White Christmas in the US is defined as 1" on-the-ground on 25-DEC. The criteria are different in other countries.
The probability of an event is determined by dividing the number of times something happened by the number of time it could have happened. If there was a white Christmas five times over 20 years...the probability is 25% in any given year.
This NCDC map is a little dated b/c it's based on the old 1961 - 1990 period of climate normals. Good enough for our purposes.
- A green Christmas; a white Easter.
- If there's thunder during Christmas week...
The Winter will be anything but meek.
- The nearer the New Moon to Christmas Day...the harder the Winter. [ed. - new moon is 24-DEC!]
- If Christmas day be bright and clear
There’ll be two winters in the year.
- Thunder in December presages fine weather.
[ed. - somewhat at odds with 'thunder during Christmas week]
- Like in December like all the year long.
[ed. - does this face forward or backward?]
The NWS has updated their white Christmas probabilities map based on the current 30-year climo period between 1981 and 2010...