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Tropical Storm Gert Forecast Discussion Number 6
2017-08-14 10:34:50| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Mon Aug 14 2017 000 WTNT43 KNHC 140834 TCDAT3 Tropical Storm Gert Discussion Number 6 NWS National Hurricane Center Miami FL AL082017 500 AM EDT Mon Aug 14 2017 Although Gert's asymmetric convective cloud pattern has improved slightly since the previous advisory, the system remains sheared with the low-level center located near the far northwestern edge of the deepest convection due to deep-layer and mid-level northerly shear of 15 kt. Although the satellite intensity estimates are a consensus T3.0/45 kt from TAFB, SAB, and UW-CIMSS ADT, the initial intensity is being held at 40 kt due to the low-level center having become more exposed since those 0600 UTC intensity estimates. Gert has maintained a north-northwestward motion or 345/08 kt. There are significant changes to the previous track forecast or reasoning. The small cyclone is expected to turn northward shortly and then turn toward the north-northeast by tonight, moving through a break in the subtropical ridge roughly midway between the U.S. east coast and Bermuda on Tuesday. By 48 h, Gert is forecast to steadily accelerate northeastward over the north Atlantic through the remainder of the forecast period ahead of a strong deep-layer trough. The latest guidance has again shifted a little bit to the west after recurvature, so the official forecast was nudged in that direction slightly, but remains on the far eastern edge of the guidance envelope. Gert should continue to be affected by moderate northerly vertical wind shear and dry mid-level air for the next 24-30 h, so only slight strengthening is anticipated despite the very warm SSTs beneath the cyclone. By 36-48h, a window of opportunity for more significant strengthening is expected when the shear decreases to less than 10 kt after the cyclone begins to recurve, with Gert possibly becoming a hurricane by 48 h. However, the intensification trend is expected to be short-lived due to the vertical wind shear shifting to southwesterly and increasing to more than 30 kt by 60 h and beyond. By 72 h, Gert will be moving over ocean temperatures of 20 deg C and colder, which should induce and sustain extratropical transition. Gert's intensity is held to near-hurricane strength at 72 h due to the system getting a baroclinic kick when the cyclone moves underneath a favorable region of the upper-level jetstream. However, rapid weakening is expected by 96 h when the cyclone is moving over 15 deg C SSTs, with the system being absorbed on day 5 by a larger extratropical low pressure system over the far north Atlantic. The official intensity forecast is similar to the previous advisory, and is lower than the consensus models IVCN and HCCA. This is due to the HWRF model creating a high bias in those models caused by an unrealistic forecast of Gert becoming a category 4 hurricane in 72 h when the cyclone will be moving over SSTs of 25 deg C or colder and in shear conditions greater than 35-40 kt. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 29.7N 72.2W 40 KT 45 MPH 12H 14/1800Z 30.9N 72.3W 45 KT 50 MPH 24H 15/0600Z 32.7N 71.9W 50 KT 60 MPH 36H 15/1800Z 34.9N 70.0W 55 KT 65 MPH 48H 16/0600Z 37.3N 66.0W 65 KT 75 MPH 72H 17/0600Z 42.7N 52.1W 60 KT 70 MPH...POST-TROP/EXTRATROP 96H 18/0600Z 49.0N 38.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 19/0600Z...ABSORBED BY LARGER EXTRATROPICAL LOW $$ Forecaster Stewart
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