The Mountain Snow Event
of 11-13 February 2006
Patrick D. Moore
National Weather Service
Photo by L. G. Sheets
Author's Note: The following report has not been subjected
to the scientific peer review process.
A major east coast winter storm produced record-setting snowfall across the
population centers of the northeastern United States during the weekend of
11-12 February, 2006 (Grumm 2006) pdf. (Click on these links to view more
information concerning impacts elsewhere along the East Coast, including
Albany NY). Before the low pressure system responsible for this high-impact
event deepened explosively off the Mid-Atlantic coast, it moved across the
midlands of the Carolinas, producing a significant amount of snow mainly
across the mountains of North Carolina after its passage (Fig. 1). The
effect of this winter storm across the western Carolinas and northeast
Georgia can be split into two phases: An initial phase associated with
the passage of the surface low (the synoptic phase), and a second phase
associated with a northwest flow at low and middle levels of the atmosphere
in the wake of the low (the northwest flow phase). The northwest flow phase
presented its own set of challenges, including the eastward extent to which
snow showers would reach and the potential contribution of shallow convection.
These facets are explored in subsequent sections.
Figure 1. Total snow accumulation in inches for the period 0000 UTC
11 February through 1800 UTC 13 February 2006. Note that sharp gradients
in accumulation across the higher terrain of western North Carolina may not
be indicated at the scale of the graphic. Click on image to enlarge.
(Click here to view a list of snow accumulation reports for the
period 11-13 February 2006
An upper-level pattern shift about one week prior to the development of the
winter storm resulted in a deep upper trough at the 500 millibar (mb) level
over the eastern half of North America, which is a pattern typical of mid
winter in the eastern United States. The days leading up to the event were
characterized by below normal temperatures across the western Carolinas.
The upper trough provided entry for a high pressure air mass of arctic
origin into the northern plains on Wednesday, 8 February. The arctic high
moved down across the mid-Mississippi Valley on Thursday, 9 February, and
pushed a surface cold front across the Southeast and over the northern
Gulf of Mexico. However, the high continued to weaken as it moved across
the Southeast Thursday night and off the coast Friday morning, 10 February,
as a secondary cold front approached from the northwest. By 1200 UTC on
10 February, the surface analysis from the Hydrometeorological Prediction
Center (HPC) showed a cold front stretching in an arc from central Illinois,
across the Ozark Plateau, to the Big Bend region of southwest Texas, with a
weak wave of low pressure on the front over north central Texas (Fig. 2,
left). Meanwhile, another weak low pressure area remained over south Texas
along the first cold front. A strong short wave at 500 mb was shown by the
Storm Prediction Center (SPC) objective analysis, diving across the northern
Plains and upper Mississippi Valley to reinforce the upper trough (Fig. 2,
Figure 2. Sea level pressure contours (mb) with HPC surface front analysis
(left) and 500 mb SPC objective analysis of height contours (hPa) (right)
for 1200 UTC 10 February. Click on each image to enlarge.
The Synoptic Phase
The low pressure system responsible for this significant event had its
origin in the two weak lows over Texas on the morning of Friday, 10 February.
Throughout the day, increasing baroclinicity ahead of the short wave dropping
down from the northern Plains, combined with upward vertical motion in the
right entrance region of a jet streak stretching across the Ohio Valley and
Mid-Atlantic regions seen at the 250 mb level, provided an environment
favorable for the development of low pressure over the northwestern Gulf Coast
region. Infrared imagery from the GOES-12 satellite (Fig. 3) shows the
development of a baroclinic leaf structure across the Arklatex and lower
Mississippi Valley regions, indicating that cyclogenesis was occurring,
although it was weak. By 0000 UTC 11 February, the two lows were organizing
into one low center over the Mississippi Delta region (Fig. 4).
Figure 3. GOES-12 enhanced infrared imagery at 0015 UTC 11 February.
Click on image to enlarge.
Figure 4. Sea level pressure contours (mb) and HPC surface fronts analysis
for 0000 UTC 11 February. Click on image to enlarge.
Forecasters expected a Miller Type-A surface low (Miller 1946) to track to
the south across central Georgia and central South Carolina before moving up
the East Coast. Forecast soundings and partial thickness values suggested
the main precipitation type issue would be a determination between rain and
snow. The Quantitative Precipitation Forecast (QPF) from the operational
Global Forecast System (GFS) and North American Mesoscale (NAM) forecast
models suggested between one-quarter and one-half inch of liquid could be
expected (Fig. 5), which would be enough to support a forecast of 3 to 5
inches of snow across the higher terrain, generally above 2000 feet. However,
periods of moderate precipitation were expected that would provide enough
cooling of low levels through melting and evaporation to bring the snow level
down closer to 1,000 feet during the early morning hours on 11 February.
