[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 112031
TCDAT2

Hurricane Ophelia Discussion Number  11
NWS National Hurricane Center Miami FL       AL172017
500 PM AST Wed Oct 11 2017

Intensity estimates for Ophelia still range wildly.  Dvorak
estimates, both subjective and objective, continue to support a much
higher intensity than other satellite-derived maximum wind
estimates.  Adding to the uncertainty, subjective Dvorak
classifications at 1800 UTC were higher than 6 hours prior, but
since that time, the ragged eye has become obscured in IR imagery.
However, a recent SSMIS pass at 1813 UTC indicated that the
convective structure of the cyclone has improved during the day, so
it wouldn't be surprising if the eye became apparent again shortly.
In an attempt to blend all available data, the initial intensity has
been increased to 65 kt, making Ophelia a hurricane. However, it
should be stressed that the uncertainty of the initial intensity is
higher than normal.

The hurricane is moving slowly toward the east and the initial
motion estimate is 090/3 kt.  The hurricane is embedded within weak
steering flow, and only a slow northeastward drift is expected for
the next 24 h.  After that time, an approaching deep-layer trough
should force Ophelia to accelerate toward the northeast.  All of the
deterministic models are in fairly good agreement on the speed and
track of Ophelia, however the various model ensembles suggest that
the uncertainty is much higher, especially regarding the forward
speed of Ophelia beyond 48 h.  The official track forecast favors
the deterministic model solutions, in part to maintain continuity
with the previous advisory.  The track forecast is therefore close
to the multi-model consensus, but much faster than the various
ensemble mean aids.

Since it isn't clear exactly how strong Ophelia is, the intensity
forecast is low confidence.  All of the intensity guidance indicates
that strengthening is likely for the next 24 to 36 hours, however
the near stationary motion of the hurricane could induce some
upwelling and limit the extent to which the hurricane may
strengthen.  Around 72 hours, the shear should begin to increase
substantially as Ophelia begins to interact with the approaching
trough, and extratropical transition will likely begin, accompanied
by a broadening of the wind field and a gradual decrease of the
maximum winds.  The global models indicate that this process will
complete by 96 h.  The new NHC intensity forecast is slightly higher
than the previous forecast, but lies on the lower end of the
intensity guidance for the first 72 h.  It is near the consensus
aids thereafter.



FORECAST POSITIONS AND MAX WINDS

INIT  11/2100Z 30.0N  36.1W   65 KT  75 MPH
 12H  12/0600Z 30.3N  35.7W   70 KT  80 MPH
 24H  12/1800Z 30.7N  35.2W   75 KT  85 MPH
 36H  13/0600Z 31.4N  34.1W   75 KT  85 MPH
 48H  13/1800Z 32.3N  31.9W   70 KT  80 MPH
 72H  14/1800Z 35.5N  24.5W   70 KT  80 MPH
 96H  15/1800Z 43.0N  16.0W   65 KT  75 MPH...POST-TROP/EXTRATROP
120H  16/1800Z 53.5N  10.5W   60 KT  70 MPH...POST-TROP/EXTRATROP

$$
Forecaster Zelinsky
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