Snow forecasting

6/3/99


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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

Author: Norman Junker

Email: norman.junker@noaa.gov

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

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