A Primer on Winter Precipitation

This primer is excerpted from Mid-Atlantic Regional Climate Impacts Summary and Outlook: Fall 2025.

Introduction

The Mid-Atlantic region lies in a climate transition zone where a range of winter precipitation types are possible. Forecasting these events in the region is "one of the most difficult challenges in operational meteorology".¹ Winter weather along the East Coast is difficult to forecast for a number of reasons, including the change in elevation from the coastal regions to the Appalachian Mountains and the influence of coastal storms.^2,3 Large scale climate circulation patterns, like ENSO (described in Part 3 above) also can influence winter precipitation in the Mid-Atlantic.

In this primer, we describe the most common kinds of winter precipitation that are possible in the region, what the different National Weather Service watches and warnings are, and how winter precipitation in the region is projected to change in the future.

Types of Winter Precipitation

Which type of precipitation falls in the winter is dependent on the temperature profile of the atmosphere from cloud-level to the ground (Figure 1).⁴ Even small variations in temperature, including where in the atmosphere these temperature changes occur, can create distinct differences in the kind of precipitation that we experience. There are four major types of precipitation:

• Snow: If the temperature of the air is below freezing (32 degrees F) from the height of the clouds all the way to the ground, that precipitation falls as snow.⁵
• Sleet: If there is a small layer of air that is above freezing in the middle layers of the atmosphere, then precipitation will start out as snow, melt slightly, then refreeze and become sleet by the time it hits the ground.⁶
• Freezing Rain: The difference between sleet and freezing rain is that the layer of air that is above 32 degrees F is thicker, so the precipitation melts completely into a liquid state. As it falls through the cold air near the ground, the droplets become supercooled and freeze into ice when they come into contact with surfaces on the ground.⁷
• Rain: The air between the clouds and the ground is warm enough (above 32 degrees F) that the precipitation does not freeze and hits the ground in a liquid form (see Figure 1).

Forecasts can also mention "wintry mix" in the winter. This is a combination of snow, sleet, freezing rain, or even regular rain.⁸ This is caused by variations in the temperature of the different layers of the atmosphere during a storm event, or across a geographic region.⁹ This kind of winter storm event is relatively common in the Mid-Atlantic region due to the dynamic between cold air that gets trapped in the area by the Appalachian Mountains interacting with warmer air along the coast.¹⁰

Winter Precipitation

Snow

Snowflakes never melt. Occurs at or below 32 degrees.

Sleet

Droplets freeze and form ice before reaching the surface. Occurs both below and above 32 degrees.

Freezing Rain

Rain freezes on contact with the surface. Occurs above 32 degrees.

Rain

Rain never refreezes. Occurs above 32 degrees.

Source: National Weather Service, https://www.weather.gov/wrn/winter_hazard_infographics

Snow can take different forms depending on how much below freezing the atmosphere is and how much moisture is in the atmosphere.¹¹ When conditions are cold and dry, then snowflakes stick together less as they fall, which creates a "powdery" snow that creates a deeper snowfall for the same amount of moisture.¹² If it is less cold and/or there is more moisture in the atmosphere, individual snowflakes stick together, creating a more dense snowfall with less accumulation, but is good for creating snowballs and snow forts.¹³ Therefore, the closer the atmosphere is to the freezing mark, the lower the snowfall accumulations will be, and the more "slushy" the snow we get.¹⁴

• Flurries are light, short duration snowfall events. Typically resulting in little to no accumulation.¹⁵
• Showers are events with varying snowfall rates over a short period of time, that potentially result in some accumulation.¹⁶
• Squalls are short bursts of intense snowfall rates. Accumulation amounts vary but can be significant. These occur more frequently in the Great Lakes region but are possible in the Mid-Atlantic.¹⁷
• Blowing Snow is when snow that is either falling or has already fallen gets blown around by strong winds, causing issues with visibility and potentially snow drifts.¹⁸
• Blizzards are storms that are characterized by a loss of visibility due to wind and blowing snow.¹⁹ Heavy snowfall rates are possible but aren't required for a storm to be called a blizzard.²⁰ They are defined as having winds of over 35 mph, resulting in visibility being reduced to ¼ mile or less for at least 3 hours.²¹

