A Discussion Of The Forecasting And Formation Of Lake Effect Snow

| January 29, 2019 @ 2:30 pm
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:

  1. The first area of concern is the time of the season which the lake will be at its warmest and first rushes of cold air approach the region.  The time of concern is usually between November and January since the lake waters have had an entire summer prior to warm up (average for this time span are usually between 40 and 50 degrees).  Development of the bands could be halted later in the season due to the lake temperatures approaching freezing and actually beginning to freeze.  At this point we are not only seeing cooler lake surface temperatures but also less area for colder air to travel over the lake.
  2. Looking at model output maps, we focus attention on the 850 millibar (mb) height, temperature, and wind direction overlays.  The 850 mb level helps to show advection (transfer) of cold air over a lake as well as the potential for steepening the lapse rate for rising air, which would allow for greater snowfall production.  In order for the production of lake effect snow to occur, there must be a temperature difference between the lake water and air at the 850mb level (nearly 1 mile above the surface) of at least 13 degrees Celsius (55.4 degrees Fahrenheit).   
  3. Another factor that plays into duration of lake effect snow is the time of day.  For example, the coldest temperatures are typically going to be found not only after the passing of a cold front, but usually throughout the night.  A band forming in the evening and lasting through the night could result in a higher strength of the band and more significant travel difficulty.
  4. Verification can be found by examining soundings (vertical profile of the atmosphere) to tell us if a strong lapse rate (change in temperature with height) exists and the strength or location of a dendritic growth zone (DGZ – used to determine the area of snowflake creation in the atmosphere).  The final variable to verify your results would be composite reflectivity which would show a projected radar image of the future that should output bands of lake effect snow if the variables are there.

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.

To learn more about other interesting winter weather topics from around the world, click here!

© 2019 Meteorologist Jason Maska

AlabamaWX is pleased to partner with the Global Weather and Climate Center team for outstanding posts about our atmosphere. Visit them at https://www.globalweatherclimatecenter.com for more great information!


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