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Hurricane Irma Forecast Discussion Number 5

2017-08-31 17:04:55| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Aug 31 2017 000 WTNT41 KNHC 311504 CCA TCDAT1 Hurricane Irma Discussion Number 5...Corrected NWS National Hurricane Center Miami FL AL112017 1100 AM AST Thu Aug 31 2017 Satellite images indicate that Irma is rapidly intensifying. Very deep convection has formed in the central dense overcast, which is now displaying a small and clearing eye. Dvorak estimates were up to 77 kt at 1200 UTC, and since the cloud pattern continues to quickly become more organized, the initial wind speed is set to 85 kt. Irma has moved somewhat south of and slower than all of the model guidance since yesterday. Consequently, it stayed longer over the warmer ocean temperatures away from the drier air to the north, possibly allowing the rapid strengthening. Irma should move over cooler waters tomorrow with some increase in mid-level dry air, so hopefully the hurricane's intensity will level off by then. In a few days, the hurricane will be moving over warmer waters with light shear shown by all of the model guidance. This should promote further strengthening of Irma, and the NHC forecast shows an extremely dangerous category 4 hurricane next week, similar to the solutions provided by the HWRF and the ECMWF models. The intensity forecast is raised considerably from the previous one due to initial trends, and is on the high end of the guidance at long range. The hurricane has turned west-northwestward at about 9 kt. This motion should continue for the next day or so before a ridge builds over the central Atlantic Ocean. This ridge should force Irma to turn westward by the weekend, and west-southwestward early next week. Guidance continues to trend southward, following the trend of the ECMWF model starting yesterday. Given the strength of the ridge and depth of the tropical cyclone, there are no obvious reasons to discount the anomalous west-southwestward motion seen in most of the guidance. Little change is made to the track forecast in short range, but the track is shifted southward and westward at long range, though not as far southwest as the overnight ECMWF and ECMWF ensemble models. FORECAST POSITIONS AND MAX WINDS INIT 31/1500Z 16.9N 33.8W 85 KT 100 MPH 12H 01/0000Z 17.5N 35.2W 100 KT 115 MPH 24H 01/1200Z 18.0N 37.1W 105 KT 120 MPH 36H 02/0000Z 18.2N 39.2W 105 KT 120 MPH 48H 02/1200Z 18.1N 41.6W 105 KT 120 MPH 72H 03/1200Z 17.0N 46.5W 110 KT 125 MPH 96H 04/1200Z 16.0N 51.0W 110 KT 125 MPH 120H 05/1200Z 16.0N 55.5W 115 KT 130 MPH $$ Forecaster Blake

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Tropical Storm Lidia Forecast Discussion Number 8

2017-08-31 16:40:00| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Thu Aug 31 2017 000 WTPZ44 KNHC 311439 TCDEP4 Tropical Storm Lidia Discussion Number 8 NWS National Hurricane Center Miami FL EP142017 900 AM MDT Thu Aug 31 2017 Conventional and microwave satellite data indicate that Lidia has become better organized this morning, with interlocking convective bands forming around a much better defined center. Satellite intensity estimates from TAFB and SAB have increased to 45 kt, and the latest CIMSS ADT estimate is close to 55 kt. The initial intensity is increased to 45 kt, and this could be conservative. The initial motion is north-northwestward or 340/8 kt between a mid-level anticyclone centered over central Mexico and a mid- to upper-level low located to the west over the Pacific. This motion should continue for the next 24-36 hours. After that time, a mid-level anticyclone over the southwestern United States should become the main steering influence and force Lidia to turn northwestward and west-northwestward on days 2 through 5. While there remains some spread in the speed, the guidance is tightly clustered on the track, and the new forecast is similar to both the previous forecast and the various consensus models. Additional strengthening is likely today before the center moves close to the Baja California peninusla, and while the chance of Lidia becoming a hurricane is low it is non-zero. Weakening is expected after 12 hours due to the center moving up the spine of the Baja California peninsula, but tropical-storm-force winds are likely to spread up the Gulf of California well east of the center. Lidia is forecast to become a remnant low by day 3 due to its interaction with land, and further weakening is anticipated over the cold waters of the California Current on days 4 and 5. Lidia is a large system accompanied by very heavy rains which are already occurring over portions of western Mexico and Baja California Sur. Regardless of how strong Lidia becomes, life-threatening flash floods and mudslides will be a significant hazard over these areas. In addition, the mountainous terrain of the Baja California peninsula could enhance the winds at higher elevations. FORECAST POSITIONS AND MAX WINDS INIT 31/1500Z 22.3N 109.7W 45 KT 50 MPH 12H 01/0000Z 23.3N 110.2W 55 KT 65 MPH 24H 01/1200Z 24.6N 111.1W 50 KT 60 MPH...INLAND 36H 02/0000Z 26.1N 112.2W 45 KT 50 MPH...INLAND 48H 02/1200Z 27.6N 113.5W 40 KT 45 MPH...INLAND 72H 03/1200Z 30.0N 117.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 04/1200Z 30.5N 119.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 05/1200Z 30.5N 122.0W 15 KT 15 MPH...POST-TROP/REMNT LOW $$ Forecaster Beven

