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Tropical Storm OLAF Forecast Discussion Number 12
2015-10-17 22:33:17| National Hurricane Center (East Pacific)
Issued at 200 PM PDT SAT OCT 17 2015 000 WTPZ44 KNHC 172033 TCDEP4 TROPICAL STORM OLAF DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 200 PM PDT SAT OCT 17 2015 There appears to have been a temporary interruption to Olaf's intensification this morning, which could be related to a couple of different factors. Microwave imagery from earlier suggested that the cyclone ingested a tongue of relatively drier air. There is also moderate northwesterly shear still affecting the system. Nevertheless, there are some signs that Olaf could be about to resume an intensification. The cyclone has maintained a small CDO, but there is now greater definition to inner-core features than 12 to 24 hours ago, and the convective bands have lengthened and gained greater curvature. While satellite classifications remain a consensus T3.0/45 kt at 1800 UTC, the initial intensity estimate of 50 kt from the previous advisory is maintained. The initial motion estimate is 265/11. The mid-level subtropical ridge steering Olaf westward should weaken soon 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 west-northwestward course at a reduced forward speed during the 2 to 3 days. After about 72 hours, a large anticyclone forming in the east-central subtropical Pacific near 140W is forecast to shift eastward while a weakness develops along farther west. Global models show Olaf moving in the flow between these two synoptic features, resulting in a northwestward and then north- northwestward turn after 96 hours. The guidance has shifted toward the left again this cycle, especially later in the forecast period, and the official track forecast is moved in that direction. Since most of the global models depicted Olaf farther north than where it currently is, it would not be surprising if further leftward adjustments are necessary. Other than some northwesterly shear over Olaf during the next day or so, the large-scale environment is conducive for Olaf to intensify. The shear could slow the rate of intensification, but it would not appear strong enough not to allow Olaf to strengthen at least at the climatological rate of development of one Dvorak T-number per day. A diminution of the shear by 36 hours, in combination with other very conducive environmental factors, should allow Olaf to strengthen to near major hurricane strength in 2 to 3 days. Olaf is also expected to become a much larger cyclone during the next several days, with an impressive expansion of the wind field depicted in global models. 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 new intensity forecast is identical to the previous one and is closest to the FSU Superensemble output and near the upper edge of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 17/2100Z 9.2N 130.5W 50 KT 60 MPH 12H 18/0600Z 9.3N 131.8W 60 KT 70 MPH 24H 18/1800Z 9.8N 133.5W 75 KT 85 MPH 36H 19/0600Z 10.3N 135.2W 90 KT 105 MPH 48H 19/1800Z 11.0N 136.9W 100 KT 115 MPH 72H 20/1800Z 12.9N 139.9W 100 KT 115 MPH 96H 21/1800Z 15.2N 142.0W 95 KT 110 MPH 120H 22/1800Z 17.3N 142.8W 85 KT 100 MPH $$ Forecaster Kimberlain
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