Winter Storms

by

Paul Kocin Winter Storm Expert

The Weather Channel

and

Dr. Gerry BellClimate Prediction Center

Outline1. Winter Weather Impacts2. Winter Forecast Issues3. Northeastern U.S. Snowfall Impact Scale4. 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

o It often occurs close to the rain/snow lineo It sometimes occurs close to the snow/ no

snow lineo 50-mile errors in location produce big

problems!

Predictability

Some storms are easy……….Affect one small region – it’s well forecastLife 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 • 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

Northeast Snowfall Impact

Scale (NESIS)

Paul J. Kocin & Louis UccelliniThe 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 – 1950-2000

• CONTOURED SNOWFALL at 4”, 10” and at20” intervals

• THOSE CONTOURS REPRESENT: (1) an AREA (2) an AFFECTED POPULATION

Feb 1978

Mar 1993

Mostly 4-10”, small 10”+

NESIS VALUESClose to 1

Small 4”, Larger area of 10”+, small 20”+

NESIS VALUESClose to 3

Even larger area of 4”+, 10”+, 20”+

NESIS VALUESGreater than 8

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

Climate Factors Influencing Winter Storms

1. El Niño/ La Niña cycle

2. North Atlantic Oscillation

El Niño and La Niña

SST(°C) and Departures

28oC is approximate threshold for deep tropical convection

28 28

ENSO Tropical Rainfall

El Niño: Tropical convection, jet stream extended eastwardLa Niña: Tropical convection, jet stream retracted westward

EL NiñoEnhancedConvection

La Niñasuppressed 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- 2001 1950’s-1960’s

++-

-

•NAO affects the Atlantic jet stream/ storm trackNAO affects the Atlantic jet stream/ storm track•Tremendous differences in poleward heat Tremendous differences in poleward heat transporttransport and temperatures at high latitudes and temperatures at high latitudes

Positive (warm) Phase of Wintertime NAO

Temperature Precipitation

Warmer Colder Wetter Drier

NAO: DJF Time Series

50 55 60 65 70 75 80 85 90 95 00

Positive: 1980’s-presentWeaker Hudson Bay LowArctic air confined to CanadaMilder Temperatures

Negative: 1950’s-1960’sStronger Hudson Bay LowMore Arctic outbreaks Cooler Temperatures

Moderate-to-StrongEl Niño’s

Negative NAO:Cooler in SE,Mid-Atlantic, NE

Positive NAO:Warmer in SE, Mid-Atlantic, NE

El Niño – NAOTemperature Composites

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

Daily NAO Index

Summary1. 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.