Jul 21 Friday 2017

Tropical Report

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Fri Jul 21 2017

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

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

$$
Forecaster Avila

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
500 AM PDT Fri Jul 21 2017

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

The National Hurricane Center is issuing advisories on Tropical
Storm Greg, located several hundred miles southwest of the southern
tip of the Baja California peninsula.

1. A broad area of low pressure centered several hundred miles south
of the southern tip of the Baja California peninsula is producing
disorganized showers and thunderstorms. Environmental conditions
are forecast to be conducive for the formation of a tropical
depression during the next couple of days before upper-level winds
become unfavorable for development. This system is forecast to move
westward to west-northwestward at around 10 mph through early next
week.
* Formation chance through 48 hours…high…70 percent.
* Formation chance through 5 days…high…80 percent.

2. Showers and thunderstorms have become better organized in
association with a tropical wave over the far eastern Pacific
southwest of Central America. Environmental conditions are
forecast to become increasingly favorable for development, and a
tropical depression is likely to form later today or tomorrow while
the system moves west-northwestward at 10 to 15 mph.
* Formation chance through 48 hours…high…70 percent.
* Formation chance through 5 days…high…90 percent.

Forecaster Blake

Tropical Storm Greg Discussion Number 16
NWS National Hurricane Center Miami FL EP072017
200 AM PDT Fri Jul 21 2017

A band of deep convection remains wrapped around the eastern half
of the circulation, but little convection is present on the western
side of the cyclone. Since the convective structure is more or
less the same as before, Dvorak estimates from TAFB and SAB are
unchanged, and the initial intensity is held at 45 kt.

Both the track and intensity forecasts are challenging. The
intensity guidance spread is high, ranging from the COAMPS-TC
model, which makes Greg a major hurricane in 3 days, to the LGEM,
which peaks at 49 kt. Based on the SHIPS diagnostics from both the
GFS and ECMWF forecast fields, the warm SST, low shear environment
would seem to support strengthening in the short term. On the
forecast track, Greg should encounter a drier and more stable
environment that should bring a halt to any intensification after
about 72 hours. The NHC forecast is close to the HWRF model for the
first 48 hours, and still shows Greg reaching hurricane strength.
After that, the HWRF and COAMPS-TC models show continued
intensification, but that seems unlikely in what is expected to be
an unfavorable environment. The NHC forecast instead follows the
trend of the statistical models late in the period and shows steady
weakening.

The center has been difficult to locate tonight, and the initial
motion is an uncertain 280/9 kt. For the first 48 hours or so, Greg
should continue on a west or west-northwest heading, below a low to
mid-level ridge. Beyond that, there is tremendous spread in the
model guidance. The GFS and UKMET positions vary by over 450 n mi
at 120 h, with the ECMWF lying in between. The GFS and the
GFS-based regional models all depict a relatively stronger Greg
making a right turn around 48 h and heading into the mid-level
ridge. On the other hand, the ECMWF and UKMET continue a weaker
Greg westward for another couple of days. Given that only modest
strengthening is forecast, the latter scenario seems somewhat more
likely at this point. The official track forecast is very close to
the ECMWF at days 4 and 5. Given the high spread in the guidance,
confidence in the track forecast beyond 48 h is low.

FORECAST POSITIONS AND MAX WINDS

INIT 21/0900Z 14.5N 118.3W 45 KT 50 MPH
12H 21/1800Z 14.8N 119.7W 50 KT 60 MPH
24H 22/0600Z 15.3N 121.8W 55 KT 65 MPH
36H 22/1800Z 15.6N 124.0W 60 KT 70 MPH
48H 23/0600Z 15.7N 126.1W 65 KT 75 MPH
72H 24/0600Z 16.0N 130.0W 65 KT 75 MPH