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