Forecasting Snow

Things to ConsiderThings to Consider

What things should be What things should be considered when making a considered when making a forecast for snow?forecast for snow?

ConsiderationsConsiderations

TemperaturesTemperatures

Moisture/PrecipitationMoisture/Precipitation

Timing (operational standpoint)Timing (operational standpoint)

ClimatologyClimatology

Location/GeographyLocation/Geography

Temperatures & MoistureTemperatures & Moisture

If there is moisture and temperatures If there is moisture and temperatures are sub-freezing, snow crystals can are sub-freezing, snow crystals can form.form.

The two factors go hand in hand.The two factors go hand in hand.

Roughly what level of the Roughly what level of the atmosphere is crucial to snow atmosphere is crucial to snow formation?formation?

850mb Level850mb Level

This level is important to forecasting This level is important to forecasting snow.snow.

Generally, if 850mb temps are below Generally, if 850mb temps are below 0°C, snow crystals can form, grow and 0°C, snow crystals can form, grow and fall.fall.

Consider a typical 850mb model Consider a typical 850mb model forecast chart…forecast chart…

0°C Line at 850mb0°C Line at 850mb

As a general rule, snow does not form As a general rule, snow does not form when 850mb temperatures are above when 850mb temperatures are above 0°C.0°C.

Snow does not necessarily fall to the Snow does not necessarily fall to the surface when 850mb temps are <0°C.surface when 850mb temps are <0°C.

What other things should be What other things should be considered?considered?

Temperature ConsiderationsTemperature Considerations

Temperatures at the surface.Temperatures at the surface.

Temperatures at other levels.Temperatures at other levels.

Thicknesses.Thicknesses.

Surface TemperaturesSurface Temperatures

Sub-freezing temperatures are favored.Sub-freezing temperatures are favored.

Snow can fall otherwise, but may not Snow can fall otherwise, but may not stick as efficiently.stick as efficiently.

Temperatures near the ground are Temperatures near the ground are below 32°F/0°C,below 32°F/0°C, what else to what else to consider?consider?

Temperatures aloftTemperatures aloft

Above freezing temperatures aloft Above freezing temperatures aloft can melt snow crystals.can melt snow crystals.

This leads to sleet, freezing rain or This leads to sleet, freezing rain or rain.rain.

Snow is favored when the entire Snow is favored when the entire column up to 850mb is below 0°C.column up to 850mb is below 0°C.

Temperature ProfileTemperature Profile

Considering various thermal Considering various thermal profiles…profiles…

ThicknessesThicknesses

What is thickness?What is thickness?

ThicknessesThicknesses Thickness is a layer of the atmosphere.Thickness is a layer of the atmosphere.

The greater the temperature, the greater The greater the temperature, the greater the thickness…the thickness…

)ln(2

112 p

p

g

TRzz

vd

Critical ThicknessCritical Thickness

So, what is a critical thickness?So, what is a critical thickness?

Critical thicknessesCritical thicknesses

Typical thickness values for a given layer, Typical thickness values for a given layer, in which the layer would likely support in which the layer would likely support snow.snow.

Generally, a virtual temperature of 0°C or Generally, a virtual temperature of 0°C or 273.15K, assumed for freezing.273.15K, assumed for freezing.

What are the main critical What are the main critical thicknesses?thicknesses?

Main Critical ThicknessesMain Critical Thicknesses

1000-500mb1000-500mb

1000-700mb1000-700mb

1000-850mb1000-850mb

850-500mb850-500mb

850-700mb850-700mb

1000-500mb Thickness1000-500mb Thickness

What is the critical thickness for What is the critical thickness for the 1000-500mb layer?the 1000-500mb layer?

)500

1000(ln

)15.273(12 mb

mb

g

KRzz d

5545 meters?

1000-500mb Critical 1000-500mb Critical ThicknessThickness

What is the actual critical What is the actual critical thickness?thickness?

Why is the critical thickness Why is the critical thickness different than the “calculated different than the “calculated one”?one”?

1000-700mb thickness1000-700mb thickness

What is the critical thickness for the What is the critical thickness for the 1000-700mb layer? 1000-700mb layer?

)700

1000(ln

)15.273(12 mb

mb

g

KRzz d

2853 meters?

1000-700mb Critical 1000-700mb Critical ThicknessThickness

What is the actual critical What is the actual critical thickness?thickness?

