January 19, 2008 Winter Storm|
...The synoptic pattern leading up to the event trended toward a pattern that
is not typically associated with major winter storms in central North Carolina...
...Light to sometimes moderate snow fell across portions of central North Carolina for several
hours during the storm. Despite the multiple hours of falling snow, the snow did not accumulate
very efficiently. Several factors are believed to be responsible for the less then expected
accumulation of snow including: generally light and inconsistent precipitation rates,
surface temperatures at or just above freezing, and rather mild soil temperatures...
...This event was another case in which forecasters utilized AMDAR aircraft soundings
to provide thermal profiles at GSO and RDU in between RAOB times. This data set is an
important resource during critical winter weather forecasts...
...In events where the surface low is weak, cold air is lacking, and there is little cold
advection, secondary factors such as diabatic processes become much more important making it
difficult to received Winter Storm Warning criteria accumulations...
...This case provides an example where the dProg/dt analysis approach with QPF and partial thicknesses
could have led to an improved forecast...
This storm followed a minor
winter storm that impacted portions of
central and western North Carolina on Thursday, January 17, 2008. The first winter
storm was moving away from the Mid Atlantic
during the evening hours of January 17. At the same time
a cold front was moving across the Mississippi Valley with
an associated 850 mb trough. Temperatures at 850 mb of
-10 deg C or below were as far south as southern Missouri. At the same time, an
Arctic cold front and Alberta
Clipper system were located over the far Northern Plains.
By the daybreak hours of Friday, January 18, the cold front had
reached the western Appalachians and colder 850 mb
temperatures were moving into the Ohio Valley. An impressive trough
was located at 500 mb over the south-central U.S. with a closed low over northern Mexico in the southern stream.
A broad southwesterly flow was present across the
southeastern U.S. A strong upper level jet at 250 mb
stretched from Texas across the Ohio Valley and into the Northeast with winds up to 150 knots.
During the late afternoon hours on Friday, the cold front crossing
the Appalachians was dissipating and the Arctic cold
front was entering the upper Mississippi Valley. A weak
surface low was developing across the western Gulf of Mexico. The 850 mb temperatures across central NC had
fallen a few degrees
in 12 hours to around 3 deg C with a west-southwesterly flow and some weak cold advection. The
500 mb closed low in the southern stream had opened up as the trough approached the Gulf
Precipitation generated in the strong southwesterly flow aloft rapidly
advanced northeast during the overnight and early morning hours on Saturday, January 19. The KGSO RAOB from
just after midnight at 06Z showed that the lower levels of the atmosphere were still dry and above freezing. The bulk of this
precipitation remained south and east of KGSO in a slightly milder air mass.
The first round of precipitation moved into central North Carolina during the predawn hours
on Saturday. This precipitation was largely driven by divergence aloft and mid level frontogenesis.
At the same time, the trough in the southern stream over
the Gulf of Mexico was shearing out as a larger northern stream trough approached the lower Mississippi Valley.
Temperatures at 850 mb across central
North Carolina had cooled slightly to around 1 deg C with a light westerly
flow still producing some weak cold advection from the Tennessee Valley where 850 mb temperatures where in the
-4 to -8 deg C range. The KGSO RAOB from
around daybreak at 12Z showed that the low level flow had become northerly and the profile had cooled.
The first round of precipitation largely remained east of Interstate 85 and the precipitation
fell as rain with surface temperatures in the upper 30s to lower 40s. The precipitation dissipated by around
midday and the break in precipitation allowed temperatures to warm into the lower to mid 40s at many locations
in central North Carolina.
A second round of precipitation developed across Georgia and South Carolina and rapidly spread northeast
into the Piedmont of North Carolina between 100 and 400 PM. During the late morning and early afternoon hours, a
cold and dry air mass was observed moving south from western Virginia into the Northwest Piedmont of North Carolina. Dew points
in this air mass dropped into the teens to lower 20s. The KGSO RAOB from
just after noon at 18Z showed that the lower levels of the atmosphere had cooled and the profile supported
snow with a possible near freezing layer near the surface. Further east at
KRDU, an AMDAR aircraft sounding from just after noon at 1724Z showed
a significant above freezing layer in the lowest 3,000 feet or so of
the atmosphere. Precipitation in the Triangle area started as rain at around 200 PM and then changed over
to wet snow between 200 and 400 PM as the melting snow aloft cooled the above freezing layer near the ground.
Eventually the melting snow aloft cooled the lower level profile to near freezing isothermal which allowed wet
snow to fall over all of the RAH CWA by the early evening hours. Despite many locations receiving several hours of
snow, the snow had a difficult time accumulating as temperatures remained just above freezing in
the 33 to 34 degrees F range. A significant
gradient in precipitation amounts also resulted in diminished
snow accumulations as the least amount of precipitation fell where the air was
coldest and most supportive of snow. All of the precipitation tapered off from southwest to northeast
during the evening hours. The Arctic cold front moved across the Appalachians during the evening hours and
began crossing central North Carolina after midnight.
After the event, snow
accumulations across the CWA were around a half inch with two areas of more
significant accumulation. The larger and more impressive area stretched from
the Northern Piedmont southwest into the northern portions of the Southern
Piedmont snow accumulations in this area ranged from 1 to 1.5 inches with localized amounts of 2 to nearly
2.5 inches in northern Durham, Orange, southern Alamance and northern Chatham
counties. Further south across the southern Sandhills and Southwestern Coastal
Plain, a localized area of snow accumulations of1 to 2 inches was located
across Scotland, Robeson, and southern Cumberland counties. The more significant snow accumulations in this area likely resulted
from heavier and more persistent precipitation rates. The snow
accumulation was visible the following morning via the MODIS visible satellite
After the event, snow accumulations across the CWA were around a half inch with two areas of more significant accumulation.
The larger and more impressive area stretched from the Northern Piedmont southwest into the northern portions of the Southern
Piedmont snow accumulations in this area ranged from 1 to 1.5 inches with localized amounts of 2 to nearly 2.5 inches
in northern Durham, Orange, southern Alamance and northern Chatham counties. Further south across the southern Sandhills
and Southwestern Coastal Plain, a localized area of snow accumulations of1 to 2 inches was located across Scotland, Robeson,
and southern Cumberland counties. The more significant snow accumulations in this area likely resulted from heavier and more
persistent precipitation rates. The snow accumulation was visible the following morning via the MODIS visible satellite imagery.
This was a very difficult storm to forecast with inconsistent model guidance, a sharp gradient in the
amount of precipitation, a complicated timing scenario between the precipitation and the arrival of colder
air, and the expected impact of diabatic processes such as cooling from evaporation and melting snow.
MODIS Visible Satellite Image Showing Snow Cover
Snow Accumulation Map
Liquid Equivalent Precipitation Map