National Weather Service United States Department of Commerce

The Winter Storm of

1-2 March 2009

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

New snow in Lincolnton, North Carolina, on 2 March 2009

After heavy snow fell overnight, the sky cleared before sunrise on the morning of 2 March, providing for spectacular scenes of new snow clinging to trees under a bright blue sky such as this in Lincolnton, North Carolina.

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

1.  Introduction

Rain and snow occurred across the western Carolinas and extreme northeast Georgia on Sunday, 1 March, and Monday, 2 March 2009, while a low pressure system moved across the southeastern United States. The precipitation began early on 1 March as rain at most locations, but snow occurred in the high elevations of the western Carolinas. The rain gradually changed to snow over a larger area during the afternoon and evening as the snow level lowered. By the time the precipitation ended during the early morning hours of 2 March, snow accumulations across the county warning area (CWA) of the Greenville-Spartanburg (GSP) Weather Forecast Office (WFO) ranged from a trace to 16 inches (Fig. 1). The most significant snowfall totals were in the high elevations east and northeast of Asheville, North Carolina, and in the Piedmont of both Carolinas. Amounts less than one inch were observed in the far western mountain counties and in the northern foothills of North Carolina. The higher snow accumulations extended northeast across the northern mountains of North Carolina and into the New River Valley of Virginia, while another swath of higher snowfall totals extended from the Piedmont Triad to south central Virginia.

Snowfall totals for 1-2 March 2009

Figure 1. Storm total snowfall for the period 1-2 March 2009. Click on image to enlarge.

Click here for a list of snowfall reports for 1-2 March 2009

One of the unique features of this event was the simultaneous occurrence of snow and thunder during the evening of 1 March. The axis of significant snow accumulation seen in Figure 1 extending from Upstate South Carolina into the Piedmont of North Carolina marked the area where snowfall rates were enhanced by lightning-producing convective storms.

2. Synoptic Overview

The sequence of events that set the stage for the winter storm can be traced to 27 and 28 February when a cold front crossed the Appalachian Mountains (Fig. 2). The front extended from the North Carolina coast southwest to the Texas coast at 1200 UTC on 28 February, as seen on the surface analysis from the Hydrometeorological Prediction Center (HPC). A cold high pressure air mass moved from central Canada into the northern Great Lakes and Great Plains. During 28 February, a weak low pressure system developed on the front over the Gulf Coast states and moved eastward.

Click here to view a 21 frame Java loop of Surface Fronts and Pressure from 0000 UTC 28 February to 1200 UTC 2 March 2009.

HPC Surface fronts and pressure analysis at 0600 UTC 1 March 2009

Figure 2. HPC surface fronts and pressure analysis at 0600 UTC on 1 March 2009. Click on image to enlarge.

The 500 mb height and wind fields at 1200 UTC on 28 February (Fig. 3) showed a ridge over the western United States and a trough over the Great Plains. A 5460 m cutoff low over Kansas and Nebraska was the dominant feature of the trough. The flow along the East Coast was generally from the west and southwest with a short wave trough moving into New England. The flow was confluent over the northern mid-Atlantic region where the southwest winds merged with a northwesterly air stream circulating around a deep low over northeastern Canada. The weak surface low on the cold front was over Georgia at 1800 UTC 28 February. A surface trough extended from the low over Georgia northwest to Missouri and Kansas where the upper low was located.

Click here to view a 28 frame Java loop of GOES-12 Water Vapor Satellite Imagery from 2345 UTC 27 February to 0345 UTC 1 March 2009.

500 mb geopotential height, temperature, and wind barbs at 1200 UTC on 28 February 2009

Figure 3. Objective analysis of 500 mb geopotential height, temperature, and wind barbs at 1200 UTC on 28 February 2009. Click on image to enlarge.

At 0000 UTC on 1 March, the Rapid Update Cycle (RUC) model (80-km grid spacing) placed a 5410 m cutoff low at 500 mb over Arkansas (Fig. 4). Low 1000-500 mb thickness values (5210 m) accompanied the deep cyclonic circulation. A wind maximum at 500 mb curved around the base of the trough with speeds in excess of 90 kt over northeast Texas. The wind maximum at 300 mb over north central Texas was 125 kt. The jet axis at 300 mb extended northeast to the mid-Atlantic coast with 90 kt wind maxima over east Tennessee and Virginia. The 500-300 mb omega analysis showed the most prominent area of upward motion was over west Tennessee.

