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Tropical Storm Rosa Forecast Discussion Number 4

2018-09-26 04:39:50| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Tue Sep 25 2018 649 WTPZ45 KNHC 260239 TCDEP5 Tropical Storm Rosa Discussion Number 4 NWS National Hurricane Center Miami FL EP202018 900 PM MDT Tue Sep 25 2018 Satellite imagery indicates that Rosa is still strengthening. The tropical storm has impressive banding features that wrap cyclonically from the northwest to northeast quadrant, though a dry slot has recently been observed infringing on the storm's developing inner-core. The most recent Dvorak intensity estimates from TAFB and SAB are 55 kt, so the initial intensity has been raised to that value. All of the intensity guidance suggests that rapid intensification (RI) is either already underway or imminent. SSMIS imagery from around 2300 UTC appeared to confirm the onset of RI with the development of a small low-level eye-like feature that has been associated with rapid intensification in past tropical cyclones. Although more recent imagery indicates this feature may have been disrupted by the aforementioned dry slot, the models suggest that this disruption may not last long. The NHC forecast therefore continues to show RI, and now calls for Rosa to become a major hurricane in about 36 h. Beyond that time, the hurricane is expected to remain in a fairly favorable environment through Friday, however, eyewall replacement cycles could limit the extent to which Rosa intensifies once it nears major hurricane strength. By the weekend, cooler SSTs and a drier surrounding environment should cause Rosa to begin weakening. The NHC intensity forecast is similar to the previous advisory, and is generally close to HCCA, especially beyond 48 h. Rosa is moving steadily west-northwestward, or 285/8 kt. A mid- level ridge to the north of Rosa should keep the cyclone on the same general heading for the next several days. There is still some large discrepancies between the global models on the strength of the ridge for the next few days, resulting in differences in the forward speed of Rosa during that time. By the end of the week, a large mid-latitude trough is expected to create a weakness in the ridge, allowing Rosa to turn northwestward, and possibly northward. The NHC track forecast will continue to follow the TVCE and HCCA consensus aids for this advisory. FORECAST POSITIONS AND MAX WINDS INIT 26/0300Z 15.1N 109.7W 55 KT 65 MPH 12H 26/1200Z 15.5N 110.9W 70 KT 80 MPH 24H 27/0000Z 16.0N 112.5W 85 KT 100 MPH 36H 27/1200Z 16.3N 114.1W 100 KT 115 MPH 48H 28/0000Z 16.6N 115.8W 105 KT 120 MPH 72H 29/0000Z 17.8N 119.0W 110 KT 125 MPH 96H 30/0000Z 20.0N 120.4W 90 KT 105 MPH 120H 01/0000Z 22.0N 121.0W 75 KT 85 MPH $$ Forecaster Zelinsky

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Tropical Storm Rosa Forecast Discussion Number 3

2018-09-25 22:54:19| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Tue Sep 25 2018 927 WTPZ45 KNHC 252054 TCDEP5 Tropical Storm Rosa Discussion Number 3 NWS National Hurricane Center Miami FL EP202018 300 PM MDT Tue Sep 25 2018 Rosa has become better organized this afternoon. An earlier 1601 UTC GMI microwave image showed impressive banding features in the south semicircle with a banding eye type feature wrapping three quarters around the center of circulation. A blend of the Dvorak intensity estimates from TAFB and SAB yield an initial intensity of 45 kt for this advisory. Although there is some modest northeasterly shear impinging in the northeast side of the cyclone, the SHIPS intensity guidance indicate that this shear will diminish in 12 hours. Subsequently, the SHIPS RI index and the DTOPS model indicate a relatively high probability of rapid intensification in 24 hours. Accordingly, the NHC forecast calls for Rosa to become a hurricane in 12 hours based on these data. Further strengthening through the 72 hour period is expected and is based on the IVCN multi-model intensity consensus. Beyond day 3, gradual weakening is expected due to increasing shear and slightly cooler waters. The initial motion is estimated to be west-northwestward, or 290 at 8 kt, and this general motion is forecast to continue through the next 3 days south of a mid-level ridge extending from northern Mexico westward over the eastern Pacific. In response to a mid-latitude trough approaching from the northwest, Rosa should turn towards the northwest to north over the weekend. It is worth noting that there is considerable along and cross-track spread in the track guidance beyond day 3. Therefore, the official track forecast follows the TVCN consensus, which lies in between the left and right global model outlier clusters. FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 15.0N 108.9W 45 KT 50 MPH 12H 26/0600Z 15.4N 110.0W 65 KT 75 MPH 24H 26/1800Z 15.9N 111.5W 80 KT 90 MPH 36H 27/0600Z 16.3N 113.1W 95 KT 110 MPH 48H 27/1800Z 16.5N 114.8W 100 KT 115 MPH 72H 28/1800Z 17.3N 118.0W 110 KT 125 MPH 96H 29/1800Z 19.2N 120.0W 100 KT 115 MPH 120H 30/1800Z 21.7N 120.8W 80 KT 90 MPH $$ Forecaster Ramos/Roberts

