Tropical Report

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Sun Sep 16 2018

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Tropical
Depression Florence, located inland over central South Carolina,
on Tropical Storm Helene, located a few hundred miles north of the
central Azores, and on Tropical Storm Joyce, located about 600 miles
west-southwest of the Azores.

1. A broad area of low pressure, associated with the remnants of Isaac,
is located a couple of hundred miles southeast of Kingston, Jamaica.
This system is producing an area of disorganized showers and
thunderstorms over the north-central Caribbean Sea. Some slow
development is possible during the next day or so as it moves
west-northwestward toward the northwestern Caribbean Sea.
Afterward, environmental conditions are forecast to become less
conducive and re-development appears unlikely. Regardless of
development, locally heavy rains and gusty winds are forecast over
portions of Hispaniola, Jamaica, and eastern Cuba during the next
couple of days.
* Formation chance through 48 hours…low…20 percent.
* Formation chance through 5 days…low…20 percent.

Forecaster Brown

HELENE QUICKLY TRANSITIONING TO AN EXTRATROPICAL LOW

Tropical Storm Helene Discussion Number 36
NWS National Hurricane Center Miami FL AL082018
500 AM AST Sun Sep 16 2018

Helene’s cloud pattern is certainly looking less and less like a
tropical cyclone this morning. Based on conventional satellite
imagery and AMSU data, Helene is quickly undergoing an extratropical
transition, and this dynamic process is expected to be completed
later today, as the cyclone moves away from the Azores. The
initial intensity is lowered to 45 kt, and is based primarily on
2326 UTC ASCAT-A wind retrievals. Cold (22C) sea surface
temperatures and strong westerly shear will continue to disrupt
Helene’s vertical structure and should cause the cyclone to
complete the extratropical transition in 12 hours, or sooner.

The initial motion is estimated to be northeastward, or 50/22 kt,
within the ferocious mid-latitude westerlies. The global models are
in remarkably good agreement with Helene accelerating northeastward
and becoming absorbed in a large baroclinic zone extending over
northern United Kingdom in 72 hours, and the official forecast
reflects this scenario.

Earlier 2326 UTC METOP-A ASCAT scatterometer data indicated that the
34 kt and 50 kt wind radii, particularly in NW and NE quadrants, had
decreased considerably. Accordingly, an adjustment was made on this
advisory.

Interests in those locations should consult products from their
local meteorological service for information about potential impacts
from Helene. Local forecasts and warnings for the UK can be found on
the website of the UK Met Office athttps://www.metoffice.gov.uk/.
Local forecasts and warnings for Ireland can be found on the website
of Met Eireann at https://www.met.ie/.

FORECAST POSITIONS AND MAX WINDS

INIT 16/0900Z 42.5N 28.4W 45 KT 50 MPH
12H 16/1800Z 44.5N 24.7W 45 KT 50 MPH…POST-TROP/EXTRATROP
24H 17/0600Z 47.4N 18.1W 45 KT 50 MPH…POST-TROP/EXTRATROP
36H 17/1800Z 50.8N 10.8W 45 KT 50 MPH…POST-TROP/EXTRATROP
48H 18/0600Z 55.1N 3.7W 40 KT 45 MPH…POST-TROP/EXTRATROP
72H 19/0600Z…ABSORBED BY A FRONT

$$
Forecaster Roberts

FLORENCE WEAKENS TO A DEPRESSION BUT FLASH FLOODING AND MAJOR RIVER FLOODING WILL CONTINUE OVER A SIGNIFICANT PORTION OF THE CAROLINAS

Tropical Depression Florence Discussion Number 68
NWS National Hurricane Center Miami FL AL062018
500 AM EDT Sun Sep 16 2018

Surface observations indicate that there are no longer any
sustained tropical-storm-force winds as the center of Florence has
moved farther inland over South Carolina. Therefore, the system is
being downgraded to a tropical depression at this time. Maximum
winds are estimated to be 30 kt. Continued gradual weakening is
likely, and the numerical guidance suggests that the cyclone
will be disorganized enough to become a remnant low in 36 hours or
so. In 72 hours, global models indicate that the system will
become an extratropical cyclone, with some strengthening due to
baroclinic processes as it moves over the Atlantic in 3-5 days.
This scenario is very similar to that from the previous advisory.

The forward speed of Florence has increased somewhat early this
morning and the motion is now near 280/7 kt. The high pressure
system that has been blocking the forward progress of Florence is
predicted to slide eastward and southeastward during the next day
or so. As a result, over the next couple of days, Florence is
expected to move northwestward, northward, and then
north-northeastward around the periphery of the high. Later in the
forecast period, Florence should accelerate east-northeastward in
the mid-latitude westerlies. The official track forecast is similar
to the previous one and close to the dynamical model consensus.

This will be the last advisory issued by the National Hurricane
Center on Florence. Future information on Florence can be found in
Public Advisories issued by the Weather Prediction Center beginning
at 11 AM EDT, under AWIPS header TCPAT1, WMO header WTNT31 KWNH,
and on the web at https://www.wpc.ncep.noaa.gov.

Key Messages:

1. Life-threatening, catastrophic flash floods and prolonged
significant river flooding are likely over portions of the Carolinas
and the southern to central Appalachians from western North Carolina
into west-central Virginia and far eastern West Virginia through
early this week, as Florence continues to move slowly inland. In
addition to the flash flood and flooding threat, landslides are also
possible in the higher terrain of the southern and central
Appalachians across western North Carolina into southwest Virginia.

2. Large swells affecting Bermuda, portions of the U.S. East Coast,
and the northwestern and central Bahamas will continue this week,
resulting in life-threatening surf and rip currents.

FORECAST POSITIONS AND MAX WINDS

INIT 16/0900Z 33.8N 81.4W 30 KT 35 MPH…INLAND
12H 16/1800Z 34.7N 82.5W 30 KT 35 MPH…INLAND
24H 17/0600Z 36.7N 83.6W 25 KT 30 MPH…INLAND
36H 17/1800Z 38.7N 82.6W 20 KT 25 MPH…POST-TROP/REMNT LOW
48H 18/0600Z 39.8N 79.5W 20 KT 25 MPH…POST-TROP/REMNT LOW
72H 19/0600Z 42.0N 68.0W 30 KT 35 MPH…POST-TROP/EXTRATROP
96H 20/0600Z 43.5N 55.0W 35 KT 40 MPH…POST-TROP/EXTRATROP
120H 21/0600Z 46.0N 40.0W 40 KT 45 MPH…POST-TROP/EXTRATROP

$$
Forecaster Pasch