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Tropical Storm Nora Forecast Discussion Number 8
2021-08-27 16:35:43| National Hurricane Center (East Pacific)
Issued at 1000 AM CDT Fri Aug 27 2021 000 WTPZ44 KNHC 271435 TCDEP4 Tropical Storm Nora Discussion Number 8 NWS National Hurricane Center Miami FL EP142021 1000 AM CDT Fri Aug 27 2021 As was expected, vertical shear appears to be decreasing, with deep convection expanding over Nora's surface center just within the past few hours. Water vapor imagery also suggests an expansion of upper-level outflow to the north of the storm. Since Dvorak data-T numbers are 3.0/45 kt from TAFB and SAB, Nora's intensity is being held at that value for now. Nora has turned toward the northwest (310/10 kt) in response to a weakness in the subtropical ridge that has developed over the U.S. southern Rockies. Little change to this steering pattern is expected during the next several days, and Nora is forecast to move toward the northwest or north-northwest for the entire 5-day forecast period. Compared to the last few days, nearly all the track models now have Nora's center staying just offshore of Cabo Corrientes, Mexico, in about 36 hours. The GFS and HMON model fields still show a landfall in that area, but their interpolated trackers--which have been shifted to begin at our estimated initial position--also stay just offshore. After 36 hours, the track guidance has shifted slightly eastward, and the new NHC track forecast now keeps Nora's center over water up into the Gulf of California from days 2 through 5. That will be a small needle to thread, however, and any future shifts in the models could increase the risk of impacts to either the Baja California peninsula or the northwestern coast of mainland Mexico through the middle of next week. SHIPS model diagnostics indicate that the northeasterly deep-layer shear over Nora has decreased to about 15 kt, and this shear is expected to decrease further to below 10 kt in about 12 hours. Along with warm waters of about 29 degrees Celsius and significant upper-level divergence (at least for the next 36 hours), these conditions should foster strengthening, and Nora is still forecast to become a hurricane on Saturday. The part of the forecast after 48 hours remains uncertain depending on how much Nora interacts with land, but based on the new forecast that keeps the cyclone over water in the Gulf of California (where SSTs are even warmer -- around 31 degrees Celsius), Nora could maintain hurricane intensity through at least day 4. The NHC intensity forecast continues to hedge on the side of the ECMWF-based SHIPS and LGEM models, since most of the other models show Nora's center interacting with land to at least some degree. The new forecast has necessitated an extension of the Tropical Storm Warning northward to San Blas, Mexico, and the issuance of a new Tropical Storm Watch north of there to Mazatlan. Key Messages: 1. Nora is forecast to strengthen to a hurricane on Saturday while passing nearby the coasts of the Mexican states of Colima, Jalisco, and Nayarit, and hurricane watches and tropical storm warnings are in effect for portions of that coastline. Interests in these areas should closely monitor the progress of Nora and subsequent 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 well as Baja California Sur. Life-threatening flash flooding and mudslides may occur. 3. Nora is forecast to move over the Gulf of California as a hurricane early next week, bringing a risk of wind and rain impacts to portions of the Mexican states of Baja California Sur, Sinaloa, and Sonora. 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 27/1500Z 15.4N 104.3W 45 KT 50 MPH 12H 28/0000Z 16.3N 105.2W 50 KT 60 MPH 24H 28/1200Z 17.9N 105.9W 60 KT 70 MPH 36H 29/0000Z 19.9N 106.5W 65 KT 75 MPH 48H 29/1200Z 21.7N 107.4W 70 KT 80 MPH 60H 30/0000Z 23.1N 108.3W 75 KT 85 MPH 72H 30/1200Z 24.2N 109.1W 75 KT 85 MPH 96H 31/1200Z 26.1N 110.7W 65 KT 75 MPH 120H 01/1200Z 28.4N 111.7W 50 KT 60 MPH $$ Forecaster Berg
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