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KF5JRV > WX 08.09.18 12:25l 153 Lines 6876 Bytes #999 (0) @ NA
BID : 20907_KF5JRV
Read: GUEST
Subj: NWS NHC Atlantic WX - Sept 08
Path: IW8PGT<CX2SA<HG8LXL<XE1FH<W9ABA<N9LYA<KF5JRV
Sent: 180908/1115Z 20907@KF5JRV.#NWAR.AR.USA.NA BPQ6.0.16
151
WTNT41 KNHC 080845
TCDAT1
Tropical Storm Florence Discussion Number 36
NWS National Hurricane Center Miami FL AL062018
500 AM AST Sat Sep 08 2018
Although Florence remains a sheared tropical cyclone, satellite
imagery during the past 6 h also indicates that the shear has
started to abate somewhat, which has allowed the dense cirrus
canopy to build back over the previously exposed low-level
circulation center. Furthermore, deep convection with overshooting
cloud tops near -80C and an abundance of lightning activity have
developed very close to the center. Based on these data along with
Dvorak intensity estimates of T3.5/55 kt from TAFB and SAB, the
initial intensity has been raised to 55 kt.
The initial motion estimate is 265/8 kt. The mid-latitude flow
across CONUS and the northern Atlantic is forecast to flatten out
and become more zonal over the next 48 h or so, resulting in the
development of a narrow east-west oriented ridge along 35/36N
latitude. This large-scale feature is expected to steer Florence
in a general westward direction during that time. By days 3-5,
however, the flow across the central and western U.S. is forecast
to buckle and become more meridional as a deep mid-/upper-level
trough over the northeast Pacific pushes inland over the western
U.S., causing downstream ridging over the northeastern U.S. and
northwestern Atlantic. The global models agree on this general
change in the synoptic-scale flow pattern, but they differ
noticeably on where a downstream mid-/upper-level high pressure cell
takes up residence over the Atlantic either to the northwest or
northeast of Bermuda. The farther west/east the high develops will
determine how far west/east Florence will eventually move and
possibly affect the U.S. east coast beyond the 5-day forecast
period. The new official forecast track is close to the previous
advisory track through 48 h, and then was nudged a little to the
left or west of the previous track, which is close to the consensus
model TVCN and is north of the corrected-consensus models FSSE and
HCCA since the bulk of the NHC model guidance lies north of those
latter two models.
The upper-level environment is expected to improve to significantly
during the next 12 h and beyond with the current 20 kt of
southwesterly shear forecast to give way to shear of less than 10
kt. By 72 h and beyond, light shear from the southeast and east
along with the development of strong upper-level outflow jets to
the north of Florence is expected to create an environment that
favors significant and possibly even rapid strengthening. The new
NHC intensity forecast has been increased over the previous advisory
in anticipation of these very favorable dynamical conditions
developing, and now shows Florence becoming a hurricane by Sunday
and a major hurricane in 3 days, followed by additional
strengthening over the very warm Atlantic waters of at least 29 deg
C that are about 2 deg C above normal right now. The consensus
models IVCN and HCCA were closely followed, which are a little
below the FSSE model.
Key Messages:
1. Regardless of Florence's eventual track, large swells are
affecting Bermuda and will begin to affect portions of the U.S.
East Coast this weekend, resulting in life-threatening surf and rip
currents.
2. The risk of other direct impacts associated with Florence along
the U.S. East Coast next week has increased. However, there is
still very large uncertainty in model forecasts of Florence's track
beyond day 5, making it too soon to determine the exact location,
magnitude, and timing of these impacts. Interests near and along
the U.S. East Coast should monitor the progress of Florence through
the weekend and ensure they have their hurricane plans in place.
FORECAST POSITIONS AND MAX WINDS
INIT 08/0900Z 24.5N 54.2W 55 KT 65 MPH
12H 08/1800Z 24.6N 55.0W 60 KT 70 MPH
24H 09/0600Z 24.6N 56.0W 65 KT 75 MPH
36H 09/1800Z 24.6N 57.2W 75 KT 85 MPH
48H 10/0600Z 24.9N 59.0W 90 KT 105 MPH
72H 11/0600Z 26.0N 64.0W 105 KT 120 MPH
96H 12/0600Z 28.0N 70.2W 120 KT 140 MPH
120H 13/0600Z 30.9N 75.8W 125 KT 145 MPH
$$
Forecaster Stewart
380
WTNT43 KNHC 080850
TCDAT3
Tropical Storm Helene Discussion Number 4
NWS National Hurricane Center Miami FL AL082018
500 AM AST Sat Sep 08 2018
Helene's structure appears to be quickly improving this morning.
The initial intensity has been increased to 40 kt based on a blend
of the latest subjective Dvorak intensity estimates and the
UW-CIMSS SATCON. Several microwave passes overnight indicate that
the cyclone has become more vertically aligned, likely due to a
reformation of the center beneath the strongest convection. In
fact, a 0733 UTC SSMIS image shows the early development of a low-
to mid-level eye, which is often a precursor to intensification.
Nearly all of the intensity guidance seems to have picked up on the
improved structure of Helene, and the models show far more
intensification than before, especially through 48 h. Consequently,
a significant change has been made to the intensity forecast, which
now calls for Helene to become a hurricane on Sunday while it is
near the Cabo Verde islands. Once the cyclone passes the islands,
the environment is expected to remain favorable for intensification
for at least 72 h. By the end of the forecast period, all of the
guidance shows some weakening due primarily to increased shear
associated with an approaching mid- to upper-level trough over the
central Atlantic. Given the recent microwave imagery signature, low
shear, and warm SSTs, its possible that the current forecast is
still too conservative, especially for the first 36 h of the
forecast, and further adjustments could be required if it becomes
clear that Helene is intensifying at a faster rate than expected.
Smoothing through the possible reformation of the center, Helene
appears to be moving west with an initial motion around 275/10 kt.
Little change was required to the track forecast, which has been
nudged only slightly south for the first 48 h of the forecast, in
line with the latest track model consensus. A westward motion
should continue for the next couple of days, as Helene is steered
by a mid-level ridge to the north. By day 5, the aforementioned
trough over the central Atlantic should force Helene to turn toward
the northwest.
FORECAST POSITIONS AND MAX WINDS
INIT 08/0900Z 13.7N 19.6W 40 KT 45 MPH
12H 08/1800Z 13.9N 21.2W 50 KT 60 MPH
24H 09/0600Z 14.1N 23.5W 60 KT 70 MPH
36H 09/1800Z 14.7N 26.3W 70 KT 80 MPH
48H 10/0600Z 15.4N 29.3W 75 KT 85 MPH
72H 11/0600Z 17.2N 35.1W 80 KT 90 MPH
96H 12/0600Z 19.0N 39.5W 70 KT 80 MPH
120H 13/0600Z 22.0N 42.5W 65 KT 75 MPH
$$
Forecaster Zelinsky
73 de Scott KF5JRV
Pmail: KF5JRV@KF5JRV.#NWAR.AR.USA.NA
email: KF5JRV@ICLOUD.COM
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