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Tropical Storm Irma Forecast Discussion Number 3

2017-08-31 04:32:24| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Aug 30 2017 000 WTNT41 KNHC 310232 TCDAT1 Tropical Storm Irma Discussion Number 3 NWS National Hurricane Center Miami FL AL112017 1100 PM AST Wed Aug 30 2017 Irma continues to become better organized, with the development of a small CDO feature and increased banding near the center. An earlier high-resolution Windsat microwave overpass showed that Irma has a tight inner core and a low-level eye-like feature was present. A Dvorak classification of T3.5 from TAFB is the basis for the initial intensity of 55 kt. Irma is expected to steadily strengthen during the next couple of days while it moves through a low-shear and moist environment, and remains over warm sea surface temperatures. After that time, slightly cooler SSTs and lower mid-level moisture may temper the intensification process. However, the statistical aids, HWRF, and most of the consensus models make Irma a major hurricane by the end of the forecast period. The NHC forecast is fairly similar to the previous advisory through 48 hours, but is above the earlier forecast thereafter. The new official intensity forecast could still be a little conservative as it remains a little below the SHIPS/LGEM guidance and the ICON consensus at days 4 and 5. Irma is moving westward at about 10 kt to the south of deep-layer ridge over the eastern Atlantic. The track forecast reasoning is about the same as the previous advisory, with Irma expected to turn west-northwestward on Thursday, then continue on that heading for a couple of days. The high pressure ridge over the central Atlantic is forecast to strengthen later this week, which is expected to result in Irma turning west-southwestward by the weekend. There is still some spread among the track models, so the NHC forecast lies near a blend of the typically reliable GFS and ECMWF, and the HFIP corrected consensus. FORECAST POSITIONS AND MAX WINDS INIT 31/0300Z 16.4N 32.2W 55 KT 65 MPH 12H 31/1200Z 16.8N 33.6W 65 KT 75 MPH 24H 01/0000Z 17.3N 35.5W 75 KT 85 MPH 36H 01/1200Z 17.7N 37.3W 85 KT 100 MPH 48H 02/0000Z 18.0N 39.5W 90 KT 105 MPH 72H 03/0000Z 17.6N 44.3W 95 KT 110 MPH 96H 04/0000Z 16.8N 49.0W 100 KT 115 MPH 120H 05/0000Z 16.2N 53.5W 105 KT 120 MPH $$ Forecaster Brown

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Potential Tropical Cyclone Fourteen-E Forecast Discussion Number 5

2017-08-30 22:36:27| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Wed Aug 30 2017 000 WTPZ44 KNHC 302036 TCDEP4 Potential Tropical Cyclone Fourteen-E Discussion Number 5 NWS National Hurricane Center Miami FL EP142017 300 PM MDT Wed Aug 30 2017 Satellite images indicate that the convection pattern of the system is becoming better organized, with a large curved band in the southern semicircle. ASCAT data from earlier indicated peak winds of 33 kt, and since the pass caught less than half of the circulation, it is assumed higher winds were not sampled. Thus the initial wind speed is set to 35 kt. However, the low still does not have a well-defined center, with the elongated circulation containing several low-level swirls, so it will stay a potential tropical cyclone. Slow strengthening is considered most likely due to the poor initial organization. If the circulation consolidates, the upper-level winds and water temperatures are still conducive to strengthening. The NHC forecast is similar to the previous one, just a bit above the model consensus. In a couple of days, the cyclone should weaken while it traverses either land or the cooler waters west of the Baja peninsula. A long-term motion of the system is 345/6. The disturbance should move on this general course around a weak ridge over central Mexico, before taking a more westward turn as it runs into a ridge over the southwestern United States. Models are in fairly good agreement on the cyclone moving very close to Baja California Sur from 36 hours to 96 hours, although again there has been a small eastward trend in the guidance. Thus, the official forecast is shifted to the east as well. A tropical storm warning has been issued for the west coast of Mexico due to the changes in the forecast. Since the disturbance still lacks a well-defined center and there is considerable spread in the model guidance, this official track forecast has greater-than-usual uncertainty. It is also important to note that very heavy rain is falling over portions of western Mexico and Baja California Sur, which is already causing threatening flash floods and mudslides. Regardless of exactly how strong the cyclone becomes, this should be a significant hazard. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 20.1N 108.8W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 31/0600Z 20.9N 109.2W 40 KT 45 MPH...TROPICAL STORM 24H 31/1800Z 22.0N 109.7W 50 KT 60 MPH 36H 01/0600Z 23.1N 110.2W 55 KT 65 MPH 48H 01/1800Z 24.3N 110.8W 50 KT 60 MPH...INLAND 72H 02/1800Z 27.0N 113.0W 40 KT 45 MPH...INLAND 96H 03/1800Z 28.5N 116.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 04/1800Z 29.0N 118.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Blake