500 mb analysis of geopotential height, 1000-500 mb thickness, and wind speed from RUC80 at 0000 UTC 1 March 2009

Figure 4. Objective analysis of 500 mb geopotential height (solid green contours, dm), 1000-500 mb thickness (dashed yellow contours, m) and wind speed (color fill, kt) at 500 mb from the RUC model at 0000 UTC on 1 March. The color table at the top shows the wind speed, shaded purple from 0 to 20 kt and green and yellow greater than 65 kt. Click on image to enlarge.

The surface analysis at 0000 UTC on 1 March showed that a center of low pressure had moved to South Carolina with very little strengthening during the preceding 12 hours. A weak surface trough extended from the low west to Arkansas where the upper-level cyclonic circulation was located. The surface analysis at 0600 UTC on 1 March indicated the weak surface low was still over South Carolina, but another weak low was evident on the front in southern Georgia. By 1200 UTC, a single surface low pressure system was near Savannah and the 5460 m cutoff low was over southern Alabama. The 1003 mb surface low had deepened approximately 5 mb since 0000 UTC and the low was more organized in response to the approach of the upper-level trough, while the surface trough that extended west northwest to northern Alabama displayed better definition.

The 95 kt wind maximum at 500 mb had rounded the bottom of the trough at 1200 UTC and extended from the northeastern Gulf of Mexico into Georgia. The 110 kt wind maximum at 300 mb was a little further west near the Louisiana coast, but the jet axis extended northeast into Georgia. The Carolinas and Georgia were in the exit region of the wind maximum approaching from the Gulf of Mexico and in the entrance region of a 100 kt wind maximum over the Mid-Atlantic States. The highly ageostrophic flow at 300 mb (Fig. 5) was accompanied by a 500-300 mb omega maximum over Upstate South Carolina and along the Carolina coast.

RUC-80 analysis of 300 mb isotachs, ageostrophic wind, and 500-300mb omega at 1200 UTC on 1 March

Figure 5. Objective analysis of 300 mb isotachs (solid blue), ageostrophic wind (yellow barbs), and 500-300 mb omega (color fill) at 1200 UTC on 1 March from the RUC-80 model. Click on image to enlarge.

Between 1200 UTC 1 March and 0000 UTC 2 March, the surface low moved from coastal Georgia to the Outer Banks, and another surface low formed just inland over the Carolinas (Fig. 6). The second low formed in the trough extending west from the coastal low and was a result of falling surface pressure associated with the deep and cold cyclonic circulation aloft that had been approaching the frontal boundary and primary low. As the 500 mb low moved from southern Alabama to central South Carolina by 0000 UTC, the 1000-500 mb thicknesses accompanying the cutoff gradually lowered across the area. As a result of the lowering thicknesses, sufficient cooling occurred to produce snow at locations that had experienced rain during most of the day.

HPC Surface fronts and pressure analysis at 1200 UTC 1 March 2009HPC Surface fronts and pressure analysis at 1800 UTC 1 March 2009

HPC Surface fronts and pressure analysis at 2100 UTC 1 March 2009HPC Surface fronts and pressure analysis at 0000 UTC 2 March 2009

Figure 6. HPC regional surface analyses at 1200 UTC on 1 March (upper left), 1800 UTC 1 March (upper right), 2100 UTC 1 March (lower left), and 0000 UTC 2 March (lower right). Click on images to enlarge.

During the next 12 hours the cutoff low at 500 mb evolved to an open, sharp trough that moved offshore. The surface low moved to a position off the northern Mid-Atlantic Coast by 1200 UTC on 2 March. The circulation around the offshore low and the high pressure system centered near the Great Lakes produced a dry northerly wind flow across the western Carolinas and extreme northeast Georgia.

Click here to view a 29 frame Java loop of a Regional Radar Reflectivity Mosaic from 0500 UTC 1 March to 0858 UTC 2 March 2009.

Click here to view a 22 frame Java loop of GOES-12 Water Vapor Satellite Imagery from 0645 UTC on 1 March to 0345 UTC on 2 March 2009.

Click here to view a 22 frame Java loop of GOES-12 Infrared Satellite Imagery from 0645 UTC on 1 March to 0345 UTC on 2 March 2009.

3. Event Details

a. Sunday morning

Around 0530 UTC on 1 March, precipitation spread into the CWA from the southwest. Rain was the dominant precipitation type, but subfreezing temperatures above approximately 6,000 ft MSL contributed to snow on the highest mountains. Precipitation continued to increase in coverage so that by 1200 UTC light rain was beginning at Asheville, and rain was occurring over nearly the entire area east of the mountains.

