Table of Contents
Snow 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
THE 50% CRITICAL THICKNESS FOR MOUNTAINS OF UTAH
Partial Thickness
Precipitation type from soundings This is the best way to determine precipitation type
The warm layer
Lower cold layer
A freezing rain sounding Note that temperature of the warm layer is above 4oC
An ice pellet sounding Note temperature of warm layer is 1-3oC
FREEZING RAIN OR SLEET THE TAU TECHNIQUE - Cys et al., 1996
Freezing drizzle
NCEP ETA PRECIPITATION TYPE ALGORITHM
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
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 DENSITY AS A FUNCTION OF TEMPERATURE. NOTE THE LOW CORRELATION SHOWN IN THE GRAPHS
SNOW RATIO TABLE FOR THE EASTERN HALF OF COUNTRY (not mountain locations)
Other Tidbits about snow to liquid ratios
Upper level aspects of major east coast snowstorms (storms that produce a significant sized area of 10” or greater snowfall
THE IMPORTANCE OF THE EASTERN CANADA UPPER LOW AND CONFLUENCE FOR EAST COAST SNOWSTORMS
Cold air damming and coastal frontogenesis
Table from Kocin and Uccellini (1990)
THE SELF-DEVELOPMENT PROCESS
The most common upper level jet pattern with snowstorms that produce a large area of 10”+.
SNOW AND THE SURFACE LOW, THE HEAVIEST SNOW FALLS
850 LOWS AND HEAVY SNOW OVER CENTRAL AND EASTERN U.S.
500 MB HEIGHTS AND 1000-500 MB THICKNESS THE HEAVIEST SNOW (CENTRAL AND EASTERN U.S.) DURING THE NEXT 12 HRS USUALLY OCCURS
COOK METHOD
Garcia technique From Cope (1996)
Garcia method (continued)
Magic Chart
Remember, snow usually occurs in mesoscale bands.
CONDITIONAL SYMMETRIC INSTABILITY
When trying to forecast CSI, make sure the cross section is normal to the 850-300 mb thickness
WHEN LOOKING FOR CSI ON A VERTICAL CROSS SECTION
CAN ALSO USE EQUIVALENT POTENTIAL VORTICITY (EPV) TO FIND CSI
CSI and heavy snow
References
References Continued
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Author: Norman Junker
Email: norman.junker@noaa.gov
Home Page: http://www.hpc.ncep.noaa.gov/research/res2.html
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