Jul 28 Friday 2017

Tropical Report

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Fri Jul 28 2017

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

1. Cloudiness and showers in the central tropical Atlantic have
decreased since yesterday. Environmental conditions have
become less favorable for tropical cyclone formation while
the system moves slowly westward for the next several days.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…low…20 percent.

Forecaster Avila

Tropical Storm Hilary Discussion Number 28
NWS National Hurricane Center Miami FL EP092017
200 AM PDT Fri Jul 28 2017

The structure of Hilary has changed little over the past several
hours, as a small area of convection persists near the center
mainly in the southeastern quadrant. Recent satellite intensity
estimates range from 45-70 kt, and based on these, the initial
intensity remains a possibly generous 60 kt.

Hilary should cross the 26C isotherm in 12-18 h, and given the
current level of convective organization, significant re-
intensification seems unlikely during that time. After that, the
cyclone is forecast to move over steadily decreasing sea surface
temperatures and into a drier air mass, and this combination should
result in steady weakening. The cyclone is now forecast to weaken
to a depression in about 72 h over 24C sea surface temperatures and
degenerate to a remnant low shortly thereafter. The new intensity
forecast shows a faster weakening than the previous forecast after
12 h, and it is in best overall agreement with the SHIPS and LGEM
models. It is possible that interaction with Tropical Storm Irwin
could lead to a faster decay than currently forecast.

The initial motion is 300/7, a little to the right of that in the
previous advisory. A mid-level ridge to the north of Hilary is
expected to steer the cyclone generally west-northwestward for 48 h
or so. Around 72 h, a westward turn appears likely as the cyclone
interacts with, and eventually absorbs Irwin. After this event,
Hilary or its remnants should resume a west-northwestward motion as
the weakening system is steered by the low-level flow. The new
forecast track is nudged north of the previous track, but it lies
south of the consensus models, which have shifted northward since
the last advisory.

FORECAST POSITIONS AND MAX WINDS

INIT 28/0900Z 18.7N 117.9W 60 KT 70 MPH
12H 28/1800Z 19.3N 119.0W 60 KT 70 MPH
24H 29/0600Z 20.3N 120.6W 55 KT 65 MPH
36H 29/1800Z 21.1N 122.6W 45 KT 50 MPH
48H 30/0600Z 21.9N 124.9W 40 KT 45 MPH
72H 31/0600Z 22.5N 129.0W 30 KT 35 MPH
96H 01/0600Z 23.5N 132.5W 25 KT 30 MPH…POST-TROP/REMNT LOW
120H 02/0600Z 25.0N 135.0W 20 KT 25 MPH…POST-TROP/REMNT LOW

$$
Forecaster Beven

Tropical Storm Irwin Discussion Number 24
NWS National Hurricane Center Miami FL EP102017
200 AM PDT Fri Jul 28 2017

Although conventional satellite imagery shows recent improvement in
the banding feature wrapping around the eastern half of the
cyclone, an ambiguity solution of a 0514 UTC ASCAT-B overpass
indicates that Irwin has weakened. Based on the scatterometer data
and a blend of the subjective Dvorak satellite estimates from TAFB
and SAB, the initial intensity is lowered to 45 kt for this
advisory. The CIMSS wind shear analysis and the SHIPS model both
indicate that moderate northeasterly shear, produced by an
anticyclone to the north, will be the primary inhibitor for any
significant strengthening prior to Irwin’s demise in 4 days.
Little change in the cyclone’s intensity is forecast before the
merging of Irwin with Tropical Storm Hilary occurs around the 96
hour period.

The initial motion estimate continues as a drift toward the
west, or 270/02 kt. The large-scale models indicate this slow
motion, or meander, continuing through the 36 hour period.
Afterward, Irwin is expected to accelerate northward around
the eastern peripheral flow of Hilary. The global and
regional models also generally agree with the merging of the two
tropical cyclones to occur in 4 days, or less. The official
forecast has been adjusted to the right of the previous track
forecast beyond 36 hours to align more closely to the TVCX and HCCA
consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT 28/0900Z 14.9N 124.8W 45 KT 50 MPH
12H 28/1800Z 14.9N 124.9W 50 KT 60 MPH
24H 29/0600Z 15.0N 124.8W 50 KT 60 MPH
36H 29/1800Z 15.7N 124.5W 50 KT 60 MPH
48H 30/0600Z 17.4N 124.5W 45 KT 50 MPH
72H 31/0600Z 22.0N 126.3W 45 KT 50 MPH
96H 01/0600Z…DISSIPATED

$$
Forecaster Roberts