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Tropical Storm Nora Forecast Discussion Number 4
2021-08-26 16:47:29| National Hurricane Center (East Pacific)
Issued at 1000 AM CDT Thu Aug 26 2021 373 WTPZ44 KNHC 261447 TCDEP4 Tropical Storm Nora Discussion Number 4 NWS National Hurricane Center Miami FL EP142021 1000 AM CDT Thu Aug 26 2021 Satellite images show that deep convection embedded within the sprawling circulation of the depression is gradually becoming better organized, and 91-GHz SSMIS data from 1117 UTC showed improved curvature to the bands, especially within the western semicircle. Subjective Dvorak estimates are now a consensus T2.5/35 kt from TAFB and SAB, while objective numbers from UW-CIMSS are running a little higher. Therefore, the depression is being upgraded to Tropical Storm Nora with 35-kt winds. The microwave data, as well as first-light visible imagery, suggest that the center may be consolidating or re-forming a little farther east of where we had been tracking it. Nora is moving slowly toward the west-northwest (296/6 kt), steered by a strong mid-level ridge located over the southern United States. This ridge is expected to dissolve over the next 24 hours as a shortwave trough moves across the Rocky Mountains, allowing Nora to turn toward the northwest and north-northwest through the weekend. Most of the differences among the track models still appear to be related to the initial location of Nora's center. The GFS continues to show the system having multiple low-level vortices, with a dominant one forming well to the east and moving inland along the southwestern coast of Mexico as early as late Friday, and many more GEFS ensemble members show a similar scenario compared to yesterday. The HWRF and HMON are also in the camp of bringing Nora's center inland over Mexico. Other models, including the ECMWF (and most of its ensemble members), the UKMET, and the consensus aids, still show Nora's center staying just offshore. The new NHC track forecast has been shifted eastward, primarily due to the adjustment of the initial position, and shows Nora's center very close to the coast of southwestern Mexico over the weekend. If Nora's center re-forms, then additional shifts in the track forecast will be likely. By early next week, Nora is likely to head towards Baja California Sur, but here is still a lot of uncertainty on the exact track at that time. Moderate northeasterly vertical shear continues to affect Nora, but this shear is expected to decrease to 10 kt or less in 36-48 hours. In addition, the storm will be moving over warm waters of 28-29 degrees Celsius and through an environment of high mid-level moisture. These factors should allow for continued strengthening, although the system's large size could be one limiting factor in how fast that strengthening happens. Nora is expected to be near or at hurricane strength when it approaches the coast of southwestern Mexico in 2-3 days. However, the intensity forecast, especially on days 3-5, hinges on whether or not the center moves inland. If it does not, Nora will have greater opportunity to strengthen while it heads toward the Baja California Peninsula. Since several of the intensity models assume a scenario where Nora moves inland, the NHC intensity forecast more closely follows the no-land versions of the GFS and ECMWF SHIPS models through day 4 to be consistent with what is shown in the track forecast. Given the high uncertainty in Nora's future track, and its roughly shore-parallel path, a larger-than-normal hurricane watch area has been issued for the southwestern coast of Mexico by the Mexican government. Key Messages: 1. Nora is forecast to strengthen to a hurricane by late Saturday while it approaches the coast of southwestern Mexico, and hurricane and tropical storm watches are now in effect for portions of that area. Interests along the southwestern coast of Mexico should closely monitor the progress of this system and updates to the forecast. 2. Heavy rain associated with Nora is expected across coastal sections of the Mexican states of Oaxaca, Guerrero, Michoacan, Colima, and Jalisco. As a result, life-threatening flash flooding and mudslides could occur. 3. Nora is forecast to be near 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 too soon to determine the magnitude and location of these potential impacts. FORECAST POSITIONS AND MAX WINDS INIT 26/1500Z 12.5N 100.8W 35 KT 40 MPH 12H 27/0000Z 13.2N 101.9W 40 KT 45 MPH 24H 27/1200Z 14.3N 103.0W 50 KT 60 MPH 36H 28/0000Z 15.7N 103.7W 55 KT 65 MPH 48H 28/1200Z 17.4N 104.5W 60 KT 70 MPH 60H 29/0000Z 19.0N 105.3W 65 KT 75 MPH 72H 29/1200Z 20.2N 106.3W 70 KT 80 MPH 96H 30/1200Z 22.3N 108.6W 75 KT 85 MPH 120H 31/1200Z 24.0N 110.5W 55 KT 65 MPH...INLAND $$ Forecaster Berg
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