Figure 5. Quantitative Precipitation Forecast (inches) from the 1200 UTC
10 February model cycle for the 6-hour period ending (a) 1200 UTC 11 February
from the NAM-80 model, (b) 1800 UTC 11 February from the NAM-80 model,
(c) 1200 UTC 11 February from the GFS-80 model, and (d) 1800 UTC 11 February
from the GFS-80 model. Click on each image to enlarge.
Precipitation developing in the increasingly moist southwest flow at low
levels across Alabama and Georgia during the early part of the evening
on 11 February was aided by a low level jet at 850 mb stretching from the
northern Gulf of Mexico to the Tennessee Valley region. Light precipitation
reached the southwestern corner of North Carolina and extreme northeast
Georgia around 0300 UTC. The warm moist upglide associated with the low
level jet translated eastward over the western Carolinas by 0600 UTC and
allowed for precipitation to spread northeast across the upstate of South
Carolina and the mountains of North Carolina.
(Click here to view a 19 frame java loop of radar reflectivity
centered on the Greer (KGSP) WSR-88D radar, depicting the
development and movement of the precipitation across the western
Figure 6. Surface low track from the HPC Surface Analysis Branch for
0300 - 2100 UTC 11 February. Sea level pressure (mb) at the low center
At the surface, the low pressure system remained relatively weak across the
Deep South during the early morning hours of Saturday 11 February, as the
center of the low moved to a position near Mobile, Alabama, at 0600 UTC and
a position near Columbus, Georgia, at 1200 UTC (Fig. 6). However, the upper
level system continued to slowly gain strength with a 531 decameter low
closing off over northern Illinois by 1200 UTC on the 500 mb analysis.
Light precipitation, forced by moist ascent in the developing warm conveyor
belt (Carlson, 1980) ahead of the deepening upper low, spread across the
Piedmont of the Carolinas between 0600 UTC and 0900 UTC. In the same time
period, radar and infrared satellite imagery showed the emergence of bands
of light precipitation over the Tennessee Valley and Cumberland Plateau,
forced by weak upward motion in a developing deformation zone to the northwest
of the surface low.
The digging upper system helped to strengthen the subtropical branch of the
jet stream with a 120 knot jet streak developing over south Texas and the
northern Gulf of Mexico by 1200 UTC. The interaction between the developing
subtropical jet and the existing strong polar jet streak over the Appalachians
and the Mid-Atlantic coast contributed to increased vertical motion ahead of
the surface low over southern Alabama and southern Georgia between 0600 UTC
and 1200 UTC. The coupling of jet streaks, combined with weak convective
instability along the Gulf Coast, provided a favorable environment for deep
convection to develop on the leading edge of the precipitation shield along
the Gulf Coast. In fact, a small linear mesoscale convective system (MCS)
developed over south Alabama at 0600 UTC and 0900 UTC (Fig. 7), which
proceeded to move quickly east across southwest Georgia and northwest Florida
by 1200 UTC, well ahead of the surface cold front which lagged across southeast
Alabama and the western part of the Florida Panhandle at that time.
(Click here to view a 16 frame java loop of radar reflectivity
centered on the Maxwell Air Force Base (KMXX) WSR-88D radar,
which shows the progression of the MCS across Alabama, Georgia,
and north Florida.)
Figure 7. Radar reflectivity (dBZ) mosaic centered on Robins AFB (KJGX)
WSR-88D at (a) 0600 UTC, (b) 0900 UTC, and (c) 1200 UTC, on 11 February.
Click on each image to enlarge.
The center of surface low pressure moved over northeast Georgia in the
morning to a position near Athens by 1500 UTC (Fig. 6). The back edge of
a weakly organized warm conveyor belt reached the western tip of North
Carolina between 1200 UTC and 1500 UTC, nearly coincident with the cold front
at 850 mb and the leading edge of the dry slot seen on water vapor satellite
imagery (Fig. 8). The eastward movement of this feature brought an end to
precipitation to the west of a line from Morganton, North Carolina, to
Greenville, South Carolina, including all of northeast Georgia, the western
part of the upstate of South Carolina, and most of the North Carolina
mountains, albeit temporarily.
Figure 8. GOES-12 enhanced water vapor at 1515 UTC 11 February.
Click on image to enlarge.
