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Hurricane Nora Discussion Number 11
NWS National Hurricane Center Miami FL EP142021
400 AM CDT Sat Aug 28 2021
The inner-core structure appears to be tightening up with Nora this
morning. Deep convection with tops colder than -70 C has been
blossoming and wrapping around the center, first around the eastern
side, but more recently in all quadrants as a distinct warm spot
denotes the center. Earlier, there was a helpful SSMIS microwave
pass at 0143 UTC that showed a closed cyan ring on the 37 GHz
channel, which is a signal often related to a formative low-level
eyewall and is often a harbinger for significant intensification.
An ASCAT-B pass at 0403 UTC also suggested the wind-radii were
gradually contracting, and also that the center position is a little
further east of the short-term track. The latest subjective Dvorak
estimates at 0600 UTC from SAB and TAFB have both increased to
T4.0/65 kt. These estimates, in addition to the recent structural
improvements in IR satellite and microwave imagery, support raising
the intensity to 65-kt for this advisory, making Nora a hurricane.
Nora continues to move a little more poleward this morning, with the
latest motion estimate at 345 degrees at 9 kt. While the track
forecast philosophy hasn't changed much over the last day, as a
weakening mid-level ridge provides a weakness for Nora to escape
north-northwestward, there has been a notable eastward shift in the
track guidance. The deterministic ECMWF run, for instance, is now
showing the hurricane brushing the shoreline of Mainland Mexico near
Cabo Corrientes in 24 hours, and is now quite similar to the latest
GFS forecast. The ECMWF ensembles have also shifted further east,
with around half of the members explicitly showing landfall over
mainland Mexico. The latest NHC track forecast has thus been shifted
eastward, especially during the first 24-48 hours of the forecast,
but I did not want to shift Nora's track as far east as the latest
HCCA and TVCE consensus aids and further eastward adjustments may
be necessary in subsequent forecasts. Afterwards, if Nora remains
intact, the guidance shows the system bending back to the northwest
and running up the gap of the Gulf of California between Baja
California Sur and Mainland Mexico.
Now that Nora appears to developing an inner core, at least steady
intensification is expected in the short-term while the hurricane
remains far enough offshore of mainland Mexico in a favorable
dynamic and thermodynamic environment. After 12 hours, the intensity
forecast becomes very tricky, due to just how much land interaction
Nora will encounter as it brushes by Mexico. Once again, a lot of
the reliable intensity guidance shows substantial land interaction
and are sharply lower after 24 hours. Because the latest forecast
track has been shifted further east, it increases the chance that
significant disruption of the storm may occur by the mountainous
terrain located near the coast of Mexico in that location. The
latest intensity forecast was lowered after 24 hours and held at
70 kt through 96 hours due to this large uncertainty tied to
possible land interaction. The latest forecast remains higher than
the majority of the guidance, but is lower than the previous
forecast. The wind radii have also been adjusted smaller after 24
hours, as the global model guidance suggests that the TC wind field
will shrink quite dramatically as the system tracks near mainland
Mexico.
Tropical Storm and Hurricane Warnings have been extended further
north along the mainland Mexico coastline given the eastward
adjustments in the latest forecast track.
Key Messages:
1. Nora is forecast strengthen today as it approaches the coasts of
the Mexican states of Jalisco and Nayarit, and hurricane warnings
are in effect for portions of that coastline. Interests in these
areas and points further north 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 Guerrero, Michoacan, Colima,
Jalisco and Nayarit, as well as Baja California Sur.
Life-threatening flash flooding and mudslides are expected. As Nora
lifts northwest, heavy rainfall will spread into the Mexican states
of Sinaloa and Sonora as well as the southwestern U.S. during the
middle to latter portion of next week.
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
intensity, confidence is not high enough to determine the magnitude
and location of these potential impacts.
FORECAST POSITIONS AND MAX WINDS
INIT 28/0900Z 17.1N 105.2W 65 KT 75 MPH
12H 28/1800Z 18.8N 105.5W 75 KT 85 MPH
24H 29/0600Z 20.8N 106.1W 75 KT 85 MPH...NEAR MAINLAND MEXICO
36H 29/1800Z 22.4N 107.0W 70 KT 80 MPH
48H 30/0600Z 23.3N 107.8W 70 KT 80 MPH
60H 30/1800Z 24.1N 108.6W 70 KT 80 MPH
72H 31/0600Z 24.9N 109.4W 70 KT 80 MPH
96H 01/0600Z 26.4N 110.6W 70 KT 80 MPH
120H 02/0600Z 28.7N 111.0W 45 KT 50 MPH...INLAND
$$
Forecaster Papin/Beven
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