TROPICAL STORM KARL DISCUSSION NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016
1100 PM AST THU SEP 15 2016
During the past six hours, a significant increase in the amount and
the organization of the deep convection has occurred near and over
the well-defined low-level circulation center. The southwestern edge
of the cold cloud canopy, characterized by tops as cold as -89C, has
continued to expand southwestward over the center despite
southwesterly vertical wind shear of 20-25 kt. The initial intensity
has been increased to 40 kt based on a constrained Dvorak satellite
estimate of 35 kt form TAFB, but with a data T-number of T3.0/45 kt
using a shear pattern. A 15/2247Z partial ASCAT-B pass indicated
several 35-36 kt surface wind vectors in the northeastern quadrant
more than 60 nmi from the center. Based on continued improvement in
the satellite presentation since the ASCAT pass, the initial
intensity is set at 40 kt, making Karl the eleventh named storm of
the season.
Karl is moving westward or 275/11 kt. The cyclone is expected to
move westward and then west-southwestward around a strengthening
Atlantic subtropical ridge for the next 48-72 hours. After that
time, the ridge is forecast to weaken some, allowing Karl to turn
back toward the west and then west-northwest by day 5. The new NHC
forecast is essentially on top of the previous advisory track, and
lies close to the multi-model consensus TVCX and the Florida State
Superensemble model.
Karl is expected to remain in a moderate to strong vertical wind
shear environment for the next 36 hours or so, which should inhibit
significant intensification. After that time, however, the shear is
forecast to decrease to around 5 kt for the remainder of the
120-h period. That would normally result in more robust
strengthening while the cyclone is moving over 28-29C SSTs.
However, Karl will also be moving through a very dry mid-level
moisture regime during that time, so the intensification trend is
forecast to be much slower than the climatological rate of 20 kt
per day, and lies close to the SHIPS intensity forecast.
The large 34-kt wind radius in the northeastern quadrant is based on
the aforementioned ASCAT wind data.
TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016
1100 AM AST WED SEP 14 2016
Satellite imagery, scatterometer data, and surface observations
indicate that the low pressure area over the Cabo Verde Islands has
developed a well-defined circulation and sufficient organized
convection to be considered a tropical depression. The initial
intensity is set at 30 kt based on the scatterometer data along
with satellite intensity estimates from TAFB and SAB.
The initial motion is 300/12. A low- to mid-level ridge should
steer the system west-northwestward and then westward during the
forecast period, with some decrease in forward speed after 36 hours
as the ridge weakens slightly. The forecast track is near the
center of the guidance envelope and close to the various consensus
models.
Some strengthening is expected during the next 12-24 hours. After
that, the cyclone is likely to encounter strong westerly vertical
wind shear while over sea surface temperatures of about 26C. This
combination should lead to weakening, and the intensity forecast
shows the system weakening back to a depression by 36 hours. Near
the end of the forecast period, the dynamical models diverge on the
evolution of the upper-level winds, with the ECMWF and UKMET
forecasting a more favorable pattern than the GFS. Based on this,
the forecast shows some re-intensification by 120 hours. An
alternative scenario is that the cyclone degenerates to a tropical
wave as forecast by the GFS. Overall, the intensity forecast is on
the low side of the guidance in best agreement with the LGEM.
Locally heavy rains and gusty winds are possible over the Cabo
Verde Islands. However, current indications are that the cyclone
will not reach tropical-storm strength until it has moved west of
the islands. Therefore, watches and warnings are not required at
this time.
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