The appearance of snow in May often surprises people who have already begun enjoying warmer weather. While late-season snowfall is an anomaly in most temperate regions, it is a meteorological possibility driven by specific atmospheric dynamics. The chance of snow decreases significantly after the spring equinox, but remnants of winter cold can still sweep far south under the right conditions. This phenomenon requires a precise and temporary alignment of cold air, moisture, and low-pressure systems to bring winter conditions back into the spring calendar.
The Atmospheric Conditions That Allow May Snow
Late spring snowfall depends on the temporary southward movement of the polar jet stream, which separates cold Arctic air from warmer mid-latitude air. When this current develops deep, wavy meanders, a trough can dip far below its typical path, pulling a mass of frigid air with it. This surge of cold air must arrive just as a moisture-laden low-pressure system moves through the region.
For snow to form, the air temperature throughout the entire column of the troposphere must be at or near freezing. A “cutoff low” is often responsible for these events; this is a low-pressure area detached from the main jet stream flow. Since it is no longer steered by prevailing westerly winds, the cutoff low can stall for several days, continuously drawing cold air and moisture into a small area and allowing precipitation to fall as snow.
Where Late Season Snowfall is Most Common
The likelihood of May snow is determined by geographic factors: high altitude and high latitude. High-elevation areas, such as the Rocky Mountains, the Sierra Nevada, and the higher peaks of the Appalachian range, frequently report May snowfall because temperatures naturally decrease with elevation. For these regions, spring snow is a recurring feature of the climate, with some mountain locations experiencing their heaviest snow months in April or May.
Regions at high latitudes, including the northern tier of the United States, the Great Lakes area, and Northern Europe, are also susceptible due to their proximity to the Arctic air source. In North America, this includes the Northern Plains and the areas surrounding the Great Lakes, where late-season cold fronts interact with moisture from the lakes. Further north, in places like Northern Sweden or Alaska, snow in May is common, as the winter season naturally extends later into the calendar year.
Historical Rarity and Frequency
While May snow is possible, its frequency drastically declines once April ends, especially in metropolitan areas at lower elevations. For the mid-latitudes, measurable snowfall in May is a statistical rarity and generally only happens during record-breaking cold outbreaks. Many major cities in the northeastern and midwestern US have their latest recorded snowfalls in late April, with May dates being exceptions.
Historical records show these late events are usually minor, often a dusting that does not accumulate significantly due to the higher angle of the sun. However, extreme events have occasionally brought measurable snow, such as the May 1966 snowfall in Chicago or the widespread May 1977 storm that impacted New England. In the high-altitude Western US, recorded snowfalls can extend into June or even July, but the frequency of a true snowstorm decreases sharply after the transition into late spring.
Immediate Effects on Plants and Daily Life
The return of cold weather and snow creates immediate problems for the natural world, which has already responded to spring warmth. Plants that have emerged from dormancy, such as young annuals, vegetable gardens, and delicate flowering trees like magnolias, are at significant risk. The freezing temperatures associated with the snow can cause intracellular water to expand, rupturing plant tissue and leading to damage.
The physical weight of heavy, wet spring snow is also a major hazard for trees that have already developed leaves. The increased surface area catches and holds the snow, leading to broken branches and structural damage. On the infrastructure side, the heavy, dense nature of late-season snow can lead to minor disruptions, including slick roads and temporary closures of mountain passes. The added weight on power lines and tree limbs increases the potential for localized power outages.