A jogger runs near the reflecting pool at the Mall while a snowstorm begins to create some problems during the morning commute on Feb. 20. (Astrid Riecken/For The Washington Post)

Every year, eager children, panicked parents, frustrated commuters and exasperated employers ask the Capital Weather Gang the same thing: Why is it so difficult to forecast snow in the D.C. region?

In most cases, weather forecasts for winter storms have advanced to the point that predictions are correct in broad strokes. But determining the details of how much snow will fall where, whether it may change to a wintry mix, how intense precipitation will be at given times, and exactly when it will start and stop remains very challenging.

Each storm is unique, and the slightest variations in storm track, temperature, jet stream interactions and atmospheric moisture can lead to dramatically different outcomes.

To make forecasting even more difficult, Washington straddles two regional climate zones: the Atlantic Ocean and Gulf Stream to the east, and the Blue Ridge and Appalachian Mountains to the west. This creates a special transition zone, with warm air on the east side and cold air to the west.

The elevation change between the low-lying areas east of Washington and the high elevations to the west also plays a critical role in snowfall forecasts. It is more likely to snow in higher elevations simply because it is colder. Sometimes, just this slight deviation in temperature can make or break a forecast.

Below, we use a variety of winter storm types to illustrate four common scenarios: a big snow, a few inches, an ice storm and a cold rain.

The big snow

The biggest snows and most ruthless blizzards are almost always caused by the kind of storm people know best: the nor’easter, named for the direction from which the wind blows onto the East Coast. A nor’easter feeds on moisture and heat from the warm Gulf Stream. On its back side, it draws down cold, Canadian air.

(The Washington Post/The Washington Post)

If the storm tracks north very close to the Mid-Atlantic coast, a nor’easter can dump a boatload of snow along the Interstate 95 corridor — crippling major cities from Richmond northward through Washington, Baltimore, Philadelphia, New York and Boston.

The D.C.-Baltimore region averages about one major snowstorm (eight inches or more of snow) every three years, according to long-term statistics. The true blockbuster events, which drop double-digit accumulations, are even rarer, typically one every decade. These storms are truly episodic events; in one recent winter season (2009-2010), our region was slammed by three blockbuster snowstorms. Yet stretches of years can go by without a single snowmaker, leading to the perception of a “snow drought.”

Snow-laden nor’easters feed off strong temperature contrasts between warm Gulf Stream air and bitterly cold Arctic air. They sometimes get their start as storm systems in the Gulf of Mexico and pull vast amounts of moisture northward.

The best snow-making potential comes when zones of storminess from the north and south merge or “phase,” and a storm develops explosively off the Mid-Atlantic coast.

Copious Atlantic moisture often feeds a heavy snow band on the back side of the storm. These bands can remain stationary for many hours, dropping snow at more than one inch per hour, sometimes accompanied by lightning and thunder.

Accurately predicting the timing, duration and snow amounts with big nor’easters remains a headache. One of the most important factors is the storm track, i.e., will Washington be on the “mild side” or “cold side” of the storm?

The sweet spot for a Washington snow bull's eye is a storm that passes parallel to the coastlines of South and North Carolina and roughly 75 miles off the Delmarva coast. Often the critical freezing line near the surface and lowest layer of the atmosphere will drape right along the I-95 corridor, meaning that a slight shift in the storm track by 10 miles or so spells the difference between an inch of cold rain, a nasty mix of precipitation or a foot of snow.

(Ian Livingston)

While critically important, it’s not just the track of the low that determines whether and how much snow will fall. Sometimes we see a near-ideal storm track for snow and rain end up with none falling, even in the heart of winter.

Most of Washington’s blockbuster snowstorms feature a strong zone of high pressure to the north, which feeds in Arctic air. Ideally, that dome of high pressure becomes established north and northwest of Washington over the Great Lakes, the Northeast and eastern Canada one to two days before the coastal storm develops.

Without that high-pressure zone in place, it is rare for Washington to receive substantial snowfall from a coastal storm. Instead, rain or a wintry mix is more likely.

