Snow and sleet mix in north Frederick on Dec. 26, 2012. By @btwarner

“Best chance of accumulation is north and west of D.C., especially at higher elevations.”

The above is a common refrain, and surely a frustrating one for metro area snow lovers. Ever wonder why higher elevations usually get more snow than lower-lying areas such as the District?

In this post, I will break down a couple of reasons why our neighbors living on higher ground often have the better chance of seeing significant snow while the rest of us are left wanting under a cold rain or mix. Then, I’ll look back at how elevation played a key role in snow accumulations for three minor winter events in late December.

The higher you go, the colder it gets.

One of the basic principles of our lower atmosphere is that temperature decreases as altitude or elevation increases. This change in temperature with height is called a lapse rate, and the average lapse rate in the lower atmosphere is 3.5°F for every 1,000 feet, or for every 1,000 feet gained in elevation the air temperature is roughly 3.5°F colder. This may not sound like much, but when it comes to D.C. area winter storms where temperatures often hover within a few degrees of freezing, this small change in temperature can be the difference between snow or rain outside your window.

Reagan National Airport is located roughly at 13 feet above sea level while Snowshoe Mountain Ski Resort in West Virginia is located around 4,700 feet. That means all things being equal Snowshoe, WV, can be up to 15°F colder; no wonder they get a lot of snow! Referring to a basic elevation map I made below, we can see how elevation increases from east to west across our region, from the coastal plain gradually upward toward the Appalachian Mountains. Areas shaded in green (D.C. and east) represent elevations from sea level up to about 200 feet, those shaded in the yellows and oranges represent elevations of 200-500 ft., reds represent 500-1,000 feet, and finally white areas (north and west of Loudoun/Frederick counties) are our highest locations at or above 2,000 feet.

Elevation map (Kathryn Prociv, CWG)

Geography: orographic uplift and the rain shadow

The Appalachian Mountains just to our west create an interesting geographical influence on our weather out east. My research at Virginia Tech investigated many of the effects the Appalachians have on Virginia weather. I’ll spare you the thesis and just say that when it comes to winter weather, the mountains can have a profound impact on snow totals. Orographic uplift is the process by which air flowing toward a mountain cannot penetrate the topographic barrier so it is instead lifted up and over. As the air is forced to rise, this causes it to cool, condense, and produce clouds; if the air is moist enough, precipitation (in the form of rain or snow) can result.

(National Weather Service Office in Wilmington, OH)

The side of the mountain where the air is pushed up is conveniently called the “windward side,” and more often than not it occurs on the western sides of the Appalachians since our prevailing winds blow from the west. Now if the windward (west) side of the Appalachians gets all of the precipitation, or feast if you will, the eastern or “leeward” side gets famine. As that same air that was pushed up the mountain then descends down the other side, it warms up and dries out making an effective “rain shadow.” The orographic uplift graphic illustrates this phenomenon.

So, when areas like Snowshoe and Canaan Valley, WV, on the western and windward slopes of the mountains cash in with the white stuff, the District and surrounding counties farther east often receive much less precipitation. The influence large mountain ranges can have on local climates is fascinating. The western U.S. contains even more dramatic examples with the Sierra Nevadas of California and Cascades of the Northwest, where elevations upward of 14,000 feet prevail. Of course the storm chasers out there (including myself) are familiar with the rain shadow of the Rockies, or the Great Plains!

Bringing it all together: Pre-New Year’s events.

We’ve discussed the two major reasons why higher elevations often receive more snow: colder temperatures, and the lifting influence of the topography producing clouds and precipitation. With these in mind, let’s examine the three minor winter events from late December (Dec. 24th, 26th, and 29th.)

The three events were quite similar in terms of storm track. In what is called a “Miller B” track, the low-pressure centers traveled roughly up the spine of the Appalachian Mountains. These types of events almost always lead to a wintry mix or snow-to-rain scenario for D.C. and surrounding counties with the freezing line cutting right through the area. These storms typically bring less snow area-wide than the “Miller A” coastal storms, but higher elevations do usually receive the highest totals.

(Kathryn Prociv, CWG)

In the chart, I compare snow totals from select locations of varying elevation for which I could find National Weather Service snow total reports for all three events. Reagan National is the lowest elevation barely above sea level, and the others increase in elevation heading farther north and west away from D.C. Generally snow accumulation increased with increasing elevation. For example, Frostburg, Md., located in Allegany County (west of Berkeley county on the map), received a good deal more snow than Reagan National (especially on the 26th). Frostburg is not only 2,000 feet higher than Reagan National making it colder by default, but it is also located in close proximity to the mountains nestled within the orographic uplift snow machine.

Referring to the snow depth maps for the three events, the highest amounts were restricted to those locations on or along the Appalachian Mountains. Given the impact of colder temperatures at higher elevations on snow accumulations, is there any wonder why Canaan Valley and Snowshoe with elevations of 3,140 feet and 4,711 feet, respectively, are the locations of popular ski resorts?

Snow depth maps for Dec. 24 (left), Dec. 26 (middle) and Dec. 29 (right) (NOAA)

Is elevation the only factor that determines which locations will get the most snow? Absolutely not! It’s critical to take into account many other factors such as storm track, depth of cold air, atmospheric moisture, etc. However, elevation often does play an important role, especially in our area and along the I-95 corridor when the storm track is inland rather than along the coast. I suggest snow lovers move to higher ground if you want to improve your chances of seeing the white stuff this winter, especially if the pattern continues to favor weaker inland systems over large coastal storms.

They say “the higher you are, the farther you fall.” Well, in the case of snow, the farther it falls, the more likely it is to turn to rain.