 hours. Significant strengthening beyond that threshold is not anticipated at this time due to the possibility of increased shear from Hilary's outflow, and the NHC forecast shows little change in intensity between days 3 and 5. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 14.8N 117.5W 50 KT 60 MPH 12H 25/0000Z 14.8N 117.9W 60 KT 70 MPH 24H 25/1200Z 15.0N 118.6W 70 KT 80 MPH 36H 26/0000Z 15.0N 119.6W 70 KT 80 MPH 48H 26/1200Z 14.9N 120.6W 70 KT 80 MPH 72H 27/1200Z 14.4N 122.2W 65 KT 75 MPH 96H 28/1200Z 15.0N 123.0W 65 KT 75 MPH 120H 29/1200Z 17.0N 124.0W 60 KT 70 MPH $$ Forecaster Berg

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