Winter weather forecasting

1/31/00


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Table of Contents

Winter weather forecasting

Snow Forecasting

Forecasting snow requires

To forecast snowfall amounts

The physical reasons that determine the amount of snow that falls over any location are

Precipitation type

Traditional ways to forecast precipitation type

1000-500 mb thickness

Partial Thickness

Precipitation type from soundings This is the best way to determine precipitation type

If there is a cold layer below the inversion, the question of precipitation type is determined by how much of the snowflake melted

Precipitation type?

PRECIPITATION TYPE?

FREEZING RAIN OR SLEET THE TAU TECHNIQUE - Cys et al., 1996

Ice forms on various nuclei at various temperatures

Variation of frequency of supercooled clouds containing ice crystals. Curves 1 and 2 (all water clouds) pertain to ordinate at left. Curves 3 and 4 (mixed cloud studies) pertain to the ordinate at right.

At what minimum cloud temperatures do crystals usually form?

Freezing drizzle

NCEP ETA PRECIPITATION TYPE ALGORITHM, AT HPC WE NOW ALSO USE ALGORITHMS DEVELOPED BY RAMER AND CYS

Snowfall intensity

ICE CRYSTALS GROW BY

The variation of crystal habit with temperature and supersaturation according to the experiments of Mason et al.

SNOWFLAKE SIZE IS ALSO DEPENDENT ON AGGREGATION

SNOWFLAKE DENSITY

WHY SHOULD I CARE ABOUT THE PHYSICAL CHARACTERISTICS OF THE SNOWFLAKES

Forecasting snow to liquid ratio Summary

For example Southern storm tracks typically are associated lower snow to liquid ratios than clipper type systems

Snow to liquid ratios vary significantly by geographic region. In Colorado the snow to liquid ratio is usually much higher than 10 to 1 ( or snow density less than .10).

SNOW RATIO TABLE FOR THE EASTERN HALF OF COUNTRY (not mountain locations)

Other Tidbits about snow to liquid ratios

Upper level aspects of snowstorms that can change the strength of the vertical motion or moisture transport into a system

THE IMPORTANCE OF THE EASTERN CANADA UPPER LOW AND CONFLUENCE FOR EAST COAST SNOWSTORMS

THE SELF-DEVELOPMENT PROCESS

The most common upper level jet pattern with snowstorms that produce a large area of 10+.

Remember, heavy snow usually occurs in mesoscale bands.

What about using empirical techniques or composites?

References and related articles

References Continued

Author: NCEP user

Email: njunker@ncep.noaa.gov

Home Page: http://www.hpc.ncep.noaa.gov/research/res2.html

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