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Tropical Storm OLAF


200 AM PDT SAT OCT 17 2015

The convective organization of Olaf has continued to improve since
the previous advisory. A ragged CDO feature has developed, but very
cold overshooting cloud tops with temperatures of -87C to -88C have
also developed near the low-level circulation center as depicted in
recent passive microwave satellite images. In addition, outer curved
band features have also improved to the north and south of the
center while the upper-level outflow pattern has expanded and become
more symmetrical. The initial intensity has been increased to 45 kt
based on a consensus satellite intensity estimate of T3.0/45 kt from
TAFB and SAB, which is supported by a UW-CIMSS ADT estimate of
T3.4/53 kt and an 0134 UTC AMSU intensity estimate of 59 kt.

The initial motion estimate is now 270/10 kt. The NHC model guidance
remains in excellent agreement on Olaf rounding the southwestern and
western periphery of a deep-layer ridge during the next five days,
accompanied by a gradual decrease in forward speed. On day 5, the
cyclone could begin to recurve and accelerate toward the
north-northeast or northeast as suggested by the GFS model, but the
preponderance of the guidance keeps Olaf south of ridge axis and
moving generally northward at that time, and this is reflected in
the official forecast. The new track forecast is similar to the
previous advisory, and essentially lies down the middle of the
guidance envelope and close to the consensus models.

Microwave imagery suggests that the aforementioned intense burst of
convection has likely tightened up the inner-core wind field and has
reduced the size of the radius of maximum winds (RMW). Given the
small RMW of about 20 nmi, the expanding outflow pattern, continued
deep-layer vertical wind shear of less than 10 kt, and mid-level
humidity values expected to increase to around 70 percent, a period
of rapid intensification is forecast to occur for the next 24 hours.
After that, the intensity forecast calls for steady strengthening
through 72 hours, followed by a leveling off in the intensity on
days 4 and 5 due to possible eyewall replacement cycles that are
impossible to forecast that far in advance. The new intensity
forecast is close to the LGEM intensity model through 72 hours, and
then is lower than the LGEM model and closer to the SHIPS model on
days 4 and 5. As mentioned in previous advisories, it would not be
surprising to see more strengthening than is currently forecast
given the very conducive large-scale environment, similar to the
96-h GFS and ECMWF model forecasts of a 938-mb and 956-mb hurricane,


INIT  17/0900Z  9.5N 128.5W   45 KT  50 MPH
 12H  17/1800Z  9.5N 129.8W   60 KT  70 MPH
 24H  18/0600Z  9.7N 131.5W   75 KT  85 MPH
 36H  18/1800Z 10.1N 133.0W   85 KT 100 MPH
 48H  19/0600Z 10.7N 134.6W   95 KT 110 MPH
 72H  20/0600Z 12.2N 137.7W  100 KT 115 MPH
 96H  21/0600Z 14.2N 140.3W  100 KT 115 MPH
120H  22/0600Z 16.3N 141.6W  100 KT 115 MPH

Forecaster Stewart