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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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