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Don Paul: Here's your lake-effect snow primer before the weekend

When weather models start to look like this, it’s time for a refresher on lake-effect snow. That image, by the way, is a depiction of lake-effect snow in place for early this coming Saturday, which I and other meteorologists have been talking about for a few days.

First, I’ll offer some perspective on this coming event. As of this writing, I’m confident in saying “no biggie” for most of the Western New York population. While conditions will be favorable for heavier amounts elsewhere in the Great Lakes, including some areas farther east off Lake Ontario, that’s not the case for the more densely populated Niagara Frontier.

It appears some portions of the Southern Tier and hilly terrain could see at least 3-5 inches or 4-6 inches from late Friday evening into Saturday which is, even for them, “no biggie.” However, amounts in that region could go higher than that range.

Note the orientation of those thin black lines in the graphic, known as isobars. Those are lines of equal barometric pressure. The wind doesn’t flow exactly parallel to isobars, but that northwest to southeast orientation implies winds from the west or west-northwest. That direction crosses a shorter “fetch” of Lake Erie, meaning less of an opportunity to pick up lake moisture. And, that direction channels most of the Lake Erie snow to ski country and the Southern Tier, rather than the Niagara Frontier.

There can also be some occasional tie-in to Lake Huron moisture in Western New York, which could help put down a very modest coating in spots on the Niagara Frontier.

For the metro area to take a real hit of heavy lake-effect, it usually requires a southwest flow, closer to 250 degrees on the compass.

That’s not indicated at all in the current models and ensembles. While there can be an occasional kink in the westerly flow brought on by a disturbance, which would briefly back the winds and send lake snow farther north, every current indication keeps the flow predominantly west or west-northwest. This is what a west flow looks like during a more potent event, when the low level wind originated from due west, or 270 degrees on the compass.

Such a low level flow steers the lake moisture over the hilly terrain of southern Erie County, parts of Wyoming County and the Southern Tier, staying south of the metro area.

WNY's first widespread lake-effect snows could come this weekend

What would make for a potent event? Ideally, you begin with a broad/synoptic pattern which favors the delivery of true arctic air over the warmer lake waters. This ensemble image shows a classic synoptic setup, with a cold trough and low closer to us with a warmer ridge near western North America. You can see the orientation with this trough shoves that arctic air right into the Great Lakes.

So, yes, the synoptic setup for Saturday does look favorable. The devil is always in the details, and when we get down to small/mesoscale level, there is again that west or west-northwest flow delivery the moisture with a shorter fetch delivering most of the moisture to the hills to the south.

Will it be cold enough for significant lake snow? Lake-effect requires a significant drop in temperature from the lake’s surface up to about 5,000 feet, called a lapse rate. Saturday’s daytime temperatures will be in the 30s, with the Lake Erie temperature currently at 52 degrees, or 11 degrees Celsius.

Models predict a temperature at 5,000 feet of about -10 degrees Celsius. That is more than cold enough for a lake response, with a temperature dropoff/lapse rate from the lake surface up to about 5,000 feet easily adequate for lake-effect snow. We typically look for a lapse rate of 13 degrees Celsius from the lake temperature to around 5,000 feet to get things going. You can do the math. The lapse rate Saturday is going to be a lot greater than 13 degrees Celsius.

In this illustration supplied by Wikipedia, you can see how snowfall rates increase with higher lapse rates.

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There also needs to be adequate humidity in the cold flow to make heavier lake snow. Sometimes, very cold air which is quite dry, even with a good lapse rate, doesn’t deliver much snow.

Winds from the surface up to around 10,000 feet should be well aligned. If the wind direction shifts more than 30 degrees on the compass with height, that is a form of wind shear which can tear apart a lake band. In this image of the Buffalo upper air sounding from Monday morning, the fifth, if you look at the little wind barbs in the image, you’ll see they are well aligned.

There is very little directional wind shear in this sounding. In a lake-effect situation, that would enhance the organization of a lake band.

Another element which can aid in lake-effect is a slight curvature to the flow, as if bottoming around a counterclockwise cyclone. Cyclonic curvature enhances lift of the moisture in the atmosphere. Note the curvature from the northwest flow off Lake Superior to the west-southwest flow coming into Western New York. That can work wonders, as seen in this NOAA GOES image.

The opposite is true when the curvature is anticyclonic, curving around the top of a high pressure ridge. That encourages sinking motion, which can “squish” the life out of a lake band.

There are some exceptions to some rules. When I said a west or west-northwest flow usually delivers little snow to the Niagara Frontier, sometimes such a flow hooks up with Lake Huron and bring distant bands into Buffalo or areas to the north.

The lake-effect forecast requires more wind direction precision than you might think.

As I wrote last December, “In addition to duration of a directional flow, there must be precision in forecasting wind direction. It’s not enough to know the wind will be ‘from the southwest.’ If our wind directional forecast is off by 5 degrees on the compass anywhere in our most densely populated area, northern Erie County, we can warn — or not warn — the wrong 100,000-plus people. If the winds are originating from 250 degrees, decades of experience tell us lake-effect will focus mainly on Buffalo out to the airport and beyond to Batavia, assuming conditions for lake-effect snow are favorable in terms of temperature, humidity, and lack of wind shear aloft. Just 5 degrees more veering to 255 degrees will focus on South Buffalo through Depew, Lancaster and some of the nearby Buffalo Southtowns. 245 degrees will reach parts of “southern” Amherst and northern Cheektowaga; 240 degrees reaches most of Amherst and Clarence … and so on. Just a very few degrees difference in direction makes a big difference. Precision such as this has been made at least partially attainable with the advent of high resolution models such as the National Weather Service WRF.”

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