Winter is just around the corner — more like upon us in some parts of the continent. For most rural folk, this means the possibility of winter storms and freezing or frozen precipitation. Worth noting, every state has had snow at some point. Even the high volcanic peaks on the Big Island of Hawaii regularly get winter snowstorms.
There are three types of winter precipitation we must deal with when temperatures drop below freezing: snow, sleet, and freezing rain. Let’s take a look at what we may have to face when these occur.
Snow is actually an ice crystal — it’s not frozen rain. Water vapor in the air (a gas) turns directly into a crystal (a solid). The six-sided flake you’ve likely seen pictures of is the most common, but there are other shapes, too. So, what do you need to know about snow?
Snow varies in moisture content and consistency. You may have heard that 10 inches of snow would melt down to 1 inch of water. That ratio actually varies a great deal. When air temperatures are below freezing but close to 32 degrees Fahrenheit, snowflakes are large, the snow is heavy and wet (great packing snow), and the ratio may drop down to 5:1. Lower temperatures mean smaller snowflakes and a lighter, drier snow with ratios as high as 30:1.
Speaking of freezing temperatures, it is never “too cold to snow.” It can snow with temperatures well below zero. But it can also snow when the surface temperature is above freezing, as the snowflakes are formed in clouds well above the surface. It has snowed at temperatures in the 40s, at least at the outset. And snow can start to accumulate even on relatively warm ground. If it’s snowing hard enough, the accumulation rate can exceed the melting rate on the ground.
How hard can it snow? Certainly, an inch or 2 per hour occurs with intense storms. Snowfall rates of more than 4 inches per hour aren’t unheard of. Some of the heaviest snowfalls occur with the “lake effect” snow bands. Cold air blowing across the still-warm waters of the Great Lakes during late fall and early winter can generate intense snow squalls on the lee shores, the shores the wind is blowing toward. Several feet of snow can fall in one event.
What about “thundersnow”? Under the right atmospheric conditions, convection (updrafts and downdrafts) can develop with temperatures below freezing. Snowfall rates are extreme.
As for the results, roads and sidewalks can become slippery once they are covered with as little as 1⁄4 inch of snow. Under the worst conditions, this occurs in minutes. With accumulations of 4 to 6 inches of wet snow, you can begin to get tree branches breaking under its weight and power lines down. Even greater amounts can cause roofs to cave in. As for driving, heavy snow reduces visibility at least to 1⁄4 mile. “Whiteout” conditions with near zero visibility are possible. This can happen even when snow isn’t falling from the sky, but rather being blown by the wind. In these situations, it’s best to avoid travel. Hunker down and enjoy a spot by the woodstove.
Sleet is frozen rain. It occurs when there are above-freezing temperatures aloft but a fairly deep layer of cold air with below-freezing temperatures at lower levels. Rain droplets fall through this layer of cold air and freeze into ice pellets before they hit the ground. Sleet is the least common winter precipitation. It will not stick to surfaces and it is less slippery than snow or freezing rain, or “glaze,” but sleet will accumulate on roads and other surfaces and can create slick spots.
Freezing rain, or glaze, consists of liquid rain that freezes on contact to the surface. Freezing rain occurs with similar situations to sleet with warmer air aloft, but in this case, the cold layer is shallow, and the water droplets don’t have a chance to solidify before they hit the ground. Freezing rain can occur with surface temperatures near zero — a very shallow layer of cold air. There have even been cases of freezing rain with the air temperature above freezing. This can happen if ground surfaces are below freezing, if it had been very cold before the rain started. How quickly the ice will accumulate depends on the rainfall rate and the temperature. Heavier rain and lower temperatures leads to more ice.
Freezing rain is the worst of the winter precipitation types. It can cover everything with a coating of ice. Roads and sidewalks become incredibly slick and dangerous with minimal ice coverage, which can occur almost instantly. The signs that warn bridges and overpasses are the first to ice over are true, because there is no relatively warm ground underneath them to keep ice from forming. Black ice occurs when meltwater refreezes on surfaces, especially at night. It’s hard to see on road surfaces and is very dangerous.
If 1⁄4 inch of ice accumulates, we start getting damage. Power lines get too heavy and break, causing widespread power outages. Tree branches will start breaking and sometimes whole trees can come down, which can take down more power lines or damage structures and vehicles. Some storms have produced ice accumulations of 2 to 3 inches. Tree damage in these cases can be massive. In the most extreme cases, whole forests can be lost.
Shallow layers of cold air can penetrate far south in winter. Ice storms can occur from Texas to Georgia. Mountains in the West tend to block shallow cold air masses, and freezing rain is rare outside some mountain valleys. Not so in the Northeast, which has the greatest occurrence of freezing rain.