In the Mid-Atlantic region, winter weather can be caused by nor'easters, which are named after the direction of their strongest winds.²² These storms form when cold arctic air moves into the region and mixes with warm, moisture-laden air over the Atlantic Ocean, creating a strong low-pressure system.²³ In addition to heavy snow and/or rain, nor'easters are known for strong winds that can be as strong as or even stronger than hurricane winds.²⁴ These winds often result in large waves that can cause beach erosion, coastal flooding, and even damage to structures along the Atlantic coast.²⁵ While they can look similar to hurricanes on satellite imagery, they are different from hurricanes due to where they form (further north) and because they are fueled by cold air in the atmosphere rather than warm air.²⁶ While nor'easters can happen any time over the course of the year, the most extreme storms tend to occur during the winter.²⁷

The jet stream air flow across the Mid-Atlantic region and into the Atlantic Ocean, combined with Northeast winds along the Atlantic coast, leads to the formation of a Nor'easter.

Source: NOAA, https://www.nesdis.noaa.gov/about/k-12-education/severe-weather/what-noreaster

Another type of storm that creates winter precipitation in the Mid-Atlantic are "Clippers".²⁸ These are very fast-moving storm systems that travel to the East Coast from the Great Lakes and Central Canada.²⁹ Generally, these storms generate less and drier/fluffier snow than nor'easters and since they tend to bring along very cold temperatures, rarely form any other type of winter precipitation than snow.³⁰

In portions of Pennsylvania and New York, lake effect snow can cause incredible snowfall rates and large amounts of snow (e.g., 2-3 inches of snow per hour).³¹ This occurs when cold air moves over the relatively warmer waters of the Great Lakes and is more common in the Great Lakes region than in the Mid-Atlantic.

Winter Precipitation Watches and Warnings

There are several types of advisories, watches, and warnings that the National Weather Service can issue in response to forecasted winter storms. These criteria can change depending on the weather forecast office or region, therefore advisories, watches, and warnings may vary across the region and certainly vary across the country.³²

• Winter Weather Advisories: This is issued when winter weather conditions are expected to cause significant disruptions and may pose hazards. However, by exercising caution, these advisory situations should remain non-life-threatening.³³
• Winter Storm Watch: This is issued 12-48 hours before a forecasted significant winter storm of any kind, including snow, sleet, or freezing rain.³⁴
• Winter Storm Warning: A Winter Storm Watch is upgraded to a warning 12-24 hours before a storm with "a combination of hazardous winter weather in the form of heavy snow, heavy freezing rain, or heavy sleet". It can also be upgraded as the event occurs.³⁵
• Blizzard Warning: This is issued in the event of a blizzard, meaning that 35 mph winds, or stronger, are happening in combination with snowfall rates or blowing snow that cause visibilities to drop to ¼ of a mile or less for three or more hours.³⁶

Source: NOAA

Three examples of winter storm products:

Winter Storm Warning: Snow, sleet, or ice expected! Take Action!

Confidence is high that a winter storm will produce heavy snow, sleet or freezing rain and cause significant impacts.

Winter Storm Watch: Snow, sleet, or ice possible! Be prepared.

Confidence is medium that a winter storm could produce heavy snow, sleet, or freezing rain and cause significant impacts.

Winter Weather Advisory: Wintry weather expected. Exercise caution.

Light amounts of wintry precipitation or patchy blowing snow will cause slick conditions and could affect travel if precautions are not taken.

