[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 200841
TCDAT3

TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL132016
500 AM AST TUE SEP 20 2016

The cloud pattern of the depression is only slightly better
organized than yesterday, with some broken convective banding over
the northeastern portion of the circulation.  Overall, however, the
system is elongated from southwest to northeast.  The initial
intensity is held at 30 kt, in agreement with Dvorak estimates from
both TAFB and SAB.  The tropical cyclone is expected to remain in a
low shear environment for the next 48 hours or so, and therefore
some strengthening is forecast during that time frame.  Later in
the period, the dynamical environment is likely to become hostile
for strengthening since shear associated with an upper-level trough
is forecast to increase to around 30 kt in 3-4 days.  Given this
expected change in the large-scale environment, weakening should
begin in 2-3 days and the system is predicted to revert to tropical
depression status by the end of the forecast period. Although the
dynamical guidance shows some relaxation of the shear at the end of
the period, the current thinking is that by that time the system
will be too disrupted to be able to recover.  The official intensity
forecast is close to the LGEM guidance through 72 hours and a little
below it at days 4 and 5.

There is considerable spread in center fixes and the initial motion
is an uncertain 295/8 kt.  A weakness in the subtropical ridge
along 30W-40W is likely to induce a more northwestward track
during the next day or so, followed by a return to a west-
northwestward motion as the ridge rebuilds a bit to the north
of the cyclone.  Late in the forecast period, the system is
forecast to turn back toward the northwest due to another weakness
in the ridge.  The official track forecast is somewhat slower and
to the right of the previous one but close to the new dynamical
model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  20/0900Z 14.4N  29.6W   30 KT  35 MPH
 12H  20/1800Z 15.4N  30.7W   35 KT  40 MPH
 24H  21/0600Z 16.6N  32.2W   40 KT  45 MPH
 36H  21/1800Z 17.5N  34.0W   45 KT  50 MPH
 48H  22/0600Z 18.4N  35.8W   50 KT  60 MPH
 72H  23/0600Z 20.8N  38.3W   45 KT  50 MPH
 96H  24/0600Z 23.4N  40.6W   35 KT  40 MPH
120H  25/0600Z 26.5N  43.5W   30 KT  35 MPH

$$
Forecaster Pasch
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