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Tropical Storm HERNAN Forecast Discussion

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WTPZ43 KNHC 281437

800 AM PDT MON JUL 28 2014

Persistent 15-20 kt of westerly shear is certainly deteriorating the
cyclone's organization this morning. Conventional satellite imagery
shows a rather shapeless cloud pattern with warming cloud tops.  A
fortuitous 0934 UTC AMSU-A microwave pass indicated an eastward
vertical tilt with the low-level center located near the western
edge of the deep convection.  A compromise of satellite intensity
estimates from TAFB and SAB along with a 0933 UTC UW-CIMSS SATCON
(which includes ADT, and AMSU intensity estimates) of 54 kt supports
an initial intensity of 55 kt for this advisory.  The large-scale
models and the statistical/dynamical intensity guidance all show a
moderate shear environment through the forecast period.
Furthermore, an increasingly stable air mass and decreasing sea
surface temperatures should ultimately weaken Hernan into a shallow
post-tropical cyclone in 48 hours.

Hernan continues to move northwestward, with an initial motion
estimate of 295/14 kt.  Global model guidance indicates that a
mid-level ridge to the northeast of the cyclone should steer Hernan
in this general motion through the 36 hour period.  Afterward, a
turn toward the west and a reduction in forward speed is forecast as
the remnant low moves within the lower tropospheric flow of the
subtropical ridge.  The NHC forecast is again adjusted slightly to
the left of the previous track beyond 36 hours, and lies between the
consensus model and the ECMWF.


INIT  28/1500Z 20.5N 115.9W   55 KT  65 MPH
 12H  29/0000Z 21.6N 117.8W   50 KT  60 MPH
 24H  29/1200Z 22.9N 120.3W   40 KT  45 MPH
 36H  30/0000Z 23.8N 122.2W   30 KT  35 MPH
 48H  30/1200Z 24.0N 123.4W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  31/1200Z 24.0N 125.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  01/1200Z 24.0N 127.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  02/1200Z...DISSIPATED

Forecaster Roberts

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Page last modified: Monday, 28-Jul-2014 14:38:03 UTC