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Tropical Storm Nora Discussion Number   7
NWS National Hurricane Center Miami FL       EP142021
400 AM CDT Fri Aug 27 2021
 
The convective structure of Nora this morning continues to look a 
bit disheveled, with the majority of the deeper convection still 
located to the south and west of the mean low-level circulation 
center due to 15-20 kt of northeasterly vertical wind shear. More 
recently, however, there are a few deeper convective tops attempting 
to develop on the northeast side of the broad vortex. A helpful 
ASCAT-A pass at 0307 UTC revealed that the circulation remains quite 
elongated, with two distinct mesovorticies, one to the southwest 
embedded in the convection, and another to the northeast. The 
current center estimate roughly splits the difference between these 
two points. The peak wind retrieval from that scatterometer pass 
was also 43-kt with a few higher rain-contaminated values. Assuming 
a bit of instrument undersampling, the current intensity is set to 
45-kt for this advisory. 
 
Nora motion continues to gradually bend rightward, with the latest 
motion estimate at 300/10 kt. As mentioned in the previous 
discussion, mid-level ridging to the north of Nora is gradually 
being eroded by a shortwave trough propagating westward across the 
Rocky Mountains. This pattern is expected to create a weakness in 
the ridge that should allow Nora to begin gaining more latitude 
later today. While the models are in decent agreement with this 
poleward turn, there still remains some spread on how sharp this 
turn will be, and whether or not the tropical cyclone will directly 
impact the southwestern coast of Mexico. The latest deterministic 
ECMWF and UKMET runs continue to forecast Nora to remain offshore 
of mainland Mexico, while the GFS and Canadian models explicitly 
show Nora making landfall in 36-48 hours. While the track guidance 
envelope has narrowed some, the multi-model consensus aids have 
changed very little this cycle. Therefore, the latest NHC track 
forecast is quite similar to the previous one, and lies close to 
the HCCA consensus aid and GFEX, which is a simple blend of the GFS 
and ECMWF forecast solutions. This forecast track still brings Nora 
close to Baja California Sur and the Gulf of California in around 
72 hours, though uncertainty by that time frame remains high, 
related to the degree of land interaction Nora will encounter prior 
to that point. 
 
Nora continues to be impacted by northeasterly vertical wind shear, 
though this shear is still forecast to subside over the next 12-24 
hours. As long as Nora stays far enough offshore, the reduction in 
shear in combination with warm 28-29 C sea-surface temperatures and 
ample deep-layer moisture favor strengthening. Therefore, the 
latest NHC intensity forecast still indicates intensification with 
Nora forecast to become a Hurricane on Saturday. This intensity 
forecast remains on the high end of the guidance envelope, 
especially after 36 hours, because a subset of the guidance aids 
(HWRF, HMON, GFS) bring Nora inland after this time, substantially 
disrupting the storm's circulation. However, since the latest NHC 
forecast track keeps Nora far enough offshore, I have elected to 
follow closer to the ECMWF and ECMWF ensemble mean intensity aids, 
which show continued intensification after 36 hours. After 72 hours, 
weakening is anticipated due to possible land interaction with the 
mountainous terrain of Baja California Sur. Needless to say, this 
intensity forecast hinges on the track of Nora remaining offshore of 
mainland Mexico, and this intensity forecast is also more uncertain 
than usual, especially after 36 hours.
 
 
Key Messages:
 
1. Nora is forecast to strengthen to a hurricane on Saturday while 
passing nearby the coast of southwestern Mexico, and a hurricane 
watch and tropical storm warning are in effect for portions of that 
coastline. Interests along the southwestern coast of Mexico should 
closely monitor the progress of this system and subsequent updates 
to the forecast.
 
2. Heavy rain associated with Tropical Storm Nora is expected 
across coastal sections of the Mexican states of Oaxaca, Guerrero, 
Michoacan, Colima, and Jalisco. Life-threatening flash flooding and 
mudslides may occur.
 
3. Nora is forecast to be near or over 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 still too soon to determine the
magnitude and location of these potential impacts.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  27/0900Z 14.6N 103.5W   45 KT  50 MPH
 12H  27/1800Z 15.3N 104.4W   50 KT  60 MPH
 24H  28/0600Z 16.6N 105.4W   60 KT  70 MPH
 36H  28/1800Z 18.5N 106.1W   70 KT  80 MPH
 48H  29/0600Z 20.6N 106.9W   75 KT  85 MPH
 60H  29/1800Z 22.1N 107.9W   80 KT  90 MPH
 72H  30/0600Z 23.2N 108.9W   80 KT  90 MPH...NEAR BAJA CAL SUR
 96H  31/0600Z 24.9N 110.4W   60 KT  70 MPH...NEAR BAJA CAL SUR
120H  01/0600Z 27.1N 111.6W   35 KT  40 MPH...NEAR BAJA CAL SUR
 
$$
Forecaster Papin/Beven
 
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