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Tropical Depression EIGHT-E (Text)


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Tropical Depression Eight-E Discussion Number   5
NWS National Hurricane Center Miami FL       EP082017
800 AM PDT Wed Jul 19 2017

The center of the depression is challenging to locate this morning
with lots of disorganized convection in the vicinity of the apparent
center.  Microwave data suggest it is on the northeast side of the
convective mass, although it doesn't agree that well with the
overnight ASCAT data.  The initial position tries to keep continuity
from the overnight advisory, since there was a direct ASCAT hit at
0600Z, but it may have to be revised when visible imagery is
available.  The initial wind speed is kept at 30 kt, based on a
blend of the TAFB/SAB fixes.

Unfortunately the forecast of the depression isn't any easier than
locating it.  Some binary interaction with the circulation of
Tropical Storm Greg is likely over the next few days since the
cyclones should be moving fairly close to one another. This pattern
would probably steer the depression southwestward, then
northwestward by day 3 as Greg passes to the north and begins to
draw the depression closer to it.  The specifics of this
interaction, however, are impossible to determine at this range
because it also depends on how strong and exactly where the two
systems are. It is becoming increasingly likely that Greg will be
the more dominant and stronger system, causing the depression to
take a more southward track, and would potentially absorb the
depression in 4-5 days.  The official forecast is shifted southward
in the short-term to account for the latest model guidance, but is
fairly close to the previous forecast at the end of the period.

The intensity forecast is also tricky because many of the global
models suggest that there is a window of lower shear between when
the northwesterly outflow-related shear from Fernanda weakens and
shifts to easterly shear related to the circulation and outflow from
Greg.  Most of the guidance, however, does not respond much to the
lower shear, suggesting little change in intensity during the next
couple of days. The new intensity forecast remains close to the
model consensus and the previous NHC prediction, though the cyclone
does have a small chance to become a tropical storm.  The majority
of the global models are showing the depression degenerating into a
trough by day 4 due to increasing shear and interaction with the
circulation of Greg, so the NHC forecast follows suit.

It should be noted that this is a very uncertain forecast due to
the unusually high number of difficult-to-predict variables.


FORECAST POSITIONS AND MAX WINDS

INIT  19/1500Z 14.5N 121.1W   30 KT  35 MPH
 12H  20/0000Z 14.0N 121.8W   30 KT  35 MPH
 24H  20/1200Z 13.3N 122.6W   30 KT  35 MPH
 36H  21/0000Z 12.6N 123.3W   30 KT  35 MPH
 48H  21/1200Z 12.3N 123.6W   30 KT  35 MPH
 72H  22/1200Z 13.0N 124.5W   25 KT  30 MPH
 96H  23/1200Z...DISSIPATED

$$
Forecaster Blake

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Page last modified: Saturday, 16-Dec-2017 12:10:06 UTC