The precipitation developed as isentropic lift strengthened in advance of the approaching low, as seen on the 295 K surface obtained from the RUC13 model (the Rapid Update Cycle model with a 13 km horizontal grid spacing) at 1200 UTC and the regional composite reflectivity radar mosaic (Fig. 7). The wind field across Upstate South Carolina and the Piedmont of North Carolina showed the flow ascending the 295 K surface (from high pressure near the surface to low pressure aloft). The precipitation coverage was quite similar to the area experiencing upward motion on that surface.

RUC-13 model objective analysis of pressure and wind on 295 K surface with composite reflectivity mosaic at 1200 UTC on 1 March

Figure 7. Objective analysis from the RUC-13 model of pressure (green contours) and wind (yellow barbs) on the 295 K surface, with composite radar reflectivity mosaic (color fill) at 1200 UTC on 1 March. Click on image to enlarge.

The 1200 UTC surface observations showed temperatures near 32o F in the extreme northeast portion of the CWA and below freezing in the higher elevations (Fig. 8). Warmer temperatures (in the 40s) existed in the far western mountain counties because high terrain blocked the westward spread of the colder surface air over the Piedmont. The observations indicated that most of the stations reporting precipitation experienced rain.

Surface observations at 1200 UTC on 1 March 2009

Figure 8. Surface observations plot at 1200 UTC on 1 March. Click on image to enlarge.

The RUC13 initial hour analysis thermal profiles at 1200 UTC on 1 March for four locations [Asheville (AVL), Charlotte (CLT), Hickory (HKY), and Greer (GSP)] showed that even though each location had surface temperatures only a few degrees above 32o F, a warm layer aloft prevented snow from reaching the ground (Fig. 9). The horizontal red line on the AVL temperature profile indicates the height of Mt. Mitchell (6,684 ft MSL). The near freezing temperature at that elevation and the subfreezing temperature above indicate that precipitation spreading across the mountains was probably in the form of snow at the highest elevations, but melting was occurring rapidly below 6,000 ft MSL.

RUC13 initial hour profile of temperature, dewpoint, and wind for AVL at 1200 UTC on 1 March 2009RUC13 initial hour profile of temperature, dewpoint, and wind for CLT at 1200 UTC on 1 March 2009

RUC13 initial hour profile of temperature, dewpoint, and wind for HKY at 1200 UTC on 1 March 2009RUC13 initial hour profile of temperature, dewpoint, and wind for GSP at 1200 UTC on 1 March 2009

Figure 9. RUC13 initial hour profiles of temperature (red), dew point (green), and wind (barbs) at 1200 UTC on 1 March for Asheville (AVL, upper left), Charlotte (CLT, upper right), Hickory (HKY, lower left), and Greer (GSP, lower right). The horizontal red line on the AVL profile indicates the height of Mt. Mitchell (6,684 ft MSL). A wind hodograph is shown in the upper left-hand corner of each image. Click on images to enlarge.

The eastward progression of the upper low center was expected to strengthen the isentropic lift to the east of the center and the low level cold advection around the western side as it acquired a more negative tilt. During the late morning hours, the strengthening of a deformation zone to the north and west of the low allowed for the development of a band of snow across north Alabama and north and west central Georgia. This band of snow was forecast to move over the western Carolinas later in the day.

b. Sunday afternoon and evening

The area east of the mountains was becoming increasingly favorable for snow in the early part of the afternoon, as outlined by a Mesoscale Discussion issued by the Storm Prediction Center. By 1800 UTC, the weak surface low was centered just west of Charleston, South Carolina, with a trough of low pressure extending northwest into northern Georgia (Fig. 6). Surface temperatures in the western Carolinas were still in the lower and middle 30s. Precipitation at the surface was falling in the form of rain nearly everywhere in the CWA, although the rain changed to snow at Asheville at 1725 UTC. The precipitation was diminishing in the southernmost counties where drier air circulating around the southern portion of the upper low was moving into the area.

The eastward progression of the deep upper-level cyclonic circulation and its associated cold air resulted in a lowering of the freezing level and downward penetration of snow in the mountains. At the same time, the cold air damming pattern east of the Appalachians contributed to cooling temperatures near the surface. The northeast surface winds advecting cold air southward from the Mid-Atlantic region were evident in the 1800 UTC surface analysis (Fig. 6). A northeast low-level jet greater than 40 kt (Fig. 10) existed at 925 mb just east of the mountains. The cold air damming was not particularly strong, and the near-surface air moving into the region was not very cold. Nonetheless, the temperatures in the lower and middle 30s were just cold enough to allow snow to reach the surface after the warm layer aloft was removed. Cooling caused by the melting of snow also likely contributed to lowering the snow level and maintaining near-freezing surface temperatures.

