Outline

1. Winter StormsbyPaul Kocin Winter Storm ExpertThe Weather Channel and Dr. Gerry BellClimate Prediction Center

2. Outline Winter Weather Impacts Winter Forecast Issues Northeastern U.S. Snowfall Impact Scale Climate factors influencing winter storms

3. Winter Weather Impacts

4. Each year, automobile accidents claim an incredible amount of lives, cause injuries and property damage.

5. The average/year of auto accidents: • 41,000 deaths, • 3 million injuries, • billions in damage. • About 15% is weather-related

6. Weather related: • 7,000 deaths • 800,000 injuries • and billions of dollars in damage per year!

7. Adverse weather is associated with over 1.5 million auto crashes per year.

8. Snow and ice make up a significant component of the dangers posed to transportation nationwide.

9. Examples: In 2001 • 5400 deaths linked to rain-slicked roads • 1100 deaths linked to snow and ice • 670 deaths linked to fog • Dept. of transportation cost estimates • Injuries, loss of life, and property damage cost an average of 42 billion dollars annually just for snow and ice!

10. Winter Forecast Issues

11. Winter forecast issues • Precipitation type forecasts can be very difficult, especially a day or more into the future • Rain vs freezing rain vs sleet vs snow • Model forecast uncertainty: • Different models can often show significantly different forecasts beyond a few days.

12. The main issue: rain vs freezing rain vs sleet vs snow

13. Problems with forecasting heavy snow • It often occurs close to the rain/snow line • It sometimes occurs close to the snow/ no snow line • 50-mile errors in location produce big problems!

14. Predictability Some storms are easy………. Affect one small region – it’s well forecast Life is GREAT Most storms are widespread, multi-day, multi-form events…………..forecasts can be good one place, Lousy others

15. More predictability • Whether big or small, winter storm predictability is very variable. • Some winter storms are amenable to prediction – even several days in advance • Other winter storms seem to be unpredictable even up to the day of the event

16. Mean = 4.80

17. 5 Top Ranked Snowfall Events Rank DATE NESIS • 1 12-14 MAR 1993 12.52 • 2 6-8 JAN 1996 11.54 • 3 15-18 FEB 2003 8.91 • 4 11-14 MAR 1888 8.34 • 5 11-14 FEB 1899 8.11

18. Northeast Snowfall Impact Scale (NESIS) Paul J. Kocin & Louis Uccellini The Weather Channel & NWS/NCEP

19. Fujita and Saffir-Simpson Scales • Measure potential for DESTRUCTION to property and loss of life from tornadoes and hurricanes

20. NESIS • Measures potential for human and economic DISRUPTION • Categories (1 thru 5)

21. SNOWSTORM IMPACT DUE TO: • SNOWFALL • WIND SPEED • TEMPERATURE • DURATION • SNOWFALL RATES • STORM INTENSITY • GROUND TEMPERATURES

22. What NESIS is • A measure that is based on the integrated effects of storm snowfall in the Northeast United States • Based on areal coverage of snowfall amounts and population affected

23. LOOKED AT 30CASES – 1950-2000 • CONTOURED SNOWFALL at 4”, 10” and at 20” intervals • THOSE CONTOURS REPRESENT: (1) an AREA (2) an AFFECTED POPULATION Feb 1978 Mar 1993

24. Mostly 4-10”, small 10”+ NESIS VALUES Close to 1

25. Small 4”, Larger area of 10”+, small 20”+ NESIS VALUES Close to 3

26. Even larger area of 4”+, 10”+, 20”+ NESIS VALUES Greater than 8

27. NESIS CATEGORIES CATEGORY NESIS VALUES # of CASES DESCRIPTION • 1 1 – 2.499 23 “NOTABLE” • 2 2.5 - 3.99 22 “SIGNIFICANT” • 3 4 – 5.99 16 “MAJOR” • 4 6 - 9.99 7 “CRIPPLING” • 5 10.0 + 2 “EXTREME” Being tested at NCDC for possible NWS operational use

28. Climate Factors Influencing Winter Storms1. El Niño/ La Niña cycle2. North Atlantic Oscillation

29. El Niño and La Niña

30. SST(°C) and Departures 28 28 28oC is approximate threshold for deep tropical convection

31. ENSO Tropical Rainfall EL Niño Enhanced Convection La Niña suppressed convection El Niño: Tropical convection, jet stream extended eastward La Niña: Tropical convection, jet stream retracted westward

32. El Niño: • Extended Jet Stream • More zonal flow over U.S. • South shift of storm track • Weaker Hudson Bay Low • Fewer arctic outbreaks • La Niña: • Retracted Jet Stream • More meridional flow • Blocking over N. Pac • Stronger Hudson Bay Low • More arctic outbreaks

33. North Atlantic Oscillation (NAO)

34. NAO affects the Atlantic jet stream/ storm track • Tremendous differences in poleward heat transport and temperatures at high latitudes NAO Air Pressure and Winds at 30,000 ft - + - + 1980’s- 2001 1950’s-1960’s

35. Wetter Drier Positive (warm) Phase of Wintertime NAO Temperature Precipitation Warmer Colder

36. NAO: DJF Time Series 50 55 60 65 70 75 80 85 90 95 00 Negative: 1950’s-1960’s Stronger Hudson Bay Low More Arctic outbreaks Cooler Temperatures Positive: 1980’s-present Weaker Hudson Bay Low Arctic air confined to Canada Milder Temperatures

37. El Niño – NAOTemperature Composites Moderate-to-Strong El Niño’s Negative NAO: Cooler in SE, Mid-Atlantic, NE Positive NAO: Warmer in SE, Mid-Atlantic, NE

38. Negative NAO establishes cold air regime and increases snow threat. Several major Northeast snowstorms have occurred during strong - NAO episodes

39. Daily NAO Index

41. Summary Winter Weather Impacts are significant. Winter storm forecasting remains a challenge, especially Rain/ snow line and snow/ no snow line. Northeastern U.S. Snowfall Impact Scale (NESIS) may be useful scale for categorizing winter storms and their impacts. Climate factors influence winter storms and snowfall throughout the U.S. Also cause year-to-year and multi-decadal cycles in winter storm activity.