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TROPICAL STORM SEYMOUR DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP202016
900 PM MDT SUN OCT 23 2016

Seymour's convective pattern has continued to improve since the
previous advisory, with the development of a mid-level eye feature
noted in 23/2250Z and 24/0107Z SSMI/S microwave images. Dvorak
classifications from TAFB and SAB are T3.0/45 kt and T3.5/55 kt,
respectively, and T3.6/57 kt from UW-CIMSS ADT. The initial
intensity is set on the low side of these estimates at 45 kt for
this advisory, which could be conservative.

The initial motion estimate is 290/13 kt. For the next 72 h, the
global models remain in excellent agreement on Seymour moving
west-northwestward along the southern periphery of a deep-layer
ridge located over Mexico. After that, the cyclone is expected to
turn northward around the western portion of the ridge and ahead of
a shortwave trough forecast to approach Seymour from the northwest
by late Wednesday. The official forecast track lies basically down
the middle of the guidance envelope, close to the multi-model
consensus TVCN.

The global and regional models indicate environmental conditions are
forecast to be quite favorable for additional strengthening to occur
during the next 3 days. In fact, due to a combination of the
vertical shear being low at around 5 kt, mid-level humidity values
being near 70 percent, sea-surface temperatures expected to be at
least 29 deg C, and the recent development of a mid-level eye, a
period of rapid intensification is forecast to occur for the next
24 hours. This trend is supported by the SHIPS and LGEM statistical
intensity models, and also by the HWRF model and the new NOAA
Corrected Consensus Model, HCCA, which makes Seymour a major
hurricane by 48 hours. By 72 hours and beyond, increasing
southwesterly vertical wind shear ahead of the aforementioned
shortwave trough and SSTs less than 25 deg C and are expected to
induce rapid weakening. The new intensity forecast lies close to a
blend of the SHIPS and LGEM models, which is significantly higher
than the previous advisory, but remains well below the peak
intensity forecast of 110 kt by HCCA and the 112-kt forecast from
the HWRF model.


FORECAST POSITIONS AND MAX WINDS

INIT  24/0300Z 14.4N 107.1W   45 KT  50 MPH
 12H  24/1200Z 14.9N 109.1W   60 KT  70 MPH
 24H  25/0000Z 15.6N 111.7W   75 KT  85 MPH
 36H  25/1200Z 16.1N 114.4W   85 KT 100 MPH
 48H  26/0000Z 16.7N 116.6W   90 KT 105 MPH
 72H  27/0000Z 18.7N 120.2W   90 KT 105 MPH
 96H  28/0000Z 22.0N 120.8W   65 KT  75 MPH
120H  29/0000Z 24.0N 120.0W   45 KT  50 MPH

$$
Forecaster Stewart

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