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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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