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Lake-Effect Snow (LES)

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Overview of the Lake-Effect Process

Occurs to the lee of the Great Lakes during the cool season

Polar/arctic air travels across a lake, picks up heat and moisture, and is destabilized

Cloud formation is enhanced by thermal and frictional convergence and upslope along lee shore

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Lake-Effect Snow Storms

Intense, highly localized snow storms that form near major bodies of water

Usually take the shape of narrow bands downwind of the shore

Can produce tens of inches of snow in a single day

Require a specific set of conditions involving the atmosphere and land & water surface

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Lake Effect Snow from space

5Lake Effect Snow from space.

SeaWifs

Nov 30, 2004

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A Lake-Effect Snow Storm on Radar

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A Lake-Effect Snow Storm on Radar

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Geographic Preferences

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Geographic Preferences

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Geographic Preferences

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Great Lakes Snowfall Climatology

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Zooming In – The Average Annual Snowfall (inches) Over the Eastern

Great Lakes

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Record Event

37.9 inches at the Buffalo Airport in 24 h

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The Lake-Effect “Season”

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Basic Concepts of Formation

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Basic Concepts of Formation

The atmosphere upwind of thelake is characterized by a verystrong temperature inversion, witharctic air near the ground. Air isblowing from the land toward thewater.

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Basic Concepts of Formation

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Basic Concepts of FormationThe warm water provides thermalenergy and moisture to theoverlying cold air – rememberthat thermal energy transportis from warm to cold. The warmair rises to form clouds. Note thatit also raises the height of thecapping inversion.

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Basic Concepts of Formation

Note how the inversion has risen in altitude and thelower-levels of the atmosphere have moistened.

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Basic Concepts of Formation

The rising air condenses to formprecipitation, and snow fallsdownwind of the shore line. Thegreater the air-water temperaturecontrast, the heavier the snowfall

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Formation of BandsLooking down the wind direction, from west to

east, the clouds tend to form into bands,usually oriented parallel to the long axis of the lake

1 2

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Formation of Bands

Note the rising and sinking motion

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Formation of Bands

Clouds are suppressed in between bands

Note the rising and sinking motion

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Formation of Bands

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Ingredient #1 for Formation

Sufficient temperature difference between the lake surface and overlying air– Represents a measure of instability, similar to

the lifted index in the context of thunderstorms– At least 13ºC difference between water and

850 mb surface– This is approximately the dry adiabatic lapse

rate between 1000 mb (surface) and 850 mb

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The Temperature Difference on a Thermodynamic Diagram

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Water Temperatures are Available

http://coastwatch.glerl.noaa.gov/cwdata/lct/glsea.png

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The State of the Water and Land is Critical

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Ingredient #2 for Formation

Sufficiently deep cold air mass at the surface–One of the most important aspects

when considering intensity– Inversion heights < 3000 ft preclude

heavy lake-effect snows– Inversion heights > 7500 ft strongly

support heavy lake-effect snows– In some cases, an inversion may not

be present or obvious

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Basic Concepts of Formation

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Ingredient #3 for Formation

Directional wind shear–Small amount of directional wind

change with height (< 30 degrees) below the inversion favors horizontal roll convection

–Highly sheared environments (> 60 degrees) disrupt and diminish the efficiency of rolls, leading only to flurries

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Ingredient #4 for Formation

Adequate Fetch–Fetch is the distance traveled by air

over water–Long fetch promotes more heating of

the air and a higher inversion–A minimum fetch of 100 miles is

needed for significant lake-effect snow

–Flow over multiple lakes can help

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Demonstration of Fetch

~70 miles

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Favorable Fetches for LE Snow

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‘Preconditioning’ by upwind lakes

SeaWiFS: Dec 5, 2000

Lake Nipigon

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‘Preconditioning’ by upwind lakes

MODIS: Dec 16, 2009

Lake Nipigon

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Ingredient #5 for Formation

Sufficiently moist upstream air–RH > 70% below the inversion favors

heavy lake-effect snow–RH < 50% usually means little snow–Often upstream RH is the factor that

kills potentially heavy lake-effect events

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Orographic Lift Can Make a HUGE Difference!

Lake Superior surface: 600 feetBrockway Mountain: 1330 feet

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Effect of Orography

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Shoreline Orientation Can Make a HUGE Difference!

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Shoreline Orientation Can Make a HUGE Difference!

Change in surfacefriction as air passes from land to water causesconvergence in theregion shown by a “+”

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Shoreline Orientation Can Make a HUGE Difference!

First bandforms in theconvergenceregion. Notedivergence“-” nearby

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Shoreline Orientation Can Make a HUGE Difference!

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This Theory in Action

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This Theory in Action

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Optimal snow growth T/RH

http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm

• Dendrites are the largest (lowest density) crystals and grow quickly• 850 mb temperatures of -10ºC or lower needed for heavy lake-effect snow

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Cyclonic circulation

Cyclonic flow at ‘subgeostrophic’ wind speeds (e.g., through a low pressure trough) increases convergence and leads to heavier snowfall – check upper air charts (e.g., 850 mb)

Cyclonic curvature (height contours curve to left downstream)

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If Atmosphere is SufficientlyUnstable, Thundersnowstorms

Can Form

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Summary – setup for LE snow Instability (dT from lake surface to 850 mb) Fetch Upstream moisture Preconditioning by upwind lakes Synoptic forcing (low pressure systems) Topography (lifting) Height of temperature inversion Low wind shear Snow/ice cover upwind Geometry of upwind lake shore