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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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