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Tropical Storm OLAF Forecast Discussion Number 10
2015-10-17 10:33:45| National Hurricane Center (East Pacific)
Issued at 200 AM PDT SAT OCT 17 2015 000 WTPZ44 KNHC 170833 TCDEP4 TROPICAL STORM OLAF DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 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, respectively. FORECAST POSITIONS AND MAX WINDS 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
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