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, 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.

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.

ALL SEVERE WEATHER EVENTS

Monthly Frequency

• Of 3515 documented events from 1950 to 2005, some form of severe weather (tornados, severe hail, and/or thunderstorm wind damage) has occurred within the WFO Blacksburg CWA in each of the 12 months.
• Severe Weather season for WFO Blacksburg CWA is late Spring and Summer (Fig. 4).
• Nearly 70% of all severe weather events occurred during the months of May, June, and July.
• The peak month for severe weather was May, where over 25% of all events occurred.

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

Hourly Distribution

• The vast majority (84%) of all severe weather events occurred between 12:00 pm and 8:00 pm (Fig. 5).
• A little over half of all severe weather events (54%) transpired between 2:00 pm and 6:00 pm, which coincides with peak surface heating. An additional 16% (567) of all severe weather events occurred during the early evening hours ( 6:00 pm to 8:00 pm) when instabilities are still high.
• A minimum number of events (about 2%) occurred from 1950 to 2005 during typical diurnal cooling from 2:00 am to 8:00 am. These events are mainly due to Mesoscale Convective Systems ( MCS) tracking from the Ohio valley in the afternoon and then into the Mid Atlantic region overnight.

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

TORNADO CLIMATOLOGY

• Only 132 tornados have been reported across the WFO Blacksburg CWA from 1950 to 2005.
• A majority of the tornados (70%) touched down east of the Blue Ridge and mainly along and south of the Virginia and North Carolina state border (Fig. 6).
• West of the Blue Ridge, 40 (30%) tornados have been reported across the mountains of southeast West Virginia, southwest Virginia, and northwest North Carolina.
• On average, two tornados touched down within the WFO Blacksburg CWA per year.

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

• 57% of the tornados occurred during the Spring and Summer months of April through July (Fig. 7).
• There is a secondary maximum of tornado (8 of 18 total for the month) occurrences in September, which is due to land-falling tropical system or their remnants.
• A few tornados have also occurred during the winter months, although none have been reported in December (1950-2005).

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

Hourly Distribution

• The majority of the tornados (71%) occurred during the afternoon and early evening hours between 12:00 pm and 8:00 pm (Fig. 8).

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

Magnitude

• The Fujita Scale (Table 1) is used to rate the intensity of a tornado by examining the damage caused by the tornadic winds.
• A majority (76%) of the 132 tornados reported between 1950 and 2005 (Fig. 9), were considered weak (F0-F1) based on the Fujita Scale (Fujita, 1981).
• The other 24% (32) were classified as strong (F2-F3) tornados.
• More strong tornados occurred in the month of April than any other month.
• About half of all tornados in April (9 of 19) were classified as strong, i.e., F2 and F3 (Fig. 10).
• As evidenced by the historical track data (Fig. 6), the stronger and longer track tornados within the WFO Blacksburg CWA are usually confined to the Piedmont and Foothill regions and along the state border of Virginia and North Carolina.
• The most recent strong tornado (F2) in the WFO Blacksburg CWA was Stewartsville , VA , in Bedford County on 17 September 2004 , and was associated with remnants from Hurricane Ivan.
• The strongest (F3) tornado in the CWA was Rockingham County North Carolina on 20 March 1998 , which produced most if its damage, including two fatalities, in Stoneville , North Carolina .
• Six additional F3 tornados occurred, including Buckingham County (June, 1966), Halifax County (March, 1969), Greenbrier County (April, 1974), Smyth County (April, 1974), Surry County (April, 1978), and Charlotte County (October, 1986).
• The Greenbrier tornado, which was part of the 1974 Palm Sunday Super Tornado Outbreak, was the most significant, with a path length covering 18 miles, resulting in over 2 million dollars in property damage.
• To this date, no violent tornados have been reported across the WFO Blacksburg CWA since 1950.

