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The 18-20 December 2009 Winter Storm

Laurence G. Lee and Patrick D. Moore
NOAA/National Weather Service
Greer, SC

Snow covers the valley of Lake Junaluska, North Carolina, on 19 December 2009.  Image taken by Bob Child.

Heavy snow fell across the mountains of North Carolina on 18-19 December 2009. Numerous locations received more than a foot of snow from the event. Image taken by Bob Child.

Author's Note: The following report has not been subjected to the scientific peer review process.

1.  Introduction
A low pressure system developed over the northern Gulf of Mexico and moved 
northeast through extreme south Georgia on Friday, 18 December, and along 
the Carolina coast on Saturday, 19 December 2009.  Snow accumulated across 
most of the mountains and foothills of North Carolina and extended across 
a large portion of the northwest Piedmont of North Carolina.  Snowfall 
totals of one to two feet were common across the central and northern 
mountains, and the northern foothills near the Blue Ridge (Fig. 1).  Most 
of extreme northeast Georgia, the area along and to the south of Interstate 
85 in Upstate South Carolina, and the southern part of the Charlotte 
metropolitan area experienced a mixture of rain, sleet, and freezing rain.
Click here to view a list of unofficial snowfall reports.
Snow accumulation for 18-21 December 2009
Figure 1.  Total snow and sleet accumulation (in inches) for the period 
18-21 December 2009 across the NWS Greenville-Spartanburg County Warning 
Area.  Click on image to enlarge.
The low pressure system continued to move north off the Mid-Atlantic coast 
on 19 December and produced significant amounts of snow over northwest 
North Carolina and western Virginia, north central North Carolina, south 
central and Tidewater Virginia, and northern Virginia and the Washington, 
DC, metro area.
Click on the above links to view storm summaries from other National 
Weather Service offices across the Mid-Atlantic region.
2.  Synoptic Developments and Storm Effects
a. 0000 UTC to 1200 UTC - 18 December 2009
At 0000 UTC on 18 December, the 500 mb analysis showed a ridge over the 
southeastern United States, a ridge along the West Coast, and a trough that 
extended from the Dakotas to south Texas (Fig.2).  Within the larger scale 
trough, short wave troughs were located over the Texas coast, Oklahoma 
Panhandle, and the northern Rocky Mountains.  The 250 mb analysis at the 
same time showed a well-defined jet structure with a large area of 80 to 
100 kt winds extending from Louisiana north over the Ohio Valley then east 
over the Atlantic Ocean.  The jet core contained a 130 kt wind maximum over 
the mid-Atlantic region (Fig. 3).  Another wind maximum (85 to 95 kt) was 
diving southward on the west side of the trough over the central and northern 
Rockies.  Yet another 250 mb wind was evident on the edge of the analysis 
area over the southwestern Gulf of Mexico.  The Gulf of Mexico wind maximum 
was the leading edge of a strong subtropical jet stream over the eastern 
North Pacific near 15N 110W.
500 mb analysis at 0000 UTC on 18 December 2009
Figure 2.  SPC objective analysis of 500 mb geopotential height, temperature 
and wind at 0000 UTC on 18 December.  Click on image to enlarge.
250 mb analysis at 0000 UTC on 18 December 2009
Figure 3.  SPC objective analysis of 250 mb isotachs, streamlines, and 
wind barbs at 0000 UTC on 18 December.  Click on image to enlarge.
The existence of multiple jet streams with wind maxima at different levels 
is a common feature of strong cyclogenesis.  For details regarding processes 
related to horizontal and along-stream wind speed variations and their 
relationships to vorticity, divergence, and vertical motion, refer to 
Uccellini and Johnson (1979), and Kocin and Uccellini (2004a).
The 0000 UTC surface analysis placed a 1006 mb center of low pressure over 
the Gulf of Mexico approximately 150 miles south of Louisiana (Fig. 4).  A 
cold front extended southward from the low and a nearly stationary front 
stretched east across central Florida.  High pressure extended from James 