RUC13 objective analysis of wind and isotachs at 925 mb at 1800 UTC on 1 March

Figure 10. Objective analysis of 925 mb isotachs (dashed contours) and wind (yellow barbs) from the RUC13 at 1800 UTC on 1 March and isotachs (shaded contours). A maximum wind speed of greater than 40 kt is located in the North Carolina Piedmont. Click on image to enlarge.

The RUC13 initial analysis thermal profiles at 1800 UTC (Fig. 11) showed that significant cooling occurred in the lower troposphere since 1200 UTC. The AVL temperature profile was below freezing except for a very shallow surface layer. The freezing level was 770 ft above the ground, so snow was able to reach the surface because insignificant melting occurred in the 33o F air. By this time, snow was rapidly accumulating above 2900 ft MSL in the mountains near Asheville, and the snow level continued to drop. The GSP and HKY profiles displayed significant cooling since 1200 UTC, but CLT retained an above-freezing layer between 910 mb and 725 mb. A comparison of the wind profiles in Fig. 11 with the corresponding profiles in Fig. 9 clearly showed the increasing depth of the surface-based cold air. Also, the winds above the cold layer backed to the south and southeast signaling the approach of the upper-level low.

RUC13 initial hour profile of temperature, dewpoint, and wind for AVL at 1800 UTC on 1 March 2009RUC13 initial hour profile of temperature, dewpoint, and wind for CLT at 1800 UTC on 1 March 2009

RUC13 initial hour profile of temperature, dewpoint, and wind for HKY at 1800 UTC on 1 March 2009RUC13 initial hour profile of temperature, dewpoint, and wind for GSP at 1800 UTC on 1 March 2009

Figure 11. As in Fig. 9, except for 1800 UTC. Click on images to enlarge.

The RUC13 objective analysis at 850 mb at 1800 UTC (Fig. 12) depicted a confluent northeast wind flow and nearly neutral thermal advection pattern over the western portion of the CWA. Strong warm air advection was still occurring in the east where the weakening, but persistent, warm layer continued on the CLT temperature profile. The objective analysis of the 295 K isentropic surface at 1800 UTC from the RUC13 (Fig. 13) showed a well-defined region of isentropic lift from northeast Georgia across upstate South Carolina into central and eastern North Carolina. The precipitation pattern on the mosaic of regional radars (Fig. 14) corresponded closely to the area of upward motion on the 295K surface. The precipitation was light and scattered across southern portions of the upstate where isentropic lift was minimal and dry air was circulating around the southern periphery of the upper-level low.

RUC13 objective analysis of 850 mb height, temperature, and wind at 1800 UTC on 1 March

Figure 12. Objective analysis of 850 mb geopotential height (green contours), temperature (solid and dashed brown contours), and wind (yellow barbs) from the RUC13 at 1800 UTC on 1 March. Click on image to enlarge.

RUC13 objective analysis of pressure, wind, and vertical motion on the 295 K surface at 1800 UTC on 1 March

Figure 13. Objective analysis of pressure (green contours), wind (barbs), and vertical motion (color fill) on the 295 K isentropic surface from the RUC13 at 1800 UTC on 1 March. The cooler colors represent downward motion and the warmer colors denote upward motion. Click on image to enlarge.

Composite reflectivity mosaic at 1800 UTC on 1 March

Figure 14. Composite radar reflectivity mosaic at 1800 UTC on 1 March. Click on image to enlarge.

At 2100 UTC the primary surface low was located near Myrtle Beach, South Carolina (MYR) (Fig. 6). A trough of low pressure extended west to a weaker low center near Augusta, Georgia (AGS). The low near AGS was the surface reflection of the cyclonic circulation aloft, which was located somewhere between AGS and Atlanta, Georgia (ATL), as seen on all levels from 925 mb through 300 mb on the RUC13 initial analysis. The steep slope of the low geopotential heights resulted in a deep layer of northeast to east winds across the western Carolinas and northeast Georgia. Snow was observed at stations in northeast Georgia as the cold air associated with the low aloft moved eastward.

The cold air damming pattern held temperatures near or just above freezing east of the mountains throughout the afternoon. Lower 1000- 500 mb thickness, related to the colder air aloft in the deep cyclonic circulation, gradually moved into the western Carolinas. The snow level lowered sufficiently so that a transition of precipitation types occurred at most locations outside the mountains. One notable exception was the North Carolina Foothills where the RUC13 925 mb temperature analysis and forecasts (not shown) indicated that a small area of relatively warm temperatures persisted throughout the afternoon. The slightly warmer temperatures delayed the onset of accumulating snow and contributed to the smaller snow totals depicted just east of the mountains in Fig. 1. Rain changed to snow at HKY around 1800 UTC, but it changed to rain again around 2000 UTC when the surface temperature increased to 36o F. Rain continued at HKY until 0047 UTC on 2 March when snow returned.

