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Tropical Storm Carlos Discussion Number   3
NWS National Hurricane Center Miami FL       EP032021
200 AM PDT Sun Jun 13 2021
 
Infrared and scatterometer satellite data indicate that Carlos 
remains a compact tropical cyclone. Two ASCAT passes between 
0400-0500 UTC revealed a radius of maximum wind of 10-12 nmi and 
tropical-storm-force winds that only extended outward about 20 nmi. 
Cloud tops colder than -50C only extend outward about 60 nmi from 
the center. The earlier ASCAT-A/-B passes contained peak surface 
wind vectors of 37 kt/40 kt, respectively, so the estimated 
intensity has been increased to 40 kt for this advisory, which 
could be a little conservative owing to the small size of the 
cyclone and possible undersampling of the scatterometer instrument.

The initial motion estimate is due west or 270/07 kt. There is 
little change to the previous NHC track forecast or reasoning. The 
latest model runs are in good agreement on Carlos turning toward the 
west-southwest later today, followed by a southwestward motion on 
Monday and Tuesday as a low- to mid-level ridge to the north of the 
cyclone builds southward. By day 3, the models diverge significantly 
when the deep-layer steering currents collapse, followed by possible 
rapid weakening. The ECMWF weakens Carlos into a shallow remnant low 
by 72 h, with the shallow vortex then being driven southwestward by 
a strong low-level ridge to the north and northeast. In contrast, 
the GFS and some of the other global and regional models keep Carlos 
stronger and vertically deeper, which results in a ridge to the east 
lifting the cyclone out toward the north. Given the large spread in 
the track guidance on days 4 and 5, the new NHC track forecast is 
similar to but slightly slower than the previous advisory track, and 
close to a blend of the TVCE and GFEX simple consensus track models.
 
Carlos' small size in combination with the relatively low deep-layer 
vertical shear of 5-10 kt for the next 36 h or so would normally 
argue for significant strengthening. However, the cyclone's 
proximity to very dry mid-level and cooler sea-surface just to its 
north and northwest is expected to result in the periodic 
entrainment of stable air for the 72 h or so, resulting in 
intermittent disruptions of the central deep convection. Thus, 
little change in strength is forecast during that time. Thereafter, 
the combination of increasing southwesterly wind shear and sub-27C 
sea-surface temperatures along the track is expected to induce 
gradual weakening. The official intensity forecast is similar to 
the previous advisory, and closely follows the IVCN and HCCA 
intensity consensus models.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  13/0900Z 11.9N 125.3W   40 KT  45 MPH
 12H  13/1800Z 11.6N 126.1W   45 KT  50 MPH
 24H  14/0600Z 11.2N 126.9W   45 KT  50 MPH
 36H  14/1800Z 10.7N 127.4W   45 KT  50 MPH
 48H  15/0600Z 10.3N 127.7W   45 KT  50 MPH
 60H  15/1800Z 10.2N 128.0W   40 KT  45 MPH
 72H  16/0600Z 10.6N 128.1W   40 KT  45 MPH
 96H  17/0600Z 12.0N 128.2W   35 KT  40 MPH
120H  18/0600Z 14.0N 127.9W   30 KT  35 MPH
 
$$
Forecaster Stewart
 
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