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Tropical Storm Irma Forecast Discussion Number 2

2017-08-30 22:36:26| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Aug 30 2017 000 WTNT41 KNHC 302036 TCDAT1 Tropical Storm Irma Discussion Number 2 NWS National Hurricane Center Miami FL AL112017 500 PM AST Wed Aug 30 2017 Irma is displaying increasingly organized symmetric and interlocking rainbands with a well-defined outflow as seen in the visible and infrared satellite imagery this afternoon. Dvorak classifications from both TAFB and SAB have gone up at 18Z, suggesting 45 and 35 kt, respectively. Given their low bias earlier compared with the ASCAT scatterometer passes, the initial intensity is bumped up to 50 kt at advisory time. Irma is embedded within very low tropospheric vertical shear currently and for the foreseeable future, as the tropical storm lies underneath the upper-level anticyclone. However, in about three days the SSTs and the mid-level humidities that Irma should start to encounter will become cooler and drier. The intensity guidance shows steady intensification for the next two to three days, then diverges in response to the low shear/not-as-favorable thermodynamical environment. The official intensity forecast is closest to the ICON consensus technique through three days, then shows no change to day five. This new forecast is higher than the previous advisory, but lower than ICON at day 4 and 5 (and thus may be conservative). Irma is moving toward the west at about 13 kt along the south side of the deep-layer Azores high. A slight turn toward the west- northwest at a slower rate of forward speed is expected during the next three days. As the high strengthens, Irma is expected to turn toward the west-southwest around day 4. There is a moderate amount of spread among the normally reliable global and mesoscale models in this scenario. The official track forecast is based upon the HFIP Corrected Consensus Approach and is nearly the same as that from the previous advisory. Despite the large overall envelope of the Irma's circulation, the earlier ASCAT passes showed a very small area of tropical-storm- force winds - only 30 nm radius. The official size forecast is based upon the RVCN - variable consensus technique. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 16.4N 31.2W 50 KT 60 MPH 12H 31/0600Z 16.8N 32.7W 60 KT 70 MPH 24H 31/1800Z 17.3N 34.5W 70 KT 80 MPH 36H 01/0600Z 17.8N 36.2W 80 KT 90 MPH 48H 01/1800Z 18.2N 38.2W 85 KT 100 MPH 72H 02/1800Z 18.2N 43.0W 90 KT 105 MPH 96H 03/1800Z 17.3N 47.8W 90 KT 105 MPH 120H 04/1800Z 16.5N 52.5W 90 KT 105 MPH $$ Forecaster Landsea

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Tropical Storm Harvey Forecast Discussion Number 42

