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Tropical Storm KARINA Forecast Discussion Number 4
2014-08-13 22:51:22| National Hurricane Center (East Pacific)
Issued at 200 PM PDT WED AUG 13 2014 000 WTPZ41 KNHC 132051 TCDEP1 TROPICAL STORM KARINA DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112014 200 PM PDT WED AUG 13 2014 Karina is experiencing some issues with dry air entrainment and modest north to northeasterly wind shear since the previous advisory, which has resulted in some erosion of the large outer convective band on the west side along with the central deep convection. However, two recent scatterometer overpasses indicate that Karina's circulation is well developed and compact, and that the radius of maximum winds has decreased to around 10-15 nmi. The initial intensity is set at 45 kt based on surface wind data of 40 kt from a 1649 UTC ASCAT-B overpass and 41 kt from a 1736 UTC ASCAT-A overpass, and assuming that the scatterometers are under-sampling the very small inner-core wind field. Karina continues to move westward or 275/13 kt. There is no significant change to the previous forecast track or reasoning. Karina is expected to maintain a general westward motion for the next 5 days as the cyclone moves along the southwestern periphery of a deep-layer subtropical ridge. The model guidance is tightly packed through 72 hours, with the models diverging significantly after that. However, the model spread has decreased on this cycle now that the new 1200 UTC ECMWF model run has shifted farther to the north, closer to the other global models and the previous NHC forecast track. The new forecast is just an update of the previous advisory track and lies close to the consensus model TVCE. Convection has developed during the past few hours within the dry slot in the eastern semicircle, and a small burst of convection has also developed very near the well-defined low-level circulation center. All of this would suggest that the aforementioned dry air issues will likely be short-lived. Over the next 3 days, the vertical shear is forecast to decrease to less than 10 kt while the cyclone remains over SSTs of 28C-29C and within a fairly moist mid-level environment. Only the proximity to cooler and more stable air just west and northwest of the cyclone precludes a near-term forecast of rapid intensification. However, the trend over the next 72 hours calls for at least steady intensification, with Karina reaching hurricane strength by 36 hours. After that, the cyclone will be moving over cooler SSTS and into a more stable air mass, which should induce gradual weakening. However, the low vertical wind shear environment is expected to result in slower weakening than normally would occur over such marginal SSTs on Days 4 and 5. The NHC official intensity forecast remains higher than the ICON intensity consensus model, and is close to a blend of the SHIPS and GFDL models. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 17.2N 112.6W 45 KT 50 MPH 12H 14/0600Z 17.3N 114.5W 50 KT 60 MPH 24H 14/1800Z 17.4N 117.0W 60 KT 70 MPH 36H 15/0600Z 17.6N 119.3W 65 KT 75 MPH 48H 15/1800Z 17.8N 121.6W 70 KT 80 MPH 72H 16/1800Z 18.2N 126.0W 75 KT 85 MPH 96H 17/1800Z 18.4N 129.6W 70 KT 80 MPH 120H 18/1800Z 18.0N 133.0W 65 KT 75 MPH $$ Forecaster Stewart
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