The February 13 event was preceded by a progressive
large amplitude upper-level wave over the central U.S. and
a surface low lifting
northeast toward the Great Lakes. A warm
front extended from the low center south across the Appalachian Mountains.
A sounding from Greensboro (GSO)
on the evening of the 12th (00 UTC on February 13)
revealed a cool, shallow layer of air near the surface,
which impeded the progression of the
warm front northward. Twelve hours later, the morning
sounding from GSO (12 UTC on February 13) shows the cold, shallow air mass lingering over the area.
The weather pattern in which a cold, shallow air mass lingers over the Piedmont region
is often called cold air damming. The damming region is the
area in which the cold, shallow, stable air mass is located. Cold air damming typically forms along the lee side of
the Appalachians, either through the funneling of cold dense air down the
East Coast by an area of high pressure to the north, or through diabatic processes that
lead to the formation of a cold stable air mass in situ. This creates a scenario where relatively cooler,
more stable air resides over western NC, while warmer, and often less stable air remains east of the
damming region. Between these two differing air masses lies a front, often referred to as the wedge front.
The location of the wedge front varies from event to event, and the front may migrate inland or retreat
towards the coast. Such conditions often
result in tremendous temperature gradients across NC, as was
the case during the overnight hours on the 12th. Temperatures across much of western NC remained in the upper 30's to lower 40's,
while temperatures just 80 miles east rose into the lower 60's.
Early on the morning of the 13th, a surface low
developed along the wedge front over Upstate South Carolina and tracked north along the coastal
warm front. As the low moved north, a band of heavy rain developed and tracked north over the North
Carolina Coastal Plain, where up to 2
inches of rain fell. Warm air was eventually
able to erode the cold near-surface layer and the wedge front.
However, temperatures were only able to recover into the mid
to upper 40's across the western and northern Piedmont.
Meanwhile, the original cold front and drier air advecting into the region from the Midwest remained
west of the Appalachians.
The combination of cooler high temperatures on the 13th and dry air upstream would prove significant
later that evening as the upper-level wave
moved into the area and more precipitation developed.
While the main storm system exited the area to the northeast,
trough was slowly pivoting around the base of the longwave trough.
The lighter colors over western NC
in the water vapor image from 21 UTC on the 13th represent
a small area of convection that developed just ahead of the shortwave.
This area of showers and thunderstorms formed in response to the increasing
mid-level lapse rates
associated with the shortwave. At around 22 UTC, the area
of precipitation was fairly small and just southwest of the NWS Raleigh
County Warning Area near Charlotte. Over the next couple of hours, the
precipitation expanded and moved into the Western Piedmont region by 00 UTC on the 14th. There were around a half
dozen lightning strikes during each hour in this period, primarily across the southern portion of the precipitation
(one hour lightning strikes ending at 23 UTC |
one hour lightning strikes ending at 00 UTC |
one hour lightning strikes ending at 01 UTC).
At the same time, drier air was beginning to move into the Northwest Piedmont. Dew point temperatures
began to plummet, and with temperatures only in the lower 40's, evaporation of the precipitation
caused significant cooling of the low-level air. Such cooling is known as evaporational
cooling, where the process of evaporating a rain drop removes heat
from the surrounding air, and subsequently cools the surrounding air. This cooling, and
moistening of the ambient air will continue until the
air becomes saturated, at which point the precipitation ceases to evaporate. The temperature
at which this occurs is known as the wet-bulb temperature.
The 00 UTC sounding from GSO shows dry
low-level air that would promote evaporation and cooling as the precipitation overspread the area.
Given the temperatures and dew points
within the dry layer, the evaporational cooling would bring most of the layer to wet-bulb
temperatures near freezing. After the layer was saturated, additional cooling was generated
from the melting of snow aloft. As snow melts, it removes heat from the atmosphere, although
it has a less significant impact then evaporation, and with moderate to heavy precipitation rates, the
role of melting becomes mores significant.
As the precipitation moved into the area, the
change over to light snow occurred at around 23 UTC on the 13th.
The first report of snow came from Hickory, NC (see observation below).
KHKY METAR 132347Z 36007KT 2 1/2SM -SN BR FEW005 SCT009 OVC050 01/00 A2969 RMK AO2 RAE02SNB02 P0006
The small area of precipitation expanded as it spread northwestward from Lexington, through the Triad area, to the North Carolina/Virginia border.
A few bands of moderate snow developed at the leading edge of the precipitation, with a southwest to northeast orientation. One band in particular developed
over northwest Person County, along the NC/VA border. With the entire system moving to the northeast, this
band gave northwest Person County an hour of moderate snow even before the main area of precipitation arrived.
At around 01 UTC, the Satellite Analysis Branch at NESDIS,
issued a Satellite Precipitation Estimates discussion (SPENES). The
full discussion and imagery is available.
The discussion noted that the vorticity center near Hickory had
been strengthening. The precipitation producing clouds associated with
the vorticity center had been rapidly expanding with increasing lift
ahead of the system producing an increase in showers across central NC.
The discussion noted some concern for the spread of the precipitation
further east and northeast and that intensities will likely pick up
with concern that moderate to heavy snow.
Snowfall rates of up to an inch per hour were reported over much of the Northern Piedmont
over the next few hours. The enhanced snowfall rates were aided, in part,
by strong forcing associated with the mid-level
shortwave and a localized area of strong upper-level
divergence. While surface temperatures were relatively warm
and the snow initially melted as it reached the surface, the intense snowfall rates quickly
overwhelmed the surface temperatures, allowing snow to first stick on grassy surfaces, and eventually
on roads. Although no major traffic problems were reported during the event, some less
traveled roads in portions of Forsyth, Guilford, and Person Counties
were quickly covered with snow.
The precipitation dissipated fairly quickly after midnight with just a few lingering
flurries persisting after 100 am.
A radar loop of KRAX reflectivity imagery during the critical portion of the event with images from every 15 minutes
between 13/2228 UTC (528 PM EDT 02/13/2008) through 14/0755 UTC (255 AM EST 02/14/2008)
Note - this loop includes 39 frames