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Hurricane Norma Discussion Number   6
NWS National Hurricane Center Miami FL       EP172023
900 PM MDT Wed Oct 18 2023
 
Norma's rapid intensification continues. Microwave imagery from late 
this afternoon indicated that a very well-defined low- to mid-level 
eye has formed with a surrounding ring of shallow to moderate 
convection associated with rapid intensification. While that eye is 
not yet visible in conventional satellite imagery, subjective 
Dvorak estimates from TAFB and SAB have increased to 77 kt and 90 
kt, respectively, while recent objective evaluations from UW-CIMSS 
range from about 75 kt to 85 kt. Based on the whole of the data, 
the intensity for Norma has been raised to 80 kt.
 
For the next 24 to 36 hours, the forecast for Norma appears to be 
relatively straightforward. Continued rapid strengthening appears 
likely tonight based on Norma's convective structure and the 
conducive surrounding environment. The hurricane should continue 
moving generally northward during that time, steered by a weak 
mid-level ridge to the east of Norma and an upper-level trough to 
its northwest. Short-term fluctuations in intensity due to eyewall 
replacement cycles will become possible starting tonight, but 
confidence in the forecast is reasonably high through 36 h.
 
After that time, the uncertainty grows considerably. Southwesterly 
upper-level flow associated with the aforementioned upper-level 
trough will continue, but the mid- to lower-level steering will 
likely become light or even switch to northerly. The resulting 
increase in shear should cause Norma to weaken, but the rate of that 
weakening remains in doubt. As long as the hurricane retains its 
vertical depth, upper-level flow should keep it moving northward. 
However, once Norma weakens to the point that its steering is 
dominated by low- to mid-level flow, its forward speed should slow 
down considerably. It is possible, therefore, that Norma will stall 
just south of Baja California peninsula, a solution favored by the 
deterministic ECMWF and a number of its ensemble members. However, 
the hurricane models (HAFS-A/B, HMON, and HWRF), which should 
resolve the structure of Norma better than a global model, all 
indicate that the hurricane will retain its depth long enough to 
reach the Baja California peninsula. The NHC track and intensity 
forecasts favor the regional models, blending their latest consensus 
with the previous forecast to try to maintain continuity. The 
updated official track forecast is slightly faster than the previous 
one, but is generally similar despite the high uncertainty.

Tropical storm or hurricane watches could be required for portions 
of the Baja California peninsula later tonight, and interests there 
should continue to monitor updates on Norma, especially since 
larger-than-normal changes to the forecast are possible due to the 
high uncertainty in this case.
 
Key Messages:
 
1. Norma is forecast to continue rapidly strengthening tonight
while it passes well offshore of southwestern Mexico.
 
2. There is a greater than normal uncertainty in the track and
intensity forecasts for Norma later this week and this weekend when
it is expected to approach the Baja California peninsula and
western Mexico. Regardless of Norma's exact track or intensity,
there is an increasing threat of wind and rainfall impacts to
southern portions of Baja California Sur by Saturday, and watches
could be required there later tonight or on Thursday.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  19/0300Z 16.4N 108.0W   80 KT  90 MPH
 12H  19/1200Z 17.2N 108.1W  100 KT 115 MPH
 24H  20/0000Z 18.3N 108.4W  100 KT 115 MPH
 36H  20/1200Z 19.3N 108.9W   90 KT 105 MPH
 48H  21/0000Z 20.6N 109.5W   80 KT  90 MPH
 60H  21/1200Z 22.1N 109.8W   70 KT  80 MPH
 72H  22/0000Z 23.0N 109.8W   65 KT  75 MPH
 96H  23/0000Z 23.5N 109.5W   60 KT  70 MPH
120H  24/0000Z 24.0N 108.5W   45 KT  50 MPH
 
$$
Forecaster D. Zelinsky
 
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