Notable recent big snowstorms

Jan. 22-23, 2016: Snowzilla, 17 to 30 inches

Feb. 5-6, 2010: Snowmageddon, 17 to 32 inches

Dec. 18-19, 2009: Snowpocalypse, 18 to 26 inches

Feb 15-16, 2003: Blizzard of 2003, 17 to 28 inches

A few inches

The vast majority of the region’s snowstorms consist of weak low-pressure systems, or a glancing blow — just a few inches but enough to wreak havoc. We see it play out every winter, often in the form of an Alberta clipper, which gets its name from the Canadian province where the storm tends to originate.

Clippers are cold enough to deliver snow if they track south of Washington, but they tend to be moisture-starved, weak and fast-moving, so we don’t often see big snow totals from them. Their accumulating snow often falls in a very narrow zone, making them difficult to forecast.

But sometimes it doesn’t take much snow for these clippers to become extremely disruptive. Subfreezing pavement plus a quick inch or two of snow falling during the morning or evening commute can paralyze our region.

On Jan. 21, 2016, a narrow band of light snow hit just after dark as air temperatures rapidly plummeted. The weight of vehicle wheels compressed and melted the snow, creating a flash freeze on area streets — and shear gridlock, or a slow-motion slip-and-slide. With roads clogged by stuck vehicles, salt and sand trucks couldn’t gain access. Many people spent an unforeseen night in their cars.

The National Weather Service now issues special commuting hazard statements when such scenarios may arise.

In many cases, however, these clippers are fairly benign. If they pass to our north, which is pretty common, they tend to draw up enough mild air for more rain than frozen precipitation.

The ice storm

Snow lovers often cheer on the arrival of a major snowstorm, but few ever root for an ice storm. The D.C. region is particularly prone to ice because of its proximity to the mountains, which act as a dam for the cold air drifting in from the north. Dense, subfreezing air pools near the ground (deepest up against the eastern mountain slopes) so precipitation that might have otherwise fallen as rain becomes supercooled and freezes instantly when it makes contact with the ground.

Meteorology aficionados and the pros refer to this scenario, in which cold air piles up east of the mountains, as “Appalachian cold air damming” or simply “the Wedge.”

Car accidents, trips to the emergency room from falls on ice, and downed tree limbs and power outages constitute a trifecta of misery when Mother Nature lays down the ice.

In ice storms, the biggest forecast challenge that often emerges is determining if and when the frozen precipitation will change to rain. Computer models often predict that temperatures will warm faster than they do, underestimating the staying power of cold, dense air. These sometimes mean that slick, hazardous conditions linger longer than forecasters anticipate.

Notable recent substantial ice storms

Feb. 10-11, 1994: Up to three inches of ice

Feb. 14, 2007: Valentine’s Day storm, four to eight inches of snow and sleet; up to three-fourths of an inch of ice

A cold rain

Snow lovers lament when their hopeful (and perhaps overhyped) snowstorm turns into a “no-storm.” Second only to the misery of an ice storm is a cold rainfall with temperatures hovering at 33 to 34 degrees.

All winter storms have the potential to wind up as a cold rain, given the right track. One type that stands out, though, is the “Appalachian runner.” This storm tracks up the spine of the Appalachian Mountains, well west of Washington, which puts most of the Mid-Atlantic in the warm sector of the storm. Cut off from the cold air, precipitation may begin as snow or a wintry mix but quickly turns to rain.

(The Washington Post/The Washington Post)

Usually, cold rain storms are straightforward to forecast; however, complications can arise near the dividing line where rain transitions to frozen precipitation.

Forecast ‘busts’

Because forecasts for winter storms in Washington are so sensitive to a degree or two and slight shifts in storm track, predictions can occasionally go awry.

Here are four of the most notable winter storm forecast busts over the past 20 years:

  • March 9, 1999: A dusting was predicted; eight to 12 inches of snow fell.
  • Jan. 25, 2000: A dusting to an inch was predicted; nine to 18 inches of snow fell.
  • Dec. 26, 2010: Up to 10 inches were forecast; not one inch fell.
  • March 6, 2013: Up to 10 inches were forecast; not one inch fell.

However, advances in weather prediction have meant these busts are far less frequent today than they were in prior decades.

Jason Samenow and Ian Livingston contributed to this report. This is an updated version of an article that first ran in 2016.