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| #1068267 (Received by flhurricane at: 4:54 PM 02.Nov.2021) TCDAT1 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. FORECAST POSITIONS AND MAX WINDS 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 |