The origin of the warmer air in the foothills was not clearly understood. Wind direction in the vertical veered from northeast to south from the surface to 500 mb and above, so an obvious downslope wind component to produce warming by subsidence did not exist. An untested hypothesis suggested that the predominant cooling in the cold air damming occurred farther to the east near the axis of the low-level jet in Fig. 10. The cooling progressed slowly along the foothills and did not lower surface temperatures close to freezing until late in the precipitation event.

The rain changed to snow in the Greenville-Spartanburg area around 2130 UTC, and the snow continued for the duration of the event. Charlotte reported a mixture of rain and snow at 1924 UTC that changed to rain before becoming a mixture of sleet and rain at 2250 UTC. Snow replaced the sleet and rain mixture at 2352 UTC, and continued until the precipitation ended after 0500 UTC on 2 March.

Click to view hourly observations and meteograms:
Station
1 March 2009
2 March 2009
Anderson
 
Asheville
 
Charlotte
 
Greenville - Downtown
 
Greenville -Spartanburg
 
Hickory
 

1) Deformation zone and trowal

The region of shearing and stretching frequently observed in the wind field to the west through north of a cyclonic circulation is one example of a deformation zone. Deformation in the wind field in some circumstances can increase the temperature gradient which leads to vertical motion that attempts to restore thermal balance. Forecasters often focus on the vertical motion in the vicinity of deformation zones to highlight areas of potentially significant precipitation. An analysis of the 500 mb height and wind (speed, direction, and streamlines) fields and omega at 2100 UTC from the RUC13 revealed significant upward motion along and just to the east of the axis of confluent streamlines west and northwest of the circulation center (Fig. 15).

RUC13 objective analysis of 500 mb wind, streamlines, and omega at 2100 UTC on 1 March

Figure 15. Objective analysis of 500 mb streamlines (solid lines with arrows), wind (yellow barbs) and omega (color fill) from the RUC13 at 2100 UTC on 1 March. Warmer colors represent upward vertical motion. Click on image to enlarge.

Perhaps a more easily understood framework for visualizing deformation zone precipitation was available by considering motions on isentropic surfaces. An axis of increasingly low pressure and the corresponding wind field extending from central North Carolina west to the mountains then south to central Georgia was noted on the analysis of the 300 K surface at 2100 UTC (Fig. 16). Air parcels at this time were climbing the sharply sloping valley on the 300K surface. The pressure decreased from approximately 680 mb near CLT to approximately 600 mb near ATL. The color shades on Fig. 16 highlight the significant upward motion that occurred north through west of the circulation center near AGS. The arc-shaped region of upward motion conformed closely to the precipitation indicated on the 2100 UTC composite radar image (Fig. 17).

RUC13 objective analysis of pressure, wind, and omega on the 300 K surface at 2100 UTC on 1 March 2009

Figure 16. Objective analysis of pressure (green contours), wind (yellow barbs) and omega (color fill) on the 300 K surface from the RUC13 at 2100 UTC on 1 March. Click on image to enlarge.

Composite reflectivity mosaic at 2100 UTC on 1 March

Figure 17. Composite radar reflectivity mosaic at 2100 UTC on 1 March. Click on image to enlarge.

The axis of low pressures curving around the north side of the circulation on the sloping 300 K surface was a signature of a trowal (trough of warm air aloft). The trowal has been described by a number of authors (e.g., Penner 1955; Martin 1999) as a key feature in understanding the structure, frontogenetical properties, and the associated three-dimensional air flow of maturing extratropical cyclones.

The ascending airstream in the trowal remained over the region until late at night as the trowal pivoted around the circulation center. This occurred because the deep cyclonic circulation moved just south of the WFO GSP CWA then turned toward the northeast. Precipitation continued until shortly after 0500 UTC on 2 March in the Greenville-Spartanburg area and until approximately 0900 UTC on 2 March at Charlotte.

The 2025 UTC infrared satellite image (Fig. 18) was enhanced to show key features of the primary conveyor belts comprising the maturing cyclone. [Refer to Carlson (1991); Moore et al. 2005; and Schultz (2001) for more detailed information regarding conveyor belts in an extratropical cyclone.] The warm conveyor belt flowed northward from the Bahamas and ascended to the comma cloud head where it curved anticyclonically and entered the high level jet stream flow. A branch of the warm conveyor belt did not rise as high as the primary branch while it curved cyclonically to wrap around the low center. This air comprised much of the trowal airstream. A much lower airstream, the cold conveyor belt, had its origins in the cold air over the Mid-Atlantic States. The cold conveyor belt flowed westward, north of the surface low pressure system, and then curved cyclonically around the low center. The dry conveyor belt originated at high levels northwest of the cyclone then descended and approached the surface to the south of the low pressure center. Some of the dry air wrapped cyclonically around the circulation while another branch curved anticyclonically and moved southeast as it neared the surface cold front.