Bay to the Carolinas.  The highest pressure (1031 mb) was east of Lake Huron.
HPC Surface fronts and pressure analysis at 0000 UTC 18 December
Figure 4.  HPC surface fronts and pressure analysis at 0000 UTC on 
18 December.  Click on image to enlarge.
Infrared satellite imagery at 2345 UTC on 17 December showed a cloud pattern 
typical of those associated with a developing low pressure system (Fig. 5).  
The western and northern edge of the growing comma pattern displayed a convex 
shape and a sharpening edge near the axis of the jet.  The coldest cloud tops 
were in the general area where moist, potentially unstable air in the warm 
conveyor belt began its ascent into the mid and upper levels.  The western edge 
of the cloud mass consisted of a well-defined margin between high, cold clouds 
in the warm conveyor belt flowing from the south and warmer clouds in dry air 
at a lower level descending from the west.  The location of these infrared 
cloud features in the right rear quadrant of a strong jet maximum was an 
indicator of cyclogenesis.
IR imagery and 250mb isotachs at 0000 UTC 18 December
Figure 5.  GOES-12 infrared satellite imagery at 2345 UTC on 17 December 
with 250 mb isotachs (kt; light blue contours) from the 80-km NAM model 
initial analysis at 0000 UTC on 18 December.  Click on image to enlarge.
Click here to view a 25 frame Java loop of a national reflectivity 
mosaic from 0600 UTC on 18 December to 0600 UTC on 19 December.
Click here to view a 25 frame Java loop of surface fronts and pressure 
analyses from 0000 UTC on 18 December to 0800 UTC on 21 December.
b. 1200 UTC to 1800 UTC – 18 December 2009
By 1200 UTC on 18 December, the surface low pressure system (1001 mb) was 
near the mouth of the Mississippi River (Fig. 6).  The surface high (1030 mb) 
was centered just north of Lake Ontario with a ridge extending southward 
through the mid-Atlantic region to the Carolinas.  The high was not 
particularly strong, but cold air damming was underway as evidenced by the 
ageostrophic surface wind (i.e., nearly normal to the isobars) and cold air 
advection from southern Virginia across both Carolinas.  Precipitation had 
spread across much of the southeastern United States north of the surface 
low and ahead of the strong upper level divergence in the entrance region 
of the 250 mb wind maximum.  The upward motion at lower levels was depicted 
by the warm conveyor belt on the 300K isentropic surface (Fig. 7).  The 40 
to 50 kt southerly winds were rapidly ascending the surface with an upward 
motion (omega) maximum over Alabama.  The dry conveyor belt (axis of strong 
northwest winds) over the northwestern Gulf of Mexico was descending along 
the same isentropic surface resulting in a region of very dry air that 
tightened the moisture gradient and contributed to further development of 
the comma cloud pattern seen in satellite imagery.
HPC Surface fronts and pressure analysis at 1200 UTC 18 December
Figure 6.  As in Fig. 4, except for 1200 UTC on 18 December.  Click on 
image to enlarge.
300K analysis at 1200 UTC on 18 December
Figure 7.  Isentropic analysis from the 80-km NAM model on the 300 K surface 
at 1200 UTC on 18 December, including pressure (mb; blue contours), wind 
(kt; barbs), vertical motion (omega, microbars sec-1; green contours and 
color fill, warmer colors denote upward motion and cooler colors show 
downward motion).  Click on image to enlarge.
A radar reflectivity mosaic at 1208 UTC showed a band of higher reflectivity 
in the developing comma head (Fig. 8), a feature often seen in maturing 
extratropical cyclones when strong forcing acts on an unstable atmosphere.  
A north-south cross section from Indiana to the eastern Gulf of Mexico 
(Fig. 9) showed a maximum in the upward motion field over Alabama.  The 
upward motion included a layer of atmosphere that was symmetrically 
unstable.  Near zero or negative values of saturated geostrophic equivalent 
potential vorticity indicated instability that could be released in vertical 
and/or horizontal motions.
NEXRAD radar reflectivity national mosaic at 1208 UTC 18 December
Figure 8.  National radar reflectivity mosaic at 1208 UTC on 18 December.  
