Eastern Pacific Basin Tropical Cyclones

Tropical Storm Pamela Forecast Discussion


390 
WTPZ41 KNHC 122053
TCDEP1

Tropical Storm Pamela Discussion Number  11
NWS National Hurricane Center Miami FL       EP162021
300 PM MDT Tue Oct 12 2021

Entrainment of very dry mid-level air from the west and northwest 
appears to have eroded Pamela's deep convection in the northwestern 
semicircle, leaving the low-level center partially exposed again 
similar to this time yesterday. Although there could still be some 
upper-level wind shear affecting the cyclone, dropsonde wind data 
from an earlier Air Force Reserve reconnaissance aircraft indicate 
that the vortex column from the surface to at least the 700-mb level 
was vertically aligned with no tilt evident. The highest aircraft 
flight-level and SFMR winds only support an intensity of about 55 
kt. However, the aircraft measured a central pressure of 989 mb on 
its second and outbound pass through the center, which was a 
decrease of 2 mb from the first pass, and would support an intensity 
close to hurricane strength. Based on the relatively low central 
pressure, and the possibility that stronger winds might exist east 
of the center in an area that wasn't sampled by the aircraft, 
Pamela's intensity has only been lowered to 60 kt for this advisory. 
Data from the aircraft indicated that the 34-kt wind radii had 
expanded in all quadrants despite Pamela's peak winds having 
weakened. Another Air Force Reserve reconnaissance aircraft is 
scheduled to investigate Pamela later tonight around 0530 UTC.

The initial motion estimate is northward, or 360/08 kt. Now that 
Pamela is on or just north of the axis of a deep-layer ridge, a 
sharp turn toward the north-northeast should occur very soon, 
followed by a northeastward motion tonight as the cyclone becomes 
embedded within southwesterly flow ahead of a broad mid- to 
upper-level trough. Pamela should then accelerate toward the 
west-central coast of Mexico tonight through Wednesday due to the 
deep-layer steering flow on the southeast side of the aforementioned 
trough. Landfall is still expected to occur along the west-central 
coast of Mexico a little before 1200 UTC Wednesday morning. After 
the hurricane moves inland, Pamela, or its remnants will continue to 
accelerate northeastward across central and northern Mexico, and 
move into Texas by Thursday night. The new NHC track model guidance 
remains tightly packed around the previous advisory track, so no 
significant changes were required.

It might not seem like it based on the downward satellite trends 
today, but the negative effects from modest vertical shear and dry 
air that have been affecting the cyclone will gradually subside once 
Pamela makes the expected northeastward turn later this evening. 
During that same time, the deep-layer shear vector is forecast to 
slowly back around to a southwesterly direction and become aligned 
with the hurricane's forward motion, which will help to reduce some 
of the effects of the shear. Surrounding dry mid-level air could 
still be a hindering factor, but this negative parameter is expected 
to be offset by Pamela moving over very warm sea-surface 
temperatures of almost 31 deg C overnight. The higher ocean heat 
content should increase the available instability during the normal 
convective maximum period tonight, enabling the cyclone to 
restrengthen to hurricane status, and continue to strengthen right 
up until landfall occurs. Although it now appears unlikely that the 
cyclone will become a major hurricane, Pamela could still become a 
category 2 hurricane prior to landfall.  After landfall occurs, 
rapid decay due to interaction with the mountainous terrain of 
Mexico is expected, with the cyclone ultimately dissipating over 
northern Mexico in a couple of days. The latest NHC intensity 
forecast lies near or above the upper end of the intensity guidance.

Although the cyclone is forecast to dissipate over the rugged
terrain of Mexico, deep moisture associated with the system's
remnants are likely to spread over north-central and northeastern
Mexico on Wednesday, and then move into portions of the
south-central United States late Wednesday or Thursday.

Key Messages:

1. Pamela is forecast to restrengthen into a hurricane before it
reaches the west-central coast of Mexico on Wednesday morning, and
life-threatening storm surge and dangerous hurricane-force winds
are expected within the Hurricane Warning area.  Residents in this
area should follow any advice given by local officials.

2. Pamela is expected to pass south of the southern tip of the Baja
California peninsula as a hurricane this afternoon and tonight, and
tropical-storm conditions are possible within the Tropical Storm
Watch area, primarily in strong gusts.

3. Heavy rains associated with Pamela are expected to move into the 
Mexican States of Sinaloa and western Durgano along with southern 
portions of Baja California Sur Tuesday and Wednesday. This will 
pose a threat of significant and life-threatening flash flooding and 
mudslides.

4. Heavy rains associated with the remnants of Pamela are expected 
across portions of central Texas and southeastern Oklahoma late 
Wednesday and Thursday. This may result in considerable flash and 
urban flooding impacts.


FORECAST POSITIONS AND MAX WINDS

INIT  12/2100Z 21.0N 109.3W   60 KT  70 MPH
 12H  13/0600Z 22.7N 107.9W   75 KT  85 MPH
 24H  13/1800Z 25.4N 105.2W   35 KT  40 MPH...INLAND
 36H  14/0600Z 28.8N 101.3W   25 KT  30 MPH...POST-TROP/REMNT LOW
 48H  14/1800Z 32.0N  97.8W   25 KT  30 MPH...POST-TROP/REMNT LOW
 60H  15/0600Z...DISSIPATED INLAND

$$
Forecaster Stewart




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