Measurable Snow with Rain Category

• Snow/rain mixtures are likely in this category with most events resulting in measurable snow during the 6 hour period. Infrequently, freezing rain or all snow occurs.   

66%      of the total events produced measurable snow during the 6 hour period

o 65% of the measurable snow events occurred with rain

o 35% of the measurable snow events were all snow

17%      of the total events were rain mixed with trace snow.

17%      of the total events were freezing rain  

         17      total events

Principal forecast issues

• Differentiating snow/rain mixtures with measurable snow vs. those with trace snow
• Potential for caa from intensifying cold air damming
• Potential for evaporative cooling

• Accounting for the infrequent “all snow” events
• Potential for near freezing isothermal layer from melting
• Accounting for infrequent freezing rain events
• Potential for in-cloud temperatures > -10 C
• Potential for waa from fast approaching surface waves

• Evaluate soundings, low level wet bulb temperatures, trends in partial thickness values, cloud microphysics, patterns of cyclogenesis and surface high configurations

Tentative findings

• The vertical temperature profiles most typically found in this category are surface based melting layers of varying degrees.
• Measurable snow events in this category likely result from low level cooling associated with cold air damming and/or evaporative cooling and less so from melting.
• Freezing rain events in this category may be due to in-cloud temperatures > -10 C

Near freezing isothermal segments in polar air masses

· Observations of sounding characteristics within 24 hours of the onset of  precipitation leading to measurable snow from rain and snow mixtures has shown that some polar air masses are characterized by near freezing isothermal segments, typically somewhere in the 900-800 mb layer.

· The presence of these segment(s) may well be an air mass precursor to the formation of a near freezing isothermal layer supporting measurable snow from mixed rain/snow events.

Footnote

· The measurable snow with rain predominant 6 hour interval p-type category is defined on the predominant p-type nomogram by the partial thickness values of 1290/1537 – 1298/1532 – 1312/1530 – 1312/1538 – 1290/1542.