GOES-12 infrared imagery at 2025 UTC on 1 March

Figure 18. GOES-12 infrared satellite image at 2025 UTC on 1 March, showing the warm conveyor belts (red), cold conveyor belt (blue), and dry conveyor belt (brown). Click on image to enlarge.

2) Thundersnow

One of the interesting features of this winter precipitation event was the occurrence of thundersnow from Upstate South Carolina northeast into the Piedmont of North Carolina. Forecasters at GSP were anticipating thundersnow and enhanced snowfall rates to move across portions of the Piedmont of the Carolinas after seeing observations of thundersnow across north Georgia and reading the Mesoscale Discussion issued by the SPC at 2040 UTC on 1 March. The first report of thundersnow was made by the observer at the Charlotte - Douglas International Airport at 2352 UTC on 1 March. Thundersnow was also observed at the Greenville- Spartanburg International Airport between 0129 UTC and 0228 UTC on 2 March. Cloud-to-ground lightning was detected between 0110 UTC and 0200 UTC. Anecdotal evidence suggested that thundersnow continued across the area from northeast of Greenville, South Carolina, to Shelby and Lincolnton, North Carolina, until at least 0300 UTC on 2 March.

Click here to view a 1.11 mb windows media video of thundersnow. The video was shot by Caitlin Rubow north of Dallas (Gaston County), North Carolina, around 9 pm on 1 March 2009. Used by permission.

Click here to view a 26 frame Java loop of Composite Reflectivity from the KGSP radar from 0045 UTC to 0230 UTC 2 March 2009.

The lightning was produced in a region of enhanced instability aloft. A cross section from the 0100 UTC RUC13 analysis (Fig. 19), extending from Kentucky southeast to a position off the South Carolina coast, depicted a significant zone of rising motion (omega) sloping upward from the South Carolina coast toward the mountains. Just east of the mountains, a small upward motion maximum coincided with an elevated zone of instability denoted by the negative values of saturated geostrophic potential vorticity. The upper air sounding taken at Greensboro, North Carolina (GSO), at 0000 UTC on 2 March (Fig. 20) showed an environment that was favorable for convectively enhanced precipitation. Although the location of the sounding was 75 to 150 miles to the northeast of the thundersnow area, it included many features seen in other thundersnow events (Market et al. 2006). The most unstable parcel level was at 734 mb, which was approximately 30 mb above the top of the frontal temperature inversion. Significant drying occurred above 650 mb which indicated a conditionally unstable layer. In fact, parcels lifted from above the top of the inversion were unstable and positively buoyant with Convective Available Potential Energy of 311 J kg-1, indicating that upright convection was possible. The most unstable parcel originated below the -1000 C level of the sounding, and its equilibrium level was above the -2000 C level, which indicated that the environment was capable of supporting cloud-to-ground lightning production as set forth by the composite thundersnow sounding of Market et al. (2006). The 700 - 500 mb temperature difference was 1900 C, which was at the upper part of the range of temperatures observed in other thundersnow events over the Southeastern United States (Hunter et al. 2001).

RUC13 cross section of omega and saturated geostrophic potential vorticity at 0100 UTC on 2 March

Figure 19. Vertical cross-section from eastern Kentucky to a point just east of Myrtle Beach, depicting omega (yellow contours) and saturated geostrophic potential vorticity (contoured shades with negative values in blue) from the RUC13 at 0100 UTC on 2 March. Click on image to enlarge.

GSO upper air observation at 0000 UTC on 2 March

Figure 20. Skew-T, log P diagram for the upper air sounding taken at GSO at 0000 UTC on 2 March. The red line is the temperature sounding and the green line is the dewpoint sounding. A hodograph is shown in the upper right corner of the figure. A table of convective parameters is included at the bottom of the figure. Click on image to enlarge.

The instability that supported the lightning-producing convective storms was probably caused by the intrusion of the dry conveyor belt from the southwest into the moist air mass. Several studies have identified the leading edge of the "dry slot" as a region of convective destabilization. Nicosia and Grumm (1999) showed that stability can decrease when the dry conveyor belt rides over moist air below. Upward motion tapping the potentially unstable air likely resulted in the development of convective updrafts sufficiently strong to produce thunderstorms and enhanced snowfall rates. The axis of relatively high snow accumulations seen in Fig. 1 more than likely owed its existence to the convectively enhanced precipitation.