The dry slot continued to move east across the foothills and Piedmont of the
Carolinas through 1800 UTC as the center of low pressure moved to a position
near Columbia, South Carolina. Precipitation ended across most of the area
to the west of a line from Greensboro, North Carolina, to Wadesboro, North
Carolina, and Lexington, South Carolina by that time. Meanwhile, the
deformation zone to the northwest of the surface low slowly reorganized as it
translated east across eastern Tennessee, with the leading edge of light snow
associated with this feature reaching the western edge of the North Carolina
mountains seen on radar at 1800 UTC.
The development of the linear MCS along the Gulf Coast area may have
contributed to the reduction of precipitation across the western Carolinas
and northeast Georgia by preventing moisture transport northward from the
Gulf of Mexico (Mahoney and Lackmann 2005). In fact, the models overestimated
the amount of precipitation by nearly a factor of two. Although the majority
of the precipitation at Asheville fell as snow through 1800 UTC, the rate at
which the snow fell was not fast enough for an accumulation of more than one
inch due to melting. Outside the mountains, the precipitation rate was not
great enough to allow cooling effects to suppress the melting level, thus the
snow level remained around 2000 feet and the precipitation fell as all rain at
the Greenville-Spartanburg Airport. Only a trace of snow fell at Hickory.
||Observed 12-hour Precipitation (liquid equivalent) ending 1800 UTC 11 February
|Greenville - Spartanburg (KGSP)
Snowfall amounts across the western Carolinas generally reflect a snow level
which remained between 2000 feet and 2500 feet during the first part of the
event (Fig. 9). Although a large part of the North Carolina Mountains received
greater than 4 inches, much of it was limited to elevations above 3000 feet,
especially the Balsams. Most of the population centers, in particular the
French Broad Valley and locations such as Bryson City and Waynesville, failed
to accumulate more than one inch.
Figure 9. Total snow accumulation (inches) for the Synoptic Phase of the
event (0000 – 2100 UTC 11 February). Click on image to enlarge.
The center of surface low pressure moved to a position near Fayetteville,
North Carolina, at 2100 UTC. As the low began to move away, the deformation
zone precipitation area skirted along the Tennessee border and moved across
the northern mountains of North Carolina. After 2100 UTC, the back edge of
the deformation zone lifted north of Avery County, North Carolina, and the
mechanism responsible for snow falling across the mountains began to change.
(Click here to view a 9 frame java loop of radar reflectivity
centered on the Morristown (KMRX) WSR-88D radar, depicting the
precipitation transition across east Tennessee and western
The Northwest Flow Phase
Radar imagery from the KMRX WSR-88D clearly showed a transition across
eastern Tennessee and western North Carolina between 1900 UTC and 2200 UTC,
as the back edge of light precipitation associated with the deformation
zone lifted northeast and precipitation redeveloped over eastern Tennessee
(Fig. 10). Nearly coincident with the transition of precipitation echoes,
the winds across the mountains of North Carolina at 850 mb veered from
southwest to northwest between 1700 UTC and 2100 UTC, after which a northwest
flow continued unabated. Observations across the mountains of North Carolina
showed the wind shift and coincident temperature drop during the early part
of the afternoon. Click the links to see meteograms at Wayah Bald (temp,
wind direction) and Bearwallow Mountain (temp, wind direction). The 850 mb
wind speed strengthened to 35 kts as the surface center of low pressure
continued to move away to the northeast across the coastal plain of North
Carolina and Tidewater Virginia at 0000 UTC during the evening of 11 February,
and eventually off the Mid-Atlantic coast by 0600 UTC, Sunday, 12 February.
The ensuing cold advection flow dropped the temperature at 850mb from -5 deg
to -9 deg Celsius between 2100 UTC 11 February and 1200 UTC 12 February along
the Tennessee border. The mechanical forcing from the northwest winds
impinging upon the higher terrain along the Tennessee border resulted in an
area of light to moderate snow, particularly over the Great Smoky Mountains
National Park, that persisted through about 1200 UTC.
Figure 10. Radar reflectivity (dBZ) mosaic centered on KMRX WSR-88D at
(a) 1856 UTC, (b) 1956 UTC, (c) 2100 UTC, and (d) 2200 UTC, on 11 February.
Click on each image to enlarge.
