ZCZC MIATCDEP4 ALL TTAA00 KNHC DDHHMM TROPICAL STORM OLAF DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 800 AM PDT SAT OCT 17 2015 Olaf's cloud pattern has not exhibited much overall change since the last advisory in conventional satellite imagery. The cyclone is characterized by a small CDO with one primary band over the western half of the circulation, though the band is detached from the main convective mass and fragmented. The convective distribution is suggestive of some northwesterly shear, as shown in GFS- and ECMWF-based SHIPS output, but a partial 1030 UTC AMSU pass indicated a mid-level eye, perhaps not completely coincident with the low-level center. Dvorak intensity estimates are consensus T3.0/45 kt, but the initial intensity is increased to 50 kt on the basis of the greater organization seen in microwave imagery. The initial motion estimate is 270/11. The mid-level subtropical ridge steering Olaf westward is about to weaken in response to the long tail of a mid- to upper-level trough near the U.S. west coast settling southward to the north of the cyclone. The weaker ridge should induce a slower forward speed with a gradual increase in latitude during the next couple of days. After 48 hours, a large anticyclone forming in the east-central subtropical Pacific near 140W is forecast to shift eastward while a weakness develops along 150W. Global models show Olaf moving in the flow between these two synoptic features, which should result in a northwestward and then north-northwestward turn after 96 hours. The official track forecast is nearly unchanged through 36 hours. After that time, the track forecast is shifted westward in agreement with a leftward- shifting multi-model consensus and a majority of ECMWF ensemble members that were farther west than the operational ECMWF solution that lies east of the multi-model consensus. The new track forecast is on the western side of the guidance envelope. Except for the northwesterly shear, there are no obvious impediments to continued strengthening for the next few days. Exactly how much the shear is a factor, however, remains to be seen, but its most likely effect would be to possibly slow the rate of intensification. According to the SHIPS output, the northwesterly shear diminishes after 36 hours, and with all other large-scale factors favoring intensification, Olaf is likely to become a major hurricane in 2 or 3 days. Late in the forecast period, once Olaf gains enough latitude, a drier environment with an increase in southerly or south-southwesterly shear should result in weakening. The official intensity forecast is near the multi-model consensus through 36 hours and generally above it after that time. FORECAST POSITIONS AND MAX WINDS INIT 17/1500Z 9.4N 129.5W 50 KT 60 MPH 12H 18/0000Z 9.5N 130.9W 60 KT 70 MPH 24H 18/1200Z 9.8N 132.5W 75 KT 85 MPH 36H 19/0000Z 10.3N 134.1W 90 KT 105 MPH 48H 19/1200Z 11.0N 135.8W 100 KT 115 MPH 72H 20/1200Z 12.6N 139.0W 100 KT 115 MPH 96H 21/1200Z 14.7N 141.4W 95 KT 110 MPH 120H 22/1200Z 16.9N 142.5W 85 KT 100 MPH $$ Forecaster Kimberlain NNNN