Let It Snow

Depending on where you are in the state of Maine, winter-like precipitation may fall anytime from September to May, a mind-boggling nine months out of the year. Average conditions are not that extreme, as some parts of the state are more likely only to see snow between October and April — a time period that is still more than half the year. The words snow, sleet, and freezing rain can bring chills to even the most seasoned traveler in Maine, as roads may become virtual skating rinks.

There are two types of storm systems responsible for most of the snow we see in Maine. The most notorious are those that follow the Atlantic track up the eastern seaboard. These coastal storms, or nor’easters, named for the northeast winds off the ocean that characterize them, are responsible for the highest snowfalls throughout the state. A nor’easter typically forms in one of two ways: first, as a single low-pressure system that forms in the western part of the Gulf of Mexico, close to Texas. These storms usually move eastward across the Gulf of Mexico, turning northward along the East Coast. Very often they seem to jump from Florida or Georgia to a point around Cape Hatteras, North Carolina, and in the process they often intensify. From there they progress up the coast, either moving out to sea before reaching Maine or moving along the Maine coastline and into the Canadian Maritimes. The second common way involves the development of a secondary storm. In this scenario, the initial storm will often move into the Northeast via the Ohio River Valley or across the Great Lakes. This storm will begin to weaken or “fill-in.” Then a secondary low-pressure system will begin to form somewhere off the mid-Atlantic coastline or possibly toward the Gulf of Maine. The first storm will transfer its energy to this second storm, which then becomes the primary coastal storm.

Regardless of the method of development, a frequent characteristic of a nor’easter is that it often intensifies or deepens very quickly. A storm whose central low-pressure drops twenty-four millibars over twenty-four hours is referred to as a meteorological bomb. This rapid deepening will produce very strong winds and very intense snowfall rates — both often a part of major nor’easters. At times these very strong storms appear to have an eye, much like a hurricane.

Of course, not all nor’easters are strictly heavy snow producers as that depends on the all-important storm track. The easiest way to predict whether you will get all snow, a mixture of everything (snow, sleet, freezing rain, rain), or just rain is to determine what side of the storm you will be on. If the storm moves to your west, you will be on the warm east side and precipitation will likely be all rain. If the storm moves east of you, then you will be on the cold west side and you may very well get all snow. If the storm moves over you, then you could easily get all types of precipitation. These are only generalizations, but here are some common scenarios:

Scenario 1: The storm is well off the coast of Maine. In this situation, western Maine and especially the west side of the mountains may get very little snow. Interior sections and the eastern slopes of the hills may get a little more snow, but the heaviest snows will be along the coast, especially in Down East sections. This would probably be an all-snow event for areas that received any precipitation.

Scenario 2: The storm is slightly off the coast of Maine. In this situation, western parts of the state and the mountains may get moderate amounts of snow with higher amounts along the eastern slopes of the hills. Interior sections may get the heaviest snow from this scenario. Coastal areas may start as snow, but then precipitation very well may change to mixed precipitation and possibly rain as warmer air is brought into midlevels of the atmosphere off the ocean. This changes the snow to mixed precipitation. However, the warmer air may penetrate farther inland than expected for a given track if the storm is very intense. Warmer air can often move up the Penobscot River, turning the snow to mixed precipitation even past Bangor.

Scenario 3: The storm moves up along the coastline. Moderate to heavy snows may blanket western Maine, especially in the mountains. Interior sections may start as snow, but would then likely change to mixed precipitation and potentially rain. Coastal areas may start as snow, but the precipitation would quickly change to mixed precipitation and very likely to all rain.

Scenario 4: The storm moves inland, but stays on the east side of the Appalachians. In this situation, western Maine may get mostly snow, although the possibility exists of a changeover to mixed precipitation. The same situation may hold true for the mountains, though higher elevations may receive all snow, which is good news for ski areas. Interior sections may start as snow, but precipitation would likely convert to mixed precipitation with the likelihood that most of the event would be rain. Coastal areas would probably just get rain in this situation.

Scenario 5: The storm moves up the west side of the Appalachians, eventually moving down the St. Lawrence River Valley. This is the worst scenario of all for snow lovers. The only part of the state that may get any snow out of this track is the mountains, and maybe a very brief period of snow in westernmost Maine. However, any precipitation that starts as snow would probably quickly change to mixed precipitation and then rain. Interior sections and coastal areas would likely get all rain out of this track.

