The scoop on snow

[Originally, I wasn’t going to include this article in Schreiben Depot; it didn’t seem to have much significance. Being a snow freak, though, I caved in and decided to post it up. This appeared in the Highlander back in 2006.]

Your Guide to Winter Weather

12/3/2006

Kenneth Burchfiel

School day or Snow day? What makes the difference

Snowstorms may appear to be very complex, and listening to meteorologists ramble on about the latest GFS readings doesn’t help. When broken down, however, it’s not hard to understand what makes the difference between a rain event and a 10-inch snowstorm. What follows are a few things that affect the amount of snow that a given storm produces.

The Temperature

At temperatures over 32 degrees, of course, snow cannot accumulate. However, temperature also influences the type of frozen precipitation that falls.

Typically, the warmer the temperature, the thicker and wetter the snow. At 30 or 31 degrees, snow is densely packed and will not accumulate very high. However, this type of snow is very heavy, meaning plows (and those shoveling snow) will have a harder time removing it. At cooler temperatures, snow takes on a powdery form and will accumulate higher. Being lighter in consistency, though, such snow is easier to plow.

Sleet and freezing rain are also possible during a snowstorm; both events are caused by differences in temperature between higher and lower altitudes of air. Sleet is formed when clouds release precipitation in the form of rain, not snow. As this rain nears the ground, it enters colder masses of air, which end up freezing it into pellets, known as sleet. Freezing rain is a similar concept: precipitation that falls as rain strikes colder ground and turns solid. Both forms of precipitation create extremely dangerous driving conditions; while they only influence final accumulations by marginal amounts, schools are often cancelled due to slick, icy road conditions developed by sleet and freezing rain.

Pre-Storm Ground Conditions

Ground conditions present at the time of snowfall can greatly impact final accumulations. Falling snow will stick easily to snow already on the ground, but will often melt if the surface it settles on is wet or above freezing. The type of road surface also plays a major role: Snow accumulates easily on dirt, sticks decently to grass and sidewalks, and has trouble collecting on roadways.

Location storms arrive from

Winter storms arriving from the Northeast will pick up abundant levels of moisture from the Atlantic Ocean, increasing the chance for a big storm. At the same time, such storms are warmer than others, as the Atlantic is much warmer than inland areas in the winter.

Storms approaching McLean from the East will often bring colder temperatures along with them. However, these storms often produce small amounts of snowfall in McLean; the reason lies in the Appalachian mountains, which act as a barrier that block off approaching storms.

Time of day in which storm approaches

Due to temperatures dropping as the sun goes down, storms usually have a better chance of producing snow in the night than in the day. However, the impact is not as great as many may think.

Winter storms usually feature large, far-reaching cloud coverage. This coverage blocks off sunlight and helps keep temperatures down during the day. In the night, however, the opposite effect takes place; heat leaving the ground is blocked by the very same clouds, making temperatures warmer than if the sky was clear. Overall, expect temperatures during storms to vary little.

Radar and its interpretation

By checking radar, students can time approaching storms and judge the intensity of approaching weather. Accuweather.com, an excellent site for radar observation, provides animated maps that can be clicked on.

Students should pay close attention to a few things when viewing radar maps. To start, it is important to check the location of the storm. Colored bands on maps show areas where precipitation is falling. After that, it should be noted what type of precipitation is falling, and where. Snow is usually colored blue, and rain is colored green. The “rain-snow” line separates these colors and shows the area where snow turns to rain, and rain to snow. In some locations, ice or a “wintry mix” may fall. This type of weather is usually colored pink or red on a radar map; it is common to see ice in between rain and snow.

The intensity of a specific area of precipitation is also important to check. Usually, darker patches on a radar screen indicate heavier precipitation in that area; lighter patches indicate less intense weather. If snow is colored gray on a radar, however, more intense patches of precipitation are usually white.

How the Mid-Atlantic region affects snowfall

Wondering how strange snowstorms can be? Look no further than Maryland.

At one location in Maryland, the average yearly snowfall is around 30 inches a year. Just 8 miles to the south, however, the average snowfall is only 20 inches a year. (Most of Fairfax County averages 20 to 23 inches a year.)

What causes this odd phenomenon? The answer lies in the strange geography of the Mid-Atlantic region.

According to G Teacher, “The lay of the land doesn’t affect us so much as the location.” Fairfax County is bordered by two prominent geographic features: the Atlantic Ocean, to the east, and the Appalachian Mountains, lying west of Fairfax County.

Unfortunately for snow lovers, says Teacher, “Our position relative to the mountains makes our winter a bit milder.” This is due to orographic precipitation, an effect in which air traveling east is forced up by the Appalachian Mountains. Water vapor in the air cools as it rises, forming clouds that precipitate on the mountain. Orographic precipitation forces winter storms to release much of their moisture on the mountains, leaving little, if any for Fairfax County. Says Teacher, “You’re not going to see these two-foot blizzards all the time.”

The Atlantic Ocean’s warm temperatures and abundant moisture also play a role in determining snowfall. On any given winter day, ocean temperatures are usually 20 to 40 degrees above land temperatures. While Fairfax County’s temperature may hover around a frosty 30 degrees, the surface of the Atlantic Ocean may be a balmy 60 or 70. Thus, winter storms running along the coast will often be much warmer than inland storms, and may produce rain, not snow as a result.

However, the moisture-rich ocean can also unleash a major winter storm. Nor’easters, storms in which warm air from the Atlantic meets colder air from northern territories and runs up along the coast, are known for their double-digit snow totals and high winds.

How the county makes decisions about snow days

Famous DC snowstorms

If you’ve lived in Fairfax County for a good portion of your life, you might remember one or two of these storms. Each was unique in its own way, and many closed schools for many days at a time. Take a walk down (a very snowy) memory lane!

February 16th, 2006

McLean received 13 inches of snow, more than half its average yearly total. Initial forecasts called for only 5-7 inches, but thundersnow, a rare event in which thunder can be heard during a storm, occurred, raising the final total.

January 25, 2000

This storm, which dumped about 14 inches of snow, arrived earlier than expected, leaving some areas unprepared and surprised.

February 17th, 2003

Known by many as the “President’s Day Storm,” this storm closed schools for days on end. Total accumulations in McLean neared two feet, and snow fell at extremely heavy rates.

January 7th, 1996

In perhaps the most well-known modern Washington area snowstorm, almost two feet of snow fell in Fairfax County. The storm is Washington’s fourth largest of all time.

March 8, 1999

8.4 inches

Although not a blockbuster event, this storm is notable for its late date. 8-inch storms, though rare in January and February, are extremely rare in March.

Source for last one: http://kiat.net/dc/weather.html

Important Links:

http://wrc.iewatershed.com/index.php?pagename=news_031205

http://www.erh.noaa.gov/lwx/Historic_Events/md-snow-avg.gif

http://www.weather.com/encyclopedia/winter/types.html

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