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Tropical Storm Rene Discussion Number  17
NWS National Hurricane Center Miami FL       AL182020
500 AM AST Fri Sep 11 2020
 
An AMSR2-GM1 microwave pass revealed that Rene's surface center is 
farther separated from the shrinking deep convection.  The 0000 UTC 
FV3 and ECMWF model soundings indicate that east-southeasterly 
30-35 kt shear near 300 mb is temporarily undercutting the 
diffluent southerly flow aloft.  The initial intensity is held at 
40 kt and is based on the subjective Dvorak estimates from TAFB and 
SAB, and an ADT estimates yielding 39 kt.
 
The shear is expected to relax during the next 36 to 48 hours which 
should allow for gradual intensification.  By mid-period, Rene is 
forecast to move into an area of increasing west-northwesterly 
shear, which should induce a weakening trend.  It's worth noting 
that the ECMWF and the FV3 Decay SHIPS show very little 
strengthening through the period and it appears to be due to 
the cyclone moving into an even more inhibiting thermodynamic 
environment, in addition to the aforementioned shear.  In fact, the 
relative humidity in the mid portions of the atmospheric is less 
than 49 percent beyond day 3.  The NHC forecast sides with the 
various intensity consensus models and is above the 
statistical-dynamical guidance.

Rene continues to move west-northwestward, or 290/9 kt within the 
southwestern periphery of a mid-tropospheric ridge extending 
from western Africa to the eastern Atlantic.  A turn toward the 
northwest on Saturday, followed by a north-northwestward and 
northward motion with a decrease in forward speed is forecast 
Sunday and Sunday night.  Early next week, a subtropical ridge is 
forecast to build to the northwest of Rene, in response to Tropical 
Storm Paulette moving northeastward over the northern central 
Atlantic.  This change in the steering pattern should cause the 
cyclone to begin a southwestward to west-southwestward drift 
through the end of the period.  The official track forecast is 
close to the previous advisory through day 4, then it's nudged a 
little south to conform with the HCCA and TVCA multi-model aids. 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  11/0900Z 19.7N  38.5W   40 KT  45 MPH
 12H  11/1800Z 20.4N  40.2W   45 KT  50 MPH
 24H  12/0600Z 21.6N  42.4W   50 KT  60 MPH
 36H  12/1800Z 23.2N  44.3W   60 KT  70 MPH
 48H  13/0600Z 25.0N  45.6W   60 KT  70 MPH
 60H  13/1800Z 26.6N  46.1W   55 KT  65 MPH
 72H  14/0600Z 27.4N  46.0W   50 KT  60 MPH
 96H  15/0600Z 26.9N  46.4W   40 KT  45 MPH
120H  16/0600Z 26.1N  48.8W   35 KT  40 MPH
 
$$
Forecaster Roberts
 
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