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Tropical Storm Leslie Forecast Discussion Number 17

2018-09-30 16:34:02| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sun Sep 30 2018 613 WTNT43 KNHC 301433 TCDAT3 Tropical Storm Leslie Discussion Number 17 NWS National Hurricane Center Miami FL AL132018 1100 AM AST Sun Sep 30 2018 Deep convection has increased a little during the past several hours in a band to the southeast of the center, but the shower activity remains quite limited elsewhere in the circulation. The latest satellite intensity estimates and a recent ASCAT pass still support an initial intensity of 45 kt. Leslie is expected to be moving into a lower wind shear environment and over warmer SSTs during the next few days. These conditions should support some gradual strengthening during that time period. After that time, however, Leslie is expected to move over cooler waters, partly induced by its own upwelling, as it heads northeastward back over its previous track. Accordingly, slight weakening is shown toward the end of the forecast period. The NHC intensity forecast is similar to the previous one and generally follows the consensus models, IVCN and HCCA. Leslie is drifting to the southwest, with the latest initial motion estimate being 230/3 kt. Leslie is caught in very weak steering currents, and a continued slow southwest to south motion is expected during the next few days. Thereafter, the models show a weak trough developing to the southwest of Leslie and that should cause the system to move northeastward at a slightly faster pace. The models have trended a little to the east this cycle, and the NHC forecast has been adjusted in that direction. The bottom line is that Leslie is forecast to meander over the central Atlantic through the forecast period. Large swells generated by Leslie when it was a strong extratropical low will continue to affect Bermuda, the east coast of the United States, and most of the Greater and Lesser Antilles for another day or two. These swells could cause life-threatening surf and rip currents. Although the swells will diminish some by mid-week, they will likely remain hazardous for the same locations through the forecast period due to Leslie's slow motion. The initial 34-kt wind radii were adjusted based on a 1257 UTC ASCAT pass. FORECAST POSITIONS AND MAX WINDS INIT 30/1500Z 33.6N 52.6W 45 KT 50 MPH 12H 01/0000Z 33.2N 53.1W 50 KT 60 MPH 24H 01/1200Z 32.9N 53.6W 55 KT 65 MPH 36H 02/0000Z 32.3N 54.0W 55 KT 65 MPH 48H 02/1200Z 31.6N 54.3W 60 KT 70 MPH 72H 03/1200Z 30.8N 54.2W 65 KT 75 MPH 96H 04/1200Z 33.0N 52.5W 65 KT 75 MPH 120H 05/1200Z 36.0N 51.5W 60 KT 70 MPH $$ Forecaster Cangialosi

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Hurricane Rosa Forecast Discussion Number 22

2018-09-30 16:33:03| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Sun Sep 30 2018 762 WTPZ45 KNHC 301432 TCDEP5 Hurricane Rosa Discussion Number 22 NWS National Hurricane Center Miami FL EP202018 800 AM PDT Sun Sep 30 2018 Rosa continues to be affected by increasing southwesterly shear and cooler waters. Most of the deep convection has been eroded over the southern semicircle of the circulation. The current intensity estimate is reduced to 65 kt in agreement with ADT estimates from UW/CIMSS. An Air Force Reserve Unit Hurricane Hunter aircraft will investigate Rosa later today to provide a better estimate of the intensity of the system. Since the vertical shear is predicted to continue to increase and SSTs should cool to below 23 deg C by Monday, additional weakening is likely before Rosa reaches the Baja California peninsula. The official intensity forecast is near the high end of the numerical guidance. The hurricane is moving just east of north, or around 010/11 kt. The track forecast reasoning is unchanged from the previous advisory package. Rosa is likely to be steered north-northeastward in the flow ahead of a large mid-level trough approaching from the west. This should take the center of the cyclone across Baja California in the Tropical Storm Warning area by Monday night. Rosa or it's remnant should move over the Desert Southwest on Tuesday. The official track forecast is similar to the previous one and roughly in the middle of the dynamical guidance. Key Messages: 1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office. 2. Tropical storm conditions are expected over portions of the central and northern Baja California peninsula on Monday, possibly spreading to the northern Gulf of California Monday night. Interests in those locations should monitor the progress of Rosa. FORECAST POSITIONS AND MAX WINDS INIT 30/1500Z 24.4N 118.6W 65 KT 75 MPH 12H 01/0000Z 25.9N 117.9W 60 KT 70 MPH 24H 01/1200Z 27.5N 116.7W 55 KT 65 MPH 36H 02/0000Z 29.3N 115.4W 45 KT 50 MPH 48H 02/1200Z 31.8N 114.0W 30 KT 35 MPH...INLAND 72H 03/1200Z 36.5N 111.5W 20 KT 25 MPH...POST-TROP/INLAND 96H 04/1200Z...DISSIPATED $$ Forecaster Pasch