Click on image to enlarge.
NEXRAD radar reflectivity national mosaic at 1208 UTC 18 December
Figure 9.  Vertical cross-section from the 80-km NAM model at 1200 UTC
on 18 December extending from Indiana (left margin) to the eastern 
Gulf of Mexico (right margin), showing vertical motion (orange contours,
solid upward and dashed downward), saturated equivalent potential 
temperature (degrees K; green contours), and saturated geostrophic
potential vorticity (PVU; color fill, where blue denotes values at or
below zero).  Click on image to enlarge.
Light snow began at the Asheville (NC) Regional Airport (AVL) around 
1200 UTC as a narrow band of precipitation moved out of north Georgia 
and Upstate South Carolina into western North Carolina ahead of the 
main area of precipitation.  An upper air sounding taken at Poga
Mountain (northern Avery County), North Carolina (Fig. 10), showed a 
layer from the surface up to about 800 mb where the temperature was 
just above 0o C, but a relatively dry sub-cloud layer below 700 mb.  
Although snow was not falling at Poga Mountain at 1200 UTC, snow 
crystals falling through the sub-cloud layer quickly brought the 
temperature sounding down around freezing due to evaporative
cooling.  Southwest flow around 850 mb suggested that warm air 
might be advected northeastward across the mountains, and in fact
the temperature around 850 mb climbed above freezing by 1500 UTC as
seen on an initial analysis profile of temperature and dewpoint from
the 13-km Rapid Update Cycle (RUC) model at AVL (Fig. 11).  By 1500 UTC, 
the precipitation changed to rain and the surface temperature increased 
from 0o C to +2o C.
Observed upper air sounding at Poga Mtn., NC, at 1200 UTC on 18 December
Figure 10.  Skew-T, log P diagram for upper air sounding taken at Poga 
Mountain, North Carolina, at 1208 UTC on 18 December.  Click on image 
to enlarge.
RUC-13 initial profile of temperature and dewpoint at AVL at 1500 UTC on 18 December
Figure 11.  Initial analysis profile of temperature, dewpoint, and wind 
from the 13-km RUC at 1500 UTC on 18 December.  Click on image to 
enlarge.
Light snow began falling at Hickory, North Carolina (HKY), just 
before 1400 UTC as the first narrow band of precipitation moved 
northward across central and western North Carolina.  The RUC13 
temperature and moisture profile at HKY did not accurately capture 
the onset of precipitation.  On the two-hour forecast valid at 
1400 UTC (Fig. 12), the near-surface layer was much too dry, but 
the model wet bulb temperature profile indicated the temperature 
in that layer would cool to below 0o C if saturation occurred.  
Of note was a small warm nose forecast near 850 mb.  Either that 
was not actually there, or snow falling from above was able to 
penetrate the layer before melting.
RUC-13 two-hour forecast temperature and dewpoint profile at HKY at 1400 UTC on 18 December
Figure 12.  Two-hour forecast profile of temperature, dewpoint, and wind 
at HKY from the 13-km RUC valid at 1400 UTC on 18 December.  Click on 
image to enlarge.
The initial narrow band of precipitation that moved north across 
the area was more than likely a result of frontogenetical forcing.  
A vertical cross-section of upward motion and frontogenesis from the 
Georgia-Florida border, through far western North Carolina, to southern 
Ohio at 1200 UTC (Fig. 13) showed positive frontogenesis aligned with 
the frontal boundary sloping from the surface in south Georgia to 
mid-levels near the Ohio River.  A frontogenesis maximum and an upward 
motion maximum were in close proximity over the mountains, which was 
the approximate location where the cross-section cut through the 
precipitation band seen on the 1400 UTC radar mosaic in Fig. 14.
NAM-40 cross section of vertical motion and frontogenesis at 1200 UTC on 18 December
Figure 13.  Vertical cross-section from southern Ohio (left margin) to 
northwest Florida (right margin) of verticsal motion (orange contours, 
solid is upward and dashed is downward) and frontogenesis (color fill; 
green, yellow, and orange shades are positive) from the 40-km NAM model 
