Jul 18 Tuesday 2017

Tropical Report

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Tue Jul 18 2017

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

The National Hurricane Center is issuing advisories on Tropical
Storm Don, located east-southeast of Barbados.

1. Disorganized showers and thunderstorms are associated with a low
pressure system located several hundred miles west-southwest of the
Cabo Verde Islands. Some gradual development of this system is
possible through mid-week while it moves toward the west-northwest
or northwest at 10 to 15 mph. After that time, environmental
conditions are forecast to become less conducive for development.
* Formation chance through 48 hours…low…30 percent.
* Formation chance through 5 days…medium…40 percent.

Public Advisories on Tropical Storm Don are issued under WMO header
WTNT35 KNHC and under AWIPS header MIATCPAT5.
Forecast/Advisories on Tropical Storm Don are issued under WMO
header WTNT25 KNHC and under AWIPS header MIATCMAT5.

Forecaster Landsea

Tropical Storm Don Discussion Number 3
NWS National Hurricane Center Miami FL AL052017
500 AM AST Tue Jul 18 2017

Don’s convective pattern has continued to improve overnight and
early this morning, including the development of a pronounced
central dense overcast (CDO) feature. There have also been periodic
bursts of deep convection very near the center, accompanied by
significant clusters of lightning activity, which is indicative of
strong updrafts in or near the radius of maximum winds. The last
recon data around 0000Z indicated peak SFMR surface winds of 41 kt.
Given the marked increase in convective organization since that
time, the initial intensity has been increased to 45 kt.

The initial motion estimate remains 280/16 kt. There is no change
to the previous track forecast or reasoning. Although the models
continue to differ some on Don’s forward speed, there is very little
cross-track difference. The models are in good agreement that the
broad Bermuda-Azores ridge to north of the cyclone will remain
strong and move little for the next several days, which should act
to keep Don moving briskly westward until dissipation occurs at
around 72 hours. The new NHC forecast track is essentially just an
extension of the previous advisory track, and is slightly north of
the multi-model consensus TVCN.

Don is expected to remain embedded within a narrow east-west zone of
low vertical wind shear for another 24-36 hours, which should allow
for some additional strengthening. Since Don is a compact tropical
cyclone, significant changes in intensity — both up and down —
can occur due to small fluctuations in wind shear and/or interaction
with the mountainous Windward Islands. By 36 hours, increasing
westerly shear is expected to cause Don to weaken while it moves
across the southeastern Caribbean Sea, and degeneration into an open
wave is forecast to occur by 72 hours. The NHC intensity forecast
is similar to the previous advisory and remains below the consensus
models ICON and IVCN. However, the intensity guidance continues to
vary widely between only showing an open wave (most of the global
models) to Don achieving hurricane strength (HWRF, ECMWF, and some
of the statistical models). As a result, confidence in the intensity
forecast remains low.

FORECAST POSITIONS AND MAX WINDS

INIT 18/0900Z 11.5N 56.2W 45 KT 50 MPH
12H 18/1800Z 11.8N 59.1W 50 KT 60 MPH
24H 19/0600Z 12.1N 63.2W 50 KT 60 MPH
36H 19/1800Z 12.4N 67.4W 50 KT 60 MPH
48H 20/0600Z 12.7N 71.6W 40 KT 45 MPH
72H 21/0600Z…DISSIPATED

$$
Forecaster Stewart

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
500 AM PDT Tue Jul 18 2017

For the eastern North Pacific…east of 140 degrees west longitude:

The National Hurricane Center is issuing advisories on Hurricane
Fernanda located well to the east of Hilo, Hawaii, and on newly
upgraded Tropical Storm Greg located a few hundred miles southwest
of Manzanillo, Mexico.

1. Satellite images indicate that organized showers and thunderstorms
continue in association with a low pressure system located about 800
miles southwest of the southern tip of Baja California. Although
upper-level winds are currently only marginally conducive for
development, only a small increase in the organization of this
system could lead to the formation of a tropical depression at any
time. This disturbance is expected to move slowly westward or
west-southwestward during the next few days.
* Formation chance through 48 hours…high…80 percent.
* Formation chance through 5 days…high…80 percent.

2. Another area of low pressure is expected to form in the eastern
Pacific Ocean south of Mexico late this week. Environmental
conditions appear conducive for some development of the system this
the weekend while it moves westward to west-northwestward at 10 to
15 mph.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…medium…40 percent.

Public Advisories on Tropical Storm Greg are issued under
WMO header WTPZ32 KNHC and under AWIPS header MIATCPEP2.
Forecast/Advisories on Tropical Storm Greg are issued under
WMO header WTPZ22 KNHC and under AWIPS header MIATCMEP2.

