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Snow Cover

The snow forecast map shows the forecast snowfall in inches for the current day.

24hr Snow Forecast

The Storm Track image shows the forecast snowfall over the next 24 hours.

48hr Snow Forecast

WSI's National Image displaying the probabilistic forecast of Light, Moderate or Heavy Snowfall Totals for the next 48 Hours in color contours.

Snow Cover
Learn About Snow Cover

The snow forecast map shows the forecast snowfall in inches for the current day. Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. The METAR code for snow is SN.

Formation

Snow forms when water vapor condenses directly into ice crystals, usually in a cloud. Floating cloud particles (ice nucleators, often of biological origin) are needed in order for snowflakes to form at temperatures above -40C. 85% of these nuclei are airborne bacteria, with dust particles making up the rest. The ice crystals which form around the ice nucleators typically have a diameter of several millimeters and usually have six lines of symmetry. A snowflake is an aggregate of such ice crystals and may be several centimeters large. The term snowflake is also used below for the symmetrical ice crystals themselves. The individual ice crystals are clear but because of the amount of light the individual crystals reflect snowflakes appear white in color unless contaminated by impurities.

Coverage

Snow remains on the ground until it melts or sublimes. In colder climates this results in snow lying on the ground all winter; when the snow does not all melt in the summer it becomes glaciers.

This is often called snowpack, especially when it does persist a long time. The deepest snowpacks occur in mountainous regions. It is influenced by temperature and wind events which determine melting, accumulation and wind erosion.

The water equivalent of the snow is the thickness of a layer of water having the same content. For example, if the snow covering a given area has a water equivalent of 50 centimetres (20 in), then it will melt into a pool of water 50 centimetres (20 in) deep covering the same area. This is a much more useful measurement to hydrologists than snow depth, as the density of cool freshly fallen snow widely varies. New snow commonly has a density of between 5% and 15% of water. Snow that falls in maritime climates is usually denser than snow that falls in mid-continent locations because of the higher average temperatures over oceans than over land masses. Cloud temperatures and physical processes in the cloud affect the shape of individual snow crystals. Highly branched or dendritic crystals tend to have more space between the arms of ice that form the snow flake and this snow will therefore have a lower density, often referred to as dry snow. Conditions that create columnar or plate like crystals will have much less air space within the crystal and will therefore be more dense and feel wetter.