Snow can fall in April across many mid-latitude regions, even though the calendar marks the start of spring. This phenomenon, often surprising, results from the atmosphere retaining its capability to produce wintry conditions. While April snow is less frequent than mid-winter events, the necessary combination of cold air and moisture can still align to create late-season snowfall. This highlights the fluid nature of atmospheric boundaries as the Northern Hemisphere transitions from winter to summer.
The Climatological Reality
The likelihood of experiencing April snow is heavily dependent on a region’s geographic position, specifically its latitude and altitude. Historically, areas in the northern half of the continental United States and Canada are far more susceptible than southern regions. Average April snowfall in a place like Madison, Wisconsin, is around two to two and a half inches. High-elevation areas, such as mountain ranges, consistently maintain a much higher probability of late-season snow well into May due to naturally lower air temperatures.
Atmospheric Mechanisms Behind Late-Season Snow
The primary mechanism for April snowfall is a significant southward displacement of the polar jet stream. The jet stream is a fast-moving, high-altitude river of air that separates the cold, dense polar air mass from warmer air masses. In April, this boundary naturally shifts northward as solar warming increases.
For snow to occur, the jet stream must develop a deep southward meander, known as a trough, which pulls Arctic or polar air into the mid-latitudes. This intrusion brings temperatures at the cloud level and the surface down to freezing, necessary for precipitation to fall as snow rather than rain. The atmospheric setup needs to be highly amplified to overcome the increasing warmth of spring. These large wobbles can sometimes lock in place for days, leading to sustained cold conditions. A low-pressure system must then track correctly, drawing moisture from the south and combining it with the cold air. This alignment of cold air and moisture is less common in spring, but it can produce heavy snowfall.
Factors Influencing Duration and Accumulation
Once April snow falls, its impact and persistence differ significantly from mid-winter snowfall. The high angle of the sun delivers much more direct solar energy to the surface than during December or January. This increased solar insolation causes rapid melting during the day, often turning accumulations into slush within hours of sunrise.
Ground temperature is another determining factor in accumulation. By April, the ground has absorbed more heat, meaning the snow melts not just from the top down due to solar radiation, but also from the bottom up due to warmer soil. April snow events often require a high snowfall rate or an overnight occurrence to accumulate significantly before daytime warming begins. The resulting snow cover rarely lasts for more than a day or two in lower elevations, distinguishing it from the prolonged snowpack of winter months.