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Tropical Storm Lowell Discussion Number  10
NWS National Hurricane Center Miami FL       EP172020
800 PM PDT Tue Sep 22 2020
 
Recent microwave satellite imagery has been extremely helpful in 
diagnosing the structure of Lowell, as well as locating the center 
of the tropical storm.   The imagery indicates the the center is 
located to the north of the bursting area of convection, and that 
the overall structure has not changed much since this afternoon.  A 
blend of the latest subjective Dvorak T- and CI-numbers and the 
earlier ASCAT data support an initial wind speed of 40 kt.  
Although the shear associated with the upper-tropospheric flow is 
low, there appears to be some stronger shear in a layer beneath that 
level which is causing the displacement of the convection to the 
south of the low-level center.  Given the current structure, only 
slight strengthening is predicted over the next 24 hours, and 
shortly after that time Lowell will be moving near an SST gradient 
and into an area of less favorable thermodynamic conditions.  As a 
result, little change in strength is shown between 24-60 h, but 
gradual weakening should begin after that time when Lowell moves 
over cooler waters and into an area of increasing southwesterly 
shear.  With Lowell moving along the SST gradient for the next 
several days, a track north of the official forecast could result in 
both a faster rate of weakening, and the system becoming 
post-tropical much sooner than indicated below, but a track farther 
south could result in slightly more intensification.  The NHC 
intensity forecast is a little above the SHIPS guidance and is in 
best agreement with the HFIP corrected consensus model.

Lowell is moving west-northwestward at 10 kt.  A west-northwestward 
motion is expected, to the southwest of a subtropical ridge near 
Baja California.  The ridge is forecast to build to the north of 
Lowell over the next couple of days which should impart a westward 
turn by late Wednesday, with a general westward heading continuing 
through the remainder of the forecast period.  Although there is 
little cross-track spread in the guidance models, there are 
significant speed differences (along-track spread) with the ECMWF 
model much faster than the GFS.  The NHC track is slightly faster 
than the previous forecast to be closer to the various consensus 
aids and the GFS and UKMET ensemble means.


FORECAST POSITIONS AND MAX WINDS
 
INIT  23/0300Z 19.4N 119.1W   40 KT  45 MPH
 12H  23/1200Z 20.0N 120.5W   40 KT  45 MPH
 24H  24/0000Z 20.4N 122.3W   45 KT  50 MPH
 36H  24/1200Z 20.7N 124.1W   45 KT  50 MPH
 48H  25/0000Z 20.9N 126.0W   45 KT  50 MPH
 60H  25/1200Z 21.0N 128.0W   45 KT  50 MPH
 72H  26/0000Z 21.0N 130.0W   40 KT  45 MPH
 96H  27/0000Z 21.0N 134.0W   35 KT  40 MPH
120H  28/0000Z 21.0N 138.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Brown
 
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