Tropical Storm Carlos Discussion Number 2
NWS National Hurricane Center Miami FL EP032021
800 PM PDT Sat Jun 12 2021
Satellite imagery indicates improved organization this evening as a
cold convective burst has expanded over the estimated low-level
circulation center. A 2032 UTC AMSR2 microwave pass indicated that
underneath this convective burst was also improved convective
banding, especially over the southern semicircle. The latest round
of Dvorak satellite estimates were T2.0/30 kt from TAFB and T2.5/35
kt from SAB. The most recent UW-CIMSS Advanced Dvorak Technique
value was also T2.5/35 kt. Favoring the higher intensity estimates,
and assuming some intensification has occurred from the earlier
scatterometer passes, Tropical Depression Three-E has been upgraded
to Tropical Storm Carlos with an advisory intensity of 35 kt.
Carlos appears to be gradually turning leftward and slowing down
this evening, with the most recent estimated motion at 270/06 kt.
The mid-level ridge north of Carlos is forecast to gradually weaken,
shift northward, and then be replaced by a deep-layer trough that
will essentially shut down the steering currents near the cyclone
after 24 h. Between 36 to 48 h the motion of Carlos is likely to
slow to a crawl, generally to the west-southwest. By 72 h, another
mid-to-upper level trough digging in from the northwest will finally
induce some northerly steering, helping Carlos to gradually
accelerate to the north. The latest track forecast is a bit west of
the previous NHC forecast owing to the current motion, but remains
very close to the HFIP corrected consensus and TCVE track consensus.
It should be noted that there remains large spread in the guidance
on how quickly Carlos slows down, with the latest GFS forecast
slowing down the cyclone almost immediately, while the CMC and ECMWF
runs show a much faster track to the west-southwest.
The intensity forecast is tricky due to both the small size of the
tropical cyclone and also the less than optimal environment
surrounding the system. While deep-layer vertical wind shear is only
expected to be low to moderate over the next 72 h, mid-level shear
out of the north to northwest is a bit higher (15-25 kt) and could
occasionally import very dry mid-level air from that direction. In
fact, ECMWF-SHIPS mid-level humidity values gradually decrease to
under 500ver the next 36 h. In addition, the very slow motion of
Carlos expected between 24-72 h could potentially upwell cooler
sea-surface temperatures than the current 27-28 C values along the
forecast track. For this reason, the latest NHC intensity forecast
is fairly conservative, forecasting a peak intensity of 45 kt
between 24-60 h, which is close to the HFIP corrected consensus and
peak intensity of the most recent HWRF run. Thereafter, as the storm
begins to gain latitude, increasingly dry, stable air in addition to
decreasing sea surface temperatures should lead to gradual weakening
beyond 60 h.
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