Tropical Report

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Fri Sep 29 2017

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

The National Hurricane Center is issuing advisories on Tropical
Storm Maria, located over the western Atlantic Ocean a few hundred
miles south of Halifax, Nova Scotia, and on Hurricane Lee, located
over the north Atlantic Ocean several hundred miles south-southeast
of Newfoundland.

1. A broad trough of low pressure interacting with an upper-level
low is producing a large but disorganized area of cloudiness and
showers. This activity extends from the northwestern Caribbean
Sea northward across western Cuba to the southern Florida peninsula.
This trough has moved slowly westward since yesterday, and a weak
low is now likely to form from this system near the west coast of
Florida during the weekend. Environmental conditions appear to be
conducive for some development of the low before the upper-level
winds become less favorable early next week. Regardless of
development, this system is likely to produce locally heavy rainfall
over portions of central and western Cuba, the Florida Keys, the
Florida peninsula during the next several days.
* Formation chance through 48 hours…medium…40 percent.
* Formation chance through 5 days…medium…40 percent.

2. A tropical wave is producing cloudiness and showers over the
northeastern Caribbean Sea and the adjacent Atlantic waters.
Although there are no signs of organization and surface pressures
are not falling at this time, conditions could become a little more
favorable for some development next week while the system moves
toward the west-northwest. This system is expected to bring
locally heavy rains over the northern Leeward Islands, including
Puerto Rico and the U.S. Virgin Islands.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…low…20 percent.

Forecaster Avila

LEE A LITTLE WEAKER WHILE MOVING FASTER TOWARD THE NORTHEAST… …EXPECTED TO DISSIPATE OVER THE NORTH ATLANTIC ON SATURDAY

Hurricane Lee Discussion Number 45
NWS National Hurricane Center Miami FL AL142017
500 AM AST Fri Sep 29 2017

Lee’s low-level center is partially exposed along the northern edge
of the convective canopy due to almost 40 kt of northerly shear.
ASCAT data from last evening showed winds as high as 70 kt, so
assuming some weakening due to the belligerent shear, the initial
intensity is lowered to 65 kt. Lee will be crossing the tight SST
gradient to the north of the Gulf Stream within the next 6-12
hours, and along with continued strong shear, this should cause the
cyclone to weaken further and become a tropical storm later today.
Global model guidance then indicates that Lee will dissipate in the
fast flow ahead of an approaching cold front by 36 hours. Even
after Lee’s circulation opens up, a swath of strong winds will
likely continue eastward toward Ireland and the United Kingdom by
days 2 and 3.

Lee is accelerating northeastward with an initial motion of 040/22
kt. The cyclone should maintain that heading with its speed
increasing further up until the time it dissipates. The track
guidance is tightly clustered for the next 24 hours before
dissipation, and the NHC forecast is mainly an update of the
previous one.

FORECAST POSITIONS AND MAX WINDS

INIT 29/0900Z 38.3N 52.4W 65 KT 75 MPH
12H 29/1800Z 41.1N 48.0W 60 KT 70 MPH
24H 30/0600Z 45.4N 39.3W 50 KT 60 MPH
36H 30/1800Z…DISSIPATED

$$
Forecaster Berg

MARIA SLIGHTLY WEAKER AND MOVING FASTER TOWARD THE EAST

Tropical Storm Maria Discussion Number 53
NWS National Hurricane Center Miami FL AL152017
500 AM AST Fri Sep 29 2017

Maria is producing a relatively small cluster of deep convection
that is displaced to the east of the center due to 15 kt of
westerly shear. Scatterometer data from last evening showed some
45-kt wind vectors south of the center, so Maria’s initial
intensity is estimated to be 50 kt, accounting for the low
resolution of the ASCAT instruments. Maria will continue to move
over waters of at least 26 degrees Celsius for at least the next
24-36 hours, although vertical shear will be increasing over the
cyclone fairly soon. The first stages of extratropical transition
are likely to begin later today, and baroclinic forcing could allow
Maria to regain a little bit of strength, which is shown by both the
GFS and ECMWF models over the next couple of days. Maria should be
fully extratropical by 48 hours and then be absorbed within a
frontal zone a little after 72 hours. Some of the global models,
such as the GFS and UKMET, show this absorption occurring sooner
than that.

Maria is accelerating eastward within the mid-latitude westerlies
with an initial motion of 085/18 kt. The cyclone should turn
east-northeastward later today, ahead of a mid-latitude trough
moving off the northeastern U.S. and Atlantic Canada coast, with
acceleration continuing up until the time Maria dissipates.
Although the ECMWF model is an outlier and has a slower solution
compared to the other guidance, the NHC track forecast remains
close to the other models and the various consensus aids.

The post-tropical intensity and wind radii forecasts at 48 and 72 h
are based on guidance provided by the NOAA Ocean Prediction Center.

FORECAST POSITIONS AND MAX WINDS

INIT 29/0900Z 37.2N 63.3W 50 KT 60 MPH
12H 29/1800Z 37.9N 59.0W 55 KT 65 MPH
24H 30/0600Z 39.8N 52.3W 55 KT 65 MPH
36H 30/1800Z 42.1N 45.0W 55 KT 65 MPH
48H 01/0600Z 45.0N 36.8W 50 KT 60 MPH…POST-TROP/EXTRATROP
72H 02/0600Z 51.4N 16.0W 40 KT 45 MPH…POST-TROP/EXTRATROP
96H 03/0600Z…ABSORBED WITHIN A FRONTAL ZONE

$$
Forecaster Berg