Winter storm, plate FB-EVT-080

What it is

A winter storm is a storm of snow, sleet or freezing rain heavy enough to be dangerous, or of several of them together: the snow that closes roads and schools, the sleet that piles up on them, the freezing rain that glazes everything it touches. Most come from large storm systems, which draw moist air into the cold, over hundreds of miles and a day or two.

The Weather Service warns it with a Winter Storm Warning when the snow, sleet or ice of one event is expected to reach the office's own criteria. It watches for it with a Winter Storm Watch when such a storm is possible, and advises with a Winter Weather Advisory when the snow or ice will fall short of the warning but still cause trouble. The criteria are each office's own, because the snow that is ordinary in one place is a danger in another: 1 in (2.5 cm) in one event across the far South, 6 in (15 cm) across much of the country, 12 in (30 cm) in the western mountains, and 18 in (46 cm) in Utah's Wasatch Mountains. Fig. B draws every zone.

What it is not

It is not a blizzard, Blizzard FB-EVT-081, which is warned for the wind and the visibility, whatever the amount; a blizzard can come with little new snow at all.

It is not an ice storm, Ice storm FB-EVT-084, warned for the ice that freezing rain builds on wires and trees, though a winter storm can bring some ice with its snow.

It is not lake effect snow, Lake effect snow FB-EVT-082, or a snow squall, Snow squall FB-EVT-086, both narrower and shorter. No Snow Squall Warning is issued while a Winter Storm Warning is in force.

Lookalikes

Blizzard: Wind and snow cutting the visibility under a quarter mile for three hours or longer, Blizzard FB-EVT-081, warned for the wind and the visibility, not the amount.
Ice storm: Freezing rain building ice on everything it touches, Ice storm FB-EVT-084, warned for the ice.
Lake effect snow: Narrow bands of snow off a lake, Lake effect snow FB-EVT-082, often under fair skies elsewhere.
Snow squall: A burst of snow and wind that passes in under an hour, Snow squall FB-EVT-086; no Snow Squall Warning is issued during a Winter Storm Warning.

Machine

Moisture drawn into the cold

A winter storm needs moisture and cold air at once. A large storm system supplies both: its winds draw moist air from the south up and over the cold air on its northern and western side, and the air, lifted along the storm's fronts, cools and drops its moisture as snow. The heaviest snow falls in bands within the broad shield of snow, often well to the cold side of the storm's center, Midlatitude cyclone life cycle FB-EVT-060.

What reaches the ground

Snow forms in the cloud. Whether it reaches the ground as snow depends on the air below. If the air is below freezing all the way down, it falls as snow. If it falls through a warm layer it melts; then, if the cold air beneath is deep enough, it refreezes as sleet, and if not, it lands as rain that freezes on contact, freezing rain. One storm can bring all three in bands from north to south, and change from one to another as the warm air advances.

Deciding on a range

A forecast of snow is a range, such as 8 to 12 inches, because the amount is uncertain. The directive decides warnings and advisories on the middle of that range, rounded up to the nearest inch, against the office's criteria; the equation below gives the rule. Fig. A draws the directive's own eight examples: with criteria of 8 in (20 cm), a forecast of 5 to 10 in earns a warning, its midpoint of 7.5 rounding up to 8, and a forecast of 4 to 8 in does not.

Fig. B The national heavy snow criteria. Snow in one event for a Winter Storm Warning, in every forecast zone of the lower 48 and Alaska, as the national map gives it; Albers equal area, Alaska drawn smaller 500 mi (805 km) BAR AS DRAWNMaximize The sheet, SVG, 11 by 17

Seattle, 4 in (10 cm)
Wasatch Mountains, Utah, 18 in (46 cm)
Denver, 8 in (20 cm)
Minneapolis, 6 in (15 cm)
Chicago, 6 in (15 cm)
Buffalo, 7 in (18 cm)
New York, 6 in (15 cm)
Washington, 5 in (13 cm)
Atlanta, 2 in (5 cm)
Dallas, 3 in (8 cm)

Fig. C The directive's clock on one storm. A watch and a warning for one storm on the directive's rules, drawn for the plate, by hours from the storm's start TIME TO SCALEMaximize The sheet, SVG, 11 by 17

The watch, at 50 percent confidence or more
Twelve hours before the start, the watch upgraded or cancelled
The warning, at 80 percent confidence or more
The storm, within 48 hours

Ingredients

A storm system drawing moisture into cold air
Air cold enough, from the cloud to the ground, for the snow to reach the ground as snow
Where a warm layer lies above cold air near the ground, sleet or freezing rain instead
Lift along the storm's fronts to wring the moisture out

Scales

time: hours to two days; an event is set by its office and runs no longer than 48 hours unless absolutely necessary
horizontal: hundreds of miles, across the cold side of a large storm system
vertical: the depth of the storm's clouds, several miles, and the layers of warm and cold air that decide what falls
orlanski: meso-alpha to synoptic

Equations

The midpoint rule

\left\lceil \frac{a + b}{2} \right\rceil \geq C

$a, b$: the low and high ends of the forecast range of snow or sleet, in
$C$: the office's warning or advisory criteria, in

Assumes Warnings and advisories for snow and sleet are decided on the average of the forecast range, rounded up to the nearest inch, against the office's criteria. The forecast is said as a range, such as 8 to 12 inches, or, where it is very uncertain, as up to 12 inches or 8 inches or more.

