Winter Weather - Winter Weather Climatology | State Climate Office of North Carolina

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

The December 4-5, 2002 Southeast Severe Ice Storm: A Climatological Comparison

Map of Stations Used in Study This study provides a wealth of information on all winter weather events observed at 18 first-order weather stations in the southeastern United States over a 55-year period, 1948-2003. Winter weather events are defined by the occurrence of measurable snow, sleet, freezing rain, and/or cold rain. Each link on the contents page allows one to access a variety of resources and analyses, including a review of precipitation types and their formation, a comprehensive climatology of the spatial and temporal distributions of winter precipitation and winter weather events, the distribution and frequency of precipitation-type transitions, and recurrence intervals and probabilities for events of varying types and intensities. A case study and climatological analysis of the December 2002 ice storm, one of the most severe and costly winter storms to affect the Southeast, is also provided. An upcoming feature of this project is the addition of a winter weather events database for each recording station used in the climatology. These databases will be available for download and may be useful research tools, as they provide a number of statistical parameters for each event, including a new winter storm magnitude typing.

Background

Winter weather (i.e. snow, sleet, freezing rain) occurs with the greatest frequency in the northern latitudes (e.g. New England and the Midwest) and higher altitudes (e.g. the Rocky Mountains ). However, such weather regularly affects the southeastern United States as far south as Georgia during each cold season. In fact, the impacts of winter weather in the Southeast have been recorded as early as the first week of October and as late as mid-April. What separates our response to winter weather in this region, particularly in the Piedmont and Coastal Plain, from that of our more northern and western neighbors, is the lack of a sufficient utility infrastructure to effectively handle these events. The result is temporary economic and social disruptions, realities that strike even at the mere threat or forecast of winter weather.

Each type of winter precipitation brings with it unique hazards. For instance, falling snow can significantly reduce visibility and create dangerous driving conditions, as well as paralyze entire communities if enough snow accumulates. Sleet (or ice pellets), can worsen the driving hazard by creating a dangerously slick surface, although sleet is less likely to adhere to trees and power lines. Perhaps the most destructive form of precipitation is freezing rain (or ice loading). Freezing rain accumulation on trees and power lines can cause them to snap, resulting in power outages and damage to homes, automobiles, and ecosystems. The fact that each frozen precipitation type occurs with some regularity throughout the Southeast is due mainly to the topography of the region as well as its geography. Continental polar air masses from Canada typically supply the cold air necessary for snow, while cold, dry air from New England entering the region can become entrained against the east slopes of the Appalachian Mountains, forming a dome (or wedge) of near-surface cold air. The moisture necessary for precipitation is brought in from the nearby Gulf of Mexico, where the thermal contrast between the cold land surface and the relatively warmer gulf waters provides a favorable environment for storm development and intensification. In these situations, if a cold dome is already in place east of the mountains, the warm frontal boundary and moisture associated with the developing storm may migrate northward over the cold dome, setting the stage for a mixed precipitation (freezing/frozen) event. In addition to identifying the atmospheric processes that produce winter weather in the Southeast, having knowledge of the spatial and temporal distributions of winter precipitation and the varying winter weather scenarios that transpire across the region is crucial in forecasting and planning for these events.

Data and Methods

Climatology: While most studies of frozen precipitation in the Southeast involve either individual storms or small groups of events, this study strives to identify climatological characteristics from a database of all frozen precipitation events across the Southeast for a 55 year period, beginning in 1948 and ending with the last winter event of the 2002-2003 season. This database includes over 70 weather variables for each event, extracted from 18 first order weather stations across the study area. In particular, recurrence intervals have been constructed for events of various intensities and precipitation types, along with a spatial analysis of various types and frequencies of precipitation-type transitions across the region.

Due to the lengths of the time series for each station and the amount of parameters included in the database, five data sources were needed. Hourly observations from all first-order weather stations were extracted from CD-ROM's produced by EarthInfo for the time period 1948 to 1997. Hourly Precipitation Data (HPD) was extracted from the National Climatic Data Center (NCDC) CD to hourly precipitation amounts. Data for the period starting in 1998 and ending in 2000, including precipitation, was extracted from NCDC Integrated Surface Hourly Weather Observations (ISHWO) CD-ROM's. Data for the remaining time period, 2001 to present, was extracted from Automated Surface Observing System (ASOS) sensors, which are maintained at airports by the National Weather Service. Daily snowfall totals (used to calculate snowfall recurrence intervals) were extracted from an online database of Cooperative Observer Network Stations (COOP) archived by NCDC.

Although most of the data is good, certain caveats prevent the construction of a complete, hourly time series due mostly to location changes and the time of construction of weather sensors. Four of the 18 stations - Athens (GA), Augusta (GA), Greer (SC), and Beckley (WV) - have time series beginning after 1948. This reduced the sample size of winter events, however, it does not have a significant impact on the climatology. Additionally, weather reports for the period 1966 to 1971, with the exception of precipitation totals, were only made every three hours. These reports were extrapolated to the intervening two hours (e.g. 1200 LST and 1400 LST reports are assumed to be the same as the 1300 LST report).

A FORTRAN program was developed to identify winter weather events in each time series file. A second program was written to link events across stations and identify climatological characteristics. Events at a given station were defined by the occurrence of measurable (0.01") precipitation and one or more observations of frozen precipitation. An event was terminated if there was more than a six-hour lapse in these conditions. A corollary to this was that events could have lapses in the requisite conditions of up to six hours.

A fourth form of winter precipitation was added to the database. Defined as "Cold Rain", these events were identified by the occurrence of rainfall and a temperature of 35° F or less. Such events provided an added perspective to understanding the processes involved in winter weather, as these events were very close to producing frozen precipitation.

To complete the database, various climatological characteristics were identified for each station and plotted on maps. These characteristics included annual frequencies and return intervals for different types of winter weather events. Special attention was given to the particulars of each event, including the types and frequencies of various phase transitions (e.g. rain to snow, snow to sleet etc.).

Winter Weather Climatologies

The December 4-5, 2002 Southeast Severe Ice Storm: A Climatological Comparison