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Tropical Storm Nora Forecast Discussion Number 7
2021-08-27 11:00:12| National Hurricane Center (East Pacific)
Issued at 400 AM CDT Fri Aug 27 2021 000 WTPZ44 KNHC 270900 TCDEP4 Tropical Storm Nora Discussion Number 7 NWS National Hurricane Center Miami FL EP142021 400 AM CDT Fri Aug 27 2021 The convective structure of Nora this morning continues to look a bit disheveled, with the majority of the deeper convection still located to the south and west of the mean low-level circulation center due to 15-20 kt of northeasterly vertical wind shear. More recently, however, there are a few deeper convective tops attempting to develop on the northeast side of the broad vortex. A helpful ASCAT-A pass at 0307 UTC revealed that the circulation remains quite elongated, with two distinct mesovorticies, one to the southwest embedded in the convection, and another to the northeast. The current center estimate roughly splits the difference between these two points. The peak wind retrieval from that scatterometer pass was also 43-kt with a few higher rain-contaminated values. Assuming a bit of instrument undersampling, the current intensity is set to 45-kt for this advisory. Nora motion continues to gradually bend rightward, with the latest motion estimate at 300/10 kt. As mentioned in the previous discussion, mid-level ridging to the north of Nora is gradually being eroded by a shortwave trough propagating westward across the Rocky Mountains. This pattern is expected to create a weakness in the ridge that should allow Nora to begin gaining more latitude later today. While the models are in decent agreement with this poleward turn, there still remains some spread on how sharp this turn will be, and whether or not the tropical cyclone will directly impact the southwestern coast of Mexico. The latest deterministic ECMWF and UKMET runs continue to forecast Nora to remain offshore of mainland Mexico, while the GFS and Canadian models explicitly show Nora making landfall in 36-48 hours. While the track guidance envelope has narrowed some, the multi-model consensus aids have changed very little this cycle. Therefore, the latest NHC track forecast is quite similar to the previous one, and lies close to the HCCA consensus aid and GFEX, which is a simple blend of the GFS and ECMWF forecast solutions. This forecast track still brings Nora close to Baja California Sur and the Gulf of California in around 72 hours, though uncertainty by that time frame remains high, related to the degree of land interaction Nora will encounter prior to that point. Nora continues to be impacted by northeasterly vertical wind shear, though this shear is still forecast to subside over the next 12-24 hours. As long as Nora stays far enough offshore, the reduction in shear in combination with warm 28-29 C sea-surface temperatures and ample deep-layer moisture favor strengthening. Therefore, the latest NHC intensity forecast still indicates intensification with Nora forecast to become a Hurricane on Saturday. This intensity forecast remains on the high end of the guidance envelope, especially after 36 hours, because a subset of the guidance aids (HWRF, HMON, GFS) bring Nora inland after this time, substantially disrupting the storm's circulation. However, since the latest NHC forecast track keeps Nora far enough offshore, I have elected to follow closer to the ECMWF and ECMWF ensemble mean intensity aids, which show continued intensification after 36 hours. After 72 hours, weakening is anticipated due to possible land interaction with the mountainous terrain of Baja California Sur. Needless to say, this intensity forecast hinges on the track of Nora remaining offshore of mainland Mexico, and this intensity forecast is also more uncertain than usual, especially after 36 hours. Key Messages: 1. Nora is forecast to strengthen to a hurricane on Saturday while passing nearby the coast of southwestern Mexico, and a hurricane watch and tropical storm warning are in effect for portions of that coastline. Interests along the southwestern coast of Mexico should closely monitor the progress of this system and subsequent updates to the forecast. 2. Heavy rain associated with Tropical Storm Nora is expected across coastal sections of the Mexican states of Oaxaca, Guerrero, Michoacan, Colima, and Jalisco. Life-threatening flash flooding and mudslides may occur. 3. Nora is forecast to be near or over the southern portion of Baja California Sur as a hurricane early next week, bringing a risk of wind and rain impacts to that area. Given the above average uncertainty in the forecast, it is still too soon to determine the magnitude and location of these potential impacts. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 14.6N 103.5W 45 KT 50 MPH 12H 27/1800Z 15.3N 104.4W 50 KT 60 MPH 24H 28/0600Z 16.6N 105.4W 60 KT 70 MPH 36H 28/1800Z 18.5N 106.1W 70 KT 80 MPH 48H 29/0600Z 20.6N 106.9W 75 KT 85 MPH 60H 29/1800Z 22.1N 107.9W 80 KT 90 MPH 72H 30/0600Z 23.2N 108.9W 80 KT 90 MPH...NEAR BAJA CAL SUR 96H 31/0600Z 24.9N 110.4W 60 KT 70 MPH...NEAR BAJA CAL SUR 120H 01/0600Z 27.1N 111.6W 35 KT 40 MPH...NEAR BAJA CAL SUR $$ Forecaster Papin/Beven
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