Cyclonic flow around the rapidly deepening surface low moving up the eastern
seaboard at 1200 UTC on Sunday, 12 February, maintained the northwest winds
at low levels across the mountains through the daytime hours, as seen on the
925 mb analysis. The character of the northwest flow precipitation changed
again during the morning as the first area of light snow weakened and moved
over the northern mountains, and new precipitation developed over eastern
Tennessee. Instead of the layered appearance of radar echoes along the western
slopes of the mountains that is typical of many northwest flow events, the
precipitation echoes that developed over northeast Tennessee after 1200 UTC
had a more cellular appearance. The upper air observation taken at 1200 UTC
at Nashville, Tennessee (KBNA), showed a nearly dry adiabatic lapse rate from
the surface to 925 mb that suggested the potential for convective instability
if surface moisture remained sufficient (Fig. 11). In fact, the unmodified
temperature sounding showed very weak amounts of Convective Available Potential
Energy (CAPE), suggesting that small changes to the profile such as cold
advection aloft or moisture advection near the surface would quickly increase
the instability of surface air parcels.
(Click here to view a 32 frame java loop of radar reflectivity
from the Morristown (KMRX) WSR-88D radar, which shows the evolution
of the Northwest Flow snow across the mountains of North Carolina.)
Figure 11. Skew-T, log P diagram for the upper air observation at Nashville
(KBNA) 1200 UTC 12 February. The thick red line is the temperature sounding
and the dashed black line is the dewpoint sounding. Wind barbs (knots) are
shown on the right. Click on image to enlarge.
The 850 mb objective analysis from the Storm Prediction Center (SPC) at 1200 UTC
(Fig. 12) showed strong cold advection, indicated by the wind barbs oriented
perpendicular to the isotherms across the Appalachians and Cumberland Plateau.
When compared to the KBNA sounding, the continued cold advection at 850 mb
suggested that boundary layer convective processes could dominate the development
of clouds and precipitation across eastern Tennessee, even with minimal amounts
of surface heating. It is hypothesized that strong cold air advection over
relatively warm ground led to thermal instability, allowing horizontal convective
rolls to develop upstream of the upslope areas along the North Carolina-Tennessee
border which contributed to snow production, as in Schultz et al. (2004). The
banded nature of precipitation echoes across northeast Tennessee by 1602 UTC
(Fig. 13, northeast of the KMRX radar site), aligned in the direction of the
northwest wind at 850 mb, agreed with the appearance of horizontal convective
rolls seen on Moderate Resolution Imaging Spectroradiometer (MODIS) satellite
imagery at 1602 UTC (Fig. 14). Farther south of the KMRX radar site, less
distinct echoes agreed with the appearance of open cellular convection over
southeast Tennessee and northwest Georgia.
Figure 12. SPC Objective Analysis at 850 mb for 1200 UTC 12 February. Wind
barbs (knots) are blue, isotherms (deg. C) are shown as dashed blue lines,
and geopotential heights (dm) are shown as solid dark gray lines. Click on
image to enlarge.
Figure 13. Composite Reflectivity (dBZ) from the Morristown, Tennesseee
(KMRX), WSR-88D at 1602 UTC (11:02 AM) 12 February. The radar site is
indicated by the red plus sign. The solid yellow lines are state boundaries
and the thin gray lines are county boundaries. Click on image to enlarge.
Figure 14. Terra MODIS image taken from 1602-1613 UTC 12 February scan.
Brighter white shades indicate more reflective (thicker) cloud cover.
Brown shades indicate bare ground. Click on image to enlarge.
In fact, by the late morning hours, the Local Analysis and Prediction System
(LAPS) analysis of CAPE showed values greater than 100 J kg-1 across much of
eastern Tennessee (Fig. 15). The weak downslope flow off the Cumberland Plateau
provided additional convective instability as solar heating kept the boundary
layer relatively warm across the Great Valley of east Tennessee.
Figure 15. LAPS analysis at 1600 UTC 12 February. The left side depicts
the analysis of CAPE with contours every 30 J kg-1 in yellow. The right side
shows a Skew-T, log P diagram at KTYS, with the temperature and dewpoint
profiles in green. Note the table of computed indices at the lower right.
Click on each image to enlarge.
By the time of the MODIS image at 1919 UTC (Fig. 16), horizontal convective
rolls are apparent across northeast Tennessee, nearly aligned in the direction
of the northwest flow at 850 mb. The radar imagery from the KMRX WSR-88D around
the time of the MODIS image also showed the appearance of horizontal convective
rolls stretching northwest to southeast across northeastern Tennessee and
intersecting the southern Appalachians (Fig. 17). The organization of the
precipitation elements may have played an important role in the variable nature
of snow accumulation across the mountains during the northwest flow phase of the
event, both in terms of providing an enhancement to precipitation in locations
where convective rolls intersected the mountains and providing a mechanism for
convective elements to persist downstream of the initial rise of terrain on the
west side of the mountains.
(Click here to view a 35 frame java loop of GOES-12 visible satellite
imagery, which shows the development of horizontal convective rolls
over northeast Tennessee and the evolution of low clouds moving up the
west side of the mountains.)
