Embedded Snow Streaks in Rain - January 19, 1998

What are they? Trace to measurable amounts of snow embedded in areas of rain.  The rain area can exist to the north and south of the area of snow.  Often referred to as “snow islands”.

In general Snow Streaks

    - Occur with weak horizontal thermal advection
    - Often with a passing weak surface wave at best
    - Jet Streak in the vicinity of location of snow - Enhance vertical velocities/precip rates
    -  Increased precip rates increases snow melt aloft
    -  Snow melts as it descends through layers just above freezing
    - Overall column is evaporatively cooled allowing freezing levels to lower

Partial Thickness Considerations

    - 850-700 MB Thicknesses typically below critical value of 1550M - Generally 1510-1540M
    - 1000-850 MB Thicknesses typically marginal or slightly above critical value of 1310M
    - 1000-950 MB Thicknesses higher than critical value of 410M - Generally 420-430M

This was a  classic case - Swath of 2-4" snow across central and eastern North Carolina with rain to the north and south. Partial thicknesses/850 MB Temps mislead forecasters in Virginia into forecasting snow. Vertical velocities were insufficient to produce snow.

    12 HR ETA Forecasts - 1000/850 Thickness - 1305 M  versus critical 1310 M value
                                             850/700 Thickness  - 1510 M  versus critical 1550 M value
                                             850 Temperatures          - 2 C