Winter Storms by Paul Kocin Winter Storm Expert The Weather Channel and Dr. Gerry Bell Climate Prediction Center

Outline 1.Winter Weather Impacts 2.Winter Forecast Issues 3.Northeastern U.S. Snowfall Impact Scale 4.Climate factors influencing winter storms

Winter Weather Impacts

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

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

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

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

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

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!

Winter Forecast Issues

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.

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

Problems with forecasting heavy snow oIt often occurs close to the rain/snow line oIt sometimes occurs close to the snow/ no snow line o50-mile errors in location produce big problems!

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

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

Mean = 4.80

5 Top Ranked Snowfall Events Rank DATE NESIS MAR JAN FEB MAR FEB

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

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

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

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

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

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

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

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

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

NESIS CATEGORIES CATEGORY NESIS VALUES # of CASES DESCRIPTION 11 – “NOTABLE” “SIGNIFICANT” 34 – “MAJOR” “CRIPPLING” “EXTREME” Being tested at NCDC for possible NWS operational use

Climate Factors Influencing Winter Storms 1. El Niño/ La Niña cycle 2. North Atlantic Oscillation

El Niño and La Niña

SST(°C) and Departures 28 o C is approximate threshold for deep tropical convection 28

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

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

North Atlantic Oscillation (NAO)

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

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

NAO: DJF Time Series Positive: 1980’s-present Weaker Hudson Bay Low Arctic air confined to Canada Milder Temperatures Negative: 1950’s-1960’s Stronger Hudson Bay Low More Arctic outbreaks Cooler Temperatures

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

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

Daily NAO Index

Summary 1.Winter Weather Impacts are significant. 2.Winter storm forecasting remains a challenge, especially Rain/ snow line and snow/ no snow line. 3.Northeastern U.S. Snowfall Impact Scale (NESIS) may be useful scale for categorizing winter storms and their impacts. 4.Climate factors influence winter storms and snowfall throughout the U.S. 5.Also cause year-to-year and multi-decadal cycles in winter storm activity.