Forecaster Landsea

Tropical Storm Greg Discussion Number 4
NWS National Hurricane Center Miami FL EP072017
300 AM MDT Tue Jul 18 2017

Deep convection has increased in both intensity and coverage during
the last several hours over and to the southeast of the center of
the cyclone. The Dvorak classifications from TAFB and SAB at 0600
UTC were 2.0/30 kt, but ADT values and satellite consensus estimates
from CIMSS at the University of Wisconsin suggest a slightly higher
intensity. Based on the improvement in convective organization
during the past few hours, the initial intensity is increased to 35
kt, making the system Tropical Storm Greg.

The northwesterly shear currently affecting Greg is expected to
gradually lessen, and the upper-level wind pattern will likely
become favorable in the 24- to 60-h time frame. Therefore, the NHC
intensity forecast continues to show modest strengthening during the
next couple of days. After that time, however, the shear is
expected to increase out of the south or southwest as the cyclone
approaches an upper-level low. This increase in shear combined with
drier air and cooler waters should end the strengthening trend and
cause weakening. The depression will likely degenerate to a remnant
low in 4 to 5 days when it moves over SSTs below 26 deg C. The NHC
intensity forecast is the same as the previous one, and is in line
with the bulk of the guidance.

Greg is moving west-northwestward at 8 kt to the south of a low- to
mid-level ridge. A westward to west-northwestward motion with some
increase in forward speed is expected during the next few days as
the ridge strengthens a little. Some of the models show Greg
interacting with the disturbance currently located about 700 n mi
to its west in 4 to 5 days, which could cause the cyclone to turn
south of due west at that time. The track models are in general
agreement, and the NHC forecast lies closest to the consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT 18/0900Z 14.6N 107.6W 35 KT 40 MPH
12H 18/1800Z 14.7N 108.7W 35 KT 40 MPH
24H 19/0600Z 15.0N 110.6W 40 KT 45 MPH
36H 19/1800Z 15.4N 112.6W 40 KT 45 MPH
48H 20/0600Z 15.8N 114.7W 40 KT 45 MPH
72H 21/0600Z 16.9N 119.7W 35 KT 40 MPH
96H 22/0600Z 17.1N 124.6W 30 KT 35 MPH
120H 23/0600Z 16.5N 129.5W 25 KT 30 MPH…POST-TROP/REMNT LOW

$$
Forecaster Cangialosi

Hurricane Fernanda Discussion Number 26
NWS National Hurricane Center Miami FL EP062017
200 AM PDT Tue Jul 18 2017

Fernanda is gradually weakening. The eye of the hurricane is no
longer apparent in satellite images, and dry slots are evident in
microwave images over the eastern portion of the circulation. The
cloud pattern now consists of a fairly circular central dense
overcast with curved bands surrounding the convective mass. The
Dvorak CI-numbers are 5.0/90 kt from TAFB and SAB, and the ADT
values from UW-CIMSS are similar. Based on these estimates, the
initial intensity is lowered a little to 90 kt.

Fernanda is still over relatively warm sea-surface temperatures, but
it is headed for cooler waters, and by 24 hours it should be
crossing the 26 deg C isotherm. These cooler waters combined with
drier air and a significant increase in southwesterly wind shear
that begins in a few days should cause the cyclone to steadily
weaken. Fernanda is predicted to be a remnant low by the end of the
forecast period when the SHIPS model shows the shear increasing to
near 30 kt and the environmental mid-level relative humidity values
below 40 percent. The NHC intensity forecast is similar to the
previous one and generally follows the intensity model consensus.

The track forecast philosophy for Fernanda remains unchanged. The
hurricane continues to move northwestward at 8 kt toward a weakness
in the ridge caused by a mid- to upper-level trough to the northeast
of the Hawaiian Islands. This general motion is expected to
continue for the next 24 hours or so. After that time, the trough
is forecast to lift out, allowing the ridge to rebuild to the north
of the weakening system. This pattern change should cause Fernanda
to turn westward to west-northwestward. Only small changes were
made to the previous NHC track forecast, and the new one lies near
the middle of the guidance envelope.

FORECAST POSITIONS AND MAX WINDS

INIT 18/0900Z 15.4N 132.8W 90 KT 105 MPH
12H 18/1800Z 16.1N 133.7W 80 KT 90 MPH
24H 19/0600Z 16.9N 134.8W 70 KT 80 MPH
36H 19/1800Z 17.4N 136.1W 60 KT 70 MPH
48H 20/0600Z 17.8N 137.6W 55 KT 65 MPH
72H 21/0600Z 18.6N 140.6W 45 KT 50 MPH
96H 22/0600Z 19.3N 143.7W 35 KT 40 MPH
120H 23/0600Z 19.9N 147.6W 25 KT 30 MPH…POST-TROP/REMNT LOW

$$
Forecaster Cangialosi

 

Weather In Cayman

Synopsis

A few isolated showers and thunder are expected across the Cayman area for the next 24 hours as an upper level low lingers over the northwest Caribbean. Radar images show isolated showers around the Cayman area moving towards the west.