Chapter 12 - Lake Effect Storms

As long as I have been interested in flying, and that goes back decades, I have read stories about the "different weather" around the Great Lakes. Having spent a few years living and working in the Rochester, New York area within five miles of Lake Ontario, I can say that the weather there is no different from other places. It's just that in winter, the storms are more intense. Lessons learned near the Great Lakes can be used in every state and province of this great continent (and presumably in any other continent), because every area has lake effect storms. I have seen one put an inch of snow down just east of the St. John's River in Florida and have driven through a dandy little one whose source of moisture was the two-mile-wide Pepacton Reservoir in southeastern New York. When the wind is out of the northwest, and when the temperature is below freezing and is at least ten degrees colder than the lake temperature, people who live along the Great Lakes know that those bands of clouds which come off the lake can put down prodigious amounts of snow.

For aviation, lake effect storms can provide extreme hazards from intense snow on final to very rapid icing buildups on the airframe during IFR operations. For meteorologists they provide insight into the formation of clouds and precipitation.

Lake effect storms aren't low pressure areas. Of course, normal low pressure areas do cross the Great Lakes and the lakes can intensify the cyclone. The storm from which Gordon Lightfoot memorialized the men who lost their lives in the song "The Wreck of the Edmund Fitzgerald" was a low pressure cyclone. If you are curious about that particular storm, Steve Ackerman of the University of Wisconsin has put together a very readable study of it at http://cimss.ssec.wisc.edu/wxwise/fitz.html. They do have some big ones on the Great Lakes.

Lake effect storms are quite different from cyclones. They are generated when cold air, usually after a cold frontal passage, flows over the warm lake water. True, there is a cold core low pressure area somewhere around, but it's generally in Eastern Canada or off the New England coast by the time the lake effect storms get going. Yet the energy sources for these storms is the same as all the rest of the weather. The lake effect storm is a microcosm of the large scale circulation. Figure 12-2 is a small part of one of the best generators, Lake Erie. The lake, unlike the others, is a shallow lake and freezes over in the winter. Lake Ontario is a deep one and there may be a few days in August when the water is warm enough to swim in at Hamlin Beach but even then one wants to swim vigorously. Most of the other Great Lakes are deep as well. Shallow or deep the water in all these lakes absorbs a lot of sunlight, and it stores it in the form of heat, although it doesn't store as much as the much bigger Mexico does.

Figure 12-2 also shows the land topography which plays a part in the development of the snow belts. If you look carefully, the land next to the lake slopes very gradually away from the lake and is quite smooth. From Buffalo, New York to Cleveland, Ohio, the south shore of Lake Erie is a wide, flat glacial plane which slopes gradually towards a ridge of rolling hills. The plane is almost as wide as the lake before it gives way to an extension of the Allegheny Plateau. Right next to the lake is the New York State Thruway. Driving this section of the thruway is perhaps the best place in the world to log time navigating these snows, but do it with at least four wheels in contact with the ground. In the lower right hand corner of the sectional notice there are hills which force some nice boosts in snowfall amounts as well.

Figure 12-3 is a cross section from southeastern Ontario through the lake and into the western Pennsylvania/New York area. Northwest is to the left and southeast to the right. As the cold northwest winds come across the warm lake they pick up a lot of moisture. The inversion layer in Figure 12-3 is a "cap" on the activity below. The bottom of the cap extends from a few thousand feet over the lake to well over 12,000 feet inland. The lowest layer is a well mixed layer, one where gusts distribute the moisture and heat throughout the layer. So the lake heats the air at the bottom of the atmosphere and adds copious amounts of water vapor to it. The gustiness mixes the warmed and more humid air upward throughout the layer capped by the inversion. The tops of these storms can be higher than the service ceiling of the planes we have in the club. If you plan to fly over one of these, make sure your turbo charger is working well and that you have oxygen for everybody.

