Lake-Effect Snow - what causes it?
Lake-effect snow typically occurs during cold outbreaks where a long fetch across a relatively warm, moist body (such as a lake or even a sea) can lead to significant accumulations of snow on the leeward side of the lake/sea. This effect can occur in many locations around the world with the right conditions, but is perhaps best known around the Great Lakes of North America where such events can leave copious amounts of snowfall, falling at rates of many inches per hour.
Cold winds blowing over the relatively warm waters of a lake will cause the air near the surface of the lake to warm. Given that the air aloft is sufficiently cold, this temperature difference through the atmosphere increases instability; the greater the temperature difference, the more unstable the air becomes. This causes the air over the lake to rise (while continuously being blown to the leeward side of the lake by the wind), and when combined with moisture evaporating from the lake, will eventually result in convective cloud development - the birth of a shower. If the air is cold enough, then these showers will fall as snow.
How lake-effect snow develops around Lake Erie in North America - click to enlarge
This convective cloud development is also aided by low-level wind convergence - winds blowing over the vast, flat lake are affected less by surface friction; however, once they reach the downwind shore, they encounter rougher terrain and an increase in surface friction, causing the winds to slow down and back, creating a "piling up" effect on the leeward side. Where this occurs, surface air is forced upwards to form deep convection, which in turn leads to heavy snow showers, and provided the wind continues to blow from the same direction then these showers will merge into rather narrow bands of persistent snow that move continuously over the same areas for many hours, if not longer, depositing significant amounts of snow along these snow belts.
The alignment of these bands of snow showers depends largely on the mean wind direction, and any slight shifts in this will cause the bands to migrate farther north or south accordingly. Meanwhile the upwind side of the lake remains relatively free of any fresh snow.
During the lake-effect snow episode of November 2014, some stunning timelapse videos were published by residents of Buffalo, NY, showing the movement of the quasi-stationary band of snow showers just to the south of the city. In the same space of time, only 1-2 inches (2.5-5cm) of snow had fallen in Buffalo itself, and yet 2 miles south of the city 60 inches (150cm) had already accumulated, highlighting just how localised and narrow these bands of persistent snow can be, causing major disruption to daily activities.
Timelapse of a band of lake-effect snow just south of Buffalo, NY, in November 2014. Video taken by Jason Holler & Joseph DeBenedictis
While this effect is well known in North America, we occasionally see some examples occurring in parts of the British Isles. When a strong east or northeasterly wind blows over the North Sea, and the airmass is sufficiently cold, bands of snow showers can affect parts of Kent and/or the Thames Estuary, known locally as the "Kent Snow Streamer" and "Thames Snow Streamer" respectively. Recent cases of these saw a Thames Streamer deposit 30cm (1 foot) of snow over the North Downs of Surrey and south London on 1st-2nd February 2009, while a Kent Streamer on 9th-10th January 2010 gave similar amounts of snow to central and eastern Kent.
Typical lake-effect snow situations in southeast England - click to enlarge
The placement of these bands is again reliant on the resultant wind direction - an easterly component will allow snow showers to funnel up the Thames Estuary and towards northern coasts of Kent and southern coasts of Essex, while a more northeasterly flow will send snow showers across more central and eastern parts of Kent instead. Similar effects are noted elsewhere across the country, such as the "Cheshire Gap Effect" whereby in a northwesterly flow frequent showers tend to filter through from Liverpool Bay and across the Cheshire Plain into the Midlands.
Dan Holley 19th November 2014