Precipitation Type Forecasts and TRENDs
The TRENDs technique, in which the
thermal structure of the atmosphere can be used to indicate
the most likely predominate precipitation type,
clearly showed a transition from snow to rain during this event.
Model forecasts of partial thickness values a before the
event proved reasonably accurate.
This technique indicated that the predominant precipitation
type would initially be snow with a brief change over to a sleet/snow/freezing
rain mix, then to mostly freezing rain for several hours
over much of the Piedmont, before the warming aloft
reached down to the ground to push surface temperatures
above the freezing mark. However, the duration of the
period of a wintry mixture ended up being very short,
typically an hour or less over much of the area. Why was this?
First, the effects of the warming aloft may have been
underestimated. An 18Z (100 PM EST) sounding from Morehead
City, NC, showed this significant warming aloft,
with temperatures over 6°C less than 2000 feet aloft.
While the Greensboro, NC, sounding at
18Z (100 PM EST) was notably colder
aloft, much of the eastern and southern Piedmont
and Coastal Plain may not have had enough
of a cold surface-based layer to overcome the warm rain
falling from above.
A second, and likely more significant factor, was the
lack of a continuous feed of low level dry air into the
area, which may have severely limited the potential for
ice accrual. To get freezing rain, temperatures must be
at or below freezing in a thin layer near the surface,
with a significant layer of above-freezing air aloft. Once a
coating of ice, or glaze, forms on surfaces near the ground,
the phase change from liquid water to ice (as
the rain freezes on contact) releases a small amount
of heat (called latent heat). The release of this heat
is often just enough to push surface temperatures that
are initially just below or near freezing up to the freezing mark.
Keep in mind that the freezing temperature is also the melting
temperature and that freezing rain without the resupply of
cold, dry air will often result in a balance where there is
some freezing of rain and some melting as well as the
temperature hovers near 32 degrees.
Historically, to get a significant coating of ice from freezing
rain in central North Carolina, such as the December 2002
ice storm, there must be a steady feed of
cold, dry low level air into the region. Typically this feed of cold,
dry air originates from with a surface high centered to our north.
As precipitation falls into it, evaporative cooling
will offset the release of latent heat and allow the ice to
build up. In this event from February 1, 2007, the high
pressure center that had
deposited the cold and dry air had
shifted well offshore and
the surface wind flow became southerly.
Once the warming aloft allowed the snow to transition to
sleet and freezing rain, the release of latent heat almost
immediately caused a changeover to mostly rain at the surface.