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Tropical Storm Lowell Discussion Number  13
NWS National Hurricane Center Miami FL       EP172020
1100 AM HST Wed Sep 23 2020
 
Unlike earlier today, deep convection has decreased since the 
previous advisory with thunderstorms containing cloud tops to near 
-70C confined to the southeastern quadrant of the circulation owing 
to modest west-northwesterly vertical wind shear and Lowell moving 
over a small cold pool/eddy. As a result, the low-level center has 
become exposed.  Dvorak current-intensity estimates from TAFB and 
SAB are T3.0/45 kt, therefore the initial intensity will remain at 
45 kt until new ASCAT surface wind data become available.
 
The initial motion estimate is 285/10 kt. Lowell lies along the 
southern periphery of a large, deep-layer subtropical ridge 
which has increased Lowell's forward speed slightly, likely due to 
the recent decreased in the vertical depth of the convection and 
circulation. The strong subtropical ridge to north of Lowell is 
expected turn the cyclone westward soon, with that general motion 
continuing through the remainder of the forecast period. The latest 
NHC track guidance envelope remains tightly packed around the 
previous forecast track, so the only change that was required 
was to increase the along-track forward speed of Lowell.
 
The current moderate northwesterly shear is forecast by the GFS and 
ECMWF to weaken sightly overnight when Lowell is forecast to move 
over a warm eddy. Thus, the intensity forecast was not lowered very 
much for the next 36 hours or so. By 48 hours and beyond, however, 
the combination of increasing deep-layer vertical wind shear of 
more than 20 kt and sea-surface temperatures decreasing to near 24 
deg C is expected to induce gradual weakening, with Lowell is 
forecast to degenerate into a remnant low by 96 hours. The new NHC 
intensity forecast is similar to the previous advisory, and follows 
a blend of the NOAA-HCCA, FSSE and IVCN intensity consensus models.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  23/2100Z 20.7N 122.1W   45 KT  50 MPH
 12H  24/0600Z 21.1N 123.5W   45 KT  50 MPH
 24H  24/1800Z 21.4N 125.4W   40 KT  45 MPH
 36H  25/0600Z 21.6N 127.4W   40 KT  45 MPH
 48H  25/1800Z 21.7N 129.6W   40 KT  45 MPH
 60H  26/0600Z 21.6N 131.9W   35 KT  40 MPH
 72H  26/1800Z 21.5N 134.2W   35 KT  40 MPH
 96H  27/1800Z 21.2N 139.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  28/1800Z 21.7N 144.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Stewart
 
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