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Hurricane Norma Discussion Number  11
NWS National Hurricane Center Miami FL       EP172023
300 AM MDT Fri Oct 20 2023
 
Earlier low-frequency microwave images indicated that significant
degradation of Norma's eye has occurred during the past few hours,
particularly in the south part of the inner core.  Although
the outflow aloft is still quite impressive, there is evidence in
the UW-CIMSS shear analysis of modest southerly shear below 250 mb
which is apparently affecting Norma's vertical structure.
Subsequently, the initial intensity is lowered to 100 kt for this
advisory and is based on a blend of the subjective and objective
intensity estimates from TAFB, SAB, and UW-CIMSS.
 
Gradual weakening should continue through the period as a result of
increasing south-southwesterly vertical wind shear produced by
an amplifying mid- to upper-level trough approaching the Baja
California peninsula, and a progressively inhibiting thermodynamic
surrounding environment with mid-tropospheric relative humidity
values of less than 50 percent.  Despite the forecast weakening
trend, Norma will likely be a hurricane while it approaches the 
southern portion of Baja California Sur on Saturday.
 
The initial motion is estimated to be north-northwestward, or 330/6 
kt.  Norma is expected to move between mid-level subtropical 
high-pressure east of the cyclone and the above mentioned trough to 
the northwest and north during the next couple of days.  By 
mid-period, Norma should turn north-northeastward to northeastward 
with a reduction in forward speed while approaching the 
west-central coast of Mexico.  The official track forecast lies 
between the TVCE simple deterministic model consensus and the HFIP 
HCCA, and is similar to the previous forecast with a slight 
adjustment to the south of it beyond the 48-hour period.

The initial wind radii have been adjusted slightly based on a 
recent METOP-B scatterometer overpass.
 
 
Key Messages:
 
1. A Hurricane Warning is in effect for the far southern portion of
the Baja California peninsula, where Norma is expected to bring
hurricane conditions on Saturday when it passes near or over the
area.  Tropical storm conditions are also expected within the 
tropical storm warning areas in Baja California Sur on Saturday.
 
2. Heavy rains from Norma will begin to impact the far southern
portions of California Baja Sur later today, continuing through
Sunday.  This rainfall may produce flash and urban flooding, along
with possible mudslides in areas of higher terrain.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  20/0900Z 18.8N 108.6W  100 KT 115 MPH
 12H  20/1800Z 19.7N 109.2W   90 KT 105 MPH
 24H  21/0600Z 21.2N 109.7W   85 KT 100 MPH
 36H  21/1800Z 22.4N 109.6W   70 KT  80 MPH
 48H  22/0600Z 23.3N 109.2W   60 KT  70 MPH
 60H  22/1800Z 23.6N 108.4W   50 KT  60 MPH
 72H  23/0600Z 23.9N 107.8W   45 KT  50 MPH
 96H  24/0600Z 25.3N 106.6W   25 KT  30 MPH...POST-TROP/INLAND
120H  25/0600Z...DISSIPATED
 
$$
Forecaster Roberts
 
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