Lake Effect Snows

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Lake Effect Snows

• Current Weather Discussion
• 16 January 2008

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What is Lake Effect Snow?

• Technically, it is a manifestation of low-level
  thermal instabilities.
• More specifically, it is snowfall that is driven
  by large-scale moist cyclonic flow of cold air
  across relatively warm lake waters.

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Lake Effect Climatology

• Most predominant downwind of the Great Lakes
• Can occur with smaller lakes
• Great Salt Lake
• Finger Lakes of New York
• Ocean effect snows are also possible
• Precipitation can be rain if not warm enough for
  snow

(image from Google Maps)
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Example October 2006

• October 12-13, 2006
• Remarkable out of season event with very wet
  snows
• Over 1,000,000 power outages
• Max snowfall totals of 24 downwind of Lk. Erie

(image from NWS BUF)
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Lake Effect Snow Basics

• What are the synoptic conditions we expect to see
  with lake effect snow?
• Broad cyclonic flow
• Low-level convergence with weak forcing for
  rising motion
• Synoptic-scale ascent can aid in band development
• Cold air mass atop relatively warm water
• Results in heat and moisture fluxes
• Frozen lakes effectively kill lake effect
• Long fetch over one or more lakes
• Weak to moderate wind shear
• Primarily for snow band type

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Lake Effect Snow Basics

• Temperature differential between lake and
  low-levels is key
• Want a gt13C differential between 850 mb and the
  lakes temperature
• This goes hand in hand with a well-mixed boundary
  layer
• Allows for heat and moisture energy to be drawn
  from the lake (fluxes)

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Lake Effect Snow Sounding

• Sounding from early February 2007

• Historic lake effect event for Lake Ontario
• Note signs of a mixed boundary layer
• 850 mb temperature -26C
• Lake temperatures 0-5C
• Thermal criteria is met with this case

Image from Univ. of Wyoming Archives
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Lake Effect Climatology

• Why do most lake effect events occur from
  November through January?
• Lakes tend to freeze over by late Jan.
• Less of an effect with Ontario and Superior, the
  deeper Great Lakes
• Becomes much tougher to draw moisture from the
  lake
• Saturation vapor pressure differences between ice
  and liquid water

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Wind Fetch Considerations

• Wind fetch plays a crucial role in low-level
  moisture acquisition and transport
• The longer the wind spends over water, the more
  likely (and efficient) lake effect snows become
• Multiple bodies of water can further aid this
  process

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Wind Fetch Considerations

• Wind fetch plays a crucial role in low-level
  moisture acquisition and transport

• The longer the wind spends over water, the more
  likely (and efficient) lake effect snows become
• Multiple bodies of water can further aid this
  process
• Thus, regions directly parallel to the long axis
  of the lakes see the greatest lake effect snows!

Base image from Google Maps
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Banding Structures

• Lake effect snows come in two flavors
• Single band
• Multiple band

Single band example Image from Univ. of Wisconsin
Multiple band example Image from Wikipedia
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Banding Structures

• Low-level wind shear is the primary
  distinguishing factor
• Consider surface-700 mb
• 0-30 shear strong, singular bands
• 30-60 shear weaker, multiple bands
• gt60 shear lake effect not likely
• Relatively weak speed shear favors single bands
• Akin to tropical cyclones and shear
• Too much shear does not allow for latent heat
  organization and banded convection to develop

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Single Band Characteristics

• Characteristics of single band structures include

• Location band tends to be parallel to low-level
  winds
• Deep mixed layer and high instability
• Weak vertical wind shear
• Relatively long fetch
• Operates as a feedback loop

Image from Penn St. Univ.

• Hydrostatically lower pressures over warm water
• Latent heat release warms column
• Warmed column lower pressure

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Multiple Band Characteristics

• Characteristics of multiple band structures
  include

• Narrow, small, and shallow in scope
• Less snowfall than with single band events
• Larger wind shears
• Relatively small fetch
• Shallower mixed layer
• Can result in weaker events despite strong
  thermal instability!

Image from College of DuPage
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Advanced Effects

• Lake effect snow forecasting isnt necessarily
  that easy, however!
• There are other, primarily geographic, features
  to take into consideration
• Presence of multiple lakes
• Frictional and Coriolis effects
• Topographic enhancement

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Multiple Lake Fetches

• Lake effect snows can be enhanced by multiple
  lake wind fetches

• Winds that pass over multiple lakes acquire more
  energy and moisture
• Added moisture makes the lake effect process more
  efficient
• The result? Enhanced snows on the eastern side of
  the Great Lakes

Base image from Google Maps
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Frictional Effects

• Recall force balances

• Friction acts as a drag on the wind speed,
  affecting the magnitude of the Coriolis force.
• In the NH, this results in cross-isobaric flow
  toward low pressure.
• Frictional effects are greater over land than
  over water.
• Result south side favored for snows!

Green divergence Red convergence Base image
from Google Maps
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Topographical Effects

• Manifest primarily in two ways
• Moving from lake to land
• Special features over land
• Low-level flow is forced to rise
• Coupled with low-level thermal instability,
  enhanced snows are possible

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Topographical Effects

• Example Upstate New York

• Downwind of Lake Ontario is the Tughill Plateau
• Near Saranac Lake
• Also known as the Adirondacks
• As wind comes off the lake, it naturally rises
  near the shore

• Uplift is further enhanced by the plateau feature
• Note that friction also plays a role
• Wind speeds faster (slower) over water (land)
• Results in enhanced convergence on east shore

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Lake Effect Summary

• Key large-scale factors
• Temperature difference between lake and top of
  boundary layer
• Depth of the mixed layer deeper is better
• Vertical wind shear, speed and direction
• Wind direction and fetch with respect to lake
  orientations
• Synoptic-scale lift and broad cyclonic flow
• Key smaller-scale factors
• Frictional convergence and Coriolis
• Local topographical features
• Enhanced moisture convergence due to multiple
  lake transport