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Tropical Storm KARINA


200 PM PDT WED AUG 13 2014

Karina is experiencing some issues with dry air entrainment and
modest north to northeasterly wind shear since the previous
advisory, which has resulted in some erosion of the large outer
convective band on the west side along with the central deep
convection. However, two recent scatterometer overpasses indicate
that Karina's circulation is well developed and compact, and that
the radius of maximum winds has decreased to around 10-15 nmi. The
initial intensity is set at 45 kt based on surface wind data of 40
kt from a 1649 UTC ASCAT-B overpass and 41 kt from a 1736 UTC
ASCAT-A overpass, and assuming that the scatterometers are
under-sampling the very small inner-core wind field.

Karina continues to move westward or 275/13 kt. There is no
significant change to the previous forecast track or reasoning.
Karina is expected to maintain a general westward motion for the
next 5 days as the cyclone moves along the southwestern periphery of
a deep-layer subtropical ridge. The model guidance is tightly packed
through 72 hours, with the models diverging significantly after
that. However, the model spread has decreased on this cycle now that
the new 1200 UTC ECMWF model run has shifted farther to the north,
closer to the other global models and the previous NHC forecast
track. The new forecast is just an update of the previous advisory
track and lies close to the consensus model TVCE.

Convection has developed during the past few hours within the dry
slot in the eastern semicircle, and a small burst of convection has
also developed very near the well-defined low-level circulation
center. All of this would suggest that the aforementioned dry air
issues will likely be short-lived. Over the next 3 days, the
vertical shear is forecast to decrease to less than 10 kt while the
cyclone remains over SSTs of 28C-29C and within a fairly moist
mid-level environment. Only the proximity to cooler and more stable
air just west and northwest of the cyclone precludes a near-term
forecast of rapid intensification. However, the trend over the next
72 hours calls for at least steady intensification, with Karina
reaching hurricane strength by 36 hours. After that, the cyclone
will be moving over cooler SSTS and into a more stable air mass,
which should induce gradual weakening. However, the low vertical
wind shear environment is expected to result in slower weakening
than normally would occur over such marginal SSTs on Days 4 and 5.
The NHC official intensity forecast remains higher than the ICON
intensity consensus model, and is close to a blend of the SHIPS and
GFDL models.


INIT  13/2100Z 17.2N 112.6W   45 KT  50 MPH
 12H  14/0600Z 17.3N 114.5W   50 KT  60 MPH
 24H  14/1800Z 17.4N 117.0W   60 KT  70 MPH
 36H  15/0600Z 17.6N 119.3W   65 KT  75 MPH
 48H  15/1800Z 17.8N 121.6W   70 KT  80 MPH
 72H  16/1800Z 18.2N 126.0W   75 KT  85 MPH
 96H  17/1800Z 18.4N 129.6W   70 KT  80 MPH
120H  18/1800Z 18.0N 133.0W   65 KT  75 MPH

Forecaster Stewart