Even your local outdoor community pool modifies the winds. In fact, that's a good place to start. Summer in the Washington, D.C. area is abysmal for a person brought up in upstate New York. The mountains just to the west of the D.C. area seem to block the drier surface air from the west and the humidity gets oppressive. I've given up looking at the high temperatures forecast for the day and concentrate on the dew point. If the dew point is over 60, it's going to be one of those days for me. Unfortunately, July, August, and the first few weeks in September rarely have dew points below 65 and frequently they are in the 70s.

It's then that the pool comes in handy. Even then, after two weeks of steamy summer weather, the pool becomes tepid. It's hardly worth the effort to go over except for the exercise. Then a few days of relief break through the hot and hazy climate and the dew points drop to a refreshing middle 50s. As this dry air blows over, the pool temperature drops drastically, and it's once again refreshing. If you've never noticed it, try to get to your pool regularly next summer, not only for the exercise, but to keep tabs on the pool temperature. Consider it an aviation safety exercise.

It takes the release of a lot of heat to cool the water 5 degrees Fahrenheit. And, that heat came out of the water in the pool. At the water's surface, much of the time there are just as many water molecules leaving the pool to the air as there are going from the air to the pool. Said another way, if more molecules leave the pool than return, the water is evaporating. If more condense on the pool's surface than leave, the overflow lets water go down the drain. After a dry spell, the lifeguards usually have to add considerable water to keep the drains that clean the surface working.

A warm pool means that the average speed of the water molecules in the pool is higher than normal. The faster ones are the ones which can break out of the surface and "evaporate" leaving the slower ones behind. If the air is also warm and muggy, the air will have just as many fast molecules as the water, and the result is no net evaporation. If a cool dry breeze comes by, most of the water molecules in the air will be to slow to break through to the pool's surface into the liquid. But the hot pool molecules will ram their way in to the air and move along with the breeze away from the pool.

The pool is acting like a big wet bulb of a sling psychrometer. You probably saw one when you were in high school, maybe even made one. If you did, the evaporating water from the wet bulb caused the thermometer reading to go down. The temperature reading on the wet bulb, called the wet bulb temperature, is roughly halfway between the air temperature and the dew point temperature. It's not exact because of differences between instruments but close. Say the temperature is 80 and the wet bulb reading was 70, then the dew point is around 60.

It works the other way as well. If the temperature is 34 degrees F and the dew point is 24, then the wet bulb temperature is 29. Since raindrops and snowflakes are wet, the temperature of the surface of the precipitation will be at the wet bulb temperature. So we can get snow falling even though the air temperature is above freezing. When this happens, the flakes are usually big and sticky and could stick to a cold window or airframe when you are slowing down on final. In winter situations like this, it's a good idea to add a line "windshield defroster on" to your pre-landing checklist, especially if you get a report that there is precipitation and the wet bulb temperature is in the neighborhood of freezing. The same idea works elsewhere. The rain during a thunderstorm is cooler than the temperature of the air it’s falling through. So, too, snowflakes will be cooler than the air temperature. You can get snowfall when the air temperature is around 35 Fahrenheit, if the wet bulb temperature is below 32 and the snow won't melt.

Lake Erie and the other Great Lakes, or any other body of water for that matter are just large pools which operate the same way, but in the late fall and winter. When cold dry air blows across warm water the water evaporates in copious quantities. As it nears the shore, the cold moist air runs into problems. The air just past the beach has been slowed by increased friction with trees, houses, fences, cars, telephone wires and the like. Figure 12-4 shows the idea. The line at the shore indicates the shape of the frictional effects on the general wind. As it slows down, it backs the air over the lake up and, just like cars in a traffic jam, the air near the beach goes anywhere it can. It can't go down, so it must go up.

