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Tropical Storm ROSLYN Forecast Discussion

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WTPZ43 KNHC 260835

200 AM PDT MON SEP 26 2016

The sheared cloud pattern in infrared satellite imagery has changed
little since the previous advisory. However, a 0304Z SSMI/S pass
indicated the low-level structure had improved markedly, with a
sharp hooked band wrapping more than half around the now
well-defined low-level circulation center. The initial intensity has
been increased to 35 kt based on a Dvorak classification of T2.5/35
kt from TAFB and the improved low-level structure indicated in
SSMI/S 37 GHZ data. This makes Roslyn the seventeenth named storm of
the 2016 eastern North Pacific hurricane season.

The initial motion estimate is 020/4 kt. The advisory position had
to be adjusted more than 30 nmi to the east of the previous advisory
track based on recent SSMI/S and AMSU microwave center fixes.
Otherwise, the previous track forecast rationale remains unchanged.
A large mid- to upper-level low currently located over the central
Baja California peninsula is expected to continue moving westward to
west-southwestward during the next day or two. The combined
southwesterly flow between that low and a deep-layer ridge located
to the east and southeast of Roslyn should keep the tropical cyclone
moving slowly north-northeastward to northeastward for the next 48
hours. After that, Roslyn is expected to weaken into a shallow
remnant low, turning northward on day 3 and then northwestward on
day 4. The new NHC track forecast is to the right of the previous
advisory track, mainly due to the more eastward initial position,
and lies close to the various consensus models.

Little if any strengthening is expected due to gradually increasing
southwesterly vertical wind shear and a very dry mid-level moisture
regime characterized by humidity values less than 40 percent. By 36
to 48 hours, the vertical shear is forecast to increase to more than
30 kt, which is expected to erode the deep convection and induce
steady weakening. As a result, Roslyn is forecast to become a
remnant low by 48 hours, but this could occur sooner. The GFS and
ECMWF models show the remnant low dissipating by the end of the
forecast period when Roslyn will be moving over sub-24C SSTs.  The
new intensity forecast is identical to the previous advisory and
closely follows the IVCN intensity consensus model.


INIT  26/0900Z 16.8N 119.5W   35 KT  40 MPH
 12H  26/1800Z 17.2N 119.2W   35 KT  40 MPH
 24H  27/0600Z 18.0N 118.5W   35 KT  40 MPH
 36H  27/1800Z 19.1N 117.6W   35 KT  40 MPH
 48H  28/0600Z 20.4N 117.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 72H  29/0600Z 22.5N 117.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  30/0600Z 24.6N 119.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  01/0600Z...DISSIPATED

Forecaster Stewart