Lake-Effect Snow

In short: lake-effect snow happens when very cold air blows across a large lake that has not frozen. The warm water heats and moistens the air from below, the air rises, and clouds build into narrow bands that dump snow on the downwind shore. Which towns get buried depends almost entirely on the wind direction, and the bands are shallow enough that radar far from the lake can miss them.

The Great Lakes are the famous home of it, but the same thing happens downwind of the Great Salt Lake, and in a smaller way off the Finger Lakes and other large water bodies when the air is cold enough.

How it forms

The ingredients forecasters check:

• A big temperature difference. A common rule from the NWS Great Lakes offices is that the lake surface should be at least about 13 °C (23 °F) warmer than the air at the 850 mb level, about a mile up. That is roughly the point where the lowest mile of air becomes unstable. Bigger differences mean more vigorous snow.
• A long fetch. Fetch is the distance the wind travels over open water. More fetch means more time to pick up heat and moisture. That is why a wind blowing down the long axis of Lake Erie or Lake Ontario can produce a single, intense band.
• Steady wind direction with height. If the wind direction changes a lot between the ground and about 10,000 ft (700 mb), the bands get torn apart. A common rule: under about 30 degrees of turning, strong organized bands; 30 to 60 degrees, weaker bands; more than 60, little more than flurries.
• A high lid. The inversion above the clouds caps how deep they grow. A lid 2 miles (about 3 km) or more up allows heavier snow, and even thundersnow.
• A cloud layer in the snow-growth range. Snowflakes grow fastest as branching crystals between about -12 and -18 °C (10 to 0 °F). When the rising air in the band spans that layer, the snow piles up fast and light, 20 inches or more from one inch of water. On a Skew-T, look for where the temperature line crosses -12 to -18 °C below the lid.
• Open water. Once a lake freezes over, the supply of heat and moisture shuts off. Shallow Lake Erie often freezes by midwinter; the deeper lakes rarely freeze completely.

Bands and where they land

Lake-effect snow falls in bands that line up with the wind:

• Single, intense bands form when the wind runs along the long axis of a lake. They can be 5 to 20 miles wide and drop 2 to 3 inches an hour or more, sometimes for a day or longer if the wind holds steady.
• Many weaker bands form when the wind crosses a lake on its short side, for example northwest winds over Lake Michigan or Superior. They look like a set of parallel streaks on radar and satellite.

A small change in wind direction moves a band by miles. A band can drift across a city for an hour and then settle over the next town for a day. That is why lake-effect forecasts name specific areas and change often, and why the NWS notes heavy snow can fall in one place while the sun shines a mile or two away.

Rising ground adds more. Air pushed up a slope is lifted again, so hills downwind of the lakes, like the Tug Hill Plateau east of Lake Ontario, pick up some of the largest snow totals in the eastern United States.

Lake-effect snow on radar

Lake-effect clouds are shallow, often topping out only a couple of miles up. The radar beam climbs as it travels (see What Radar Can’t See), so:

• Close to the radar, bands show as long, narrow streaks of echo coming off the lake.
• Far from the radar, the beam can pass over the top of the band. The radar shows light snow or nothing while the ground gets inches an hour. A neighboring radar may see it better. The Montague, New York radar (KTYX) sits on the Tug Hill Plateau largely so the bands off eastern Lake Ontario stay in view.
• Snow reads low. A band dropping an inch of snow an hour can show only 20 to 30 dBZ, because that inch is about a tenth of an inch of water, and ice returns less energy than water. Judge a snow band by what it piles up, not by the rain colors. See How to Read Weather Radar.

In MesoRadar: Reflectivity on the Local tab, from the nearest radar, and Rain / Snow / Ice on the National tab, the mosaic built from the lowest beam of every radar, so a band missed by one site often shows from another. Choosing a radar view.

Measuring and forecasting it

What fell. NOAA’s SNODAS analysis blends snow observations with a model into a daily national map. It is the quick answer for how deep the snow is. In MesoRadar, 24-Hour Snowfall is the day’s rise in that depth, so snow that settles or melts during the day is not counted, and a fluffy lake-effect dump can read several inches below what a cleared snowboard measured. Because it is an analysis on a grid, a very narrow band can also come out smoothed. For the real total, check the NWS snowfall reports (Public Information Statements) for the area.

What is coming. High-resolution models like the HRRR and RRFS can draw individual lake-effect bands. Their total for a region is often useful. Their placement of a band, to the town, is much less reliable, because a few degrees of wind direction moves it miles. Snow totals also depend on how much snow each inch of water makes. MesoRadar’s HRRR and ECMWF rows assume 10 inches of snow per inch of water; the RRFS rows use the model’s own snowfall, which lets the ratio vary. Lake-effect snow is often fluffier, 20 to 1 or more, so the HRRR number can run well under what piles up.

What the NWS issues. A Lake Effect Snow Warning is the NWS warning for heavy lake-effect snow. Because the bands are narrow, the area under the warning is often a few counties, and totals can vary from a few inches to several feet inside it. Ahead of an event, the NWS issues a Winter Storm Watch, and lighter lake-effect snow comes as a Winter Weather Advisory; the separate lake-effect watch and advisory were folded into those in 2017. A short, intense burst moving across roads can instead get a Snow Squall Warning, a storm-based warning; one tagged significant sets off Wireless Emergency Alerts.

In MesoRadar: the Snow tab. Accumulated Snow (SNODAS) has 24-Hour Snowfall and Snow Depth; the forecast groups run from Next 6 hr (HRRR) to Next 5 days (ECMWF), free, with RRFS between; Next 10 days (NBM) is Pro. Lake Effect Snow Warning is one of the warning types Pro lets you pick for the places you save. Snow in the app.