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Post-Tropical Cyclone Leslie Forecast Discussion Number 9

2018-09-25 16:40:18| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Tue Sep 25 2018 676 WTNT43 KNHC 251439 TCDAT3 Post-Tropical Cyclone Leslie Discussion Number 9 NWS National Hurricane Center Miami FL AL132018 1100 AM AST Tue Sep 25 2018 Leslie has become a post-tropical cyclone. Conventional and microwave satellite imagery show that Leslie's surface circulation has become elongated along an intruding baroclinic zone indicated by a recent 1158 UTC ASCAT-A pass. Any deep convection appears to be developing due to dynamic forcing. A rather large stratocumulus cloud shield and associated cooler, stable air is quickly advecting into the area as well. The initial intensity is maintained at 30 kt for this last advisory. Leslie is forecast to strengthen during the next couple of days due to baroclinic processes. The official forecast, based on the GFS and European global models, and the SHIPS statistical intensity guidance, shows an increase of the sustained winds to hurricane force in 2 days, around the same period that the shear drops below 10 kt. The non-tropical low is then forecast to move south of the strong upper-level westerlies and detach from the frontal boundary. According to the global models and the Florida State Cyclone Phase forecast, Leslie should make a transition from a frontal system to an asymmetric shallow warm-core seclusion. Beyond day 3, guidance suggests that Leslie will acquire subtropical characteristics and a more symmetric, deepening warm-core low while meandering in weaker steering currents. The intensity forecast follows this scenario and is based on the aforementioned global and statistical models. Leslie is now moving eastward at a slightly faster forward motion, about 10 kt. A turn to the north is expected in 36 hours followed by a slower westward motion as Leslie slips south of the mid-latitude westerlies. Through the remaining portion of the forecast, Leslie should move at a slower speed generally westward as a ridge builds to the north. NHC forecast is nudged toward the TVCN consensus through 48 hours, then follows more closely to the GFEX (GFS/ECMWF) consensus model. This is the last advisory issued by the National Hurricane Center on Leslie. Additional information on this system can be found in High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFAT1, WMO header FZNT01 KWBC, and available on the Web at https://ocean.weather.gov/shtml/NFDHSFAT1.shtml. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 31.6N 44.4W 30 KT 35 MPH...POST-TROPICAL 12H 26/0000Z 32.1N 42.5W 40 KT 45 MPH...POST-TROP/EXTRATROP 24H 26/1200Z 33.6N 40.1W 50 KT 60 MPH...POST-TROP/EXTRATROP 36H 27/0000Z 35.6N 39.9W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 27/1200Z 36.1N 41.7W 65 KT 75 MPH...POST-TROP/EXTRATROP 72H 28/1200Z 35.5N 45.3W 60 KT 70 MPH...SUBTROPICAL STORM 96H 29/1200Z 34.6N 48.1W 55 KT 65 MPH...SUBTROPICAL STORM 120H 30/1200Z 34.4N 49.6W 50 KT 60 MPH...SUBTROPICAL STORM $$ Forecaster Roberts

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Tropical Storm Rosa Forecast Discussion Number 2