at 1200 UTC on 18 December.  Click on image to enlarge.
Regional composite reflectivity mosaic at 1400 UTC on 18 December
Figure 14.  NWS composite radar reflectivity mosaic at 1400 UTC on 
18 December.  Click on image to enlarge.
Soon after the preliminary band of precipitation affected the western
Carolinas, the main area of precipitation spread northward across the 
region.  A brief period of light rain was observed at Anderson, South
Carolina (AND), during the passage of the initial precipitation band, 
but the steady rain began at approximately 1400 UTC.  The onset of 
rain at Greenwood, South Carolina, also occurred near 1400 UTC.  The 
rain began at the Greenville-Spartanburg Airport (GSP) at approximately 
1500 UTC.  The 13-km RUC profile in Fig. 15 indicated the surface wet 
bulb temperature was 0o C, but a rather deep warm layer between 925 mb 
and 825 mb melted the snow falling from above.
RUC13 initial analysis profile of temperature and dewpoint for GSP at 1500 UTC on 18 December
Figure 15.  13-km RUC model initial analysis profile of temperature, 
dewpoint, and wind for GSP at 1500 UTC on 18 December.  Click on image 
to enlarge.
c.  1800 UTC - 18 December 2009 to 
    1200 UTC - 19 December 2009
At 1800 UTC, surface low pressure was centered over northwest 
Florida and precipitation had spread over nearly all of northeast
Georgia and the western Carolinas (Fig. 16).  Snow was falling at 
both AVL (33o F) and Hickory (HKY; 31o F), and rain was observed at 
GSP (34o F).  Continuous precipitation began at Charlotte (CLT) 
around 1800 UTC.  The precipitation started as rain, but a mixture 
of sleet and rain began at 2005 UTC as the surface temperature 
lowered from 37o F to 34o F.  The 13-km RUC profile at CLT valid 
at 1900 UTC identified a deep warm layer just above the surface.  
The strong easterly wind flow seen in the lower portion of the 
wind profile was responsible for spreading the warm air into the 
area.
Regional composite reflectivity mosaic at 1800 UTC on 18 December
Figure 16.  As in Fig. 14, except for 1800 UTC on 18 December.  
Click on image to enlarge.
The upper-level wind analysis at 1800 UTC showed the primary 250 mb 
jet maximum over New England (Fig. 17) with the axis of strong wind 
extending west and southwest to Tennessee.  A secondary wind maximum 
was along the Carolina coast.  The strongest upper-level divergence 
in the 300-500 mb layer (Fig. 18) was located along the Tennessee -
North Carolina border and extended south along the Georgia - South
Carolina border.  Beneath the upper-level divergence, strong ascent 
was occurring.  The 300K isentropic analysis at 1800 UTC showed a 
well-defined, strong wind flow climbing the isentropic surface with 
the maximum upward motion centered over Georgia and extending into 
the western Carolinas (Fig. 19).
NAM40 250 mb heights and isotachs at 1800 UTC on 18 December
Figure 17.  40-km NAM model 250 mb geopotential height (dam; green 
contours), wind barbs (kt), and isotachs (kt; beige contours and color 
fill) at 1800 UTC on 18 December.  Click on image to enlarge.
NAM40 500-300 mb average height, wind, and divergence at 1800 UTC on 18 December
Figure 18.  40-km NAM model 500-300 mb average geopotential height 
(dam; green contours), wind barbs (kt), and divergence (s-1; beige 
contours and color fill) at 1800 UTC on 18 December.  Click on image 
to enlarge.
NAM40 300 K analysis of wind, pressure, and omega at 1800 UTC on 18 December
Figure 19.  40-km NAM model isentropic analysis on the 300 K surface 
at 1800 UTC on 18 December showing pressure (mb; green contours), 
wind (kt; barbs), and omega (color fill, where warmer colors represent 
upward vertical motion).  Click on image to enlarge.
A cross section from central Florida to southern Wisconsin (Fig. 20)
highlighted the sloping region of frontogenetical forcing extending 
from the surface in south Georgia to near 500 mb over Tennessee and 
Kentucky.  The atmosphere’s response to frontogenetical forcing was
a circulation that produced ascending air on the warm side of the 
front and subsiding air on the cool side of the front.  In this 
particular situation, that circulation resulted in the tall tower 
