Lake-Effect Snow
• A “Fair weather” event
– Occurs after the passage of a mid-latitude
cyclone
– Areas downwind of relatively warm lakes
can receive *incredibly heavy snow*
while nearby areas have improving
weather
Up on the roof!
Where is my car?
Snowed in!
Ten foot Wall of Snow, ‘77
Up on the roof again ‘77
Sea of snow and cars, ‘77
Average Temps: Lake Vs. Air
Unstable season
LAKE EFFECT: Conceptual Model
Green/Red arrows?
LAKE EFFECT: Conceptual Model
Snow intensity: Factors?
How cold is air? (Temp. diff Lake to Air)
How deep is cold air?
How long is air over lake (fetch)?
LAKE EFFECT: Conceptual Model
<-North
Narrow bands of heavy snow
2-20 miles wide, 50-250 miles long
South->
SEASONAL SNOWFALL
PA Average Annual Snowfall
92” 120”
50” 65”
47”
5”
41”
10”
19”
0”
Red number is amount that’s lake-effect snow
TERRAIN AND PRECIPITATION
LAKE EFFECT: Conceptual Model
LAKE-EFFECT SNOW “LONG FETCH”
Bands oriented parallel to the mean wind from sfc to 850mb
LAKE-EFFECT SNOW “SHORT FETCH”
Bands oriented parallel to the mean wind from sfc to 850mb
LAKE-EFFECT SNOW: WESTERLY WIND
MULTIPLE BANDS
Short fetch
SINGLE BAND
Long fetch
COLD AIR
Bands oriented parallel to the mean wind from sfc to 850mb
LAKE-EFFECT SNOW: NORTHWEST WIND
COLD AIR
Bands oriented parallel to the mean wind from sfc to 850mb
Great Lake Snow Bands
Great Lake Snow Bands?
Great Lake Snow Bands?
Great Lake Snow Bands?
Great Lake Snow Bands
LAKE-EFFECT SNOW BAND
CURTAIN OF DEATH!!!
LAKE-EFFECT SNOW
Lake-Effect Snow
• Location  wind direction/speed
• Intensity 
– Stability
• Tlake-T850mb>13C, less if synoptic support
• Larger the difference, generally heavier the snow
• Depth of cold air … consult mr. sounding!
 Deeper cold air  Deeper cloud  Heavier snow!
– Fetch
• Longer distance/time  more modification  heavier snow
• Need ~40 mi. of fetch, over 120mi.can bring intense snows
• Single bands vs. multiple bands
– Wind Shear
• Little or no directional shear  heavier snow
– Topography
• Upslope regions favored for heavier snow
– Best events have synoptic-scale support
• Approaching 500mb shortwave
– “lifts inversion” ->> deeper convection
• 700mb RH >70% …. Good indicator of synoptic support
• Right conditions can bring >3”/hour rates !!!
– Distance inland: multiply 850mb wind speed by ~1.5
Where else does LE occur?
CLASSIC “BIG” LAKE EFFECT:
Surface pattern
Cold Advection
Canadian High
- cp, Arctic air
* TLAKE-T850mb>=13C
-”general” rule
* Occurs if -8 to -12 C diff. in certain situations
CLASSIC “BIG” LAKE EFFECT:
Surface pattern
CLASSIC “BIG” LAKE EFFECT:
500mb pattern
500mb Pattern
LAKE EFFECT with Synoptic Support
High RH is a good indicator of “Synoptic” support
Buffalo Snow: Christmas 2001
Buffalo Snow: Christmas 2001
Buffalo Snow: Christmas 2001
Lake-Effect?
Lake-effect?
WINTER WX: LAKE EFFECT
• IMPORTANT ELEMENTS:
– Caused by cold air over warm lake (* Tlake-T850mb>13C)
• * less if synoptic support
– 850mb T’s < -7C, If warmer -> rain
– Bands // to mean wind from sfc to 850mb
– Longer fetch -> more intense the snow
• Need ~40 mi. of fetch, over 120mi.can bring intense snows
–
–
–
–
Single bands: more intense, wind // to long axis of lake
Multiple bands: less intense, wind // to shorter axis of lake
Low vertical wind shear is favorable
Best events have synoptic-scale support
• Approaching 500mb shortwave
– “lifts inversion” ->> deeper convection
• 700mb RH >70% …. Good indicator of synoptic support
• Right conditions can bring >3”/hour rates !!!
– Distance inland: multiply 850mb wind speed by ~1.5
Lake effect: NGM model
Lake effect: ETA coarse model
Lake effect: ETA fine res