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Hurricane Irma Forecast Discussion Number 13

2017-09-02 16:56:18| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 02 2017 000 WTNT41 KNHC 021456 TCDAT1 Hurricane Irma Discussion Number 13 NWS National Hurricane Center Miami FL AL112017 1100 AM AST Sat Sep 02 2017 Hurricane Irma continues to display an eye within a small central dense overcast in the visible and infrared imagery from the GOES and METEOSAT satellites. Dvorak current intensity numbers from TAFB, SAB, and the CIMSS ADT have not changed, so the intensity remains 95 kt. The environment in which Irma is embedded shows mixed signals for the next few days. While the vertical shear remains low through Monday, the SSTs are lukewarm and mid-level humidities are dry. (Indeed an overnight SSMIS microwave pass showed a distinct finger of dry air wrapping around the south side of Irma not far from its inner core.) However, after the waters warm and the atmosphere moistens at days 3-5, the shear is forecast to go up. The guidance has substantial spread between Category 2 and 4 by the end of the forecast period. The official intensity forecast is based upon a blend of the LGEM/DSHP statistical techniques and the HWRF dynamical model and is unchanged from the previous advisory. Irma is finally moving slightly south of due west at 13 kt. A large, building Azores-Bermuda high should turn Irma toward the west-southwest at a slower rate of speed during the next two days. By days 3 to 5, Irma should be rounding the southwestern periphery of the high and start turning back toward the west-northwest. The track guidance is in tight agreement with this scenario and the official forecast is a simply an update from the previous advisory. Irma is a small hurricane, as observed by ship BATFR17 and the overnight ASCAT scatterometer pass with tropical-storm-force winds extending out at most about 60 nm. The official size forecast is based upon the RVCN multi-model consensus technique. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it moves closer to the Lesser Antilles early next week, producing rough surf and rip currents. Irma could also cause dangerous wind, storm surge, and rainfall impacts on some islands, although it is too soon to specify where and when those hazards could occur. Residents in the Lesser Antilles should monitor the progress of Irma through the weekend and listen to any advice given by local officials. 2. It is much too early to determine what direct impacts Irma will have on the Bahamas and the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 18.8N 43.3W 95 KT 110 MPH 12H 03/0000Z 18.5N 45.0W 95 KT 110 MPH 24H 03/1200Z 18.0N 47.4W 100 KT 115 MPH 36H 04/0000Z 17.4N 49.5W 105 KT 120 MPH 48H 04/1200Z 17.1N 51.6W 110 KT 125 MPH 72H 05/1200Z 17.4N 56.0W 110 KT 125 MPH 96H 06/1200Z 19.2N 60.6W 115 KT 130 MPH 120H 07/1200Z 22.0N 66.0W 115 KT 130 MPH $$ Forecaster Landsea

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Tropical Storm Lidia Forecast Discussion Number 16

2017-09-02 16:54:17| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Sat Sep 02 2017 000 WTPZ44 KNHC 021454 TCDEP4 Tropical Storm Lidia Discussion Number 16 NWS National Hurricane Center Miami FL EP142017 900 AM MDT Sat Sep 02 2017 Lidia has become less organized since the previous advisory, with satellite imagery suggesting that the low-level and mid-level centers have become decoupled. In addition, the coverage of deep convection has diminished significantly, and there are no recent observations of sustained tropical-storm-force winds. The initial intensity will be lowered to 35 kt based on the premise that these winds are still occurring over the Gulf of California. Lidia should continue to weaken as the center moves over the cold waters west of the northern Baja California peninsula, and the system is expected to degenerate into a remnant low in 24 h or less. The NHC forecast will continue to show a remnant low at 72 h, but as noted in the previous advisory it is unclear whether this will be Lidia or another low pressure area that absorbs the remnants of Lidia. The initial motion is 315/10 along the south side a mid-tropospheric high centered over the southwestern United States, and this motion should continue for the next 36 h or so. After that, a turn toward the west-northwest is expected due to interaction with a mid- to upper-level low currently seen in water vapor imagery well to the west of the Baja California peninsula. The new forecast track is an update of the previous track and lies in the center of the tightly-clustered track guidance. Despite the ongoing and forecast weakening, the heavy rainfall threat will continue over northwestern Mexico and the Baja California peninsula through Sunday. Moisture from the cyclone should spread over portions of the southwestern United States through the weekend. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 27.9N 114.3W 35 KT 40 MPH...ON COAST 12H 03/0000Z 28.9N 115.6W 30 KT 35 MPH...OVER WATER 24H 03/1200Z 30.2N 117.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 04/0000Z 31.4N 119.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 04/1200Z 32.1N 121.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 05/1200Z 33.5N 126.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 06/1200Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Lidia Forecast Discussion Number 15