Earlier on, you learned that air going upward cools according to the adiabatic lapse rate. When the temperature of the rising air reaches the dew point, condensation starts to occur forming clouds. Since the jamming is continuous, cloud after cloud forms with one following the other like coaches in a train. The air is almost thrown upward and clouds form fast. Since it takes some time for snow to form, the droplets are supercooled, just the recipe for lots of heavy rime icing. Once snow starts to form, the scavenging process proceeds but there can still be a lot of icing there.

Each cloud drops a load of snow, how much depends on the temperature in the snow forming part of the cloud. If the temperature is around -15^o C, the most efficient snowflake forming temperature, the cloud will produce flakes in profusion. If the temperature is higher, more of the cloud drops will evaporate along the edges and icing can be more severe. The best place to be when you are flying in one of these is in the clear areas between snow bands.

Forecasters have found that the upper lakes - Superior, Huron, Michigan - tend to have slightly different characters to these cloud and snow bands from Erie and Ontario. The upper lakes tend to have bands which start near the North shore and move parallel to each other toward and past the South shore. They tend to be smaller and more numerous than those on the lower lakes. Lakes Erie and Ontario tend to have only single bands which move along the long axis of the lakes. The bands tend to be wider and longer and tend to stretch out well over the downwind shore. When the wind is flowing along the longest part, these two lakes will often have a single snow band which extends the entire length of the lake and goes well onshore. It is often curved as the winds aloft will move the clouds with it as the clouds grow upward.

Wind shear, the change in wind with height, is a pretty good determining factor. If all else is good for storms (including strong surface winds), if the wind changes direction more than 60 degrees over the first 12,000 feet, lake effect storms will probably not occur. If the wind turns 30 degrees or less, the storms will probably be multiple bands. In between, the storms will most likely be a single band of snow clouds, one after another. Other techniques are being tested to try to predict with a statistical benefit the locations of the snow bands before the storms start. The people who plow our roads and runways would like to plan their shifts better. Figure 12-5 shows a satellite picture of snow bands on Erie and Ontario. Both bands laid down over 20 inches of snow.

Once established, these bands can lay down a foot of snow in a matter of hours. Figure 12-6 shows one of these snow bands. The bands are remarkably stable in position above the ground. On each side of the bands, the sky is often a clear blue, sometimes "severe" clear. The bands of clouds will lay down a two-foot-deep band of snow 15 or so miles wide but on either side, there will be no snow on the ground. The intensity and depth of snow is a function of a number of things but primarily on the fetch or amount of clear water the air has blown over. For the eastern lakes, take into account if the air has been preconditioned by blowing over Georgian Bay of Lake Huron as it frequently does.

Stories have been told of caravans headed by snow plows which form up along the road just before one of these bands. The story I heard was that the plows lined up three across the road, lights flashing and engines idling. When sufficient cars backed up behind, the plows took off, diesel engines roaring and chains clanking on bare pavement. The line hit the snow like a football line during a run off-center. When the plows made it through the line of snow, they pulled off the road to the center and the cars behind went on through. They then crossed the median to line up behind the "conga line" in the other lane. I have never participated in one. I have learned never to park the car pointed into the wind when a Lake Effect Storm is forecast. I did one night, and snow drifted into the engine compartment and plugged it full. To get it started, we had to tow it to a heated garage and let it melt out. Only then would the engine have enough spark to keep it running. That storm put down enough snow that schools were closed for a full week. They plowed the roads the next day, but the school buses couldn't make the corners because of the snow banks.

Once formed, the band of falling snow produced by a lake effect snowstorm usually does not move much until the wind changes direction. To see why, let's look at the processes which are going on. If the wind speed is slow, clouds may form out over the lake where the updrafts are sufficiently moist. When conditions are right, the snow falls back into the lake. More likely, the bands will start to form as the updraft is bunched up as it reaches the eastern shore. Once the latent heat is released, the roll circulation is intensified with downdrafts (read clear blue sky) between bands and moisture being concentrated and more snowfall in the bands.