Although the environment supported upright convection during the evening hours, individual convective towers were not discernable on the Weather Surveillance Radar at the Greenville-Spartanburg International Airport (the KGSP radar). The precipitation was loosely organized in two bands stretching from southwest to northeast (Fig. 21). One band was located from extreme northeast Georgia, across Oconee County, South Carolina, to the eastern part of the North Carolina Mountains across Transylvania, Henderson, and Buncombe counties. The other band was situated across the southern half of Greenville County, South Carolina, and stretched across Spartanburg and Cherokee counties, to Cleveland and Lincoln counties in North Carolina. It was the second band in which the thundersnow occurred. The thundersnow band contained some reflectivity greater than 40 dBZ on the 0.5 degree scan, generally at a level below 2,000 feet AGL. The higher reflectivity probably resulted from aggregates of partially melted dendritic snow crystals. Above that level, reflectivity was less than 40 dBZ and radar echo tops showed little variation across the band, thus it was difficult to detect stronger convective cells that might produce lightning.

KGSP radar reflectivity at 0.5, 1.3, and 2.4 degrees, and echo tops, at 0101 UTC on 2 March

Figure 21. KGSP base reflectivity at (A) 0.5 degrees, (B) 1.3 degrees, and (C) 2.4 degrees, and (D) echo top, for the 0101 UTC volume scan on 2 March. The color table on the left is for the reflectivity images and the color table on the right is for the echo top image. The radar images were created using GRLevel2 Analyst. Click on image to enlarge.

c. Sunday night and Monday morning

The two low pressure systems seen in Fig. 6 moved northeast away from the Carolinas late Sunday night and early Monday morning. By 1200 UTC 2 March, a single surface low pressure was centered off the New Jersey coast. The 500 mb cutoff low had evolved into an open, sharp trough that was moving offshore. In the wake of the storm, the sky cleared across the western Carolinas and extreme northeast Georgia allowing the Aqua MODIS satellite to capture the image in Fig. 21. The image was obtained during an early afternoon pass, so warming temperatures had already eliminated the snow in locations where only minor accumulations occurred. Close examination of the figure revealed the lack of significant snow in the far western portion of the CWA and in the North Carolina Foothills. Lesser snow accumulations were also seen in the French Broad River Valley in the vicinity of Asheville. Smaller snow accumulations in the valley were probably caused by above freezing surface temperatures in the lower elevations that delayed the downward penetration of snow and, perhaps, by a precipitation shadow affect.

Aqua MODIS image at 1830 UTC on 2 March

Figure 22. Aqua MODIS image from approximately 1830 UTC 2 March 2009. The early afternoon sun caused melting where only small amounts of snow accumulated the previous afternoon and evening. Nearly snow-free areas are in the foothills and in the lower elevations of the far western counties. The French Broad River valley in the vicinity of Asheville is near the center of the image. Image: Space Science and Engineering Center, UW-Madison. Image overlays: National Environmental Modeling and Analysis Center, UNC Asheville. Click on image to enlarge.

4. Summary

A low pressure system traveling across the southeastern United States from 28 February until 2 March 2009 produced rain and snow across portions of the western Carolinas and extreme northeast Georgia. Snow accumulations ranged from a trace in far western counties to 16 inches at Mt. Mitchell, North Carolina. Some Piedmont locations in North and South Carolina received 6 to 10 inches of snow. Two sources of cold air contributed to the transition from rain to snow while the low pressure system traversed the area: 1) Cold air accompanying the deep upper-level low, and 2) Near-surface cold air supplied by the high pressure system near the Great Lakes. Vertical motion related to highly ageostrophic flow aloft contributed to the significant and widespread precipitation. The duration of precipitation was enhanced by the slow eastward progression of the trowal airstream as it pivoted around the upper-level low. Warmer air along the Foothills delayed the transition to snow thus reducing snow accumulations. Only small snowfall totals were observed in some of the far western counties where the westward penetration of the cold air damming air mass was blocked by higher terrain. Snowfall rates in the Piedmont were enhanced Sunday evening by an unstable atmosphere that developed as the deep upper-level low moved just south of the area.

Photo Gallery

View from Poga Mtn, near Flat Springs community in northern Avery County, North Carolina on 2 March

The view from Poga Mountain, near the Flat Springs community in northern Avery County, North Carolina, on the morning of 2 March 2009. Image by Dr. Baker Perry.

