1 Lake-Effect Snow (LES)

2 Overview of the Lake-Effect Process n Occurs to the lee of the Great Lakes during the cool season n Polar/arctic air travels across a lake, picks up heat and moisture, and is destabilized n Cloud formation is enhanced by thermal and frictional convergence and upslope along lee shore

3 Lake-Effect Snow Storms n Intense, highly localized snow storms that form near major bodies of water n Usually take the shape of narrow bands downwind of the shore n Can produce tens of inches of snow in a single day n Require a specific set of conditions involving the atmosphere and land & water surface

4 Lake Effect Snow from space

5 Lake Effect Snow from space. SeaWifs Nov 30, 2004

6 A Lake-Effect Snow Storm on Radar

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8 Geographic Preferences

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10 Geographic Preferences

11 Great Lakes Snowfall Climatology

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

13 Record Event 37.9 inches at the Buffalo Airport in 24 h

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15 The Lake-Effect “Season”

16 Basic Concepts of Formation

17 Basic Concepts of Formation The atmosphere upwind of the lake is characterized by a very strong temperature inversion, with arctic air near the ground. Air is blowing from the land toward the water.

18 Basic Concepts of Formation

19 Basic Concepts of Formation

20 Basic Concepts of Formation The warm water provides thermal energy and moisture to the overlying cold air – remember that thermal energy transport is from warm to cold. The warm air rises to form clouds. Note that it also raises the height of the capping inversion.

21 Basic Concepts of Formation

22 Basic Concepts of Formation Note how the inversion has risen in altitude and the lower-levels of the atmosphere have moistened.

23 Basic Concepts of Formation

24 Basic Concepts of Formation The rising air condenses to form precipitation, and snow falls downwind of the shore line. The greater the air-water temperature contrast, the heavier the snowfall

25 Formation of Bands Looking 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

26 A Lake-Effect Snow Storm on Radar 1 2

27 Formation of Bands Note the rising and sinking motion

28 Formation of Bands Clouds are suppressed in between bands Note the rising and sinking motion

29 Formation of Bands

30 Ingredient #1 for Formation n 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

31 The Temperature Difference on a Thermodynamic Diagram

32 Water Temperatures are Available

33 The State of the Water and Land is Critical

34 Ingredient #2 for Formation n 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

35 Where’s the Beef?

36 Basic Concepts of Formation

37 Ingredient #3 for Formation n 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

38 Ingredient #4 for Formation n 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

39 Demonstration of Fetch ~70 miles

40 Favorable Fetches for LE Snow

41 ‘Preconditioning’ by upwind lakes SeaWiFS: Dec 5, 2000 Lake Nipigon

42 ‘Preconditioning’ by upwind lakes MODIS: Dec 16, 2009 Lake Nipigon

43 Ingredient #5 for Formation n 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

44 Orographic Lift Can Make a HUGE Difference! Lake Superior surface: 600 feet Brockway Mountain:1330 feet

45 Effect of Orography

46 Shoreline Orientation Can Make a HUGE Difference!

47 Shoreline Orientation Can Make a HUGE Difference! Change in surface friction as air passes from land to water causes convergence in the region shown by a “+”

48 Shoreline Orientation Can Make a HUGE Difference! First band forms in the convergence region. Note divergence “-” nearby

49 Shoreline Orientation Can Make a HUGE Difference!

50 This Theory in Action

51 This Theory in Action

52 Optimal snow growth T/RH Dendrites are the largest (lowest density) crystals and grow quickly 850 mb temperatures of -10ºC or lower needed for heavy lake-effect snow

53 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)

54 If Atmosphere is Sufficiently Unstable, Thundersnowstorms Can Form

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