SEVERE WEATHER CLIMATOLOGY FOR THE WFO BLACKSBURG VIRGINIA
COUNTY WARNING AREA

by Robert Stonefield and James Hudgins

 

The National Weather Service ( NWS) definition of a severe thunderstorm ( NWS, 2002) includes one or more of the following criteria: a tornado, hail three-quarters of an inch in diameter or larger, and/or convective winds of at least 50 knots (58 mph) or convective winds which cause damage, including power lines and large trees being blown down. This local severe weather climatological study provides forecasters an improved understanding of the type and frequency of seasonal and diurnal severe weather events, and better prepares them to anticipate such events.

 

The combination of topography, meteorological forcing and warm low-level moisture can aid the development of strong to severe thunderstorms across WFO (Weather Forecast Office) Blacksburg’s County Warning Area (CWA). Warm moist air advecting from the Gulf of Mexico and the Atlantic Ocean is common in the summer months, and when weak frontal boundaries cross WFO Blacksburg’s CWA during maximum surface heating time ( 2:00 to 6:00 pm), organized severe thunderstorms may develop. Other organized convection, such as a Mesoscale Convective System ( MCS), moving from the Ohio or the Tennessee Valleys can bring severe weather into the area at any hour, day or night. Most of these MCS’s weaken by the time they get to the WFO Blacksburg’s CWA either due to loss of instability in the evening or because of typically lower instabilities in the higher elevations. Unorganized convection can also produce severe weather from orographically induced, “pulse” storms, or those that form along convergence zones across the mountains and lee side of the Blue Ridge. In addition to severe weather from deep convection in the spring and summer, the remnants of tropical systems can often produce tornados.

 

The National Weather Service’s primary responsibility is to provide severe weather warnings “for the protection of life and property.” The WFO’s are tasked with issuing severe weather warnings for their area of responsibility or County Warning Area (CWA). The NWS Forecast Office located in Blacksburg, Virginia (RNK) has forecast and warning responsibility across Southeast West Virginia, Southwest Virginia, and Northwest North Carolina (Fig. 1). WFO Blacksburg CWA includes 40 counties and 11 independent cities (separate local governments not affiliated with any particular county). These counties cover an area from the Appalachians in the west, across the Blue Ridge Mountains, to the Piedmont in the east.

 

Figure 1. WFO Blacksburg County Warning Area

Figure 1. WFO Blacksburg, VA, (RNK) County Warning Area (white border) and regional/topographical map indicating geographical areas (scale in thousands of feet).

 

The data included in this study were compiled from 1950 to 2005. Data for this paper were collected from Local Storm Data publications and the National Climatic Data Center (NCDC) Storm Events database located in Asheville, North Carolina. Tornado intensity and track was also supplemented via the NWS Storm Prediction Center ( SPC) archived database which includes tornado data from 1950 to 2004. This study consists of 3515 documented severe weather events across the WFO Blacksburg’s CWA between 1950 and 2005. All of the times are referenced to Eastern Standard Time.

 

The topography of the WFO Blacksburg CWA (Fig. 1) is characterized by a rapid increase in elevation from southeast to northwest, starting from less than 1000 feet in the Piedmont, to mountainous terrain of 3200 to 5000in the higher elevations of the Blue Ridge and Appalachian mountains of western Virginia (VA), southeastern West Virginia (WV), and northwestern North Carolina (NC).

 

WFO Blacksburg’s CWA is approximately 20000 square miles and has a population of roughly 1.7 million. A majority of the population (1.1 million) resides in counties (Fig. 2) along and east of the Blue Ridge in Central Virginia, and Northwest North Carolina. The largest population centers east of the Blue Ridge are the independent cities of Roanoke (ROA), Lynchburg (LYH), and Danville (DAN), Virginia. Along and west of the Blue Ridge, the majority of the population lives in river valleys, particularly in the New River Valley of Virginia and the Greenbrier Valley of Southeastern West Virginia.

 

Figure 2. County population in thousands by county (white) based on 2000 Census Data

Figure 2. County population in thousands by county (white) based on 2000 Census Data.

 

A majority of the WFO Blacksburg CWA is comprised of rural farmland or is heavily forested, and therefore sparsely populated, with only a few moderate populated cities. Thus, most counties have a very low population density, with a few obviously influenced by higher populated centers (Fig. 3). This uneven distribution of people across the CWA can lead to skewing of observed severe weather events toward the more heavily populated areas, especially before the Skywarn volunteer spotter network was increased significantly in the mid-to-late 1990s.

