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 South-><-North Narrow bands of heavy snow 2-20 miles wide, miles long

SEASONAL SNOWFALL

PA Average Annual Snowfall 92” 41” 19” 47” 120” 5” 10” 50” 65” 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 COLD AIR Bands oriented parallel to the mean wind from sfc to 850mb MULTIPLE BANDSSINGLE BAND Short fetchLong fetch

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

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 * TLAKE-T850mb>=13C -”general” rule Canadian High - cp, Arctic air Cold Advection * 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

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 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