Photo: 24-Hour Accumulated snowfall map showing lake effect snows bringing accumulation to lake adjacent areas. Map generated from pivotalweather.com on December 28, 2017.
As December has come and gone and we are knee deep in Meteorological Winter, the discussion of winter weather in the Midwest usually starts with strong cyclones passing through the continental United States bringing anything from blizzards to the north and strong thunderstorms to the south. We can steer our focus into something on a smaller scale (or mesoscale) and focus on the Great Lakes region, specifically on lake effect snow.
The importance of understanding lake effect snow lies in the fact that it is convectively induced (similar to thunderstorms in the summer), thus is often very intense compared to other systems. The snow-to-liquid ratio for lake effect snow can range from 15 to up to 30 inches of snow per one inch of liquid, compared to 10:1 for other systems as a general ratio. Bands of lake effect snow can fall at intense rates of 2-3 inches in the matter of one hour and can be very localized in nature based on the size of the snow band. Snowfall forecasts can often be misleading if we are unaware of the ratio during a given system. The general rule of thumb with these snows is when cold air passes over a generally warmer lake, the temperature differential causes rising motions over the lake producing a cloud and eventual strong snow.
When forecasting lake effect snow, there are multiple aspects to consider besides cold air over a warm lake:
Lake effect snow bands are different from most other snows that there are usually more intense snowfalls in a short time period. These storms can also occur during large scale high pressure, which can be deceiving for a storm to pass in a moment’s notice, only to have the sun reappear.
Observing temperatures at 850mb is a common forecast tool because the convective cloud tops for these systems usually is between 700 and 850mb. Convection refers to the ability of motion in warmer air to rise and cooler air to sink which in the atmosphere can create varied forms of severe weather. Warm waters in contact with cold air allow for CAPE (Convective Available Potential Energy) to be generated (usually around 300-400 J/KG) in a shorter layer called the boundary layer (roughly a mile above the surface) that allows for quick rising motions to create these systems. This can be compared to summertime convection which can reach and exceed 200mb (6-7 miles) into the atmosphere where CAPE values have values in the thousands and allow for higher and faster updrafts (rising motion).
Photo: 850mb Temperature, Height and Wind map showing cold air over the Great Lakes Region ideal for producing lake effect snows. Image generated from pivotalweather.com on December 29, 2017.
Based on the forecasting factors, travelers will need to be aware of changing conditions based on their proximity to lakes. Short term forecasts and nowcasting is crucial to understanding where these bands could hit and who can be affected. Snows like these can mean significant accumulation to nothing in as little as 40 miles distance. Ultimately, preparation will be important as these storms can be deceiving since they can occur during high pressure systems, meaning that a strong winter storm can pass by with high pressure behind it, but still bring more accumulation near a lake. Awareness of the situation is important along with staying up to date with changes throughout the day.
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© 2019 Meteorologist Jason Maska