What seems to be required for significantly below average temperatures...
 

*Strong* winds from a cold direction (northerly, or easterly in winter)
- as today.

By contrast, the real below-average-temperature killer appears to be a
combination of persistent sun and light winds, no matter what the
airmass and time of year. Such conditions appear to produce above
average conditions no matter what the time of year, if very cold at
night... Winter 1991/92, and February and November 2001 spring to mind.

Nick