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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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