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Tropical Storm Epsilon Forecast Discussion Number 4
2020-10-20 04:39:45| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Mon Oct 19 2020 000 WTNT42 KNHC 200239 TCDAT2 Tropical Storm Epsilon Discussion Number 4 NWS National Hurricane Center Miami FL AL272020 1100 PM AST Mon Oct 19 2020 Water vapor imagery indicates that Epsilon has been interacting with a shear line/dissipating cold front from the north and with a negatively tilted upper-level trough from the south. Furthermore, a pronounced dry slot has developed in the eastern semicircle, which has severed the convective band that had been wrapping about three-fourths of the way around the circulation. Overall, the cloud pattern more closely resembles that of an occluded extratropical low, with a small inner-core tropical feature. A 20/0025Z ASCAT-A pass indicated a small fetch of mostly straight-flow 40-kt winds located 60-90 nmi northeast of the well-defined surface center. Given the distance from the low-level center and lack of any significant curvature to those winds, undersampling is probably not occurring. Therefore, the initial intensity is being held at 40 kt for this advisory, which is consistent with satellite classifications of T2.5/35 kt from TAFB, SAB, and UW-CIMSS ADT, and a 19/2202Z SATCON estimate of 42 kt. Epsilon is a large cyclone with gale-force or tropical-storm-force winds extending outward more than 250 nmi in the northern semicircle. The initial motion estimate is 360/02 kt. No significant changes were made to the previous track forecast or reasoning. Epsilon is expected to meander within weak steering currents well to the southeast of Bermuda for the next 12 h or so. By late Tuesday, a ridge is forecast to build to the north and east of the cyclone, forcing Epsilon generally toward the northwest through Friday. As an upper-level trough and associated frontal system approach the cyclone, Epsilon is expected to turn sharply northeastward between the trough and the ridge by late Friday, and accelerate northeastward thereafter over over the north Atlantic. The latest NHC model guidance is coming into better agreement, with a tight clustering of he various consensus models lying essentially along the previous advisory track. On the forecast track, Epsilon should make its closest approach to Bermuda on Friday. Epsilon is forecast to remain over sea-surface temperatures (SST) of at least 27C the next 36 h or so and, when coupled with 200-mb temperatures of -55C, sufficient instability will exist to continue to allow for deep convection to be generated both in the inner- and outer-core regions of the cyclone. Thus, gradual strengthening is forecast during that time. However, by 48 h and continuing through 72 h, SSTs cool to near 26.5C and the depth of the relatively warm water becomes quite shallow, as indciated by upper-ocean heat content values dropping to near zero by 72 h. The large and expansive wind field should result in cold upwelling both ahead of and beneath the inner-core wind field, which is likely to temper the intensification process. This may be reflected in the past couple of HRWF runs which no longer make Epsilon a hurricane by the time the cyclone approaches Bermuda on Friday. For now, the intensity forecast has only been lowered slightly since there may be some baroclinic interaction with an approaching upper-level trough that could offset the cooler waters. Epsilon could be undergoing extratropical transition by the 120-h forecast period, but for now the system will be shown as still being tropical since it will be located over marginal SSTs near 26C at that time. Key Message: 1. Epsilon is forecast to be at or near hurricane strength when it approaches Bermuda late this week. While it is too soon to determine the exact details of Epsilon's track and intensity near the island, there is a risk of direct impacts from wind, rainfall, and storm surge on Bermuda, and interests there should closely monitor the progress of Epsilon. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 25.3N 55.3W 40 KT 45 MPH 12H 20/1200Z 26.1N 55.5W 45 KT 50 MPH 24H 21/0000Z 27.5N 56.7W 50 KT 60 MPH 36H 21/1200Z 28.3N 58.5W 55 KT 65 MPH 48H 22/0000Z 29.1N 59.8W 60 KT 70 MPH 60H 22/1200Z 30.4N 60.8W 65 KT 75 MPH 72H 23/0000Z 31.3N 61.5W 70 KT 80 MPH 96H 24/0000Z 32.9N 62.5W 75 KT 85 MPH 120H 25/0000Z 36.4N 60.3W 75 KT 85 MPH $$ Forecaster Stewart
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