Regardless of the track taken, once the storm passes north to northeast of Maine, there is often a period of instability in the atmosphere, with very strong and cold northwest winds. It is possible that northern areas could easily get another inch or two the day after the major event from snow showers and snow squalls generated by this instability. Northernmost Maine can pick up several inches or more after the primary storm has moved into the Maritimes.

The second major type of storm that brings snow to Maine is the Alberta Clipper. This system typically forms around Alberta, Canada, from where it often dips into the northern part of the United States. It then moves quickly through the Great Lakes and into New England. Since these are fast-moving storms, they usually drop no more than eight inches of snow and that is usually in the area close to the track of the central low-pressure system. Snowfall totals often drop off quickly away from the center of the storm. There are times, however, when an Alberta Clipper will explode once it reaches the Gulf of Maine. When that happens, the storm can be very intense, essentially becoming a nor’easter and bringing heavy snows and strong winds to parts of Maine.

There are occasions where atmospheric conditions bring a strong flow of air off the ocean and significant snowfall along the coast, much like the lake-effect snows that hit areas east of the Great Lakes, such as Buffalo. This situation can produce snowfall rates of three to four inches per hour.  However, the airflow off of the water that produces ocean-effect snows rarely attains the intensity and persistence that produces the mind-boggling snowfalls in areas around the Great Lakes. Most ocean-effect situations drop only a few inches of snow over an extended period of time. Ocean-effect snow forms when very cold air moves over the open water, producing significant evaporation of ocean water. The vapor will condense and precipitation occurs as the air moves over land and encounters greater friction from the surface.

The snowiest month statewide is January, with an average of twenty-seven inches. December is the second snowiest, averaging twenty-one inches, followed by February’s nineteen inches.

March is just a bit lower than February, but then November and April average only about seven to eight inches and most of that snow falls in the central interior and northern parts of the state. Likewise, the less than one inch that occurs during October and May is primarily restricted to northern parts of the state. Only a few tenths of an inch of snow falls in the state during September and that is usually in the north. However, some very big snowstorms can occur in early spring because of the great contrast in air masses, as warm, humid air from subtropical parts of the Atlantic Ocean begins to encroach into the cold, dry winter air still residing over much of the United States.

This can set the stage for some major winter storms in late April and even early May.

As winter approaches there is often much anticipation for that first snow of the year. The first measurable snowfall of the year occurs usually during the last week of October over most of the upper third of the state. Coastal sections receive their first measurable snow about a month later than The County, that is, the last week in November, or just in time for a “White Thanksgiving.” The first snow of at least one inch comes about a week later, with the northwestern-most part of the state receiving its first inch or greater snow the first week of November and coastal parts of the state receiving their first inch or more the first week of December.

As anticipated as the first snowfall of the year may be, the last snow of the year is often downright annoying as your thoughts turn to spring. For the most part, both the last one inch or greater snowfall and the last measurable snowfall occur in April, except for the southwestern-most corner of the state. That part of the state, including the towns of Sanford and Kennebunkport, receives its last snow the last week of March. The overall distribution of the last one inch or greater snow is fairly regular across the state, but the last measurable snowfall distribution is very sporadic. This is not surprising, as some late-season storms may have a small burst of snow at the end as colder air infiltrates into the state behind a cold front. This can produce a dusting of snow in many places. The exception is the western mountains and northern parts of The County. Those areas receive their last inch or more of snow the third week of April.

One of the biggest questions in early and mid-December, and even as early as late November around Thanksgiving, is will there be a white Christmas. Snow on the ground during the holiday shopping  season seems to add something to the experience; it puts you in the mood, so to speak. Certainly, living in Maine gives almost everyone in the state better than a 50-50 chance of having a white Christmas. A white Christmas, as defined by the National Weather Service, is at least one inch of snow on the ground sometime during the twenty-four-hour period. In other words, it could start snowing late at night and technically still could qualify as a white Christmas. The other aspect of a white Christmas is that it does not have to be snowing on Christmas Day to qualify — although some purists may argue that point.

Excerpted from Conditions May Vary [1]b [1]y Gregory A. Zielinski; Down East Books; May 2009; paperback; 256 pages, $15.95.