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Tropical Storm Marie Discussion Number   4
NWS National Hurricane Center Miami FL       EP182020
900 PM MDT Tue Sep 29 2020
 
Infrared and passive microwave imagery indicate that Marie has 
continued to become better organized despite moderate northeasterly 
vertical wind shear of about 15 kt. The deep convection is now 
primarily confined to a single tightly curved band that wraps 
around the west side of the circulation. A late-arriving 29/1718Z 
ASCAT-C scatterometer pass revealed that Marie has a very tight 
inner-core circulation with a radius of maximum winds (RMW) of only 
10-15 nmi and a small 34-kt wind radii extending outward to only 
about 20 nmi The scatterometer data also showed some 34-38 kt 
surface wind vectors in all quadrants. A blend of the scatterometer 
wind data and subjective Dvorak intensity estimates of of T2.5/35 kt 
and T3.0/45 kt from SAB and TAFB, respectively, the intensity has 
been increased to 40 kt.
 
The initial motion estimate is 275/11 kt. As mentioned in the 
previous discussion, Marie's forecast track remains fairly 
straightforward. The cyclone is expected to move generally westward 
to west-northwestward for the next few days as it skirts the 
southern periphery of a deep-layer ridge to the north. Thereafter, a 
slower motion toward the northwest is forecast as Marie moves into a 
weakness in the ridge. The new NHC track forecast is similar to but 
a little to the left of the previous advisory track, and lies 
down the middle of the tightly packed consensus track model 
guidance envelope.
 
Although the GFS- and ECMWF-based SHIPS intensity models have backed 
off significantly on predicting rapid intensification (RI) on this 
cycle, several environmental and cyclone structural parameters are 
still favorable for RI to occur. For about the next 72 hours, 
sea-surface temperatures (SST) are forecast to remain above 28C, 
alone with mid-level humidity values above 70 prevent and low wind 
shear values around 5 kt. Those environmental conditions coupled 
with Marie's small RMW should result in RI beginning in about 24 
hours and continuing until the 60-h time period.  As result, Marie 
is forecast to strengthen into a hurricane in about 24 hours and 
possibly become a major hurricane in 60-72 hours. Rapid weakening 
is expected on days 4 and 5 due to SSTs less than 26C, likely 
significant cold upwelling owing to Marie's expected large and 
strong wind field, and increasing southwesterly vertical shear of 
20-25 kt. The NHC official intensity forecast is similar to the 
previous advisory, and closely follows a blend of the Decay-SHIPS 
and LGEM statistical models and the corrected-consensus HCCA model.
 
Marie's 12-ft sea radii remain larger than would be expected for a
small tropical storm due to a large fetch of southerly swell 
originating from the Southern Hemisphere.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  30/0300Z 13.8N 110.4W   40 KT  45 MPH
 12H  30/1200Z 14.1N 112.9W   50 KT  60 MPH
 24H  01/0000Z 14.6N 115.9W   60 KT  70 MPH
 36H  01/1200Z 15.0N 118.8W   75 KT  85 MPH
 48H  02/0000Z 15.5N 121.4W   90 KT 105 MPH
 60H  02/1200Z 16.5N 123.8W  105 KT 120 MPH
 72H  03/0000Z 17.6N 125.9W  100 KT 115 MPH
 96H  04/0000Z 19.8N 129.3W   80 KT  90 MPH
120H  05/0000Z 21.4N 132.1W   60 KT  70 MPH
 
$$
Forecaster Stewart
 
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