Working form With criteria of 8 in (20 cm), a forecast of 5 to 10 in (13 to 25 cm) has a midpoint of 7.5 in, rounded up to 8: a warning. A forecast of 4 to 8 in (10 to 20 cm) has a midpoint of 6: no warning. With criteria of 6 in (15 cm), 3 to 6 in has a midpoint of 4.5, rounded up to 5: no warning.

Signatures

surface: the pressure falling as the storm nears; the wind from the east or north ahead of it; snow beginning; often heavy for hours
satellite: a large comma of cloud around the storm; its head over the cold air
radar: broad areas of snow; with heavier bands within them; a bright band where snow melts into rain
sounding: the temperature below freezing from the cloud to the ground for snow; a warm layer aloft over cold air for sleet or freezing rain

Numbers

Quantity	Value, and the kind of number it is
Winter Storm Warning	Snow, lake effect snow where a region chooses, ice or sleet meeting the office's event criteria; or a combination with at least one of them meeting its criteria; or a storm forecast to have high impact though it will not meet the criteriaStandard, Directive 10-513
Winter Storm Watch	Conditions favorable for a winter weather event to meet or exceed the office's event Winter Storm Warning criteriaStandard, Directive 10-513
Winter Weather Advisory	One or more winter hazards meeting the office's 12 or 24 hour advisory criteria but not the warning's, or based on impactStandard, Directive 10-513
When a watch is issued	At 50 percent confidence or more that the event will meet the warning criteria, with as much lead time as possible; not within 12 hours of the startStandard, Directive 10-513
When a warning or advisory is issued	At 80 percent confidence or more, when the weather is occurring, imminent or highly likelyStandard, Directive 10-513
How often they are renewed	A watch at least every 12 hours; a warning or advisory at least every 6 to 8 hoursStandard, Directive 10-513
Impact-based criteria	A warning may be issued for less snow than the criteria early in the season, or at a critical time such as rush hourStandard, Directive 10-513
How long an event runs	Set by the office, and not over 48 hours unless absolutely necessaryStandard, Directive 10-513
From one zone to the next	The criteria change by no more than 1 in (2.5 cm), except where elevation or climate requiresStandard, Directive 10-513
Warning criteria in the lower 48	From 1 in (2.5 cm) of snow in one event, across the far South from Texas to Georgia and Florida and in parts of Arizona and California, to 12 in (30 cm) in the western mountains and 18 in (46 cm) in Utah's Wasatch MountainsStandard, Winter Storm Warning Criteria for Heavy Snow
The most common criteria	6 in (15 cm), in 1,072 of the 3,850 zones of the lower 48Standard, Winter Storm Warning Criteria for Heavy Snow
Higher ground	In 80 zones a higher figure above an elevation, such as 8 in (20 cm) above 5,000 ft (1,524 m) in northern ArizonaStandard, Winter Storm Warning Criteria for Heavy Snow
Warning criteria in Alaska	From 6 to 24 in (15 to 61 cm)Standard, Winter Storm Warning Criteria for Heavy Snow

How the station sees it

The cooperative observers measure the snowfall and the depth on the ground once a day, and the airport stations report the temperature, the wind and the weather falling every minute. Where a Winter Storm Warning, Watch or Winter Weather Advisory is in force, the places it covers carry it, and each alert has its own report.

Cooperative observers: the snowfall and the depth on the ground once a day
Airport weather stations: the temperature, the wind and the weather falling, every minute
Severe Alerts: every Winter Storm Warning and Watch and Winter Weather Advisory in force, each with its own report

How it is warned

A Winter Storm Watch is issued, with as much lead time as possible, when there is a 50 percent chance or more that a storm will meet the warning criteria; it is renewed at least every 12 hours. It is not issued within 12 hours of the storm's start: by then the office cancels it or turns it into a warning or an advisory. A Winter Storm Warning or a Winter Weather Advisory is issued when the confidence reaches 80 percent or more, when the weather is occurring, imminent or highly likely, and is renewed at least every 6 to 8 hours until the storm ends. Fig. C draws the clock on one storm.

A warning can be issued for one kind of weather, such as heavy snow or ice; for a mix in which at least one meets its criteria; or for a storm that will fall short of the criteria but strike at a bad time, early in the season or in the rush hour. Each event is set by the office and runs no longer than 48 hours unless absolutely necessary. Across the lower 48, the criteria change by no more than an inch from one zone to the next, except where elevation or climate requires; in 80 zones of the West a higher figure holds above a stated elevation.

A Winter Storm Warning for heavy snow means the snow will make travel dangerous: travel only in an emergency, and if you must, keep an extra flashlight, food and water in the vehicle.

VTEC	Phenomenon	The alerts that carry it
`WS`	winter storm	Winter Storm Warning; Winter Storm Watch
`WW`	winter weather	Winter Weather Advisory

Every alert in force now

Sources

National Weather Service. Directive 10-513, WFO Winter Weather Products Specification.
National Weather Service. Winter Storm Warning Criteria for Heavy Snow, Directive 10-513 Appendix D.

Definition after the Directive 10-513, WFO Winter Weather Products Specification. Plate FB-EVT-080, revision 1, 2026-10-17. The number is permanent; cite it.

Winter storm