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