Source: National Weather Service, https://www.weather.gov/safety/winter-ww

There are additional winter weather warnings for events such as ice storms, snow squalls, lake-effect snowfall events, and cold/freezing temperatures. More information about all of types of watches and warnings are available via the local National Weather Service offices. For example: https://www.weather.gov/ctp/wwaCriteria has watch, warning, and advisory criteria for the areas served by the State College, Pennsylvania office, as well as surrounding offices.

Winter Precipitation Impacts

The impacts associated with winter storms vary depending on the kind of precipitation, surface temperatures, and winds. As shown in previous Climate Summaries (e.g., Winter 2024-2025), winter storms can result in school closures, flight cancellations, slippery roads, and power outages due to downed trees and power lines. Additionally, the conditions can contribute to car accidents and intense traffic impacts, such as the January 3, 2022, storm that resulted in hundreds of cars stuck in an over 24-hour traffic jam along a 50-mile stretch of Interstate 95 in northern Virginia.³⁷

Generally, most injuries that are associated with winter storms are not caused by the storm itself, but rather traffic accidents, hypothermia, or heart attacks and other injuries from shoveling snow.³⁸ About 70 percent of injuries that are related to snow and ice happen in car accidents.³⁹ Therefore, it is important to pay attention to winter storm watches and warnings and avoid travel during and after storm, until roads are deemed safe.

How Is Winter Precipitation Changing?

Recent observations of winters in the Mid-Atlantic indicate a trend towards warming temperatures, below-normal seasonal snowfall totals, and geographic and temporal variability in snowfall.^40,41,42,43 The winter of 2024-2025 exemplified this variability: while northern portions of the Chesapeake Bay watershed, including Pennsylvania and parts of New York, received below-normal snowfall, areas of central and southeastern Virginia recorded 100-200% of normal snowfall, and Washington, D.C. (Reagan National Airport) observed its snowiest day in six years, despite finishing the season with below-average total snowfall.⁴⁴ In both the 2023–2024 and 2024–2025 winters, mid-season snow events ended streaks of little to no winter snow in parts of the region. For example, a January 2024 snowstorm ended Lynchburg, Virginia's streak of 728 consecutive days without at least one inch of snow.⁴⁵

A previous climate summary data tool for the Winter of 2020–2021 explored total annual snowfall amounts and how they have varied across the region during the recent historical period (1981-2019).⁴⁶ This interactive tool showed that while Maryland, Delaware, and portions of New York and Pennsylvania saw increases in snowfall over that time period, Washington, D.C. saw a decrease in annual snowfall.⁴⁷

Generally, over the past thirty years, the Northeast region has seen an increase in the frequency of heavy snowfall events.^48,49 This is due to a combination of more frequent polar vortex disruptions that allow cold air from the Arctic to travel down the East Coast and warming waters in the western Atlantic Ocean that provide ample moisture to help intensify these storms and increase winter precipitation amounts.⁵⁰

Given how difficult it is to forecast winter storms even in the near-term, it is understandably difficult to determine how climate change will impact winter precipitation in the future. The Fifth National Climate Assessment showed that climate models generally indicate that winter temperatures will get warmer in the Northeast region, causing less snowfall and an overall shorter snow season.⁵¹ More winter precipitation has been falling as rain than in the past, and this trend is expected to continue.⁵²

However, global climate models generally do not have high enough resolution to simulate the kinds of storms (e.g., nor'easters) that bring much of the winter precipitation to the Mid-Atlantic.^53,54 They also don't do a good job with representing the larger scale circulation patterns we would need to see to understand how the variations we are currently seeing in the polar vortex may change in the future.⁵⁵ Recent work has suggested that there may be more extreme cold events caused by dips in the polar vortex, which could also result in a further increase in extreme winter weather events.⁵⁶

Preparedness Resources

• Prepare! Don't Let a Winter Storm Take You by Surprise: https://www.weather.gov/safety/winter-before
• Winter Weather: https://www.vaemergency.gov/threats/winter-weather
• Winter Preparedness at Home: https://www.weather.gov/lwx/winter_home-pr