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Tropical Storm Leslie Forecast Discussion Number 16

2018-09-30 10:55:57| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Sep 30 2018 408 WTNT43 KNHC 300855 TCDAT3 Tropical Storm Leslie Discussion Number 16 NWS National Hurricane Center Miami FL AL132018 500 AM AST Sun Sep 30 2018 Convection associated with the tropical storm has become more fragmented over the past 12 hours or so, and now consists of several broken bands primarily over the southeastern portion of the circulation. Satellite intensity estimates have not changed much since the previous advisory, and the initial intensity is held at 45 kt, which is in agreement with earlier ASCAT data and a recent UW/CIMSS SATCON estimate of 42 kt. The recent warming of the clouds tops may be associated with cooler waters caused by upwelling beneath the large, slow-moving tropical storm. Leslie, however is forecast to move southwestward toward somewhat warmer waters and a more favorable upper-level environment during the next 2 to 3 days. This conditions should allow for gradual strengthening, and the NHC forecast follows the intensity guidance by bringing Leslie to hurricane strength in about 72 hours. Later in the period, Leslie is likely to move back over some of the cooler upwelled waters along its previous track, which could result in gradual weakening. Recent satellite fixes show that Leslie is moving west- southwestward or 240 degrees at 4 kt. The tropical storm is caught between a pair of mid-level ridges and a slow southwestward to south-southwestward motion is expected over the next 2 to 3 days. Around mid-week, a shortwave trough moving off the coast of the northeastern United States is expected to help lift Leslie slowly northward at days 4 and 5, but the models have trended toward a somewhat slower solution. Despite the storm's expected slow motion, the spread in the track guidance becomes fairly large by days 4 and 5, and it appears that Leslie is likely to meander over the Central Atlantic for quite some time. Large swells generated by Leslie when it was a strong extratropical low will affect Bermuda, the east coast of the United States, and most of the Greater and Lesser Antilles through at least the early part of this week. These swells could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 33.8N 52.5W 45 KT 50 MPH 12H 30/1800Z 33.4N 53.2W 45 KT 50 MPH 24H 01/0600Z 32.9N 53.8W 50 KT 60 MPH 36H 01/1800Z 32.5N 54.2W 55 KT 65 MPH 48H 02/0600Z 31.9N 54.5W 55 KT 65 MPH 72H 03/0600Z 30.7N 54.7W 65 KT 75 MPH 96H 04/0600Z 32.4N 54.1W 65 KT 75 MPH 120H 05/0600Z 35.5N 53.0W 60 KT 70 MPH $$ Forecaster Brown

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Tropical Storm Sergio Forecast Discussion Number 4

2018-09-30 10:54:29| National Hurricane Center (East Pacific)

Issued at 400 AM CDT Sun Sep 30 2018 130 WTPZ41 KNHC 300854 TCDEP1 Tropical Storm Sergio Discussion Number 4 NWS National Hurricane Center Miami FL EP212018 400 AM CDT Sun Sep 30 2018 Microwave and infrared satellite images, along with scatterometer wind data, indicate that Sergio has continued to become better organized, with strong convection now more concentrated near the center. However, scatterometer data showed that the inner-core wind field is still rather loose and not particularly well-defined quite yet. The initial intensity has been increased to 45 kt based on a Dvorak satellite intensity estimate of T3.0/45 kt from TAFB and SAB, which is supported by a 0536 UTC ASCAT pass that contained a few 45-kt vectors east of the center. The initial motion estimate is 270/10 kt. There is no significant change to the previous forecast track or reasoning. For the next 48 hours or so, Sergio should move generally westward along the southern periphery of a large, deep-layer subtropical ridge that extends from Mexico westward across the eastern and central Pacific. By 72 hours, a broad mid- to upper-level trough is forecast to drop southward and southeastward over the southwestern U.S. and the northeastern Pacific, producing a break in the ridge which will allow Sergio to move toward the northwest at a slower forward speed through 120 hours. The new NHC track forecast is similar to but slightly south of the previous advisory track, and lies close to an average of the consensus models TVCE, HCCA, and FSSE. Sergio is forecast to remain in a low-shear, warm-water, and high-moisture environment for the next 36 hours or so, during which time rapid intensification is expected. In the 48- to 72-hour period, the vertical shear is forecast by the GFS and ECMWF global models to increase from the northeast at around 25 kt, which should act to cap the strengthening process, and possibly even induce some weakening. By 96 and 120 hours, the shear is expected to decrease to 5 kt or less, which would favor re-strengthening. However, due to the uncertainty in how much Sergio's inner-core wind field will be disrupted by the aforementioned strong shear, the intensity forecast is simply leveled off at 95 kt at 72 hours and beyond. The official intensity forecast is slightly higher than the previous advisory and closely follows the IVCN consensus model, which is lower than the more robust HCCA and FSSE models, which bring Sergio to near category 4 strength in 36-48 hours. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 12.0N 104.3W 45 KT 50 MPH 12H 30/1800Z 12.0N 105.9W 60 KT 70 MPH 24H 01/0600Z 11.9N 108.2W 75 KT 85 MPH 36H 01/1800Z 11.6N 110.4W 90 KT 105 MPH 48H 02/0600Z 11.7N 112.4W 100 KT 115 MPH 72H 03/0600Z 13.0N 115.9W 95 KT 110 MPH 96H 04/0600Z 15.2N 118.6W 95 KT 110 MPH 120H 05/0600Z 17.1N 120.8W 95 KT 110 MPH $$ Forecaster Stewart