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

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

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

Associated with Tropical Systems

• During hurricane season, tropical systems or their remnants can track through the Mid-Atlantic region and affect the Blacksburg CWA (Hudgins, et. al 2005).
• 17% (22) of all the tornados reported since 1950 have come from these tropical entities (Fig. 11).
• 20 of 22 (90%) tropical-induced tornados were classified as weak (F0-F1) and touched down in the Piedmont of North Carolina and South Central Virginia.
• 2 of 22 (10%) tropical-induced tornados were of the stronger F2 variety and affected Stewartsville in Bedford County and Fieldale in Henry County on September 17, 2004 (remnants of Hurricane Ivan).
• The most notable tropical tornado outbreak was associated with remnants of Hurricane Ivan ( September 17, 2004). Five tornados tracked across the Foothills and Piedmont of Virginia, as well as Northwest North Carolina on this day.

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

HAIL CLIMATOLOGY

Monthly Frequency

• Of the1456 severe hail events (hail diameter exceeds 0.75 inches) across the WFO Blacksburg CWA since 1950, 87% of the reports occurred from April through July (Fig. 12).
• May is the peak month for the occurrence of severe hail, with 37% of the events reported.
• October through February is the least likely time to expect severe hail with only 13 events since 1950.
• December is the only month with no reports of severe hail.

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

Hourly Distribution

• The data showed that a vast majority of severe hail events (89%) usually occurred between 12:00 pm and 8:00 pm (Fig. 13).
• 59% occurred during the peak surface heating hours of 2:00 pm and 6:00 pm.

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

Magnitude

• Hail of 0.75 to1.00 inch constitutes 78% of all severe hail reports in the WFO Blacksburg CWA (Fig. 14).
• Only 20% of the severe hail measured between one and two inches.
• Very large hail, with a diameter of greater than 2 inches, was rare but did occur 2% of the time between 1950 and 2005.
• The largest reported hail on record in the WFO Blacksburg CWA was softball size (4.0 inches), occurring 2 June 1998 in Boone, NC ( Watauga County).

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

DAMAGING WIND CLIMATOLOGY

Monthly Frequency

• The minimum wind speed for a thunderstorm to be considered severe is 58 mph (50 knots).
• The majority (82%) of the severe convective wind events occurred during the late Spring and Summer months of May through August (Fig. 15).
• A similar distribution curve compared to hail is observed, however, severe wind events maximize a little later in the summer with a peak in July.

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

Hourly Distribution

• The diurnal distribution of convective severe winds were similar to that of severe hail, with the peak corresponding to maximum heating between 12:00 pm and 8:00 pm (Fig. 16).
• 82% of severe wind events occurred during this time, and 50% occurred between the hours of 2:00 pm and 6:00 pm.

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.

• If a storm becomes severe, 55% of the events are from convective winds, 41% are from hail, and less than 4% are tornados, of which, 17% are from tropical systems.

• Tornados that touch down in the WFO Blacksburg CWA are generally east of the Blue Ridge and near the North Carolina and Virginia border.

• On average, two tornados touch down within the WFO Blacksburg CWA per year.

• Tornados developing within the WFO Blacksburg CWA usually occur between the months of April and July. There is a secondary peak of tornado touch downs in September. No tornados have been reported in the month of December.

• The majority (71%) of the tornados occur between 12:00 pm and 8:00 pm with a diurnal peak between 2:00 pm and 6:00 pm (49%).

• Weak tornados account for 76% of all the tornado events. Of the strong tornados (24%), most occur during the month of April. No violent tornados have been reported within the CWA.

• Ninety percent of the tropical cyclone-induced tornados are weak tornados.

• Severe hail occurs most often between the months of April and July (87%). The peak month for severe hail is May. No severe hail was reported in the month of December.

• The majority (89%) of severe hail occurs between 12:00 pm and 8:00 pm with a diurnal peak between 2:00 pm and 6:00 pm (60%).

• Severe hail of 0.75 to 1.00 inch constitutes 78% of all severe hail reports.

• Severe thunderstorm wind events are most likely to occur between the months of May and August (82%), with a seasonal peak in July.

• The majority (82%) of severe thunderstorm damaging winds occur between 12:00 pm and 8:00 pm, with a diurnal peak between 2:00 pm and 6:00 pm (50%).

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]