Winter precipitation typically occurs with winter storms. These are the low pressure areas you see on the weather map or, as they are officially called, “extratropical cyclones.” A cyclone is a low pressure area, and extratropical means it formed outside of the tropics. These surface systems are tied in aloft with the jet stream, which separates cold, polar, or arctic air to the north from warm, tropical air to the south. It’s the temperature contrast that provides energy for the low. Sometimes a particularly strong winter storm is called a “bomb,” having undergone bombogenesis in which the storm system strengthens very rapidly.
Aside from precipitation, these winter storms typically produce strong winds, which by themselves can knock down trees and cause widespread power outages as well as wreaking havoc on all forms of transportation. How strong can winter storms get? In October 1977, a storm in the Bering Sea had the lowest central pressure of 27.35 inches of Mercury (Hg) ever recorded in North America for a non-tropical system. Hurricane force winds battered Adak, Alaska, for over 12 hours, with a peak recorded gust of 125 mph. An even stronger storm, Braer, occurred in the North Atlantic in January 1993. With a barometric pressure under 27 inches Hg, it would have qualified as a Category 5 hurricane in a tropical setting.
Winter storms often follow preferred tracks depending on the overall flow pattern in the atmosphere. Typically, when one half of the country is getting hit repeatedly by a series of storms, the other half is dry. The Pacific Ocean is a breeding ground for storms. Usually they either stay to the north, hitting the Pacific Northwest, or they stay to the south and hit California. This year (2017) was unusual and welcome with storms hitting up and down the coast, alleviating California’s long-time drought. (The Sierra Nevada received 29 feet of snow in just over 20 days at one point last winter.)
Storms — at least their surface reflection — tend to dissipate in the western mountains and then reform in the lee of them. Some may stay south and become “gulf lows,” bringing precipitation to the South and Southeast. Some, the “Colorado lows,” may head north from the central Rockies toward the Great Lakes. With a very northern storm track, lows form in the lee of the Canadian Rockies, “Alberta lows,” and race southeast across the northern tier of states. Typically, they are moisture lacking but can usher in very cold, arctic air. And if the prevailing weather pattern is favorable, we can get storms coming from various directions that will eventually move up the East Coast. There are the classic “nor’easters,” which can bring heavy precipitation and strong winds all the way up to the Canadian Maritimes.
Can winter storms be forecast in advance? Yes. You can find the official winter weather warnings that can be issued by the National Weather Service on Page 37. Major storms can be predicted days in advance, sometimes a week or more. However, forecasting specific amounts of winter precipitation for one location is difficult, even in short range.
You’ve probably heard names given to winter storms. The Weather Channel (TWC) started doing this in November 2012, and it can be picked up by media outlets. The NWS (National Weather Service) does not do this. TWC feels that names make it easier for the public to track a particular storm. The NWS believes winter storms vary too much in intensity and effects locally.
One term you’ve probably heard in the past several winters is “polar vortex.” It’s used as an “Armageddon-esque” description of a coming cold spell. However, it’s not a storm. In fact, it’s not found anywhere near the Earth’s surface. It’s an upper-level low-pressure area associated with the coldest air aloft. In the summer, it stays near the North Pole. In winter, it migrates southward, often remaining in central Canada. If it drops farther south, it will be accompanied by an arctic outbreak but not necessarily much winter precipitation. It’s been around for thousands of years, but rarely does it cross the U.S. border.
The NWS has a number of advisories it will send out when winter weather threatens a region. The NWS issues “watches” for potential events up to 48 hours in advance. A “warning” implies an imminent or occurring event.
Winter storm watch/warning: a more general advisory for heavy snow, damaging ice, or a combination of both with strong winds.
Heavy snow warning: snowfall of 4 inches in 12 hours or 6 inches in 24 hours; snowfall criteria will vary by region with lower amounts in the south and higher amounts to the north.
Freezing rain advisory: if accumulation less than 1⁄4 inch is expected.
Ice storm warning: expected damaging ice accumulations of over 1⁄4 inch.
High wind watches/warnings: potentially damaging sustained winds of 40 mph or greater, or wind gusts of 58 mph can be expected for at least an hour.
Blizzard warning: snow and/or blowing snow reducing visibility to 1⁄4 mile or less, and winds of 35 mph or greater. No temperature requirement.
For 30-plus years, Ed Brotak taught thousands of college students about the weather, and helped hundreds of them pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife — also a meteorologist — and his two daughters, who vow never to be “weather weenies.” He still goes outside when he hears thunder.
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