From Don Paul, a primer on lake-effect snow - The Buffalo News

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From Don Paul, a primer on lake-effect snow

“Cold air over warm lake waters equals lake-effect snow.” Right? Not entirely, and there is not nearly enough information there for a useful forecast, especially a geographically targeted forecast.

Here are some basic principles, before we get down to the nitty-gritty. The greater the difference in temperature between the lake surface and the atmosphere at about 5,000 feet/850 millibar pressure surface, the more buoyant/unstable the relatively milder, moist air just above the lake becomes. As I say on the air, it becomes “bubblier.” That dropoff in temperature is called the lapse rate.

LES-ConceptualModel-NOAA-525px

As a rough rule of thumb, we like to see a lapse rate of 13 degrees Celsius from the lake to 5,000 feet to get lake effect going, but that is not ironclad. However, when that lapse rate goes much beyond 13 degrees Celsius, the instability increases and can occasionally range all the way to extreme. Extreme instability was part and parcel of the November storms of two years ago. During the October 2006 storm, we saw a lapse rate of 20 to 22 degrees.

We also like to see air that is not exceedingly dry to begin with, before it crosses the lake. Arctic air is important, but if it is moisture-starved, and the flow is curved by flowing around a ridge of high pressure (anticyclonic) rather than curved by flowing around low pressure (cyclonic), it will have a tendency to sink. Sinking air “squishes” the clouds, and reduces or eliminates lake-effect convection.

Next, we like to see a flow that is fairly uniform in direction from the lake up to 5000 ft/850 millibar pressure surface. If the winds shift more than 30 degrees in direction on the compass with height in the first 5,000 feet, that’s wind shear which can tear the lake-effect band apart. In other words, if the surface flow is originating from 250 degrees on the compass, but the flow at 5,000 feet is originating from 290 degrees, little lake effect will develop and what does will be multiple band and poorly organized. If the shear to 10,000 feet is greater than 60 degrees, that’s even worse. And, the arctic air has to be deep. It can’t be shallow, with warmer air aloft for true lake effect to set up.

Lake_Effect_Snow_over_Michigan-480pxAs for wind direction, here are some broad rules of thumb concerning northerly, northwester, western and southwestern winds near the surface and targeted regions for lake effect. In the image to the left, the striation of the lake bands really depicts the shifting wind directions from northwest over Lake Superior to west-southwest over Lake Erie.

However, if a bloke like me is trying to make a useful and detailed forecast for location of lake-effect band(s), I need to know far more than the approximate low-level wind direction. “Southwest” just isn’t enough. I need to know the precise likely wind direction in degrees, as on a compass.

From decades of experience and research, we know a wind originating from 240 degree steers lake-effect snow toward a large portion of Amherst, Clarence and Akron, generally missing Buffalo. A 250-degree wind hits the biggest populace, including most of Buffalo and out through Cheektowaga and the airport, sometimes reaching to Batavia. Just a 5 degree shift to 255 degrees can spare downtown and north Buffalo and focus on South Buffalo, Lancaster and Depew.

Illustration of a compass faceAs the wind direction veers a wee bit more westerly, the band can focus more on the densely populated Southtowns; a bit more and it reaches more rural portions of southern Erie below the southtowns and Wyoming County. Due west, or 270 degrees, takes the snow directly into the Southern Tier; 275 or 280 degrees may target the far Southern Tier (Jamestown) more than Silver Creek. The rougher, hillier terrain to the south forces more lift of the moist air, and this frictional lift can squeeze out lots more snow. That’s why Mayville is SO much snowier than lakeshore Westfield in Chautauqua County.

Anyway, I hope you get the picture. In the more densely populated part of Western New York, if your wind direction forecast is off by more 5 degrees you may warn (not warn) the wrong 100,000-plus people.

Here is a high resolution GOES satellite image of the heart of the November 2014 storms, when the lower level wind was averaging 255-260 degrees in origin. You can see the vast expanse of Lake Erie the arctic wind was traveling with that orientation.

That distance is known as the fetch; the distance over the water. The greater the fetch, the more moisture can be picked up and lifted upward. A 250-degree wind is maximum fetch for Lake Erie and the metro area. A 290 degree wind is short fetch, and that means less moisture delivered to the far southwest corner of Western New York. If you look at the northern Great Lakes in the image, you’ll note a curvature in the flow, with a northwest flow originating off Lake Superior.

That kink in the flow is a short wave of low pressure in the mid-level flow. As a short wave approaches from the west, the low level winds often back from west to south-southwest, which will shift bands northward. If the backing is very sharp, that implies wind shear which will at least temporarily disrupt the band.

Once the short wave passes, winds will veer back to from south-southwest to west or northwest, steering the band back to the south. These oscillations of the band will tend to diminish accumulations in any one location because of shortened “residence time” over any one location. If the band doesn’t stay over your house for long, you obviously receive less snow.

During the November storms, we had a long residence time with a very long fetch and extreme instability combining to make a “perfect lake-effect storm.” Predicting the timing and extent of those oscillations can be meteorological murder. High-resolution models help a great deal, but overreliance on models can lead to trouble as well. We also have to constantly eyeball satellite and radar imagery upwind from Western New York to observe reality vs. model output. In addition, we have to watch wind speeds. Low-level winds that are too strong, such as sustained winds of 40-50+ mph,  can also tear apart lake bands.

In general a west or northwest wind does not favor the Buffalo metro area or much of the Niagara Frontier for a lot of lake-effect snow. On occasion, though, a cold west-northwest flow can make a hookup with Lake Huron and develop a skinny band of significant lake snow that steers into parts of the Niagara Frontier. A northwest flow in Western New York usually results in multiple, thinner bands of lake snow, as opposed to the sometimes monstrous single bands that can develop off a southwest or west-southwest flow.

Incidentally, winters with below average temperatures do not guarantee more lake-effect snow for the Niagara Frontier. In some cold winters, the average track for storm/low pressure systems tends to bring northwest or west-northwest low level winds following the storms’ cold frontal passage, rather than the more productive west or southwest winds behind the cold front.

And one last item: In the early part of the lake-effect snow season (which includes this weekend), the warmth of the lake can make accumulating snow within several miles of the lake much less likely than locations farther inland, away from warmer waters.

Enough for now? There’s more, but we’ll save that for a future article.

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