The 1000-700mb thickness maps are not as commonly available on sites such as NCEP, E-Wall, Unisys and others…

1000-850mb thickness1000-850mb thickness

What is the critical thickness for the What is the critical thickness for the 1000-850mb layer? 1000-850mb layer?

)850

1000(ln

)15.273(12 mb

mb

g

KRzz d

1300 meters

1000-850mb thickness1000-850mb thickness

The low-level column is below The low-level column is below freezing.freezing.

The critical thickness for this layer is The critical thickness for this layer is 1300m or 130dm.1300m or 130dm.

Arguably the best indicator.Arguably the best indicator.

The blue line is NOT the critical thickness!

Other Views of Other Views of TemperaturesTemperatures

Soundings/Skew-TsSoundings/Skew-Ts

Text extracted outputText extracted output

Others?Others?

PrecipitationPrecipitation

If there is enough moisture – If there is enough moisture – precipitation.precipitation.

As a general rule, what are As a general rule, what are typical liquid equivalent to typical liquid equivalent to accumulated snowfall ratios?accumulated snowfall ratios?

Water to Snowfall RatiosWater to Snowfall Ratios

The typical standard is a 10:1 ratio.The typical standard is a 10:1 ratio.

How might ratios be higher or How might ratios be higher or lower than 10:1?lower than 10:1?

Ground TemperaturesGround Temperatures

How might ground temperatures How might ground temperatures affect accumulated snowfall?affect accumulated snowfall?

Timing of SnowfallTiming of Snowfall

What times of the year is What times of the year is snowfall favored in the tri-state snowfall favored in the tri-state region?region?

How does time of the year affect How does time of the year affect snowfall?snowfall?

Time of DayTime of Day

Time of day is important to snowfall Time of day is important to snowfall forecasting, especially early/late forecasting, especially early/late season.season.

The timing is also crucial for traffic, The timing is also crucial for traffic, schools, businesses, etc.schools, businesses, etc.

ClimatologyClimatology

What types of events typically What types of events typically bring the heaviest snowfall to bring the heaviest snowfall to the tri-state region?the tri-state region?

Famous Nor’EastersFamous Nor’Easters

http://www.meteo.psu.edu/~gadomski/NARR/1993/us0313.http://www.meteo.psu.edu/~gadomski/NARR/1993/us0313.phpphp

http://www.meteo.psu.edu/~gadomski/NARR/1996/us0108.http://www.meteo.psu.edu/~gadomski/NARR/1996/us0108.phpphp

http://www.meteo.psu.edu/~gadomski/NARR/2006/us0212.http://www.meteo.psu.edu/~gadomski/NARR/2006/us0212.phpphp

Other Snow EventsOther Snow Events

What other types of storm What other types of storm systems or events bring snow to systems or events bring snow to our area?our area?

Other Snow EventsOther Snow Events

Clipper systemsClipper systems

Lake-effect snow (ocean effect as well)Lake-effect snow (ocean effect as well)

TroughsTroughs

Warm fronts (overrunning)Warm fronts (overrunning)

Cold frontsCold fronts

Location & GeographyLocation & Geography

Proximity to heat sourcesProximity to heat sources

Urban areas (urban heat effect*)Urban areas (urban heat effect*)

Coastal locationsCoastal locations

COLD AIR MASS

WARMER OCEAN

Marine layer Marine layer Marine

layer

Location & GeographyLocation & Geography

ElevationElevation

How does elevation effect snowfall?How does elevation effect snowfall?

(consider temperatures and geography)(consider temperatures and geography)

Location to Air MassesLocation to Air Masses

Cold (generally) northwest side of a Cold (generally) northwest side of a low favors the heaviest snowfall.low favors the heaviest snowfall.

Frontal boundaries and wind flow.Frontal boundaries and wind flow.

Moist air mass vs. dry air mass.Moist air mass vs. dry air mass.

Forecasting SnowfallForecasting Snowfall

1.1. Know temperatures through the Know temperatures through the column.column.

2.2. Consider the precipitation amounts.Consider the precipitation amounts.

3.3. All snow or mixed precipitation?All snow or mixed precipitation?

4.4. Estimate snowfall ratios.Estimate snowfall ratios.

5.5. Keep climatology in mind.Keep climatology in mind.

6.6. Factor in model trends and biases.Factor in model trends and biases.

7.7. Note seasonal or recent patterns.Note seasonal or recent patterns.

VerificationVerification

What What couldcould go wrong with a go wrong with a snowfall forecast?snowfall forecast?

How do snowfall forecasts How do snowfall forecasts sometimes go wrong?sometimes go wrong?