What does this symbol mean?
It seems to be the symbol &quot;M&quot; for &quot;missing cloud information&quot;. Note that CWIJ has a circle for cloud coverage that is slightly bigger than the standard circle size used for most other airports.
Southeast of CWIJ, there is CWIC which has the symbol M as well, but there the edge of the circle coincides with the lower part of the symbol shown in the CWIJ circle.
  

Another example can be found with CXDK (normal size circle) and CXLL (big circle size)

There is another station towards the east (CYBK) that has a bigger circle as well, and it has a 6/8 cloud coverage symbol that does not fully fill the circle. For comparison, PAUM has a standard circle size with the same 6/8 cloud coverage symbol filling all the way to the circle's edge.
  
This leads me to believe that the software behind aviationweather.gov has a symbol library which includes cloud coverage symbols designed for a certain standard circle size. Some airport are represent by a circle which is bigger than the standard, and this leads to the strange representation of the symbol 'M' seen at CWIJ.