Jul 24 Monday 2017

Tropical Report

Tropical Weather Outlook
Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Mon Jul 24 2017

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

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

$$
Forecaster Blake

Hurricane Hilary Discussion Number 12
NWS National Hurricane Center Miami FL EP092017
400 AM CDT Mon Jul 24 2017

Hilary’s overall cloud pattern continues to become better organized,
although inner-core convection is still fluctuating somewhat.
There is well-defined upper-level outflow, particularly over the
northwest and northeast quadrants of the circulation. Dvorak
T-numbers from both TAFB and SAB are now 4.0, which supports
upgrading the system to the fourth hurricane of the 2017 eastern
North Pacific season. Hilary is expected to continue traversing
a warm ocean with weak vertical shear for the next couple of days,
which favors continued strengthening. Rapid intensification is a
distinct possibility, since the SHIPS Rapid Intensification Index
(RII) shows a 60 percent chance of a 30 kt increase in intensity
during the next 24 hours. The NHC forecast is in line with the
RII guidance, and is close to the HFIP Corrected Consensus
intensity forecast through 72 hours, and a little below it
thereafter when SSTs beneath the cyclone are expected to cool below
26 deg C.

A west-northwestward motion continues, near 300/7 kt. The flow on
the southern periphery of a large 500 mb ridge that extends westward
from the southwestern United States should be the main steering
mechanism for the next several days. A slight strengthening of the
flow should lead to some increase in the forward speed of Hilary
over the next few days. There is still the possibility of some
binary interaction with Tropical Storm Irwin to the west, but for
now this interaction is not expected to be very significant within
the forecast period. The official track forecast is similar to the
previous one and is a blend of the simple and corrected dynamical
model consensus predictions.

FORECAST POSITIONS AND MAX WINDS

INIT 24/0900Z 14.0N 103.8W 65 KT 75 MPH
12H 24/1800Z 14.4N 104.8W 80 KT 90 MPH
24H 25/0600Z 15.1N 106.3W 95 KT 110 MPH
36H 25/1800Z 15.8N 108.0W 110 KT 125 MPH
48H 26/0600Z 16.4N 110.0W 115 KT 130 MPH
72H 27/0600Z 17.5N 114.0W 105 KT 120 MPH
96H 28/0600Z 18.5N 117.5W 90 KT 105 MPH
120H 29/0600Z 19.5N 121.0W 75 KT 85 MPH

$$
Forecaster Pasch

Tropical Storm Irwin Discussion Number 8
NWS National Hurricane Center Miami FL EP102017
200 AM PDT Mon Jul 24 2017

The convective structure of Irwin has improved quite a bit this
evening. A recent AMSU microwave pass at 0541 UTC suggests that
deep convection is wrapping nearly all the way around a developing
mid-level eye. It isn’t yet clear if this feature is aligned with
the low-level center. Current intensity estimates from TAFB, SAB,
and the UW-CIMSS ADT at 0600 UTC were all 45 kt, so that value has
been set as the initial intensity.

Determining the initial motion of Irwin has been tricky, since
recent microwave fixes of the center between 2100 UTC yesterday and
0600 UTC today suggest that the center has barely moved during that
time. It is assumed that at least some westward motion is still
occuring, and the initial motion is a rather uncertain 270/5 kt.
In the short term, Irwin should continue slowly westward to the
south of a low- to mid-level ridge. At days 4 and 5, there remains
a fair amount of uncertainty in the degree to which Irwin’s track
will be affected by Hurricane Hilary to the east, with the global
models still generally showing more and the regional models less
interaction. Although the NHC track forecast has been adjusted
slightly to the north at day 5, it assumes that only a little
interaction will take place, and remains south of the GFS and
ECMWF consensus.

The environment in the short-term still appears favorable for
intensification and the guidance is a little higher for this
advisory. The NHC forecast now calls for Irwin to become a
hurricane within 24 hours. After about 2 days, outflow from Hilary
should begin to reach Irwin, resulting in an increase in vertical
shear. The ECMWF model in particular shows a large increase in
shear at days 3-5, and the NHC forecast therefore begins to show
gradual weakening at that time. The intensity forecast is near the
HFIP Corrected Consensus, HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT 24/0900Z 14.8N 117.4W 45 KT 50 MPH
12H 24/1800Z 14.8N 117.9W 55 KT 65 MPH
24H 25/0600Z 14.8N 118.6W 65 KT 75 MPH
36H 25/1800Z 14.8N 119.4W 70 KT 80 MPH
48H 26/0600Z 14.8N 120.5W 70 KT 80 MPH
72H 27/0600Z 14.4N 122.0W 65 KT 75 MPH
96H 28/0600Z 14.5N 123.1W 60 KT 70 MPH
120H 29/0600Z 15.7N 124.1W 60 KT 70 MPH

$$
Forecaster Zelinsky

Tropical Storm Greg Discussion Number 28
NWS National Hurricane Center Miami FL EP072017
200 AM PDT Mon Jul 24 2017

ASCAT passes at 0542 and 0636 UTC suggest that Greg has weakened a
little. The highest wind vector in either pass was 36 kt in the
northeast quadrant, and no winds exceeding 32 kt were observed in
any other quadrants. Based on the ASCAT data, and sampling
considerations, the intensity has been decreased to 40 kt.

The ASCAT passes were very helpful in locating the center of Greg
and the initial motion estimate is 285/8 kt. No significant change
has been made to the NHC track forecast. There remains fair
agreement between the models that Greg will continue west-northwest
or northwestward, steered by an extension of a mid-layer ridge
centered to the northwest. Once the tropical storm weakens to a
remnant low, the low-level flow should turn the remnant circulation
back toward the west or west-southwest until it dissipates.

Greg is not expected to recover from the dry air that is
contributing to its current weakening trend. The vertical shear is
expected to increase as well in about 24-36 hours, which should
cause further weakening. The new forecast is a little lower than
the previous one, mainly to account for the lower initial
intensity. A few models, especially the GFS, suggest that Greg
could open into a trough within the 5-day forecast period, but this
isn’t supported by the ECMWF, UKMET, or HWRF. Out of respect for
those models, the forecast currently calls for Greg to remain a
closed circulation throughout the forecast period, but it would not
be surprising if it dissipated by day 5.

FORECAST POSITIONS AND MAX WINDS

INIT 24/0900Z 14.4N 132.1W 40 KT 45 MPH
12H 24/1800Z 14.8N 133.5W 40 KT 45 MPH
24H 25/0600Z 15.4N 135.0W 40 KT 45 MPH
36H 25/1800Z 16.2N 136.4W 35 KT 40 MPH
48H 26/0600Z 16.9N 137.8W 30 KT 35 MPH
72H 27/0600Z 17.4N 140.8W 25 KT 30 MPH…POST-TROPICAL
96H 28/0600Z 17.0N 144.1W 25 KT 30 MPH…POST-TROP/REMNT LOW
120H 29/0600Z 16.0N 147.0W 20 KT 25 MPH…POST-TROP/REMNT LOW

$$
Forecaster Zelinsky