Snow falling near the Clemson Marina at 5:15 pm on 1 March 2009Snow falling near the Clemson Marina at 5:15 pm on 1 March 2009

Snow falling near the Clemson Marina at 5:15 pm on 1 March 2009Snow falling near the Clemson Marina at 5:15 pm on 1 March 2009

Light snow falls near the Clemson Marina around 5:15 pm on Sunday, 1 March 2009. Images by Ted Paskiewicz.

View at WFO GSP on the morning of Monday, 2 March 2009View at WFO GSP on the morning of Monday, 2 March 2009

View at WFO GSP on the morning of Monday, 2 March 2009View at WFO GSP on the morning of Monday, 2 March 2009

This was the view outside the National Weather Service Office at the Greenville-Spartanburg Airport on the morning of 2 March 2009. Officially, 4.4 inches fell at the GSP Airport during the event.

View near Lincolnton, NC on the morning of Monday, 2 March 2009View near Lincolnton, NC on the morning of Monday, 2 March 2009

More views of the new-fallen snow around Lincolnton, North Carolina, just after sunrise on 2 March 2009.

New snow in the Five Forks area of Simpsonville, SC on the morning of Monday, 2 March 2009New snow in the Five Forks area of Simpsonville, SC on the morning of Monday, 2 March 2009New snow in the Five Forks area of Simpsonville, SC on the morning of Monday, 2 March 2009

New snow on the morning of Monday, March 2, in the Five Forks area of Simpsonville, South Carolina.

References
Bennetts, D. A., and B. J. Hoskins, 1979: Conditional symmetric instability -
     A possible explanation of frontal rainbands. Quart. J. Roy. Meteor. Soc., 
     105, 945-962.

Carlson, T. N., 1991:  Mid-Latitude Weather Systems.  Harper-Collins Academic, 
     London, UK, 507 pp.

Hunter, S. M., S. J. Underwood, R. L. Holle, and T. L. Mote, 2001:  Winter 
     lightning and heavy frozen precipitation in the southeast United States.  
     Wea. Forecasting, 16, 478-490.

Market, P. S., A. M. Oravetz, D. Gaede, E. Bookbinder, A. R. Lupo, C. J. 
     Melick, L. L. Smith, R. Thomas, R. Redburn, B. P. Pettegrew, and A. E. 
     Becker, 2006:  Proximity soundings of thundersnow in the central United 
     States.  J. Geophys. Res., 111, D19208. 

Martin, J. E., 1999: Quasigeostrophic forcing of ascent in the occluded sector 
     of cyclones and the trowal airstream.  Mon. Wea. Rev., 127, 70-88.

McCann, D.W., 1995: Three-Dimensional Computations of Equivalent Potential 
     Vorticity.  Wea. Forecasting, 10, 798-802.

Moore, J. T., 1993:  Isentropic Analysis and Interpretation.  National Weather 
     Service Training Center, Kansas City, MO, 99 pp.

Moore, J. T., Ng, S., and C. E. Graves, 2005:  The role of conveyor belts in 
     organizing processes associated with heavy banded snowfall.  Preprints, 
     21st Conference on Weather Analysis and Forecasting, Washington, DC, 
     Amer. Meteor. Soc.

Nicosia, D. J., and R. H. Grumm, 1999: Mesoscale band formation in three major 
     northeastern United States snowstorms.  Wea. Forecasting, 14, 346-368.

Penner, C. M., 1955:  A three-front model for synoptic analyses.  Quart. J. Roy. 
     Meteor. Soc., 81, 89-91.

Schultz, D.M., 2001: Reexamining the cold conveyor belt.  Mon. Wea. Rev., 129, 
     2205-2225.

Acknowledgements

Blair Holloway prepared the snowfall map using ArcView GIS. The surface fronts and pressure analyses were obtained from the Hydrometeorological Prediction Center surface analysis archive. The thermal profiles in Figs. 9 and 11 were created using the Rawinsonde Observation Program version 5.8 for Windows. The upper air analyses and sounding plot in Fig. 20 were obtained from the Storm Prediction Center. The surface observations plot, regional radar reflectivity mosaics, and satellite imagery were obtained from the RAP - Real Time Weather Data page maintained by the University Corporation for Atmospheric Research. The radar imagery in Fig. 21 was created using the GRLevel2 Analyst software package. Hourly observations were obtained from the National Climatic Data Center. The meteogram plots were obtained from the Plymouth State College Weather Center. The Aqua MODIS image was obtained from the Space Science and Engineering Center at the University of Wisconsin at Madison, with overlays from the National Environmental Modeling and Analysis Center at the University of North Carolina at Asheville.

Special thanks are given to Eric Thomas (WBTV Charlotte), Michael Dross, and Justin Rubow for providing the unusual thundersnow video clip.