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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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