The storm this past weekend has been given many names: “Snowmageddon”, “Kahunageddon” , “Snowpocalypse”. In Baltimore, December through January was the fifth snowiest period on record, and this January was the first in five years to have above normal snowfall. The first week of February has already dumped a total of 29.7 inches of snow with a deviation of -4.4 degrees from the average daily temperature.
So what is the science behind our winter weather?
One of the major contributing factors is the variation in atmospheric pressure in the Northern Hemisphere. We’re currently experiencing negative phases in both the Arctic Oscillation and the North Atlantic Oscillation indices, measurements of variations in atmospheric pressure. A “negative phase” in these atmospheric pressure patterns indicates relatively high pressure over the polar region and low pressure at midlatitudes, causing the frigid air in the stratosphere to move south into the continental United States. This means more snow for the Midwestern states and the East Coast while Greenland sees warmer winter temperatures.
The Arctic Oscillation index was largely positive in the early 1990s, resulting in relatively warmer winters in the continental United States, but the past year has shown a shift to the negative phase:
Data from NOAA National Centers for Environmental Prediction and averaged.
The reasons behind the shift in pressure patterns are unknown and may be random. It may be comforting to know that based on previous data, there’s bound to be a shift to the positive any year now. And these certainly aren’t the coldest winters Baltimore has ever had. In both 1899 and 1934, February temperatures in Baltimore reached as low as -7oF.
May that be a comfort to you as you watch the snow fall tonight.
