Forecasting Snow

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

Considerations Temperatures Temperatures Moisture/Precipitation Moisture/Precipitation Timing (operational standpoint) Timing (operational standpoint) Climatology Climatology Location/Geography Location/Geography

Temperatures & Moisture If there is moisture and temperatures are sub-freezing, snow crystals can form. If there is moisture and temperatures are sub-freezing, snow crystals can form. The two factors go hand in hand. The two factors go hand in hand. Roughly what level of the atmosphere is crucial to snow formation? Roughly what level of the atmosphere is crucial to snow formation?

850mb Level This level is important to forecasting snow. This level is important to forecasting snow. Generally, if 850mb temps are below 0°C, snow crystals can form, grow and fall. Generally, if 850mb temps are below 0°C, snow crystals can form, grow and fall. Consider a typical 850mb model forecast chart… Consider a typical 850mb model forecast chart…

0°C Line at 850mb As a general rule, snow does not form when 850mb temperatures are above 0°C. As a general rule, snow does not form when 850mb temperatures are above 0°C. Snow does not necessarily fall to the surface when 850mb temps are <0°C. Snow does not necessarily fall to the surface when 850mb temps are <0°C. What other things should be considered? What other things should be considered?

Temperature Considerations Temperatures at the surface. Temperatures at the surface. Temperatures at other levels. Temperatures at other levels. Thicknesses. Thicknesses.

Surface Temperatures Sub-freezing temperatures are favored. Sub-freezing temperatures are favored. Snow can fall otherwise, but may not stick as efficiently. Snow can fall otherwise, but may not stick as efficiently. Temperatures near the ground are below 32°F/0°C, what else to consider? Temperatures near the ground are below 32°F/0°C, what else to consider?

Temperatures aloft Above freezing temperatures aloft can melt snow crystals. Above freezing temperatures aloft can melt snow crystals. This leads to sleet, freezing rain or rain. This leads to sleet, freezing rain or rain. Snow is favored when the entire column up to 850mb is below 0°C. Snow is favored when the entire column up to 850mb is below 0°C.

Temperature Profile Considering various thermal profiles… Considering various thermal profiles…

Thicknesses What is thickness? What is thickness?

Thicknesses Thickness is a layer of the atmosphere. Thickness is a layer of the atmosphere. The greater the temperature, the greater the thickness… The greater the temperature, the greater the thickness…

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

Critical thicknesses Typical thickness values for a given layer, in which the layer would likely support snow. Typical thickness values for a given layer, in which the layer would likely support snow. Generally, a virtual temperature of 0°C or K, assumed for freezing. Generally, a virtual temperature of 0°C or K, assumed for freezing. What are the main critical thicknesses? What are the main critical thicknesses?

Main Critical Thicknesses mb mb mb mb mb mb mb mb mb mb

mb Thickness What is the critical thickness for the mb layer? What is the critical thickness for the mb layer? 5545 meters?

mb Critical Thickness What is the actual critical thickness? What is the actual critical thickness? Why is the critical thickness different than the “calculated one”? Why is the critical thickness different than the “calculated one”?

mb thickness What is the critical thickness for the mb layer? What is the critical thickness for the mb layer? 2853 meters?

mb Critical Thickness What is the actual critical thickness? What is the actual critical thickness?

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

mb thickness What is the critical thickness for the mb layer? What is the critical thickness for the mb layer? 1300 meters

mb thickness The low-level column is below freezing. The low-level column is below freezing. The critical thickness for this layer is 1300m or 130dm. The critical thickness for this layer is 1300m or 130dm. Arguably the best indicator. Arguably the best indicator.

The blue line is NOT the critical thickness!

Other Views of Temperatures Soundings/Skew-Ts Soundings/Skew-Ts Text extracted output Text extracted output Others? Others?

Precipitation If there is enough moisture – precipitation. If there is enough moisture – precipitation. As a general rule, what are typical liquid equivalent to accumulated snowfall ratios? As a general rule, what are typical liquid equivalent to accumulated snowfall ratios?

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

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

Timing of Snowfall What times of the year is snowfall favored in the tri-state region? What times of the year is snowfall favored in the tri-state region? How does time of the year affect snowfall? How does time of the year affect snowfall?

Time of Day Time of day is important to snowfall forecasting, especially early/late season. Time of day is important to snowfall forecasting, especially early/late season. The timing is also crucial for traffic, schools, businesses, etc. The timing is also crucial for traffic, schools, businesses, etc.

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

Famous Nor’Easters

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

Other Snow Events Clipper systems Clipper systems Lake-effect snow (ocean effect as well) Lake-effect snow (ocean effect as well) Troughs Troughs Warm fronts (overrunning) Warm fronts (overrunning) Cold fronts Cold fronts

Location & Geography Proximity to heat sources Proximity to heat sources Urban areas (urban heat effect*) Urban areas (urban heat effect*) Coastal locations Coastal locations

COLD AIR MASS WARMER OCEAN Marine layer Marine layer Marine layer

Location & Geography Elevation Elevation How does elevation effect snowfall? How does elevation effect snowfall? (consider temperatures and geography)

Location to Air Masses Cold (generally) northwest side of a low favors the heaviest snowfall. Cold (generally) northwest side of a 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 Snowfall 1. Know temperatures through the column. 2. Consider the precipitation amounts. 3. All snow or mixed precipitation? 4. Estimate snowfall ratios. 5. Keep climatology in mind. 6. Factor in model trends and biases. 7. Note seasonal or recent patterns.

Verification What could go wrong with a snowfall forecast? What could go wrong with a snowfall forecast? How do snowfall forecasts sometimes go wrong? How do snowfall forecasts sometimes go wrong?