Sometimes many bands occur, other times only a few or even one band will form. How many and where they will form may depend on a little change in wind, the strength and height of the inversion above the lake, and the temperatures of the air and lake. The forecasters along the lakes have grown quite adept at forecasting these snowy flows; however, in other states lake effect storms are rare so the forecasters may not be quite as quick to pick them up. Since the flows are on smaller scales than the computer simulations (but perhaps not for much longer), you need to be on your toes when flying just downwind of large bodies of water when the temperature is below freezing and the water is free of ice.

The eastern shore of Lake Ontario has perhaps the worst situation possible. If the winds aloft are right, they can swing across Lake Superior, down Lake Huron, across part of Lake Erie and most of Lake Ontario and end up rising over the Tugh Hill Plateau and then the Adirondack Mountains. One storm in 1988 deposited 70 inches of snow in two days. According to one of the observers, Gerald Morczk, this was one of the heaviest snows in his location that he could remember. This location averages 225 inches of snow annually. It is small wonder that the DPWs in that region have big plows on big trucks to handle it. Fortunately for the guys on the plows, the snow from a lake effect snow is usually not dense. About 20 inches of that snow, when melted, made one inch of water. Normally 10 inches of snow will melt to one inch of water, as long as it's "dry" snow. Figure 12-7 is a map of the total snowfall of the storm which was in the satellite image of Figure 12-5 as measured by cooperative observers.

Perhaps the greatest expanse of lake effect storms occurs over the biggest body of water near the eastern U.S., the Atlantic Ocean. Figure 12-8 is a satellite picture taken after a cold front had pushed out into the Atlantic. The cold air flowing from the Northwest out over the warmer Atlantic Ocean water gets easily organized into roll clouds. Data taken near these clouds indicates bases are around 3,000 feet and the shortest distance between the rolls seen on the satellite pictures is of the order of 100 miles.

If you look closely enough at Figure 12-8 the bands which extend outward from New York City, Philadelphia, and some other major cities are more pronounced than the other ones. It may be, not proven yet, that the heat rising from the cities will organize the cellular motion. We've spotted this enough times that it's not a coincidence; yet, it doesn't always work.

In Figure 12-8, the sky over Pennsylvania is so clear you can see the snow cover on the ground. There are clouds coming off Georgian Bay, moving over Ontario and becoming more intense over Western Lake Ontario. These clouds did produce some light lake effect snow along the New York and Pennsylvania shores.

Even on calm cool days, standing or running water has an effect on the air above it. The Potomac River sometimes has a mirroring river of cloud or haze suspended above it. The thin cloud above the rivers in the early morning light is fascinating and makes for good photography. Even if you can't see it, there is a good chance that on a quiet clear morning, there will be some very tiny droplets suspended in the air just above a lake or a river. One of my instructors told of coming into Dulles Airport early in the morning on a cold fall day. The winds were calm and he had no problem holding the ILS glide slope. The ILS approach for runway 19L goes just over the Potomac, a beautiful pastoral scene. He said that after he had established his approach, he was watching the glorious morning. As he was gliding across the Potomac he was jolted out of complacency when a layer of ice suddenly coated his windshield.

Fortunately, he was able to get the defroster on, remain on the glide slope, and check his progress through the Piper's little open window. (Visual contact with the ground probably saved him from vertigo.) The ice started to melt off just as he was over the inner marker, and so he made a normal landing. I think that's a little too exciting for me. I now add windshield heat to my pre-landing checklist whenever the temperature is in the neighborhood of freezing and I'm near a river even on a clear day.

All of these phenomena have a common thread. Cold air flows over warm water. The cold air picks up the fastest molecules at the surface and transports them away from the surface. After the air becomes moist and the air is forced upward the air cools to the dew point. The moisture then condenses on dust or salt nuclei to form fog or cloud. If there is sufficient moisture, and the air is forced upward enough, precipitation occurs. If you are in the area when these storms occur, try to stay VFR as much as possible, and if your destination is beneath one, plan to land in zero-zero visibility.

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