Icing - Freezing Rain with Trace Frozen Category

• Largely consists of freezing rain, often mixed with a trace of frozen during the 6 hour period. Infrequent exception is significant snow associated with a deep near freezing isothermal layer.

70%      of the total events were predominantly freezing rain

15%      were predominantly snow

15%      rain w/trace snow

26 total events

Principal forecast issues

• Surface wet bulb temperatures define southern & eastern most extent of icing area

• High potential for significant icing
• Freezing rain as a self limited process vs. sustained low level cold advection
• Evaluate factors favoring formation of deep near freezing isothermal layer
• Weak thermal advection (baggy highs, flat lows)
• Northwest quadrant of mature cyclone
• Moderate to heavy precipitation rates & surface temperatures within several degrees of 0 C (precipitation banding)
• Near snow/rain boundary on the snow side

Tentative findings

• In this predominant p-type category, deep near freezing isothermal layers are most likely associated with 850/700 mb partial thickness values from 1550-1557 m

Vertical temperature profiles associated with the “Icing - Freezing Rain with a trace of frozen” predominant type category

• Prominent melting layer (>1560) above a relatively shallow surface based sub-freezing layer (1291-1300) characterized nearly 40% of the soundings.
• Significant near freezing isothermal layer characterized 12% of the soundings
• Melting layer (1550’s) above a surface based sub-freezing layer (1293-1303) characterized 48% of the soundings.

Prolonged Freezing Rain

• Requires an ongoing source of low level cold air advection typically associated with a surface cold air high, ridging into the freezing rain area
• Without an ongoing cold air advection source, freezing rain is a “self limiting process” due to warming associated with the latent heat of freezing.
• As temperatures near 30 F, the ratio between liquid and ice accrual is no longer 1:1. Once temperatures reach 32 F (point at which water freezes and ice melts), there is no longer any significant ice accrual.
• Prolonged freezing rain storms may occur without a surface low via isentropic lift. 
• “Icing - Freezing Rain with a trace of frozen” is the predominant p-type category most likely to be associated with prolonged freezing rain events.

Freezing Rain at Tree-top level

• Ground level temperatures and dew points near or just above freezing can be unrepresentative of colder temperatures and drier dew points at tree top level.
• Though more research is needed, temperatures in the upper to mid 20s supporting heavy icing in trees, can occur while ground level temperatures remain near freezing. 

Footnotes

• The “Icing - Freezing Rain with a trace of frozen” category is defined on the predominant p-type category by the partial thickness values of 1292/1550 – 1303/1550 – 1299/1580* - 1290/1580*.
• Freezing rain events infrequently can occur with the 850-700 mb thickness values >1580 m.  Until more cases have been documented, it is recommended that users simply extrapolate the current category boundaries for “Icing” from 1580 to 1590