A Winter Weather Climatology for the Southeastern United States

Christopher Fuhrmann and Charles E. Konrad, II

Department of Geography
The University of North Carolina at Chapel Hill

• Introduction [1]
• Summary of Storm Meteorology [2]
• Precipitation Distribution and Recurrence Intervals [3]
• Concluding Remarks [4]

Introduction

Early in the winter season of 2002-2003, many areas across the Southeast were crippled by the effects of a powerful winter storm. In the Piedmont region, this storm quickly became known as the "December Ice Storm" with many communities experiencing record amounts of freezing rain. New records were also set across the Carolinas and Virginia in regards to power outages and their durations, traffic accidents, school closing durations, and fatalities resulting from an extreme weather event (Hurricane Hugo in 1989 set the previous record for regional power outages as reported by Duke Energy). Many communities across the Southeast, particularly across the Charlotte Metro and Triangle, NC areas, were left paralyzed for days, some over a week. While the effects of the storm have garnered significant attention in regards to the economic and social inconveniences they caused, the climatological significance of this storm has not been assessed. This type of analysis is critical in assessing probabilities for a recurrence of such a crippling storm.

Summary of Storm Meteorology

1. (Map 1 [5]) Cold, dry air enters the Southeast via a cold front associated with an arctic high pressure cell to the northwest. As the cold air approaches the Gulf coast, a baroclinic environment becomes established on the southwest side of the high pressure, increasing the likelihood for enhancement of cyclogenesis (note the weak area of low pressure developing over New Mexico).
2. (Map 2 [6]) High pressure has now moved over New England and is feeding a dome of cold, dry air entrained against the east slopes of the Appalachian Mountains (i.e. cold air damming). An area of low pressure is now located over Louisiana with a warm front bringing moisture and above-freezing temperatures in the mid-levels of the troposphere over the dome of cold surface air.
3. (Map 3 [7]) As the storm is reaching its peak across the Southeast, secondary cyclogenesis is occurring off the coast of North Carolina. This synoptic feature has been shown to produce specific regions of precipitation type dominance with a snow and sleet mix in the mountains and the northwest portion of the region, freezing rain extending in a broad band oriented parallel to the mountains in the Piedmont, and mainly rain mixed with small amounts of freezing rain along the coastal plain.

Precipitation Distribution and Recurrence Intervals

Freezing Rain

• Nearly 75% of the sampled stations recorded measurable amounts of freezing rain
• Heaviest amounts were confined to the North Carolina Piedmont (eastern half)
• Above-average amounts in the mountains of Tennessee, West Virginia, and Virginia
• Freezing rain totals for most stations equaled or exceeded that of a storm experienced between one and four times each decade
• The most significant recurrence intervals were at Bristol, TN (28 years) and Raleigh, NC (most freezing rain from a single storm since 1948)

Sleet

• More than 25% of the sampled stations recorded measurable amounts of sleet
• Heaviest amounts occurred in the Piedmont and Shenandoah Valley of Virginia
• Average amounts in the North Carolina Piedmont
• Sleet totals for most stations equaled or exceeded that of a storm experienced every one to five years
• The most significant recurrence interval was Bristol, TN (18 years) with 0.01"

Snow (given as a measure of water equivalence)

• Half of the sampled stations recorded measurable snowfall
• Heaviest amounts occurred within the Virginia Piedmont and the southwestern Appalachian Mountains
• Snowfall amounts were not climatologically significant, as all stations reporting snow saw recurrence intervals of every year or multiple times per year

Rain

• Half of the sampled stations recorded measurable rainfall
• Heaviest amounts occurred within the Tennessee River Valley, the South Carolina Piedmont, and the North Carolina and Virginia coastal plains
• Of the nine stations reporting measurable rainfall, only three had recurrence intervals over one year: Norfolk, VA , Knoxville, TN , and Greenville-Spartanburg, SC

Concluding Remarks

The distribution of precipitation across the Southeast was indicative of Appalachian cold air damming and secondary coastal cyclogenesis, resulting in:

• An icy mixture of precipitation in the Piedmont
• Rain and snow in the Appalachian Mountains
• Rain mixed with smaller amounts of freezing rain along the coast

This storm was generally freezing rain dominant throughout the Southeast, with above-average amounts occurring at most every station that reported ice accumulation. Raleigh, NC more than doubled its previous record for freezing rain totals from a single storm (February 2, 1996 with 0.69 inches) and Bristol, TN saw an anomalously high amount of freezing rain. The latter was due primarily to a thin layer of sub-freezing air that became trapped near the valley floor along the Tennessee Plateau. Fortunately, while freezing rain was generally confined to the Piedmont, the greatest amounts of rainfall occurred in the mountains, along the coast, and generally in areas void of significant freezing rain. If these areas were superimposed, the effects of freezing and thawing with an icy glaze and standing water could have resulted in an even more substantial ice load. Sleet was confined mainly to central and western Virginia. Bristol, TN saw a minimal amount of sleet, but a long recurrence interval, indicating that sleet events are rather uncommon along the Tennessee Plateau. Future work will investigate the atmospheric features responsible for the record-breaking amounts of freezing rain experienced across the region.