of upward motion seen in Fig. 21.  Also shown in the figure was the 
saturated geostrophic potential vorticity which was an indicator of 
instability in the atmosphere.  The blue shade in the portion 
of the upward motion maximum where the saturated equivalent potential 
temperature increased with height indicated the potential for enhanced 
precipitation to occur in slantwise motion.  Indeed, that is what was 
seen in the radar mosaic (Fig. 22) over middle Tennessee.
NAM40 cross section of frontogenesis and saturated equivalent potential temperature at 1800 UTC on 18 December
Figure 20.  40-km NAM model vertical cross section from Wisconsin (left 
margin) to central  Florida (right margin) showing saturated equivalent 
potential temperature (degrees K; green contours) and frontogenesis 
(color fill) at 1800 UTC on 18 December.  Click on image to enlarge.
NAM40 cross section of vertical motion, saturated equivalent potential vorticity, and saturated equivalent potential temperature at 1800 UTC on 18 December
Figure 21.  As in Fig. 20, except showing vertical motion (white 
contours, solid is upward and dashed is downward), saturated equivalent 
potential temperature (degrees K, green contours), and saturated 
equivalent potential vorticity (PVU; color fill).  Click on image 
to enlarge.
NEXRAD radar reflectivity national mosaic at 1808 UTC 18 December
Figure 22.  As in Fig. 8, except for 1808 UTC.  Click on image to enlarge.
A broad view of the lower tropospheric thermal structure at 1800 UTC 
is seen in Fig. 23.  The 1000-850 mb thickness pattern revealed two 
key features:  The nose of cold air just east of the Appalachians 
and the tight thermal gradient in the frontal boundary extending 
from the eastern Gulf of Mexico to the South Carolina coast.  The 
850-700 mb thickness pattern depicted the northward progression of 
warm air that is the result of the 40 to 50 kt southerly wind in 
that layer.
NAM40 thickness in the 1000-850 mb layer and 850-700 mb layer at 1800 UTC on 18 December
Figure 23.  40-km NAM model thickness in the 1000-850 mb layer (m, blue 
lines) and 850-700 mb layer (m, red lines), and 850-700 mb mean wind 
(kt; standard barbs) at 1800 UTC on 18 December.  Click on image to 
enlarge.
Initial hour profiles from the 40-km NAM model identified details 
of the thermal structure at AVL, HKY, GSP, and CLT at 1800 UTC 
(Fig. 24).  The AVL sounding was clearly a snow sounding with 
both the temperature and wet bulb temperature below 0o C through 
the entire profile.  (Saturation is not indicated in the primary 
dendritic snow crystal growth region (-12o C to -18o C), but we 
know from radar data that precipitation-size particles were 
present.)  The model temperature profile at HKY was similar to 
Asheville’s in that the temperature and wet bulb temperature 
profiles were subfreezing except for a slightly above freezing 
surface temperature.  In actuality, the 1800 UTC surface 
temperature was 31o F and snow was falling.  The GSP profile 
displayed an even warmer and thicker layer of above freezing air 
from 850 to 700 mb.  Below 850 mb, the wet bulb temperature was 
below 0o C, but the surface temperature was a degree or two above 
freezing.  The continuous precipitation that began as rain just 
before 1700 UTC mixed with sleet at 2000 UTC.  The 1800 UTC 
model sounding at CLT showed temperatures warmer than 0o C in 
most of the layer from the surface to 800 mb.  However, the wet 
bulb temperature was below 0o C.  This indicated the initial 
precipitation would more than likely arrive at the surface as 
rain, but as evaporation cooled the air, freezing or frozen 
precipitation would eventually be observed.  A very important 
feature in the CLT profile was the warm nose between 800 and 
850 mb.  The corresponding wet bulb temperature was also above 
0o C, so the warm layer was likely to persist while the air below 
cooled to below freezing.  The strong southeast wind in the 800 to 
850 layer indicates that the warm air will not only persist, but 
could become even warmer.   The rain that started at CLT just 
before 1800 UTC became a rain and sleet mixture at 2005 UTC.

