The probability of a white Christmas -- with snow at least an inch deep on Christmas Day -- is low for many areas of the United States, forecasters say. While northern New England, the Upper Midwest, the Rocky Mountains and Intermountain West have a 75 percent of a white Christmas, New York City, Philadelphia and the District of Columbia have less than a 25 percent chance, AccuWeather.com Wednesday. Regional differences in normal December snowfall and temperatures are both critical factors in what areas can expect a white Christmas, AccuWeather said. \"It tends to stay colder across the northern tier during the day and night, so when snow falls, it\'s less likely to melt,\" AccuWeather meteorologist Alex Sosnowski said. Further south, temperatures play a more important part, forecasters said. \"By the time Christmas comes around, there is a pronounced temperature difference from north to south [across the Midwest],\" AccuWeather meteorologist Jim Andrews said. \"The \'refrigeration\' needed to keep the snow from melting is less reliable in Chicago compared to somewhere like International Falls, Minn.\"