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Tropical Storm ROSLYN Forecast Discussion (Text)


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000
WTPZ43 KNHC 262048 CCA
TCDEP3

TROPICAL STORM ROSLYN DISCUSSION NUMBER   6...CORRECTED
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP182016
200 PM PDT MON SEP 26 2016

CORRECTED STATUS AT 72H

Although the center of Roslyn has become farther separated
from the deep convection this afternoon due to strong
southwesterly shear, recent ASCAT data indicate that the
tropical cyclone is stronger than previously estimated.  The
1756 UTC ASCAT overpass revealed 40-45 kt winds, and a much
larger area of 34-kt winds than analyzed earlier.  Based on
these data, the initial intensity has been set at 45 kt, and
the 34-kt wind radii have been expanded accordingly.

Increasing southwesterly vertical wind shear and gradually
decreasing SSTs should cause weakening, but given the large size of
the system the spindown process is likely to be more gradual than
average.  In 24 to 36 hours, much cooler waters and a more stable
airmass are expected to cause Roslyn to degenerate into a remnant
low and to dissipate in 3 to 4 days.  The NHC intensity forecast has
been adjusted upward in the early portion of the forecast to account
for the higher initial intensity, but is unchanged thereafter.  The
official forecast shows a slower rate of decay than the SHIPS
guidance, but it is general agreement with the trend in the global
models.

Roslyn has not moved much during the past 24 hours, and the best
estimate of the initial motion is 035/2 kt.  The models insist that
Roslyn will begin moving northeastward within deep layer
southwesterly flow between a mid-level ridge to the southeast and a
large upper-level low over Baja California.  In a couple of days,
the cyclone should turn northward, then northwestward once it
weakens and is steered by the low-level flow.  The new NHC track is
similar to the previous advisory and is near a consensus of the
ECMWF and GFS models.

FORECAST POSITIONS AND MAX WINDS

INIT  26/2100Z 16.8N 118.8W   45 KT  50 MPH
 12H  27/0600Z 17.3N 118.4W   40 KT  45 MPH
 24H  27/1800Z 18.4N 117.7W   35 KT  40 MPH
 36H  28/0600Z 19.8N 117.3W   30 KT  35 MPH
 48H  28/1800Z 21.2N 117.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  29/1800Z 23.3N 118.8W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  30/1800Z...DISSIPATED

$$
Forecaster Brown


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Page last modified: Monday, 26-Sep-2016 20:48:51 UTC