000
WTNT41 KNHC 312042
TCDAT1
 
Subtropical Storm Wanda Discussion Number   4
NWS National Hurricane Center Miami FL       AL212021
900 PM GMT Sun Oct 31 2021
 
Wanda has struggled a bit this afternoon, as it appears that some 
dry air has entrained into its circulation. Deep convection is now
mainly confined to a band wrapping around the eastern semicircle of 
the cyclone. Remarkably, the 3 ASCAT overpasses today missed 
the core of Wanda, however, the latest Dvorak intensity estimates 
from TAFB, and the UW-CIMMS ADT and SATCON remain consistent, and 
indicate an initial advisory intensity of 45 kt.

The storm made a rather abrupt halt in its eastward motion this 
morning, and for much of the day has been drifting westward, with an 
initial motion of 90/1 kt. Wanda is expected to make a slow, nearly 
180 degree turn to the south, east, then northeast through Tuesday 
as it pivots around a mid-tropospheric trough. A faster northeast to 
north motion is expected through Wednesday as ridging builds to the 
east of the cyclone. The model guidance is in reasonable agreement 
on this scenario, and there were only minor tweaks to this portion 
of the NHC track forecast. Thereafter, the guidance has made quite a 
startling change from the previous runs, and is now indicating that 
a blocking ridge may build to the north of Wanda by late this week. 
This is a major shift in the guidance from previous runs that showed 
the cyclone accelerating northeastward into the mid-latitudes. The 
latest NHC track forecast tries to adjust for this change by 
indicating a slower forward speed and less poleward motion late in 
the 5-day forecast period, but this remains well north and east of 
the bulk of the guidance. There is also a large amount of spread now 
in the guidance beyond 72 h, and therefore, that portion of the 
track forecast is of low confidence. 

The upper-trough supporting the subtropical nature of Wanda is 
forecast to lift out to the northeast by Monday. In addition, over 
the past day or so, with each passing advisory cycle the cyclone has 
taken on a little more of the appearance of a tropical cyclone 
versus a subtropical cyclone. Model guidance is in good agreement on 
this transition occurring soon, as per the latest FSU phase space 
forecasts. Therefore, Wanda is expected to transition to a tropical 
cyclone by Monday. The recent ingestion of dry air may have ended 
the chance for further strengthening of the storm, and the latest 
NHC intensity forecast shows no change in strength over the next few 
days. Beyond 72 h, the intensity forecast becomes murky as there 
is now a high level of uncertainty as to where Wanda may be headed 
later this week. The previous NHC forecast had the cyclone moving 
over much cooler waters late this week. However, the latest shift in 
the track guidance suggests the cyclone could remain over the 
current 24 degrees C waters throughout the next 5 days. Regardless 
of the differing tracks and resulting sea surface temperatures 
below, the cold pool of air in the upper troposphere that has been 
giving Wanda a chance to maintain its deep convection will begin to 
vanish by midweek, therefore, weakening is indicated in the 
official NHC intensity forecast. The latest NHC intensity forecast 
was decreased slightly from the previous one, and is close to the 
various intensity consensus solutions. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  31/2100Z 36.3N  43.2W   45 KT  50 MPH
 12H  01/0600Z 35.7N  43.0W   45 KT  50 MPH
 24H  01/1800Z 35.2N  42.2W   45 KT  50 MPH...TROPICAL CYCLONE
 36H  02/0600Z 35.6N  41.1W   45 KT  50 MPH
 48H  02/1800Z 36.8N  40.2W   45 KT  50 MPH
 60H  03/0600Z 38.6N  39.7W   45 KT  50 MPH
 72H  03/1800Z 40.7N  39.4W   45 KT  50 MPH
 96H  04/1800Z 44.1N  37.0W   45 KT  50 MPH
120H  05/1800Z 45.4N  32.0W   40 KT  45 MPH...POST-TROPICAL
 
$$
Forecaster Latto