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Tropical Storm Wanda Discussion Number  15
NWS National Hurricane Center Miami FL       AL212021
300 PM GMT Wed Nov 03 2021
 
Wanda appears well organized this morning, with visible satellite 
imagery indicating cyclonically curved convective bands wrapping 
practically all the way around the circulation center. However, the 
cloud top temperatures associated with this activity are not very 
cold, peaking between minus 50 to 55 C in the deepest convection. An 
ASCAT-A scatterometer pass at 1021 UTC also did not show much change 
in the peak derived winds (39-kt), although the 34-kt radii have 
expanded a bit. Subjective Dvorak satellite estimates this morning 
also remain unchanged at CI 2.5/35 kt from TAFB and CI 3.0/45 kt 
from SAB. The current intensity is held at 45 kt for this advisory, 
assuming some undersampling in the scatterometer data.
 
The storm is moving a little faster to the north-northeast at 015/9 
kt. The short-term track philosophy is fairly straightforward, as 
Wanda continues to be steered generally northward by a narrow 
mid-level ridge centered to the east of the cyclone. The track 
guidance remains in good agreement on this evolution over the next 
24 hours. Thereafter, the spread in track solutions increases quite 
dramatically, owing to uncertainty related to the degree of 
influence an approaching shortwave trough to the north has on Wanda. 
For now, both the deterministic ECMWF and GFS solutions show this 
shortwave trough bypassing the cyclone to the north, and in its wake 
a narrow but sharp mid-level ridge develops west of Wanda, resulting 
in the cyclone being steered back southward between 48-72 hours. 
However, the spread in track solutions from both the GFS and ECMWF 
ensemble members is very large at this time frame. In fact, the 06z 
GFS ensemble track spread at 60 h is more than 500 n mi, far greater 
than the average forecast track errors for that period. Finally, a 
stronger shortwave trough is expected to pick up Wanda in the latter 
portion of the forecast period, resulting in an abrupt acceleration 
to the northeast. The official track forecast is similar to the 
previous one for the first 24 hours, but has been nudged a little 
south between 48-72 h following the consensus aids. Given the high 
spread of ensemble solutions beyond 36 h, the track forecast beyond 
that time is of low confidence.
 
While Wanda's winds have not increased over the past 24 hours, its 
structure on satellite imagery has improved, with convective banding 
wrapping around the low-level center, aided by low vertical wind 
shear over the system. Even though Wanda is over marginally warm 
sea-surface temperatures (SSTs) that decrease further along the 
track over the next 36 hours, this negative factor will be offset by 
cooling upper-level temperatures overhead. The trough forecast to 
bypass Wanda to the north could also provide a bit of baroclinic 
enhancement aiding ascent over the cyclone. Most of the intensity 
guidance responds to this environment by showing some slight 
strengthening and the latest NHC forecast now shows a peak intensity 
of 50 kt in the 36-48 h time-frame. Afterwards, some increase in 
shear and decreasing mid-level moisture may lead to weakening, 
though this could be offset by Wanda moving back over warmer SSTs as 
it tracks back south. The official intensity forecast is a bit 
higher than the previous one, but ultimately still shows Wanda 
losing tropical characteristics by the end of the forecast period as 
it merges with an approaching mid-latitude trough.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  03/1500Z 38.8N  39.8W   45 KT  50 MPH
 12H  04/0000Z 40.1N  39.7W   45 KT  50 MPH
 24H  04/1200Z 41.4N  39.6W   45 KT  50 MPH
 36H  05/0000Z 41.9N  38.9W   50 KT  60 MPH
 48H  05/1200Z 40.9N  37.5W   50 KT  60 MPH
 60H  06/0000Z 39.2N  36.6W   45 KT  50 MPH
 72H  06/1200Z 38.1N  36.6W   45 KT  50 MPH
 96H  07/1200Z 40.1N  33.2W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  08/1200Z 47.1N  27.1W   45 KT  50 MPH...POST-TROP/EXTRATROP
 
$$
Forecaster Papin
 
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