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.
3% measurable snow w/rain
30 total events
- 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)
- Freezing rain events in this category will typically be
accompanied by temperatures close to freezing, limiting the impact of
icing at tree top level due to sustained cold air advection and colder
temperatures just above ground
modifying and very shallow arctic airmass
very cold ground (rare in central North Carolina)
profiles associated with “freezing rain or rain” category
melting layer above a relatively shallow surface based sub-freezing layer
characterized nearly 60% of the soundings and resulted in freezing rain.
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.
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.
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.
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
anticipating the location of sufficient precipitation rates from the
passing jet maximum is the most challenging aspect for capturing this
relatively infrequent event.
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
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
such is not the case for the upper boundary of the freezing or rain
category (i.e., 1310-1550 – 1315*/1580)
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.