Shallow Convective Snow Bands
Just before daybreak on March 2, 2009 as a
area of precipitation was moving away from
central North Carolina, forecasters began to
notice bands of snow
developing and extending southward across central
North Carolina (loop of KRAX WSR-88D reflectivity imagery
and loop of TRDU TDWR reflectivity imagery).
The bands were also visible across southern Virginia and northern North Carolina on the
visible satellite imagery available just after daybreak.
The snow was expected to be on the wane during the early morning
hours as a low pressure system developed off the Mid Atlantic
coast and a 1041 mb high pressure system moved into the western Great Lakes and extended
into the Mississippi Valley. These systems combined to produce a moderate pressure gradient
that resulted in steady northerly winds across central North Carolina. The northerly flow combined with
a cold air mass over the Great Lakes and Ohio Valley produced cold advection across the Carolinas
and Virginia both at 850 mb and at the
surface. At the same time, air temperatures in the
mid to upper 20s were moving across
4 inch soil temperatures in the 39 to 41 degree range in the Triangle area
(ground temperature map |
soil temperature text product).
The base reflectivity radar loop from the krax WSR-88D shown to the right shows the
appearance of the bands after a larger area of snow moves northeast away from central
North Carolina. In the wake of the larger precipitation area, numerous bands of light
precipitation appear. During the loop which runs from 1004 UTC through 1431 UTC on
March 2, 2009, the narrow bands consistently extend from north to south as the number and
character of the bands themselves evolve. The krax base reflectivity image
from 1044 UTC shows numerous bands extending southward from near the Virginia
border to just south of the Triangle area. By 1239 UTC, the band structure became
fairly regular with north south
bands separated by approximately 5 km (3 miles). The narrow bands became more difficult to find
as a slightly larger area of precipitation associated with an approaching short wave moved southeast
across the region and changed the narrow bands into larger and broader areas of precipitation.
A previous study, Snowbands
during the Cold-Air Outbreak of 23 January 2003 (Schultz, D.M., D.S. Arndt, D.J. Stensrud, and J.W. Hanna, 2004)
investigated similar events and hypothesized that
the bands were produced through two processes: 1) thermal instability
in the planetary boundary layer that produced horizontal convective rolls (HCR's) over widespread areas, and 2)
lake-effect processes downstream of small lakes which produced localized bands. The horizontal convective rolls
are a strong candidate for the production of the bands since the bands were associated with
a strong cold-air outbreak, at times they were regularly spaced over
a large area, and occurred within the planetary boundary layer. The study noted that the updrafts associated
with HCR's in the planetary boundary layer can saturate and produce parallel cloud bands or cloud streets
if the updrafts are sufficiently deep and moist. HCR development can be result of either thermal or
dynamic instabilities. The March 2, 2009 event was associated with cold advection over
a relatively warm land surface suggesting that the thermal instability mechanism likely produced the HCR.
The same study noted that for this mechanism, the buoyancy provides the
energy for the circulations, and the vertical wind shear organizes the circulations into bands provided the
instability does not become too great. Finally, HCR's due to the thermal instability mechanism are
frequently observed in slightly unstable environments with some sensible heat flux from the surface.
In the study by Schultz, Arndt, Stensrud, and Hanna, they
state that HCR's in the planetary boundary layer form when the
environmental lapse rate is near neutral or slightly unstable,
the mean wind speed in the roll layer exceeds
some relatively small value (2–5 m/s), the vertical
wind shear is nonzero, and a modest value of surface
sensible heat flux exists (Atkinson and Zhang, 1996).
The four panel image of RUC analysis soundings shown to the right (click on
the image to enlarge) for KRDU on March 2 at 09 UTC, 12 UTC, 15 UTC, and 18 UTC
shows that the boundary layer became slightly unstable. In fact, the
RUC soundings at 12 UTC and
at 15 UTC indicates that a layer
at or just above the surface which was around 4,000 to feet deep had become unstable with
the top of the boundary layer at around 850 mb or 5,000 feet.