NAM40 initial hour model sounding for AVL at 1800 UTC on 18 DecemberNAM40 initial hour model sounding for HKY at 1800 UTC on 18 December

NAM40 initial hour model sounding for GSP at 1800 UTC on 18 DecemberNAM40 initial hour model sounding for CLT at 1800 UTC on 18 December

Figure 24.  40-km NAM model initial hour profiles of temperature 
(solid red line), dewpoint (dashed blue line), wet bulb temperature
(green line) and wind (barbs) for AVL (top left), HKY (top right), 
GSP (bottom left), and CLT (bottom right) at 1800 UTC on 18 December.  
Click on images to enlarge.
The surface analysis at 2100 UTC placed two 997 mb low pressure 
centers over south Georgia.  The analysis indicated occlusion 
was underway with the primary low just south of Albany, Georgia, 
and the triple point low just west of Brunswick, Georgia.  A 
nearly stationary front extended from the triple point low 
northeast along the Carolina coast.  A cold front stretched 
southward through Florida.  The 1026 mb high pressure center over 
eastern Canada extended south into the Carolinas and continued to 
display a weak cold air damming pattern in the isobars.
During the afternoon, the upper level trough over the south central 
United States and the short wave trough over the Gulf Coast moved 
steadily eastward.  The Carolinas remained in the entrance region 
of the strong 250 mb jet over the northeastern states.  Another 
250 mb wind maximum over the Gulf of Mexico was moving northeast 
toward Florida.  The low levels of the atmosphere over the western
Carolinas continued to experience isentropic lift in what appeared 
to be the rising branch in a dual jet circulation (Kocin and 
Uccellini 2004; pp. 117-120) between the two upper level wind 
maxima.  The double jet structure was evident in a six-hour forecast
cross section of isotachs and vertical motion valid at 0000 UTC
on 19 December (Fig. 25).  Of interest was the tower of upward 
motion (orange and red shades) between the two jets that extended
from the surface all the way to 250 mb.  The presence of a 
southeasterly low-level jet was highlighted by the 850 mb wind 
maximum of 50 to 70 kt extending from the Atlantic into the central 
Carolinas (Fig. 26).  The 850 mb jet was the means by which 
considerable moisture was transported into the region of low-level 
convergence then subsequently lifted in the upward motion seen in 
the cross-section.
NAM40 cross section 6-hr forecast of isotachs and vertical motion valid at 0000 UTC 19 December
Figure 25.  40-km NAM model six-hour forecast vertical cross-section 
from western Quebec (left margin) to Cuba (right margin) showing 
isotachs (kt; blue contours), saturated equivalent potential temperature 
(degrees K; green contours), and vertical motion (color fill) valid at 
0000 UTC on 19 December.  Click on image to enlarge.
NAM40 6-hr forecast of 850 mb isotachs and wind speed valid at 0000 UTC 19 December
Figure 26.  40-km NAM model six-hour forecast of wind speed (contours 
and barbs) and isotachs (color fill) valid at 0000 UTC on 19 December.  
Click on image to enlarge.
The greatest precipitation rates occurred generally between 
1800 UTC on 18 December and 0000 UTC on 19 December.  This period 
coincided with a favorable superposition of the strong, convergent 
low-level flow transporting moisture into the western Carolinas 
while upper-level divergence was crossing the area.  The resulting 