2018-09-25 16:34:48| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Tue Sep 25 2018 060 WTPZ45 KNHC 251434 TCDEP5 Tropical Storm Rosa Discussion Number 2 NWS National Hurricane Center Miami FL EP202018 900 AM MDT Tue Sep 25 2018 The cloud pattern of the tropical cyclone has become better organized, with developing convective banding features. Dvorak intensity estimates are 35 kt and 45 kt from SAB and TAFB, respectively, and the current intensity estimate is set at 40 kt. Thus the system is being upgraded to a tropical storm. Conditions favor continued strengthening, with Rosa likely to remain in an environment of warm waters, low shear, and a very moist mid-level air mass for the next several days. The official intensity forecast is a little higher than the previous one, but below some of the objective guidance. By late in the forecast period, a gradual weakening trend should commence as the system begins to move over slightly cooler waters. The storm is moving west-northwestward, or 285/8 kt. A mid-level ridge is forecast by the global models to remain in place to the north of Rosa through the middle part of the forecast period. This would likely maintain a generally west-northwestward motion for the next 72 hours or so. By days 4-5, the ridge is predicted to weaken which should result in a gradual turn toward the northwest and north-northwest. The official track forecast is similar to that from the previous advisory and is close to the corrected dynamical model consensus, HCCA. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 14.7N 108.0W 40 KT 45 MPH 12H 26/0000Z 15.2N 109.1W 50 KT 60 MPH 24H 26/1200Z 15.8N 110.6W 60 KT 70 MPH 36H 27/0000Z 16.1N 112.1W 70 KT 80 MPH 48H 27/1200Z 16.4N 113.7W 80 KT 90 MPH 72H 28/1200Z 17.1N 116.7W 95 KT 110 MPH 96H 29/1200Z 18.8N 118.9W 90 KT 105 MPH 120H 30/1200Z 21.0N 120.0W 85 KT 100 MPH $$ Forecaster Pasch

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Tropical Depression Twenty-E Forecast Discussion Number 1

2018-09-25 10:43:46| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Tue Sep 25 2018 831 WTPZ45 KNHC 250843 TCDEP5 Tropical Depression Twenty-E Discussion Number 1 NWS National Hurricane Center Miami FL EP202018 300 AM MDT Tue Sep 25 2018 The low pressure system that NHC has been monitoring off the coast of southwestern Mexico has now become a tropical depression, the twentieth of the eastern North Pacific 2018 season. ASCAT data from around 0400 UTC indicated that the system had a well-defined center and maximum winds in the 25-30 kt range. In addition, satellite images show deep convection increasing in coverage and intensity near the center. The initial wind speed is set at 30 kt, based on the ASCAT data, but this is a little below the latest Dvorak estimate from TAFB. Since the depression has only recently formed, the initial motion is an uncertain 280/7 kt. Mid-level ridging to the north of the system should keep the depression on a westward to west- northwestward path at about the same forward speed for the next few days. Thereafter, the models show the ridge breaking down due to a large cut off deep-layer low moving toward the west coast of the United States. In response, the cyclone is expected to turn to the northwest this weekend. The models agree on this overall scenario, but there is a fair amount of spread by the end of the forecast period. The NHC track forecast lies near the middle of the guidance envelope and close to the best performing models, the consensus aids. The environmental conditions appear favorable for the depression to strengthen. SSTs are expected to remain quite warm beneath the cyclone for the next several days while vertical wind shear values are around 10 kt or less. These conditions combined with a moist airmass should allow for at least steady strengthening during the next 3 to 4 days. Some weakening is possible by the end of the forecast period due to slightly cooler SSTs and an increase in shear. The NHC intensity forecast shows a slower rate of strengthening than the guidance, since the system is still in the developing stage, but predicts peak winds near the IVCN consensus model. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 14.5N 107.2W 30 KT 35 MPH 12H 25/1800Z 14.9N 108.2W 35 KT 40 MPH 24H 26/0600Z 15.5N 109.8W 40 KT 45 MPH 36H 26/1800Z 15.8N 111.2W 55 KT 65 MPH 48H 27/0600Z 16.1N 112.6W 70 KT 80 MPH 72H 28/0600Z 16.9N 115.6W 90 KT 105 MPH 96H 29/0600Z 18.3N 118.1W 90 KT 105 MPH 120H 30/0600Z 20.7N 119.5W 85 KT 100 MPH $$ Forecaster Cangialosi

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