2020 Hurricane Season (Atlantic & East Pacific)

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

hatrickpatrick wrote: »

Eta is now on the cusp of Category 3 status and the NHC anticipates it reaching Cat 4 before making landfall in Nicaragua tomorrow night. Yet another explosive intensification at very short notice.

@Gaoth, I know you rightly put a lot of RI trends down to the increasing frequency of recon flights in recent years, but would you agree that even within that context 2020 has been very much an outlier in terms of how often we've seen RI events? It seems like every storm which reaches hurricane status in either the Caribbean or Gulf this year has been a candidate for subsequent RI at some stage in its forecasting cycle, and prior to this year I'd have said that RI events were a relatively rare phenomenon.

Even the different probability models used to predict RI events - RI15, RI25 and RI35, corresponding to how large a wind speed increase (measured in KT) is possible during the next 24 hours - used to only appear in the spaghetti charts on an extremely rare basis, and you always knew when they showed up that the storm in question had the potential for an explosive deepening. This year, they've appeared on an almost continuous basis, and while this could be down to the probability threshold being lowered from previous years (I don't know off the top of my head what probability is required before the output of these models is included in the intensity forecasts), the fact that RI has followed fairly consistently suggests that it's not model bias, but that this year has indeed featured a seriously abnormal rate of explosive deepening events relative to the rest of this decade.

In this case, the RI25 and RI30 models are included in the spaghetti suite, indicating that the probability of a rapid intensity increase between 25KT and 30KT has reached a threshold considered likely enough to include these models in the forecasts. Again, I don't remember off the top of my head what the threshold is, but I haven't heard of it being adjusts in recent years and it used to be exceptionally rare to encounter these models among the model suites.

Akrasia wrote: »

There is a very big difference between a depression experiencing RI to become a storm, and a tropical storm intensifying to become a hurricane, and between a cat 1 hurricane rapidly intensifying to become a major hurricane

The most dangerous storms are those that are already powerful cyclones with an established eyewall, but are not forecast to intensify beyond TC or low Cat 1 hurricane, that unexpectedly rapidly intensify close to landfall and hit land as a major hurricane with very short notice.
Hurricanes like Harvey, or Michael, where people went to bed expecting a cat 1 or 2 hurricane and woke up facing Cat 4 winds coming at them with no time to evacuate

Rapid intensification of depressions into weak cyclones will always be the majority of instances of RI. If a storm changes from a weak depression to become a TC at 34kts within 24 hours, that counts as RI, but it's absolutely not the same as an established TC going through RI to become a major hurricane within 24 hours.

And the scale of RI that Eta is demonstrating now, at this time of the year, is extremely rare and the people of Nicoragua must be really worried right now at what they will face in the coming days and weeks, just as the people in the Phillipines are facing months of recovery after Typhoon Goni rapidly intensified to a Super typhoon increasing from 30mph sustained winds to 175mph in just 2.5 days

045
URNT12 KNHC 031214
VORTEX DATA MESSAGE AL292020
A. 03/11:46:10Z
B. 13.63 deg N 083.06 deg W
C. 700 mb 2575 m
D. 938 mb
E. 050 deg 13 kt
F. CLOSED
G. C8
H. 97 kt
I. 234 deg 5 nm 11:44:30Z
J. 100 deg 110 kt
K. 028 deg 10 nm 11:38:30Z
L. 103 kt
M. 155 deg 6 nm 11:50:30Z
N. 271 deg 118 kt
O. 168 deg 12 nm 11:52:30Z
P. 11 C / 3047 m
Q. 19 C / 3053 m
R. NA / NA
S. 12345 / 07
T. 0.02 / 1 nm
U. AF307 0629A ETA OB 08
MAX FL WIND 118 KT 168 / 12 NM 11:52:30Z

302
URNT12 KNHC 030403
VORTEX DATA MESSAGE AL292020
A. 03/03:10:10Z
B. 14.03 deg N 082.61 deg W
C. 700 mb 2435 m
D. 925 mb
E. 290 deg 14 kt
F. CLOSED
G. C7
H. 135 kt
I. 127 deg 3 nm 03:07:00Z
J. 232 deg 127 kt
K. 135 deg 4 nm 03:06:30Z
L. 125 kt
M. 043 deg 5 nm 03:13:30Z
N. 135 deg 125 kt
O. 043 deg 5 nm 03:13:30Z
P. 11 C / 3037 m
Q. 20 C / 2998 m
R. 3 C / NA
S. 12345 / 07
T. 0.02 / 0.5 nm
U. AF304 0429A ETA OB 15
MAX FL WIND 137 KT 193 / 5 NM 02:18:00Z
MANY MESOVORTICES ROTATING ON INSIDE EYEWALL.