Figure 16. As in Fig. 14, but for Aqua MODIS at the 1919-1930 UTC
(2:19 - 2:30 PM) 12 February scan. Click on image to enlarge.
Figure 17. As in Fig. 13, at 1918 UTC (2:18 PM) 12 February.
Click on image to enlarge.
The loss of daytime heating after 2200 UTC spelled an end to the convective
organization of precipitation echoes seen on the KMRX radar. Coverage of
precipitation decreased significantly by 0100 UTC 13 February and was limited
mainly to a persistent band across Haywood County, North Carolina, which itself
weakened by 0500 UTC. The production of light snow gradually waned during the
early morning hours of Monday, 13 February, as low level moisture dwindled and
northwest winds diminished, with the event essentially ending around sunrise.
The extent of the snow accumulation was revealed on satellite imagery as
cloudiness decreased across the southern Appalachians on Monday morning.
A Terra MODIS image from the 1644 UTC scan showed the eastern edge of the
snow fall as the transition between white and brown shades near the Blue
Ridge Escarpment, although clouds still obscured some of the snow pack across
east Tennessee, north Georgia, and the mountains of North Carolina (Fig. 18).
A later image from the 1824 UTC Aqua MODIS scan reveals the true extent of the
snow cover, as clouds have completely dissipated across the southern
Appalachians (Fig. 19). Note how accumulating snow was limited almost
entirely to elevations above 2000 feet, with very little snow cover present
across the upper Little Tennessee River valley and the middle and upper French
Broad River valley.
Figure 18. As in Fig. 14, but for Terra MODIS image from the 1644-1657 UTC
(11:44 AM- 1157 AM) 13 February scan. Click on image to enlarge.
Figure 19. Aqua MODIS image from the 1824-1837 UTC (1:24 - 1:37 PM) scan.
Note the contrast between the white shades indicating snow cover and the
brown shades near the Blue Ridge Escarpment. Click on image to enlarge.
The greatest snow accumulations from the Northwest Flow phase of the event
were observed along the northwest facing slopes of the higher elevations
along the Tennessee border, including reports of over three feet along the
Cherohala Skyway in western Graham County and drifts of five to six feet in
the parking lot at Newfound Gap (Fig. 20). Although there is a definite
elevation dependency seen in the snow accumulation, many valley locations in
the shadow of the high peaks near the Tennessee border, such as Robbinsville,
Cherokee, Sylva, and Burnsville, still received significant amounts. The
organization of precipitation noted by the horizontal convective rolls seen
on satellite and radar imagery may have contributed to the downstream transport
of snow from the main production area where the terrain rises quickly along
the Tennessee border.
Figure 20 Snow accumulation (inches) for the Northwest Flow Phase (2100 UTC
11 February to 1200 UTC 13 February). The graphic may not indicate sharp
gradients across the higher terrain. Click on image to enlarge.
Photo courtesy of Ron and Nancy Johnson at www.Tailof theDragon.com
The author wishes to thank Larry Lee (Science and Operations Officer, NWS Greer)
for his assistance with locating references and providing a critical review of
the manuscript. Jonathan Blaes (Information Technology Officer, NWS Raleigh)
provided a list of web pages where archived weather data could be located.
Rick Neal (Information Technology Officer, NWS Greer) archived the event and
assisted with loading the data on the Weather Event Simulator. Last, but not
least, this page would not have been possible without the help of Neil Dixon
(webmaster, NWS Greer), who set up the html framework for the page and provided
numerous pointers on how to accomplish most of the html coding.
Carlson, T. N., 1980: Airflow through midlatitude cyclones and the
comma cloud pattern. Mon. Wea. Rev., 108, 1498-1509.
Grumm, R. H., 2006: The Megalopolitan snowstorm of 11-12 February 2006:
Problems with uncertainty. Unpublished manuscript. 17 pp.
Mahoney, K. M., and G. M. Lackmann, 2005: The effects of organized
upstream convection on downstream precipitation. Preprints,
21st Conf. on Weather Analysis and Forecasting/17th
Conf. on Numerical Weather Prediction, Washington, D. C.,
Amer. Meteor. Soc., CD-ROM, 3.1.
Miller, J. E., 1946: Cyclogenesis in the Atlantic coastal region
of the United States. J. Meteor., 3, 31-44.
Schultz D. M., D. S. Arndt, D. J. Stensrud and J. W. Hanna, 2004:
Snowbands during the cold-air outbreak of 23 January 2003.
Mon. Wea. Rev., 132, 827–842.