Freezing Rain or Rain Category

• With few exceptions, consists of freezing rain or rain for the 6 hour period.

57%    of the total events were freezing rain.

40%    rain

  3%    measurable snow w/rain

 30      total events

Principal forecast issues

• Use surface wet bulb temperatures to differentiate freezing rain vs. rain

• Nuisance vs. significant icing event
• Freezing rain as a self limiting process vs. sustained low level cold air advection (evaluate configuration of surface high)  

Tentative findings

• Freezing rain events in this category will typically be accompanied by temperatures close to freezing, limiting the impact of icing. Exceptions:

· Significant icing at tree top level due to sustained cold air advection and colder temperatures just above ground

· Slowly modifying and very shallow arctic airmass

· Preexisting very cold ground (rare in central North Carolina)

Vertical temperature profiles associated with “freezing rain or rain” category

• Prominent melting layer above a relatively shallow surface based sub-freezing layer characterized nearly 60% of the soundings and resulted in freezing rain.
• Slightly above freezing isothermal layer(s) or a shallow but an above surface sub-freezing layer beneath a melting layer characterized 40% of the soundings and resulted in a cold rain.

Significant icing associated w/ a shallow but very cold arctic air mass

• The TREND technique will not likely account for freezing rain events associated with a shallow surface based, and very cold arctic air mass.
• If the very shallow sub-freezing layer is characterized by temperatures in the 20’s (or colder) and dew points in single digits (or lower), significant freezing rain events can occur. 
• Though more likely in the Plains than in the Mid Atlantic or Southeast, heavy precipitation rates from favorable jet kinematics falling into an insitu Arctic air mass can deposit heavy icing before the latent heat of freezing limits ice accrual.
• Monitoring the trends in the surface temperatures and dew points of the insitu Arctic Air mass will be more helpful to the precipitation type forecast than the 1000-850 thickness values when the Arctic air mass is very shallow
• However, anticipating the location of sufficient precipitation rates from the passing jet maximum is the most challenging aspect for capturing this relatively infrequent event.

Footnotes

• The” freezing rain or rain” predominant p-type category is defined on the predominant p-type nomogram by the partial thickness values of 1310/1550 – 1315*/1580 - 1299/1580 – 1302/1550
• TRENDS empirical data base has consistently shown that as the melting layer aloft (850-700 mb thickness) increases, there is a corresponding decrease in the surface based sub-freezing (1000-850 mb) values to support freezing or frozen precipitation.
•  However such is not the case for the upper boundary of the freezing or rain category (i.e., 1310-1550 – 1315*/1580)
• The tendency to force symmetry upon data which does not support has been avoided here, but is a systematic flaw to the nomogram approach. 
• The current best guess is that the reversed slope is reflective of a secondary frequency of freezing rain associated with the far southern end of cold air damming Arctic air masses.