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Tropical Storm WANDA

Tropical Storm Wanda Discussion Number  12
NWS National Hurricane Center Miami FL       AL212021
900 PM GMT Tue Nov 02 2021
Late-arriving 1201 UTC ASCAT-B scatterometer data indicated that a 
small patch of 40-kt winds was located about 30 nmi east and 
southeast of the well-defined center. Owing to known undersampling 
by the scatterometer instrument for tropical cyclones that possess a 
small radius of maximum of winds (RMW), those 40-kt winds support 
the previous advisory intensity of 45 kt. That 45-kt intensity 
estimate has been maintained for this advisory based on Wanda having 
developed a small, closed eye-like feature in shallow convection, 
with a curved band of deeper convection having recently developed 
in the RMW where those 40-kt ASCAT surface winds were detected. 
Wanda's overall convective organization has improved with more and 
tighter curved low-level cloud lines now evident in visible 
satellite imagery.
The initial motion estimate is northward, or 010/07 kt. There remain 
no significant changes to the previous forecast track or reasoning 
through 60 h. Thereafter, however, the latest model guidance has 
made a significant southward shift in the track on days 3-5. For the 
next couple of days, Wanda is forecast to move slowly poleward ahead 
of an approaching mid- to upper-tropospheric trough. As the trough 
weakens and lifts out to the north of the cyclone on day 3, Wanda is 
expected to move slowly eastward before being forced southeastward 
to southward by a narrow shortwave ridge that is forecast to trail 
the aforementioned trough. The GFS and UKMET models show Wanda 
merging with a frontal system on day 4 and lifting out to the 
northeast, whereas the ECMWF drives the cyclone farther south ahead 
of the cold front and turns the system into a  convective-free 
post-tropical cyclone on day 5. For now, the official track forecast 
remains similar to the previous advisory track through 60 h, and 
then shows Wanda turning sharply southeastward on day 4, which is 
well to the right of the previous forecast track. The cyclone is 
then forecast to lift out to the northeast on day 5 as an 
extratropical low pressure system after merging with a cold front. 
The official forecast track is similar to the tightly packed 
consensus models through 60 h, and then is to the left or north of 
the consensus aids in the 72-120-h period. There is 
lower-than-normal confidence in the forecast track after 72 h due 
to the major difference between the GFS and ECMWF model solutions.

Wanda's robust low-level circulation should be able to continue to 
mix out occasional dry air intrusions, allowing for inner-core 
convection to redevelop and resultant slight strengthening to occur 
during the next 24-48 h. However, if a band of moderate convection 
ends up developing around the aforementioned eye-like feature, then 
Wanda could strengthen a little more than currently forecast. By day 
3 and beyond, slow weakening is expected due to a sharp increase in 
vertical wind shear and a decrease in mid-level moisture. Based on 
what now appears to be stronger baroclinic forcing in the 60-96-h 
period, the status of Wanda has been changed to a tropical cyclone 
on day 3, with the day 4 and 5 statuses indicating extratropical 
transition due to merger with a frontal system. The new NHC 
intensity forecast is basically just an update of the previous 
advisory forecast, and remains close to an average of the intensity 
consensus models IVCN, NOAA-HCCA, and FSSE.
INIT  02/2100Z 36.6N  40.4W   45 KT  50 MPH
 12H  03/0600Z 37.9N  40.0W   50 KT  60 MPH
 24H  03/1800Z 39.5N  39.8W   50 KT  60 MPH
 36H  04/0600Z 41.1N  39.7W   45 KT  50 MPH
 48H  04/1800Z 41.8N  39.4W   45 KT  50 MPH
 60H  05/0600Z 41.8N  38.0W   45 KT  50 MPH
 72H  05/1800Z 40.0N  37.5W   45 KT  50 MPH
 96H  06/1800Z 39.0N  36.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  07/1800Z 43.0N  29.5W   40 KT  45 MPH...POST-TROP/EXTRATROP
Forecaster Stewart