Jul 27 Thursday 2017

Tropical Report

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Thu Jul 27 2017

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

1. Disorganized cloudiness and showers extending from the Cabo
Verde Islands southwestward across the Atlantic are primarily
associated with a tropical wave. Slow development is possible over
the next several days while the system moves westward or
west-northwestward at 5 to 10 mph.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…low…30 percent.

Forecaster Avila

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
500 AM PDT Thu Jul 27 2017

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

The National Hurricane Center is issuing advisories on Hurricane
Hilary, located several hundred miles southwest of the southern tip
of the Baja California peninsula, and on Tropical Storm Irwin,
located about 1000 miles west-southwest of the southern tip
of the Baja California peninsula.

1. A tropical wave, located several hundred miles south of Manzanillo,
Mexico, is producing limited shower and thunderstorm activity.
Significant development is not anticipated during the next few days
due to strong upper-level winds produced by Hurricane Hilary.
Although upper-level winds could become more conducive this weekend
as Hilary weakens and moves farther away, overall environmental
conditions are expected to remain only marginally conducive for
development. The system is expected to move west-northwestward at 5
to 10 mph during the next several days.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…low…20 percent.

Forecaster Zelinsky

Hurricane Hilary Discussion Number 24
NWS National Hurricane Center Miami FL EP092017
200 AM PDT Thu Jul 27 2017

Hilary has lost organization due to the effects of moderate
northerly vertical shear, with recent Windsat and GPM overpasses
indicating that the center is now near the northwestern edge of the
main convective area. Subjective satellite intensity estimates are
75-90 kt, while estimates based on microwave sounding data and
the CIMSS ADT are 55-65 kt. Based on a blend of these, the
initial intensity is reduced to a possibly generous 75 kt.

The large-scale models forecast Hilary to move under a cyclonic
shear axis between two upper-level high pressure areas during the
next 24-48 h. This evolution may allow the current shear to
decrease, but it is not an ideal environment for a tropical
cyclone. Thus, the intensity forecast calls for little change in
strength during this time, which is in good agreement with the
intensity guidance. After 48 h, Hilary should move over cooler sea
surface temperatures, and it is likely to significantly interact
with Tropical Storm Irwin. The large-scale models differ on the
details, but they agree that the combination of the interaction, dry
air, and the presence of cool water should cause Hilary to quickly
weaken as a result. Thus, the new intensity forecast now calls for
the system to degenerate to a remnant low by the end of the forecast
period.

The initial motion is 290/10. For the first 48 h or so, Hilary
should be steered generally west-northwestward by the subtropical
ridge. After that, the track forecast becomes very uncertain due
to the interaction with Irwin, where the latter cyclone is likely
to move around the eastern and northern sides of Hilary before a
possible merger. The Canadian model merges the two cyclones in
about 72 h, while the GFS finishes the merger at about 120 h. On
the other hand, the ECMWF delays the merger until after the forecast
period, while the UKMET keeps the two systems separate. The new
track forecast will not attempt to show the details of the
interaction, but it will show more of a westward turn and changes in
speed during the interaction than the previous forecast. Overall,
the new track is close to the consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 17.3N 115.1W 75 KT 85 MPH
12H 27/1800Z 17.7N 116.5W 70 KT 80 MPH
24H 28/0600Z 18.4N 118.2W 70 KT 80 MPH
36H 28/1800Z 19.1N 119.9W 70 KT 80 MPH
48H 29/0600Z 19.8N 121.7W 70 KT 80 MPH
72H 30/0600Z 21.0N 126.0W 60 KT 70 MPH
96H 31/0600Z 21.5N 130.5W 45 KT 50 MPH
120H 01/0600Z 22.0N 133.0W 30 KT 35 MPH…POST-TROP/REMNT LOW

$$
Forecaster Beven

Tropical Storm Irwin Discussion Number 20
NWS National Hurricane Center Miami FL EP102017
200 AM PDT Thu Jul 27 2017

Overall, Irwin’s cloud pattern has change little during the past
several hours. An earlier 0218 UTC SSMIS microwave pass shows a
considerable structural vertical tilt toward the northwest due to
the impinging southeasterly shear produced by Hilary to the east.
Experimental GOES-16 channel 9 mid-level water vapor imagery
indicates drier air intruding in the northeast quadrant of Irwin
with subsequent convective erosion in that portion of the cloud
canopy. The Dvorak satellite intensity estimates from SAB and TAFB
support maintaining the initial intensity at 50 kt for this
advisory.

The dynamical intensity models indicate that the moderate
southeasterly shear will back more toward the northeast during the
next 12-24 hours as Hilary invades from the northeast. Beyond the
24 hour period, the intensity forecast becomes a bit problematic.
The hurricane models, the HFIP Corrected Consensus, the Florida
State Superensemble, and the GFS/ECMWF global models all show
Irwin strengthening back into a hurricane under much improved
diffluent flow aloft. The statistical-dynamical intensity models,
on the other hand, show only slight strengthening. As a compromise
for this forecast package, the official forecast shows modest
strengthening, then gradual weakening after 48 hours.

The initial motion is estimated to be west-southwestward, or 255/05
kt. As Hilary continues to moves closer and passes to the
north of Irwin, a significant reduction in Irwin’s forward speed,
essentially drifting in a cyclonic fashion, is expected during the
next 24-36 hours. After that time, the cyclone is forecast to
accelerate northward and then northwestward up the eastern and
northern portion of Hilary’s peripheral circulation. The
deterministic guidance remains in fair agreement with this synoptic
pattern scenario, and the NHC forecast is similar to the previous
one and is based on a blend of the TVCX and HCCA consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 14.7N 123.8W 50 KT 60 MPH
12H 27/1800Z 14.6N 124.4W 50 KT 60 MPH
24H 28/0600Z 14.5N 124.8W 55 KT 65 MPH
36H 28/1800Z 14.3N 124.7W 55 KT 65 MPH
48H 29/0600Z 14.7N 124.3W 55 KT 65 MPH
72H 30/0600Z 17.8N 124.4W 50 KT 60 MPH
96H 31/0600Z 22.0N 128.0W 40 KT 45 MPH
120H 01/0600Z 24.5N 133.5W 35 KT 40 MPH

$$
Forecaster Roberts