Tropical Depression Greg Discussion Number 36
NWS National Hurricane Center Miami FL EP072017
200 AM PDT Wed Jul 26 2017
Even with the help of microwave imagery, the center of Greg is
difficult to locate due to the disorganization of the associated
convection. The best estimate is that the center is to the
southeast of the remaining convection. The initial intensity
remains 30 kt for this advisory, which could be generous. A
combination of continued southerly to westerly shear, a dry air
mass, and cool sea surface temperatures along the forecast track
should cause the cyclone to stop producing convection and degenerate
to a remnant low in about 24 h. The low is subsequently expected
to weaken to a trough in 72-96 h.
The initial motion is 305/12. Low- to mid-level troughing north of
the depression should maintain a northwestward or west-northwestward
motion for the next 12-24 hours. After that, the remnant low
should turn west-southwestward in the trade winds. The forecast
track lies near the model consensus and is similar to the previous
advisory.
Tropical Depression Greg Tropical Cyclone Update
NWS National Hurricane Center Miami FL EP072017
625 AM PDT Wed Jul 26 2017
...GREG CROSSES INTO THE CENTRAL PACIFIC...
Satellite imagery indicates that Tropical Depression Greg has
crossed 140W longitude and is now located in the central North
Pacific basin. As a result, the next advisory, which will be issued
at 800 AM PDT/1500 UTC, will use the following central Pacific basin
WMO and AWIPS headers:
Tropical Storm Greg Discussion Number 25
NWS National Hurricane Center Miami FL EP072017
800 AM PDT Sun Jul 23 2017
Greg's convective pattern is looking a little better this morning,
with two distinct bursts of thunderstorm activity located near and
to the west of the center. The latest Dvorak intensity estimates
range from T2.0 to T3.0, and the initial intensity will remain 45
kt since the cyclone's structure has improved somewhat during the
past several hours.
Vertical shear over Greg is less than 10 kt at the moment, but the
cyclone's biggest challenge is dry air in the surrounding
environment. GOES-16 lower-level water vapor imagery shows that
Greg is running into a very dry low- to mid-level air mass to its
west, and this environment will likely curtail strengthening. Greg
is forecast to maintain its intensity for another 24 hours,
followed by gradual weakening due to the dry air, increasing shear,
and cooler sea surface temperatures after 48 hours. This scenario
is captured by all of the intensity guidance, and Greg is likely to
degenerate into a remnant low by 96 hours.
A little more southward adjustment of the center location was needed
based on additional ASCAT and other microwave data received since
the last advisory. Still, low- to mid-level ridging to the north
of Greg is expected to maintain a westward motion through 36 hours.
The ridge is forecast to weaken after 36 hours, which should allow
the cyclone to gain a little more latitude on days 2 through 4.
The low-level trade winds are then expected to push the remnant low
west-southwestward by day 5. Except for a southward adjustment in
the track forecast to account for the initial position, the model
guidance is in good agreement, and no other significant changes
were required from the previous advisory.
Tropical Storm Greg Discussion Number 4
NWS National Hurricane Center Miami FL EP072017
300 AM MDT Tue Jul 18 2017
Deep convection has increased in both intensity and coverage during
the last several hours over and to the southeast of the center of
the cyclone. The Dvorak classifications from TAFB and SAB at 0600
UTC were 2.0/30 kt, but ADT values and satellite consensus estimates
from CIMSS at the University of Wisconsin suggest a slightly higher
intensity. Based on the improvement in convective organization
during the past few hours, the initial intensity is increased to 35
kt, making the system Tropical Storm Greg.
The northwesterly shear currently affecting Greg is expected to
gradually lessen, and the upper-level wind pattern will likely
become favorable in the 24- to 60-h time frame. Therefore, the NHC
intensity forecast continues to show modest strengthening during the
next couple of days. After that time, however, the shear is
expected to increase out of the south or southwest as the cyclone
approaches an upper-level low. This increase in shear combined with
drier air and cooler waters should end the strengthening trend and
cause weakening. The depression will likely degenerate to a remnant
low in 4 to 5 days when it moves over SSTs below 26 deg C. The NHC
intensity forecast is the same as the previous one, and is in line
with the bulk of the guidance.
Greg is moving west-northwestward at 8 kt to the south of a low- to
mid-level ridge. A westward to west-northwestward motion with some
increase in forward speed is expected during the next few days as
the ridge strengthens a little. Some of the models show Greg
interacting with the disturbance currently located about 700 n mi
to its west in 4 to 5 days, which could cause the cyclone to turn
south of due west at that time. The track models are in general
agreement, and the NHC forecast lies closest to the consensus aids.
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