Figure 3. Population density (persons per square mile by county) (white) based on 2000 Census Data

Figure 3. Population density (persons per square mile by county) (white) based on 2000 Census Data.

 

ALL SEVERE WEATHER EVENTS

 

Monthly Frequency

 

Figure 4. Total number of severe weather events by month from 1950 to 2005

Figure 4. Total number of severe weather events by month from 1950 to 2005.

 

Hourly Distribution

 

Figure 5. Severe thunderstorm wind events by time (1950-2005)

Figure 5. Severe thunderstorm wind events by time (1950-2005).

 

 

TORNADO CLIMATOLOGY

 

Figure 6. Historical tornado tracks (red lines) from 1950 through 2004

Figure 6. Historical tornado tracks (red lines) from 1950 through 2004.

Yellow highlighted area depicts WFO Blacksburg CWA.

(Note: Data for 2005 was not available for plotting at the time of this publication. Data courtesy of the Storm Prediction Center , Norman , OK ).

 

 

Monthly Frequency

 

Figure 7. Tornado events by month (1950-2005)

Figure 7. Tornado events by month (1950-2005).

 

Hourly Distribution

 

Figure 8. Tornado events by hour (1950-2005)

Figure 8. Tornado events by hour (1950-2005).

 

Magnitude

 

 

Fujita

Scale

Wind Speed (mph)

Tornado

Damage

F0

40-72

Light

F1

73-112

Moderate

F2

113-157

Considerable

F3

158-206

Severe

F4

207-260

Devastating

F5

261-318

Incredible

Table 1. Fujita Damage Scale. (from Fujita, 1981).

 

 

Figure 9. Tornados by the Fujita Scale (F0-F5) (1950-2005)

Figure 9. Tornados by the Fujita Scale (F0-F5) (1950-2005).

 

Figure 10. Strong (F2-F3) tornados across WFO Blacksburg CWA (1950-2005)

Figure 10. Strong (F2-F3) tornados across WFO Blacksburg CWA (1950-2005).

 

 

Associated with Tropical Systems

 

 

Figure 11. The number of tornados with tropical systems by county (1950-2005)

Figure 11. The number of tornados with tropical systems by county (1950-2005).

 

 

HAIL CLIMATOLOGY

 

Monthly Frequency

 

 

Figure 12. The distribution of hail events by month (1950-2005)

Figure 12. The distribution of hail events by month (1950-2005).

 

 

Hourly Distribution

 

Figure 13. The distribution of hail events by hour (1950-2005)

Figure 13. The distribution of hail events by hour (1950-2005).

 

Magnitude

 

Figure 14. The distribution of hail by size (1950-2005)

Figure 14. The distribution of hail by size (1950-2005).

 

 

DAMAGING WIND CLIMATOLOGY

 

Monthly Frequency

 

 

Figure 15. Severe thunderstorm wind event distribution by month (1950-2005)

Figure 15. Severe thunderstorm wind event distribution by month (1950-2005).

 

Hourly Distribution

 

 

 

 

Figure 16. The distribution of severe thunderstorm wind events by hour (1950-2005)

Figure 16. The distribution of severe thunderstorm wind events by hour (1950-2005).

 

 

CONCLUSIONS

 

The severe weather climatology for the WFO Blacksburg CWA is a historical resource for forecasters to use to increase their awareness prior to a severe weather event. Knowledge of the type and frequency of seasonal and diurnal severe weather events, as well as the local topography and demographics will greatly enhance severe weather warning decisions for the protection of life and property.

 

 

 

 

 

 

 

 

 

 

 

 

 

ACKNOWLEDGMENTS

 

The authors would like to thank Steve Keighton, Science and Operations Officer, for his insight and review of the document, as well as Rosemary Auld and Ken Johnson of the Eastern Region Headquarters, Scientific Services Division for their reviews.

 

REFERENCES

 

Fujita, T. T., 1981: Tornados and downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38, 1511-1534.

 

National Weather Service, 2002: National Weather Service Instruction10-511. [Available on-line at http://www.nws.noaa.gov/directives/sym/pd01005011curr.pdf]

 

Hudgins, J. E. et al., 2005: Climatology of Heavy Rainfall Associated With Tropical Cyclones Affecting the Central Appalachians, NOAA Tech Memo, ER-98, 86 pp. [Available from NWS Eastern Region Headquarters, Scientific Services Division, Bohemia, NY]