1. 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

2. Up on the roof!

3. Where is my car?

4. Snowed in!

5. Ten foot Wall of Snow, ‘77

6. Up on the roof again ‘77

7. Sea of snow and cars, ‘77

8. Average Temps: Lake Vs. Air
Unstable season

9. LAKE EFFECT: Conceptual Model
Green/Red arrows?

10. 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)?

11. LAKE EFFECT: Conceptual Model
<-North
South->
Narrow bands of heavy snow
2-20 miles wide, miles long

12. SEASONAL SNOWFALL

13. PA Average Annual Snowfall
92”
120”
50”
65”
47” 5”
19”
41” 0”
10”
Red number is amount that’s lake-effect snow

14. TERRAIN AND PRECIPITATION

15. LAKE EFFECT: Conceptual Model

16. LAKE-EFFECT SNOW “LONG FETCH”
Bands oriented parallel to the mean wind from sfc to 850mb

17. LAKE-EFFECT SNOW “SHORT FETCH”
Bands oriented parallel to the mean wind from sfc to 850mb

18. LAKE-EFFECT SNOW: WESTERLY WIND
MULTIPLE BANDS
SINGLE BAND
Short fetch
Long fetch
COLD AIR
Bands oriented parallel to the mean wind from sfc to 850mb

19. LAKE-EFFECT SNOW: NORTHWEST WIND
COLD AIR
Bands oriented parallel to the mean wind from sfc to 850mb

21. Great Lake Snow Bands

22. Great Lake Snow Bands?

23. Great Lake Snow Bands?

24. Great Lake Snow Bands?

25. Great Lake Snow Bands

26. LAKE-EFFECT SNOW BAND

27. CURTAIN OF DEATH!!!

28. LAKE-EFFECT SNOW

29. 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

30. Where else does LE occur?

31. 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

32. CLASSIC “BIG” LAKE EFFECT: Surface pattern

33. CLASSIC “BIG” LAKE EFFECT: 500mb pattern

34. LAKE EFFECT with Synoptic Support
High RH is a good indicator of “Synoptic” support

35. Buffalo Snow: Christmas 2001

36. Buffalo Snow: Christmas 2001

37. Buffalo Snow: Christmas 2001

38. Lake-Effect?

39. Lake-effect?

40. 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

41. Lake effect: NGM model

42. Lake effect: ETA coarse model

43. Lake effect: ETA fine res