2017-09-02 10:53:12| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Sat Sep 02 2017 000 WTPZ44 KNHC 020853 TCDEP4 Tropical Storm Lidia Discussion Number 15 NWS National Hurricane Center Miami FL EP142017 300 AM MDT Sat Sep 02 2017 Lidia's convection has a rather amorphous pattern in infrared satellite imagery this morning, but earlier microwave images indicate that the storm still has some banding features. The center has moved inland again and is now located over northern Baja California Sur. Since the center is over land and cloud tops are gradually warming, the advisory intensity is set at 40 kt, which is slightly below the average of the latest Dvorak CI numbers. The strongest winds are likely occurring well to the north and east of the center over the Gulf of California. Continued weakening is expected since Lidia will be moving over the rugged terrain of the Baja California peninsula and then over the cold waters off the peninsula's west coast. Lidia should lose its deep convection over the cold waters by 36 hours, at which point it would become a remnant low. The global models generally keep Lidia as a separate and distinct vorticity maximum through at least 48 hours, but they then show it merging or becoming absorbed by a larger area of vorticity by day 3. The NHC forecast continues to show a day 3 point for continuity, but whatever surface low exists at that time is likely to be a new and separate feature. Lidia continues to move northwestward (320/10 kt) along the southern edge of a mid-tropospheric high centered over the southwestern United States, and this motion should continue for the next 36 hours. A bend toward the west-northwest is expected after that time due to Lidia's interaction with a mid-/upper-level low currently located well to the west of the Baja California peninsula. Lidia should re-emerge off the west coast of the Baja California peninsula later today and then gradually move away from land over the next few days. Despite weakening, the heavy rainfall threat will continue over northwestern Mexico and the Baja California peninsula through Sunday. Moisture from the cyclone could reach the southwestern United States over the weekend. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 27.2N 113.6W 40 KT 45 MPH...INLAND 12H 02/1800Z 28.3N 114.8W 35 KT 40 MPH...OVER WATER 24H 03/0600Z 29.6N 116.6W 30 KT 35 MPH 36H 03/1800Z 30.8N 118.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 48H 04/0600Z 31.9N 120.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 05/0600Z 33.1N 125.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 06/0600Z...DISSIPATED $$ Forecaster Berg

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Hurricane Irma Forecast Discussion Number 12

