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Tropical Storm Rina Discussion Number   4
NWS National Hurricane Center Miami FL       AL192017
1100 PM AST Mon Nov 06 2017

Despite the low-level center being exposed to the west of the deep
convection, a pair of ASCAT-A and ASCAT-B passes between 00Z and
01Z showed an area of 30-35 kt winds east of the cyclone's center.
Based on the ASCAT data, the depression is upgraded to a tropical
storm with the initial intensity set at 35 kt. The rather
disorganized cloud pattern is consistent with the approximately 20
kt of westerly shear analyzed by UW-CIMSS and the SHIPS model. The
environment is only expected to be marginally conducive for
intensification via diabatic processes, with the shear expected to
remain near 20 kt and the SSTs cooling along the forecast track.
After 24 to 36 hours, cooling upper-tropospheric temperatures and
increasing upper-level divergence suggest that Rina will maintain
its intensity as it begins extratropical transition, which should be
complete between by 72 hours. The new NHC intensity forecast is
similar to the previous one and is close to the HWRF and HCCA aids.
The official forecast shows Rina dissipating by 96 hours in
agreement with global model fields.

The exposed low-level center and ASCAT passes result in high
confidence in the initial position, and Rina has begun to move
more steadily, with an initial motion estimate of 010/06. Rina
should continue to accelerate northward and north-northeastward
during the next 36 to 48 hours between a mid-level ridge to the east
and an upper-level trough to the west. After 48 hours, Rina is
forecast to accelerate further as it enters the mid-latitude
westerlies. The track model guidance has shifted left this cycle,
and the along-track spread is quite large by 72 hours, with the GFS
more than 500 n mi northeast of the ECMWF at that time. The new NHC
track forecast has been adjusted westward and a bit slower, and lies
close to a blend of the GFS and ECMWF solutions to the east of the
TVCA multi-model consensus. Confidence in the details of the track
forecast is lower than usual given the large model spread.

FORECAST POSITIONS AND MAX WINDS

INIT  07/0300Z 30.4N  49.9W   35 KT  40 MPH
 12H  07/1200Z 32.4N  49.6W   40 KT  45 MPH
 24H  08/0000Z 35.4N  49.0W   45 KT  50 MPH
 36H  08/1200Z 39.0N  48.0W   45 KT  50 MPH
 48H  09/0000Z 42.3N  46.2W   45 KT  50 MPH
 72H  10/0000Z 51.0N  36.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
 96H  11/0000Z...DISSIPATED

$$
Forecaster Brennan

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