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Tropical Storm Epsilon Forecast Discussion


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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