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Tropical Storm Lidia Forecast Discussion Number 7

2017-08-31 10:48:59| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Thu Aug 31 2017 000 WTPZ44 KNHC 310848 TCDEP4 Tropical Storm Lidia Discussion Number 7 NWS National Hurricane Center Miami FL EP142017 300 AM MDT Thu Aug 31 2017 Scatterometer data and microwave images indicate that Lidia has a deformed circulation, with what appears to be two lobes of vorticity rotating around a common center. The unmanned NASA Global Hawk flew very near the centroid of the circulation last evening and provided a better estimate of Lidia's central pressure (about 998 mb at the time). In addition, a dropsonde released to the east of the center measured a mean boundary layer wind of 45 kt, which equates to a surface wind a little over 35 kt. Assuming the dropsonde did not sample the highest winds, and given that deep convection has become more organized into two distinct clusters since that time, the initial intensity is raised to 40 kt. Lidia is moving north-northwestward, or 335/7 kt, between a mid-level high centered over central Mexico and a cyclonic gyre located to the west over the Pacific. This motion should continue for the next 36 hours, but then after that time, high pressure over the southwestern United States should force Lidia to turn northwestward and west-northwestward on days 2 through 5. Most of the spread in the track models is related to Lidia's forward speed, and there is a notable disparity between the faster GFS and slower ECMWF models. The new NHC track forecast is a little faster than the previous one to better match the speeds of the TVCN multi-model consensus and the HFIP Corrected Consensus Approach (HCCA). Even though Lidia is in an environment of low shear and over very warm waters, the storm's large size and lack of an inner core are likely to prevent fast strengthening before the center reaches the Baja California peninsula. Since the ambient environment is favorable, the NHC forecast continues to call for modest strengthening and still shows a peak intensity of 55 kt in about 24 hours, which is in line with HCCA. Weakening is expected after 24 hours due to the center moving up the spine of the Baja California peninsula, but tropical-storm-force winds are likely to spread up the Gulf of California well east of the center. Lidia is forecast to become a remnant low by day 3 due to its interaction with land, and further weakening is anticipated over the cold waters of the California Current on days 4 and 5. Lidia is a large system accompanied by very heavy rains which are already occurring over portions of western Mexico and Baja California Sur. Regardless of how strong Lidia becomes, life-threatening flash floods and mudslides will be a significant hazard over these areas. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 21.3N 109.4W 40 KT 45 MPH 12H 31/1800Z 22.3N 109.9W 50 KT 60 MPH 24H 01/0600Z 23.6N 110.5W 55 KT 65 MPH 36H 01/1800Z 25.1N 111.4W 45 KT 50 MPH...INLAND 48H 02/0600Z 26.7N 112.6W 40 KT 45 MPH...INLAND 72H 03/0600Z 29.2N 115.9W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 04/0600Z 30.0N 119.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 05/0600Z 30.5N 121.5W 15 KT 15 MPH...POST-TROP/REMNT LOW $$ Forecaster Berg

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Tropical Storm Irma Forecast Discussion Number 4

