[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 070837
TCDAT3

Tropical Storm Gabrielle Discussion Number  15
NWS National Hurricane Center Miami FL       AL082019
500 AM AST Sat Sep 07 2019

Gabrielle's cloud pattern has temporarily degraded over the past
several hours.  With the vertical shear pattern shifting from the
east, the shrinking, shapeless, convective mass is displaced to the
west of the surface circulation.  A blend of the Dvorak subjective
satellite T-numbers yields a current intensity estimate of 35 kt,
and a UW-CIMSS ADT estimate as well as an AMSU-A analysis come
in at 39 kt.  The initial intensity is generously above these data
and is set at 45 kt out of respect for the earlier scatterometer
wind analysis.

Gabrielle has moved to the northeast of the upper cut-off low which
has been producing a moderate to strong southerly shear
environment over the past several days.  The upper wind pattern in
this quadrant of the upper low is more easterly, and although still
quite strong, a little more diffluent. This slightly more favorable
flow aloft, along with increasingly warmer sea surface temperatures
in the region, should allow Gabrielle to gradually strengthen during
the next 2 to 3 days.  Toward the end of the forecast period,
weakening is forecast to commence as the cyclone turns northeastward
and quickly moves within deep-layer strong southwesterly flow
associated with an approaching mid-latitude baroclinic frontal
zone.  Interaction with the frontal system as well as the cyclone
traversing significantly decreasing oceanic temperatures (less than
20C), should induce a rather rapid extratropical transition around
day 5.  The NHC intensity forecast is an update of the previous one
is above all of the guidance with the exception of the HWRF.

The initial motion is estimated to be northwestward, or 310/15 kt.
Azores high pressure to the northeast of Gabrielle is forecast to
build over the north central Atlantic, causing the cyclone to turn
west-northwestward later today.  Afterward, southwesterly flow
downstream from the aforementioned front should force Gabrielle to
turn northward and northeastward on Sunday and Monday, respectively.
A slight adjustment to the left of the previous forecast was made
through the entire forecast in order to align more with the NOAA
HFIP HCCA model.

Some adjustments to the wind radii were made based on a 06 Sep
2320 UTC ASCAT-A/B ambiguity analysis.


FORECAST POSITIONS AND MAX WINDS

INIT  07/0900Z 30.9N  42.1W   45 KT  50 MPH
 12H  07/1800Z 31.9N  44.3W   50 KT  60 MPH
 24H  08/0600Z 33.3N  46.6W   55 KT  65 MPH
 36H  08/1800Z 35.4N  47.9W   60 KT  70 MPH
 48H  09/0600Z 38.0N  47.3W   65 KT  75 MPH
 72H  10/0600Z 43.6N  40.3W   75 KT  85 MPH
 96H  11/0600Z 50.7N  26.7W   65 KT  75 MPH
120H  12/0600Z 59.4N   7.9W   55 KT  65 MPH...POST-TROP/EXTRATROP

$$
Forecaster Roberts
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