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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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