2017-08-31 10:44:57| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Aug 31 2017 000 WTNT41 KNHC 310844 TCDAT1 Tropical Storm Irma Discussion Number 4 NWS National Hurricane Center Miami FL AL112017 500 AM AST Thu Aug 31 2017 Irma continues to become better organized with a developing CDO and a few banding features. The overall cloud pattern is fairly symmetric, with fair upper-level outflow over all but the southeastern portion of the circulation. The current intensity is set at 60 kt, which is the mean of Dvorak estimates from TAFB and SAB. The environment ahead of Irma appears to be mostly conducive for intensification, with increasingly warm SSTs. Vertical shear is expected to remain low as an upper-level trough to the west-northwest of the tropical cyclone lifts out to the north and west. The only noticeable inhibiting factor is slightly drier mid-level air in 2-5 days, but this will probably not be much of a deterrent for strengthening. The official intensity forecast follows the latest consensus of the models, and is similar to the previous NHC prediction. Based on geostationary satellite fixes, the initial motion is just slightly north of west or 280/10 kt. Most of the track guidance shows a west-northwestward motion for the next day or so. Irma will remain situated to the south of a well-defined mid-tropospheric ridge through the forecast period. Much of the guidance indicates that Irma will turn toward a slightly south of westward heading in a couple of days, presumably in response to some building of high pressure to the north and northwest. The official track forecast also shows this, and remains on the southern side of the guidance suite. This is mainly a blend of the GFS and ECMWF solutions. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 16.5N 32.9W 60 KT 70 MPH 12H 31/1800Z 17.0N 34.2W 70 KT 80 MPH 24H 01/0600Z 17.6N 36.0W 80 KT 90 MPH 36H 01/1800Z 18.0N 37.9W 85 KT 100 MPH 48H 02/0600Z 18.2N 40.1W 90 KT 105 MPH 72H 03/0600Z 17.5N 45.0W 100 KT 115 MPH 96H 04/0600Z 16.7N 49.5W 105 KT 120 MPH 120H 05/0600Z 16.0N 53.5W 105 KT 120 MPH $$ Forecaster Pasch

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Tropical Storm Lidia Forecast Discussion Number 6

2017-08-31 04:37:53| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Wed Aug 30 2017 000 WTPZ44 KNHC 310237 TCDEP4 Tropical Storm Lidia Discussion Number 6 NWS National Hurricane Center Miami FL EP142017 900 PM MDT Wed Aug 30 2017 Satellite images indicate that the circulation, although broad, is more consolidated, and convection has become more concentrated near the center. However, there is no evidence of an inner core at this time. Data from a unmanned NASA Global Hawk aircraft currently in the area of Lidia, and observations from the Mexican Navy automatic weather stations, were very helpful in determining the structure of the cyclone. The best estimate of the initial intensity is 35 kt. Lidia is within a very favorable environment of low shear, and the ocean is warm. The only factor against a higher intensification rate appears to be the large size of the cyclone. The NHC forecast calls for gradual strengthening during the next 12 to 24 hours, and Lidia has a chance to be near hurricane strength before the cyclone begins to interact with the high terrain of the Baja California peninsula. After that time, gradual weakening should then begin. The overall circulation is still moving toward the north-northwest or 345 degrees at 6 kt. Lidia is being steered by the flow between a weak ridge over Mexico and a weak cyclonic gyre to the west. This pattern will persist, and the cyclone should continue on the same general track for the next 2 days. Once the cyclone reaches the central portion of Baja California peninsula, a ridge to the north is forecast to amplify, and this flow pattern will force a weaker Lidia on a more west-northwest track toward cold waters. It is important to emphasize that Lidia is a large system accompanied by very heavy rains which are already occuring over portions of western Mexico and Baja California Sur. These rains are already causing threatening flash floods and mudslides and regardless of exactly how strong Lidia becomes, this should be a significant hazard. FORECAST POSITIONS AND MAX WINDS INIT 31/0300Z 20.7N 109.2W 35 KT 40 MPH 12H 31/1200Z 21.6N 109.5W 45 KT 50 MPH 24H 01/0000Z 22.8N 110.0W 55 KT 65 MPH 36H 01/1200Z 23.8N 110.6W 45 KT 50 MPH...INLAND 48H 02/0000Z 25.5N 111.7W 40 KT 45 MPH...INLAND 72H 03/0000Z 28.0N 114.3W 25 KT 30 MPH...INLAND 96H 04/0000Z 29.0N 117.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 05/0000Z 29.5N 119.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Avila

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