deep vertical motion produced moderate to heavy rain and snow 
during the afternoon and early evening.  Snow was the predominant 
form of precipitation at both AVL and HKY during the afternoon.  
The rain and sleet mixture at GSP changed to freezing rain around 
2100 UTC.  A rain and sleet mixture at CLT became all rain at 
approximately 2300 UTC.
By the time the occluding low pressure centers (996 and 997 mb) 
approached the Georgia coast at 0000 UTC on 19 December, the 
precipitation was diminishing across northeast Georgia and the 
western part of Upstate South Carolina.  The dual jet circulation 
was propagating to the northeast so the best low level isentropic 
lift, divergence aloft, and associated strong upward motion were 
moving from central North Carolina toward the mid-Atlantic coast.  
The strong warm air advection just above the surface-based cold 
air finally modified the HKY temperature profile to the extent 
that snow changed to freezing rain around 0130 UTC.
The vertically-pointing RENCI (RENaissance Computing Institute) 
MicroRain Radar (MRR) located in Newton, North Carolina (about 
12 miles southeast of HKY) provided an interesting view of the 
event.  A time vs. height display of radar reflectivity (Fig. 27)
showed the passage of the initial band of precipitation (seen in 
Fig. 14) between 1300 UTC (0800 EST) and 1500 UTC (1000 EST).  
Very little precipitation occurred until the best period of low-
level convergence, upper-level divergence, and upward motion 
crossed the area during the afternoon and evening.  Moderate to 
heavy snow was observed at HKY between 2100 UTC (1600 EST) and 
2300 UTC (1800 EST).
Micro Rain Radar vertical profile of reflectivity from 0500 UTC to 2300 UTC 18 December
Figure 27.  Vertical profile of radar reflectivity from MRR at Newton, 
North Carolina, from 0500 UTC to 2300 UTC on 18 December.  Horizontal 
axis is time (EST).  Vertical axis is height above ground level (ft).  
Click on image to enlarge.
The radar derived particle fall velocity product from the MRR 
gave additional information about precipitation type in the 
column above the radar (Fig. 28).  The downward-sloping (from 
left to right) blue shades near the left-hand margin of the image 
were probably associated with snow gradually descending to the 
surface.  The yellow and red shades indicated faster fall speeds 
between approximately 1500 UTC (1000 LT) and 1630 UTC (1230 LT) 
that corresponded to a period when freezing rain was reported 
at HKY.  The near-surface blue shades between 1630 UTC (1230 LT) 
and 0100 UTC (2000 LT) were the time during which moderate to 
heavy snow was observed at HKY.  The intervals of "green fingers" 
reaching the ground were probably periods of heavy snow.  The 
yellow and red colors after approximately 0000 UTC (1900 LT) 
occurred in conjunction with the transition of precipitation at 
HKY from snow to freezing rain.
Micro Rain Radar vertical profile of fall velocity from 0500 UTC to 2300 UTC 18 December
Figure 28.  Vertical profile of radar-derived particle fall velocity 
from MRR at Newton, North Carolina, from 0500 UTC to 2300 UTC on 
18 December.  Horizontal axis is time (EST).  Vertical axis is height 
above ground level (ft).  Click on image to enlarge.
Click to view hourly observations and meteograms:
Station
18 December 2009
19 December 2009
Anderson
 
