Jul 20 Thursday 2017

Tropical Report

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Thu Jul 20 2017

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

Tropical cyclone formation is not expected during the next 5 days.

$$
Forecaster Beven

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
500 AM PDT Thu Jul 20 2017

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

The National Hurricane Center is issuing advisories on Tropical
Storm Fernanda, located more than a thousand miles east of Hilo,
Hawaii, on Tropical Storm Greg, located several hundred miles south-
southwest of the southern tip of the Baja California peninsula, and
on Tropical Depression Eight-E, located more than a thousand miles
southwest of the southern tip of the Baja California peninsula.

1. Showers and thunderstorms centered about 600 miles south of
Manzanillo, Mexico, are showing some signs of organization in
association with a broad low pressure system. Environmental
conditions are expected to be conducive for development of this
system while it moves westward to west-northwestward at 10 to 15
mph, and a tropical depression is expected to form well southwest of
Mexico over the weekend.
* Formation chance through 48 hours…high…70 percent.
* Formation chance through 5 days…high…90 percent.

2. Disorganized cloudiness and showers in the far eastern Pacific
near and west of Central America are associated with a tropical
wave. Environmental conditions are forecast to become increasingly
favorable for development, and a tropical depression could form by
early next week while the system moves west-northwestward at 10 to
15 mph.
* Formation chance through 48 hours…low…10 percent.
* Formation chance through 5 days…medium…60 percent.

Forecaster Blake

Tropical Storm Greg Discussion Number 12
NWS National Hurricane Center Miami FL EP072017
300 AM MDT Thu Jul 20 2017

An ASCAT pass at 0458 UTC revealed that Greg is a little weaker than
previously estimated. The maximum winds were 31 kt, limited to a
small area 50 n mi due north of the center of circulation. Since the
time of the ASCAT pass, the cloud pattern of Greg has improved
considerably, and a band of deep convection now wraps nearly all the
way around the circulation. The initial intensity is set at 35 kt,
assuming at least a little undersampling in the ASCAT pass, and
allowing for some increase in the winds due to the increase in
convection.

It is not yet clear if the recent increase in the organization of
Greg is temporary or the beginning of a longer-term trend. It is
worth noting that around 0000 UTC yesterday, the tropical storm
featured a similar band that weakened after about 6 hours. There is
a fair amount of spread in the intensity guidance. Notably, the
LGEM model shows a peak intensity of only 43 kt while the HWRF peaks
at 77 kt. Given that the environment appears at least somewhat
conducive for strengthening, the official forecast still shows Greg
reaching hurricane strength in a few days and is very close to the
intensity consensus IVCN.

The initial motion is again 270/9 kt. No significant changes were
made to the first 72 hours of the track forecast, and Greg is still
expected to be steered generally westward by a strong ridge to the
north. At days 4 and 5 there is more spread in the guidance, with
some models turning the cyclone toward the northwest and others
keeping it on a westward heading. For now, the forecast splits the
difference between the GFS and ECMWF models, both of which show a
slowdown and slight turn toward toward the northwest at the end of
the forecast period.

FORECAST POSITIONS AND MAX WINDS

INIT 20/0900Z 14.1N 114.7W 35 KT 40 MPH
12H 20/1800Z 14.3N 116.1W 40 KT 45 MPH
24H 21/0600Z 14.7N 118.1W 50 KT 60 MPH
36H 21/1800Z 15.1N 120.3W 55 KT 65 MPH
48H 22/0600Z 15.4N 122.4W 60 KT 70 MPH
72H 23/0600Z 15.6N 127.1W 65 KT 75 MPH
96H 24/0600Z 15.8N 131.5W 55 KT 65 MPH
120H 25/0600Z 16.5N 135.0W 45 KT 50 MPH

$$
Forecaster Zelinsky
Tropical Depression Eight-E Discussion Number 8
NWS National Hurricane Center Miami FL EP082017
200 AM PDT Thu Jul 20 2017

It has been difficult to locate the center this evening, and the
circulation of the depression appears to be limited to a shallow
layer near the surface. The cloud pattern is unchanged and deep
convection remains limited to a single band in the southwestern
quadrant. Satellite intensity estimates remain largely unchanged
since the previous advisory, and the initial intensity is held at
25 kt.

There is no change to the intensity forecast thinking. A combination
of shear from an upper-level low to the north and interaction with
Tropical Storm Greg to the east should prevent the depression from
gaining any organization, causing the system to gradually weaken.
Most of the dynamical guidance suggests that the cyclone will no
longer have a well-defined center within the next 48 hours or so,
and only a slight decrease in convective organization would cause
the system to become a remnant low. A WindSat overpass around
0200 UTC suggested that the circulation was already becoming
elongated, so dissipation could occur sooner than indicated.

The initial motion remains 245/5 kt. The depression is still
expected to be advected generally southwestward, around the
circulation of Tropical Storm Greg. The track models have all
shifted slightly westward, so the NHC track forecast has been
adjusted a little in that direction, but otherwise no significant
changes have been made.

FORECAST POSITIONS AND MAX WINDS

INIT 20/0900Z 13.7N 122.6W 25 KT 30 MPH
12H 20/1800Z 13.2N 123.2W 25 KT 30 MPH
24H 21/0600Z 12.5N 123.9W 25 KT 30 MPH
36H 21/1800Z 12.0N 124.3W 25 KT 30 MPH…POST-TROP/REMNT LOW
48H 22/0600Z 11.6N 124.6W 20 KT 25 MPH…POST-TROP/REMNT LOW
72H 23/0600Z…DISSIPATED

$$
Forecaster Zelinsky