Why Winter Weather Is Difficult to Forecast

School bus in the snow
Wintertime precipitation is one of the trickiest forecasts to get right. John Foxx/Stockbyte/Getty Images

We've all experienced it at one time or another... anxiously awaiting the arrival of the three to five inches of snow in our forecast, only to awaken the following morning to find a mere dusting on the ground.

How could meteorologists get it so wrong?

Ask any meteorologist, and he'll tell you that wintertime precipitation is one of the trickiest forecasts to get right.

But why?

We'll take a look at the number of things forecasters consider when determining which of the three main winter precipitation types—snow, sleet, or freezing rain—will occur and how much of each will accumulate. Next time a winter weather advisory is issued, you may have a newfound respect for your local forecaster.

01
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A Recipe for Precipitation

overrunning.jpg
© 2007 Thomson Education

In general, precipitation of any kind requires three ingredients:

  • A moisture source
  • Air lift to produce clouds
  • A process by which to grow cloud droplets so they'll become large enough to fall

In addition to these, frozen precipitation also requires below freezing air temperatures.

While it may sound simple enough, getting the right mix of each of these ingredients is a fragile balance that often depends on timing.

A typical winter storm setup involves a weather pattern known as overrunning. During winter, cold polar and arctic air is ushered into the United States when the jet stream dips southward out of Canada. At the same time, southwesterly flow streams relatively warm, moist air in from the Gulf of Mexico. As the leading edge of the warm air mass (the warm front) encounters the cold and denser air at low levels, two things happen: low pressure formation occurs at the boundary, and the warm air is forced up and over the region of cold. As the warm air rises, it cools and its moisture condenses into precipitation-inducing clouds.

The type of precipitation these clouds will produce depends on one thing: the temperature of air at levels high up in the atmosphere, down low at ground level, and in-between the two.

02
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Snow

Vertical temperature profile for snow. NOAA NWS

If low-level air is extremely cold (such as is the case when arctic air masses enter the U.S.), overrunning won't greatly modify the cold air that's already in place. As such, temperatures will remain below freezing (32°F, 0°C) from the upper atmosphere all the way down to the surface and precipitation will fall as snow.

03
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Sleet

Vertical temperature profile for sleet. NOAA NWS

If the incoming warm air mixes with the cold air enough to form a layer of above-freezing temperatures at mid-levels only (temperatures at high and surface levels are 32°F or below), then sleet will occur.

Sleet actually originates as snowflakes high up in the cold upper atmosphere, but when the snow falls through the milder air at mid-levels, it partially melts. Upon returning to a layer of below-freezing air, precipitation re-freezes into ice pellets.

This cold-warm-cold temperature profile is one of the most unique, and is the reason why sleet is the least common of the three winter precipitation types. While the conditions that produce it may be fairly uncommon, the light tinkling sound of it bouncing off of the ground is unmistakable!

04
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Freezing Rain

Vertical temperature profile for freezing rain. NOAA NWS

If the warm front overtakes the region of cold, leaving below freezing temperatures at the surface only, then precipitation will fall as freezing rain.

Freezing rain first starts out as snow, but melts completely into rain when falling through a deep layer of warm air. As the rain continues to fall, it reaches the thin layer of below-freezing air near the surface and supercools — that is, cools to below 32°F (0°C) but remains in liquid form. Upon hitting the frozen surfaces of objects like trees and power lines, the rain drops freeze into a thin layer of ice. (If temperatures are above freezing throughout the atmosphere, precipitation will of course fall as a cold rain.)

05
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Wintry Mix

The above scenarios tell which precipitation type will fall when air temperatures stay well above or well below the freezing mark. But what happens when they don't?

Anytime temperatures are expected to dance around the freezing mark (generally anywhere from 28° to 35°F, or -2° to 2°C), a "wintry mix" may be included in the forecast. Despite public dissatisfaction with the term (it's often viewed as a forecast loophole for meteorologists), it's actually meant to express that atmospheric temperatures are such that they're unlikely to support only one precipitation type during the forecast period.

06
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Accumulations

Tiffany Means

Deciding whether or not inclement weather will occur—and if so, what type—is only half of the battle. Neither of these are much good without an accompanying idea of how much is expected.

To determine snow accumulations, both the amount of precipitation and the ground temperature must be taken into account.

Precipitation amount can be gathered from looking at how moist air is at a given time, as well as the total amount of liquid precipitation expected over a certain period of time. However, this leaves one with the amount of liquid precipitation. In order to convert this into the amount of corresponding frozen precipitation, the liquid water equivalent (LWE) must be applied. Expressed as a ratio, LWE gives the amount of snow depth (in inches) it takes to produce 1" of liquid water. Heavy, wet snow, which often occurs when temperatures are right at or just under 32°F (and which everyone knows makes for the best snowballs), has a high LWE of less than 10:1 (that is, 1" of liquid water will produce approximately 10" or less of snow). Dry snow, which has little liquid water content due to extremely cold temperatures throughout the troposphere, can have LWE values of up to 30:1. (An LWE of 10:1 is considered average.)

Ice accumulations are measured in increments of tenths of an inch.

Of course, the above is only relevant if ground temperatures are below freezing. If they're above 32°F, anything that hits the surface will simply melt!

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Means, Tiffany. "Why Winter Weather Is Difficult to Forecast." ThoughtCo, Mar. 28, 2018, thoughtco.com/winter-weather-difficult-to-forecast-3444527. Means, Tiffany. (2018, March 28). Why Winter Weather Is Difficult to Forecast. Retrieved from https://www.thoughtco.com/winter-weather-difficult-to-forecast-3444527 Means, Tiffany. "Why Winter Weather Is Difficult to Forecast." ThoughtCo. https://www.thoughtco.com/winter-weather-difficult-to-forecast-3444527 (accessed May 23, 2018).