Asheville
 
Charlotte
 
Greenville -Spartanburg
 
Hickory
 
3.  Storm Wrap-up
The last of the precipitation forced by the surface low exited 
the GSP forecast area prior to 1200 UTC on 19 December when the 
low pressure system was near the Outer Banks.  Another burst of
snow occurred along the Tennessee border during the early morning
of 20 December as northwest flow forced moisture up the west 
slopes of the Appalachians. The low subsequently followed a track 
to the northeast that placed it off the southern New England coast 
on the morning of 20 December.  In the wake of the storm, snow 
covered most of the North Carolina portion of the GSP forecast 
area, as seen in the Aqua MODIS image taken from 1842 UTC to 
1854 UTC on 20 December (Fig. 29).  The greatest depth was at 
Mount Mitchell State Park in Yancey County, where 28 inches fell.  
Other significant accumulations included 24 inches near Buladean 
(Mitchell County), 24 inches about ten miles west of Robbinsville 
(Graham County), and 22 inches at Little Switzerland in McDowell 
County.  The total snowfall at Asheville was 10.1 inches and 
Hickory had 5 inches.
Aqua MODIS image at 1854 UTC 20 December
Figure 29.  Aqua MODIS satellite image from 1842 UTC to 1854 UTC on 
20 December.  The white shades across North Carolina are mainly snow 
cover, although cloudiness associated with northwest flow is present 
along the Tennessee border.  Click on image to enlarge.
The storm had significant impacts to the north of the GSP forecast
area.  A swath of very heavy snow extended from western North 
Carolina through western Virginia and West Virginia to southern 
New England (Fig. 30).  A large area of 20 to 30 inch accumulations 
occurred in the central Appalachians.  The Northeast Snowfall Impact 
Scale (NESIS; Kocin and Uccellini 2004) classified this event as 
"Major."  It ranked 23rd on a list of 37 high impact snowstorms 
that had affected the region from southern Virginia to New England 
by that date.
Preliminary Snowfall and NESIS rating for 18-21 December 2009 Winter Storm
Figure 30.  Preliminary snowfall analysis and NESIS rating of the 
18-21 December 2009 East Coast snowstorm.  Click on image to enlarge.
4.  Photo Gallery 
Here are some images submitted to the NWS from this event.

Snowfall at 3800 feet above Lake Junaluska at 1130 am on 18 DecemberSnowfall at 3800 feet above Lake Junaluska at 150 pm on 18 December

Snowfall at 3800 feet above Lake Junaluska at 530 pm on 18 DecemberSnowfall at 3800 feet above Lake Junaluska at 500 am on 19 December

Series of snow images taken by Bob Child, location near Lake Junaluska 
(Haywood County), North Carolina at 3800 ft elevation, at 1130 am 
(upper left), 150 pm (upper right), 530 pm (lower left), and 500 am 
(lower right).  Click on image to enlarge.

Snowfall at 3800 feet above Lake Junaluska on 19 DecemberSnowfall at 3800 feet above Lake Junaluska on 19 December

More snow images taken by Bob Child, location near Lake Junaluska 
(Haywood County), North Carolina at 3800 ft elevation, on the morning 
of 19 December 2009.  Click on image to enlarge.
Snowfall in Cleveland County, NC on 19 December 2009
Image of snowfall in Cleveland County, North Carolina, on the morning 
of 19 December 2009.  Click on image to enlarge.
References
Keeter, K.K., and J.W. Cline, 1991: The Objective Use of Observed 
     and Forecast Thickness Values to Predict Precipitation Type 
     in North Carolina.  Wea. Forecasting, 6, 456–469.

Kocin, P. J., and L. W. Uccellini, 2004: A Snowfall Impact Scale 
     Derived From Northeast Storm Snowfall Distributions.  Bull. 
     Amer. Meteor. Soc., 85, 177-194.

Kocin, P. J., and L. W. Uccellini, 2004:  Northeast Snowstorms. 
     Vol. 1: An Overview.  Meteor. Monogr., No. 54, Amer. Meteor. 
     Soc., 296 pp.

Uccellini, L.W., and D.R. Johnson, 1979: The Coupling of Upper 
     and Lower Tropospheric Jet Streaks and Implications for the 
     Development of Severe Convective Storms.  Mon. Wea. Rev., 
     107, 682–703.
Acknowledgements
The snow accumulation map was prepared by Blair Holloway, NWS.
The upper air analyses were obtained from the Storm Prediction 
Center.  The surface fronts and pressure analyses were obtained 
from the Hydrometeorological Prediction Center.  The national 
radar mosaics and the NESIS snowfall analysis map were obtained 
from the archive at the National Climatic Data Center.  The 
upper air sounding taken at Poga Mountain was obtained from Dr.
Douglas Miller, University of North Carolina at Asheville.  The 
NAM and RUC profiles were created using the Rawinsonde Observation
(RAOB) program (Version 5.8) for Windows.  The MRR images were 
obtained from RENCI.  The Aqua MODIS image was obtained from the
archives at the Space Science and Engineering Center at the 
University of Wisconsin.  The meteograms were obtained from 
the Plymouth State College.


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