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Tropical Storm Marie Discussion Number   3
NWS National Hurricane Center Miami FL       EP182020
300 PM MDT Tue Sep 29 2020
 
A WindSat microwave image from earlier this morning indicated that 
the tropical cyclone's low-level circulation was becoming better 
defined, with perhaps the formative stage of a cyan ring seen in the 
37-GHz channel.  Visible images also show the center tucked just 
beneath recent bursts of deep convection, and Dvorak estimates have 
risen to T2.0 and T2.5 from TAFB and SAB, respectively.  These data, 
along with ambiguity analyses of recent ASCAT-B and -C scatterometer 
passes, indicate the depression has strengthened to Tropical Storm 
Marie with maximum winds of 35 kt.

Marie's future track is probably the most straightforward part of 
the forecast.  A mid-tropospheric high anchored over the 
southwestern United States is steering Marie westward with an 
initial motion of 275/10 kt.  This high will remain the main driver, 
forcing the cyclone westward or west-northwestward for the next 3 
days.  By days 4 and 5, Marie is likely to reach a break in the 
ridge and should slow down a bit and turn toward the northwest. 
There are no notable outliers among the track guidance, and the 
small spread among the models yields higher-than-normal confidence 
in the track forecast.  The new NHC forecast is very similar to the 
previous prediction, and is close to the TVCE and HCCA consensus 
aids.

The intensity forecast is a little more challenging, but mostly 
because a good proportion of the guidance suggests that Marie will 
intensify significantly during the next few days.  Relatively low 
deep-layer shear, warm ocean waters, and favorable upper-level 
divergence all favor strengthening, and several of the various 
SHIPS Rapid Intensification thresholds are several times higher 
than their climatological means.  In addition, all of the dynamical 
models, the consensus aids, and the GFS-based SHIPS model bring 
Marie to hurricane strength within 24 hours.  Given these signals, 
the NHC intensity forecast has been raised from the previous one 
and lies near or just below the intensity consensus in order to 
maintain some continuity.  But given what is shown by some of the 
better-performing intensity models, I would not be surprised if 
subsequent forecasts show a faster rate of intensification or 
a higher peak intensity.  Weakening is expected by days 4 and 5 due 
to cooler waters and increasing southwesterly shear.

Marie's 12-ft sea radii are larger than would be expected for a 
small, just-developing tropical storm due to a large fetch of 
southerly swell originating from the Southern Hemisphere.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  29/2100Z 13.6N 109.5W   35 KT  40 MPH
 12H  30/0600Z 13.9N 111.6W   45 KT  50 MPH
 24H  30/1800Z 14.6N 114.5W   60 KT  70 MPH
 36H  01/0600Z 14.9N 117.5W   75 KT  85 MPH
 48H  01/1800Z 15.3N 120.3W   90 KT 105 MPH
 60H  02/0600Z 16.0N 122.7W  100 KT 115 MPH
 72H  02/1800Z 17.2N 124.9W  100 KT 115 MPH
 96H  03/1800Z 19.6N 128.5W   85 KT 100 MPH
120H  04/1800Z 21.4N 131.6W   60 KT  70 MPH
 
$$
Forecaster Berg
 
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