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Tropical Depression Fifteen-E Discussion Number   3
NWS National Hurricane Center Miami FL       EP152021
300 AM MDT Wed Sep 08 2021
 
The circulation of the depression has continued to become better 
defined, and there has been a substantial increase in deep 
convection within bands to the north and northeast of the center.  
However, the inner core remains slightly displaced from this 
activity, and a combination of subjective and objective intensity 
estimates supports maintaining the cyclone as a 30-kt depression 
for this advisory.

The system has either been meandering or its center re-formed 
yesterday, and the long-term motion is a drift toward the 
north-northwest (335 degrees) at about 3 kt.  This motion is likely 
due to the depression being caught within weak steering currents 
between a mid-level low west of the Baja California peninsula and 
troughing extending across Mexico to the Gulf of Mexico.  A strong 
mid-tropospheric high over the western United States should 
gradually cause the system to move faster toward the northwest 
during the next 2 to 3 days, bringing it very close to Baja 
California Sur in about 48 hours.  Of the track models, the GFS and 
COAMPS-TC models are the solutions which bring the cyclone's center 
closest to Baja California Sur, while the ECMWF and UKMET models 
have the farther-offshore tracks.  The new NHC track forecast 
splits the difference among these solutions and is a little east 
of the previous forecast during the first 36 hours to account for 
recent short-term motions.  After 36 hours, it falls more or less 
in line with the previous prediction and is very close to the 
consensus aids, HWRF, and HMON.

There are counteracting factors at play regarding the depression's 
future intensity.  Although deep-layer shear is forecast to be low, 
the GFS suggests that moderate to strong mid-level shear could 
develop below the outflow level during the next 24-48 hours.  Sea 
surface temperatures will be plenty warm for strengthening during 
the next 48 hours, yet upper-level divergence is not expected to be 
ideal.  Since the system's circulation appears to be improving, 
steady strengthening is forecast during the first 48 hours, and the 
depression could become a short-lived hurricane while it passes the 
Baja California peninsula.  This forecast is near the upper end 
of the guidance envelope, close to the GFS, HWRF, COAMPS-TC, and 
HCCA solutions.  The cyclone will be moving over colder waters in 
2-3 days, which should cause quick weakening and a degeneration to 
remnant low status by day 4.
 
 
Key Messages:

1.  Tropical Depression Fifteen-E is expected to strengthen to a 
tropical storm later today, and it could become a short-lived 
hurricane while it passes just to the southwest of southern 
portions of Baja California Sur Thursday and Friday.  A Tropical 
Storm Watch is in effect for southern portions of Baja California 
Sur, and interests there should continue to monitor the progress of 
the depression.

2.  Heavy rains associated with Tropical Depression Fifteen-E are 
possible across southern portions of Baja California Sur Thursday 
through Friday.  This will pose a threat of flash flooding and 
mudslides.

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  08/0900Z 18.5N 107.1W   30 KT  35 MPH
 12H  08/1800Z 19.1N 107.6W   40 KT  45 MPH
 24H  09/0600Z 20.0N 108.4W   50 KT  60 MPH
 36H  09/1800Z 21.1N 109.6W   60 KT  70 MPH
 48H  10/0600Z 22.3N 111.2W   65 KT  75 MPH
 60H  10/1800Z 23.3N 112.8W   55 KT  65 MPH
 72H  11/0600Z 23.9N 114.5W   45 KT  50 MPH
 96H  12/0600Z 24.1N 117.1W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  13/0600Z 23.6N 119.6W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Berg
 
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