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Tropical Storm Marie Discussion Number   6
NWS National Hurricane Center Miami FL       EP182020
900 AM MDT Wed Sep 30 2020
 
Marie's satellite presentation has been gradually improving.  The
center is embedded beneath a central dense overcast feature, and
the convective band within the western semicircle has become a
little more pronounced and continuous.  In addition, a mid-level eye
has begun to form, as observed in a 1200 UTC SSMIS microwave 
pass.  Dvorak intensity estimates range from 45-55 kt from 
TAFB and SAB, and the most recent SATCON estimate was 53 kt (at 
around 0900 UTC).  Based on these numbers and the continued 
improvement of Marie's structure, the initial intensity is set at 55 
kt.

The stage appears set for Marie to rapidly intensify during the 
next couple of days.  Water vapor imagery indicates that the 
easterly shear over the cyclone has continued to decrease and 
should be generally low for the next 3 days, and upper-level 
divergence will also be in place during that period to help 
ventilate the storm.  The thermodynamics are also favorable for 
fast strengthening, highlighted by sea surface temperatures of 
28-29 degrees Celsius and plenty of moisture in the surrounding 
environment.  Due to these conditions, the NHC forecast explicitly 
shows rapid intensification during the next couple of days, with a 
peak intensity likely occuring sometime between 48 and 60 hours.  
The peak intensity shown in the official forecast has been nudged 
upward slightly from the previous prediction, following the trends 
in the intensity guidance, however it's noteworthy that even this 
forecast is 5-10 kt lower than the solutions provided by the HCCA 
consensus aid and the COAMPS-TC model.  In about 3 days, cooler 
waters and then increasing shear (especially on days 4 and 5) 
should induce a gradual weakening trend.

Marie is moving westward (275/14 kt) to the south of a strong 
mid-tropospheric high centered near the U.S./Mexico border.  This 
ridge is expected to maintain Marie on a westward or 
west-northwestward trajectory for the next 2-3 days.  Even though a 
mid- to upper-level low near 130W longitude has caused a break in 
the ridge, the models have trended toward this feature weakening 
during the next few days.  This has in turn caused many of the 
models to trend westward with Marie's track after day 3, as was 
noted in the previous advisory, but there is also greater spread 
than there was yesterday.  The NHC track forecast remains closest to 
the TVCE and HCCA consensus aids, near the middle of the guidance 
envelope, which is bracketed on the right side by the GFS (and its 
ensemble mean) and the left side by the ECMWF (and its ensemble 
mean) by the end of the forecast period.
 
The 12-ft sea radii remain larger than would be expected for a
tropical storm of Marie's size due to a large fetch of southerly 
swell originating from the Southern Hemisphere.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  30/1500Z 14.2N 113.8W   55 KT  65 MPH
 12H  01/0000Z 14.3N 116.1W   65 KT  75 MPH
 24H  01/1200Z 14.5N 118.9W   85 KT 100 MPH
 36H  02/0000Z 15.1N 121.5W  100 KT 115 MPH
 48H  02/1200Z 15.9N 123.9W  110 KT 125 MPH
 60H  03/0000Z 16.9N 125.9W  115 KT 130 MPH
 72H  03/1200Z 18.0N 127.6W  100 KT 115 MPH
 96H  04/1200Z 19.8N 130.7W   75 KT  85 MPH
120H  05/1200Z 21.1N 133.4W   55 KT  65 MPH
 
$$
Forecaster Berg
 
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