2017-09-02 10:50:43| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sat Sep 02 2017 000 WTNT41 KNHC 020850 TCDAT1 Hurricane Irma Discussion Number 12 NWS National Hurricane Center Miami FL AL112017 500 AM AST Sat Sep 02 2017 Irma appears to have weakened a little during the last several hours. The eye has become cloud filled once again, and the convective pattern is not as impressive as it was yesterday. A blend of the latest Dvorak classifications from TAFB/SAB and ADT values from CIMSS at the University of Wisconsin support lowering the initial wind speed a little to 95 kt. It is interesting to note that a ship (BATFR17) passed within 50 n mi to the west of the center of Irma and has only reported winds of about 40 kt, indicating that the core of Irma is compact. The observed fluctuations in strength during the past day or so are likely to continue for about another day while Irma remains over marginally warm waters and in fairly close proximity to dry air. Eyewall replacement cycles, like the one observed yesterday, could occur, but forecasting the timing and duration of these are not possible. After 24 hours, Irma is expected to move over progressively warmer waters and into a more moist environment. These more favorable conditions combined with low to moderate wind shear should allow the hurricane to strengthen. The NHC intensity forecast follows the consensus aids HCCA and IVCN, and it is fairly similar to the previous advisory. Irma is now moving due west at 12 kt. A subtropical high pressure system to the north of the hurricane is expected to strengthen and build westward during the next couple of days. This pattern should cause Irma to move west-southwestward during that time. Thereafter, a turn back to the west and then west-northwest is predicted in the 3-5 day time period when Irma moves on the south and southwest sides of the high. Although the models agree on the overall scenario, there remains about 200 n mi north-south spread among the best-performing models on day 5. The NHC track forecast has been adjusted to the south at the longer-range points, and it is about halfway between the latest runs of the GFS and ECMWF models. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it moves closer to the Lesser Antilles early next week, producing rough surf and rip currents. Irma could also cause dangerous wind, storm surge, and rainfall impacts on some islands, although it is too soon to specify where and when those hazards could occur. Residents in the Lesser Antilles should monitor the progress of Irma through the weekend and listen to any advice given by local officials. 2. It is much too early to determine what direct impacts Irma will have on the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 19.0N 41.8W 95 KT 110 MPH 12H 02/1800Z 18.8N 43.7W 100 KT 115 MPH 24H 03/0600Z 18.3N 46.1W 100 KT 115 MPH 36H 03/1800Z 17.7N 48.3W 105 KT 120 MPH 48H 04/0600Z 17.1N 50.4W 110 KT 125 MPH 72H 05/0600Z 17.0N 54.7W 110 KT 125 MPH 96H 06/0600Z 18.4N 59.3W 115 KT 130 MPH 120H 07/0600Z 21.0N 64.5W 115 KT 130 MPH $$ Forecaster Cangialosi

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Hurricane Irma Forecast Discussion Number 11

2017-09-02 04:42:42| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Sep 01 2017 000 WTNT41 KNHC 020242 TCDAT1 Hurricane Irma Discussion Number 11 NWS National Hurricane Center Miami FL AL112017 1100 PM AST Fri Sep 01 2017 After developing a nearly clear eye during the afternoon hours, Irma appears to have once again temporarily peaked. A WindSat pass around 2100 UTC hinted at the beginning of another eyewall replacement cycle, which would be consistent with the observed cooling of the eye. However, the small size of the hurricane's inner core relative to the resolution of the microwave instrument makes it impossible for me to say for sure. The initial intensity has been lowered slightly to 100 kt, but it should be stressed that this is probably just another fluctuation, in what will likely be a long string of small changes in intensity over the next several days. Since we do not have the ability to predict such changes, the NHC forecast shows very gradual intensification throughout the forecast period, given the warm SSTs and increasing moisture content along the forecast track. The NHC forecast is near the intensity consensus, but slightly favors the higher dynamical models. The initial motion remains 275/12 kt. The hurricane has moved a little farther north than previously expected, and the track has been adjusted in that direction. Overall, the reasoning behind the track forecast has not changed, and Irma is still expected to turn west-southwestward on Saturday due to a building ridge over the central Atlantic. After about 72 h, there remains a large north-south spread in the guidance, with the GFS continuing to show a weaker ridge (and a northern track), while the ECMWF shows a stronger ridge/southern track. The ECMWF has performed better for Irma thus far, so my forecast continues to favor that solution. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it approaches the Lesser Antilles early next week, producing rough surf and rip currents. Irma could also cause dangerous wind, storm surge, and rainfall impacts on some islands, although it is too soon to specify where and when those hazards could occur. Residents in the Lesser Antilles should monitor the progress of Irma through the weekend and listen to any advice given by local officials. 2. It is much too early to determine what direct impacts Irma will have on the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 19.1N 40.5W 100 KT 115 MPH 12H 02/1200Z 19.0N 42.3W 100 KT 115 MPH 24H 03/0000Z 18.5N 44.6W 105 KT 120 MPH 36H 03/1200Z 17.9N 46.9W 110 KT 125 MPH 48H 04/0000Z 17.3N 49.0W 110 KT 125 MPH 72H 05/0000Z 16.8N 53.3W 110 KT 125 MPH 96H 06/0000Z 18.0N 57.5W 115 KT 130 MPH 120H 07/0000Z 20.5N 62.5W 115 KT 130 MPH $$ Forecaster Zelinsky

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