The winds in this layer
ranged up to around 30 knots and they were from the north-northwest with very little directional
shear. Wind speeds were light at the surface and then increased in speed up to around 3,000 feet.
While there was very little backing of the
winds observed in the lower portion of the RUC analysis soundings, backing was noted above 850 mb
in the frontal inversion.
Cold advection was noted in the SPC 850 mb temperature advection
analysis and in the SPC 3 hour surface temperature change analysis.
Some sensible heat flux should be expected with
surface air temperatures ranging between 26
and 30 degrees at 12 UTC and soil temperatures
ranging between 39 and 41 degrees at 12 UTC. Following a similar methodology used in
the study by Schultz, Arndt, Stensrud, and Hanna, the difference between the ground temperature at 4 cm
below the native vegetation (TS04) and the air temperature (TAIR) was calculated. Across the Northern
Piedmont where the snow bands occurred, the ground temperatures
at 12 UTC ranged between 39 and 41 degrees while the air temperatures
at 12 UTC ranged between 26 to 30 degrees. For example, at Oxford in Granville County,
the ground temperatures at 12 UTC was 39 degrees while the air temperatures at 12 UTC was 26 degrees,
a difference of 13 degrees. Similar differences between the ground and air temperatures were observed
in the Northern Piedmont with differences of 13 degrees at Reedy Creek in western Wake
County and 13 degrees at Clayton in northwestern Johnston County. Goldsboro reported a
difference at 12 UTC of 10 degrees.
The bands of light snow during the morning and into the midday hours on Monday generally produced
only light snow accumulations with most locations in the Northern Piedmont reporting an
additional light coating of snow on Monday morning. The snow was rather persistent
through much of the morning across portions of the area (observations from Raleigh-Durham
and Henderson) with light snow falling for several hours
in North Raleigh with only a light dusting of new snow observed. This period of snow was generally
unanticipated and was likely a surprise to many residents in the area.
Another interesting aspect of the snow bands is that some of the bands appear to have at least a limited
connection with the relatively large lakes that straddle the North Carolina - Virginia border
including Kerr Lake and Lake Gaston. The reflectivity loops from the
KRAX WSR-88D radar located 10 miles southeast of Raleigh
and the TRDU Terminal Doppler Weather Radar (TDWR)
located just north of the Raleigh-Durham International Airport can be stopped and advanced to allow
improving viewing of the phenomenon. While many of the bands do no appear to originate or even be
located downwind of the lakes, there are a couple of bands that may be enhanced by the additional
flux of moisture heat from the relatively mild lake surface.
Oftentimes the precipitation on the back edge of a departing winter cyclone ch0nages from
light snow to a light drizzle or freezing drizzle as the deep moisture exits and the
only lingering moisture is shallow and below the favored growth zone for dendritic ice
crystals between -12 ° C and -18 ° C. The RUC analysis soundings from March 2 at KRDU
indicate that for much of the morning after daybreak the low level moisture was generally confined below
(warmer) the -10 ° C to -12 ° C layer (RUC soundings for KRDU on March 2 at
09 UTC, 12 UTC,
15 UTC, and 18 UTC.
The moisture appeared to be sufficient at 09 UTC with
considerable drying noted at 12 UTC. The low level moisture
extended up to -14 ° C at 15 UTC with only some slight drying
in the sub -10 ° C layer at 18 UTC.
The bands of snow that moved across the Northern Piedmont of North Carolina
during the morning of March 2, 2009 were an unusual event. The
bands may be a result of thermal instability in the boundary layer that produced horizontal convective
rolls (HCR's). The instability is largely the result of large scale cold advection over a relatively warm
ground that produces a modest sensible heat flux. The HCR's may have been enhanced by additional fluxes of sensible heat and moisture from the
relatively large lakes near the North Carolina - Virginia border including Kerr Lake and Lake Gaston. It
is somewhat uncommon in winter across central North Carolina to get the conditions necessary for snow producing
HCR's including a boundary layer that is very moist, increasingly unstable, and sub freezing.