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Hurricane Rosa Forecast Discussion Number 21

2018-09-30 10:40:27| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Sun Sep 30 2018 841 WTPZ45 KNHC 300840 TCDEP5 Hurricane Rosa Discussion Number 21 NWS National Hurricane Center Miami FL EP202018 200 AM PDT Sun Sep 30 2018 Infrared and microwave satellite images indicate a continued erosion of Rosa's inner-core structure since the previous advisory due to southwesterly vertical wind shear near 20 kt along with colder water beneath the hurricane and entrainment of drier mid-level air in the southwestern quadrant. The intensity was assessed as 80 kt at 0600 UTC based on the high-end estimates from TAFB and UW-CIMSS ADT and SATCON, but since then the rapid erosion of the eye feature and strong northeastward tilt to the vortex column noted in microwave imagery suggests a lower estimate of 75 kt for the advisory intensity. This initial motion estimate is 005/10 kt. Rosa is forecast to continue moving northward around the western edge of a deep-layer ridge for the next 24 h or so, followed by a turn toward the north-northeast on Tuesday as a mid-/upper-level trough approaches from the west. As the low- and upper-level circulations continue to decouple, Rosa should essentially maintain its current forward speed until landfall occurs in 36-48 hours due to the cyclone not being influenced by the faster deep-layer steering flow. The new NHC track forecast is a little slower than the previous advisory track, and closely follows the consensus models HCCA and IVCN. A 72-hour forecast position continues to be provided for continuity purposes, but Rosa's surface circulation is likely to dissipate before that time over northwestern Mexico or southern Arizona, with the mid-level remnants continuing northward across the Desert Southwest and Intermountain West. Rosa is now moving over waters colder than 25 deg C, with colder water near 22 deg C ahead of the cyclone just prior to landfall. The combination of increasing wind shear, cooler waters and drier and more stable air being entrained from the west should result in steady or even rapid weakening of the cyclone until landfall occurs. The official forecast follows the sharp weakening trend indciated in the previous advisory, which is supported by the latest intensity guidance. Rosa is expected to devolve into an exposed low-level center with the associated deep convection being sheared off to its north and northeast by the time it is nearing the Baja California coast on Monday. However, it will take some time for the circulation to spin down, and Rosa is still expected to bring tropical-storm-force winds to portions of Baja California in 36-48 hours. Key Messages: 1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office. 2. Tropical storm conditions are expected over portions of the central and northern Baja California peninsula on Monday, possibly spreading to the northern Gulf of California Monday night. Interests in those locations should monitor the progress of Rosa. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 23.3N 118.8W 75 KT 85 MPH 12H 30/1800Z 24.8N 118.3W 65 KT 75 MPH 24H 01/0600Z 26.5N 117.2W 50 KT 60 MPH 36H 01/1800Z 28.3N 116.0W 45 KT 50 MPH 48H 02/0600Z 30.6N 114.5W 35 KT 40 MPH 72H 03/0600Z 37.2N 110.8W 20 KT 25 MPH...POST-TROP/INLAND 96H 04/0600Z...DISSIPATED INLAND $$ Forecaster Stewart

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