2017-08-30 22:35:26| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Wed Aug 30 2017 000 WTNT44 KNHC 302035 TCDAT4 Tropical Storm Harvey Discussion Number 42 NWS National Hurricane Center Miami FL AL092017 400 PM CDT Wed Aug 30 2017 Harvey is gradually weakening as the center moves farther inland, with the winds near the center diminishing and the central pressure rising to near 998 mb. However, scatterometer data and surface observations indicate a band of 30-35 kt winds over the Gulf of Mexico well to the southeast of the center, and this is the basis for keeping Harvey a tropical storm. The cyclone should weaken to a depression in less than 12 hours, and continued slow weakening is anticipated until the cyclone dissipates completely between 72-96 h. It should be noted that the forecast weakening will not eliminate the risk of continued heavy rainfall and flooding along Harvey's path, although the system's faster motion will keep subsequent rainfall totals well below what occurred over southeastern Texas. Harvey is wobbling back and forth around an overall motion of 020/7. The cyclone is located on the northwestern side of a low- to mid-tropospheric level ridge over the western Atlantic and eastern Gulf of Mexico, which should steer it north-northeastward and then northeastward across the Lower Mississippi and Tennessee Valleys during the next 72-96 h until the system dissipates. Key Messages: 1. Ongoing catastrophic and life-threatening flooding will continue across southeastern Texas. While the threat of heavy rains has ended in the Houston/Galveston area, catastrophic and life- threatening flooding will continue in and around Houston, Beaumont/Port Arthur, eastward into southwest Louisiana for the rest of the week. Isolated storm totals have reached 50 inches over the upper Texas coast, including the Houston/Galveston metropolitan area. Please heed the advice of local officials. Do not attempt to travel if you are in a safe place, and do not drive into flooded roadways. Refer to products from your local National Weather Service office and the NOAA Weather Prediction Center for more information on the flooding hazard. A summary of rainfall totals compiled by the Weather Prediction Center can be found at: www.wpc.ncep.noaa.gov/discussions/nfdscc1.html 2. The flood threat has spread farther north and east, and additional rainfall accumulations of 3 to 6 inches with isolated amounts up to 10 inches are expected from southwestern Louisiana and the adjacent border of eastern Texas northeastward into western Kentucky through Friday. Please heed the advice of local officials and refer to products from your local National Weather Service office and the NOAA Weather Prediction Center for more information on the flooding hazard in these areas. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 30.8N 93.1W 35 KT 40 MPH...INLAND 12H 31/0600Z 31.8N 92.4W 30 KT 35 MPH...INLAND 24H 31/1800Z 33.5N 91.0W 25 KT 30 MPH...INLAND 36H 01/0600Z 35.1N 89.1W 25 KT 30 MPH...INLAND 48H 01/1800Z 36.4N 87.1W 25 KT 30 MPH...INLAND 72H 02/1800Z 38.6N 83.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 03/1800Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Irma Forecast Discussion Number 1

2017-08-30 16:53:22| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Wed Aug 30 2017 000 WTNT41 KNHC 301453 TCDAT1 Tropical Storm Irma Discussion Number 1 NWS National Hurricane Center Miami FL AL112017 1100 AM AST Wed Aug 30 2017 Satellite images indicate that the low pressure area in the far eastern Atlantic has become much better organized since yesterday, with many curved bands around the center. ASCAT data showed peak winds of about 42 kt, and after considering the small size of the circulation and some undersampling due to the resolution of the instrument, the initial wind speed is set to 45 kt. Global models indicate that the upper-level winds are likely to be favorable for strengthening of Irma during the next several days. However, Irma will be moving over more marginal water temperatures and into drier mid-level conditions, which should temper the intensification rate. The NHC solution is a blend of the intensity consensus and the statistical-dynamical hurricane models SHIPS and LGEM. At the end of the period, the forecast could turn out to be conservative if the very conducive environment shown in most of the global models emerges. The initial motion estimate is 280/11. A ridge over the eastern Atlantic is forecast to steer Irma westward over the next few days. Thereafter, the ridge builds southwestward, which will likely cause the storm to move, somewhat unusually, toward the west-southwest. The official forecast puts more weight on the global models than the regional hurricane models, which appear to have a northward bias on this cycle. Thus, the NHC track prediction is on the southwestern side of the guidance envelope, although not as far in that direction as the ECMWF or its ensemble mean. FORECAST POSITIONS AND MAX WINDS INIT 30/1500Z 16.4N 30.3W 45 KT 50 MPH 12H 31/0000Z 16.7N 31.9W 50 KT 60 MPH 24H 31/1200Z 17.3N 33.8W 55 KT 65 MPH 36H 01/0000Z 17.9N 35.7W 60 KT 70 MPH 48H 01/1200Z 18.2N 37.6W 65 KT 75 MPH 72H 02/1200Z 18.7N 41.9W 70 KT 80 MPH 96H 03/1200Z 17.7N 46.5W 75 KT 85 MPH 120H 04/1200Z 17.0N 51.0W 80 KT 90 MPH $$ Forecaster Blake

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