Severe Weather Climatology for the Charleston, WV WFO County Warning Area

A SEVERE WEATHER CLIMATOLOGY FOR THE CHARLESTON, WV
WFO COUNTY WARNING AREA

Hunter Coleman

National Weather Service Office
Charleston, West Virginia

1. INTRODUCTION

The National Weather Service's (NWS) primary responsibility is to provide the public with information on impending severe weather that threatens life and property. A forecaster can be better prepared to anticipate and react to the timing, strength and nature of severe weather, with a general understanding of a local severe weather. This is the motivation for conducting this local study.

The NWS Weather Forecast Office (WFO) in Charleston, WV has warning and forecast responsibility for 49 counties (Figure 1). This combination of counties, referred to as the County Warning Area (CWA), includes four counties in northeast Kentucky, two counties in southwest Virginia, nine counties in southeast Ohio and thirty four counties in West Virginia.

The Charleston, WV CWA is comprised of 21,963 square miles. The terrain across the CWA is quite variable. The higher mountain peaks of the Appalachian Mountains, with some elevated inhabited valleys, define the eastern third of the CWA. The highest point within the CWA, Snowshoe Mountain (4765 ft), lies within Pocahontas County. Across the middle third of the CWA, west of the Appalachians, the terrain transitions into very steep, densely wooded hilltops and carved river valleys. A slope of 400-500 ft from any hilltop to river is not uncommon. Finally, the western third of the CWA is made of rolling hills and is more agricultural in land use, with the main stem Ohio River flowing through it. The lowest point in the CWA is along the Ohio River in Greenup County, KY (495 ft) near South Shore.

2. DATA

The National Climatic Data Center (NCDC) in Asheville, NC provides online access to severe weather data all across the country. Weather offices send local storm reports from a severe weather event to the Storm Prediction Center (SPC), located in Norman, Oklahoma. The storm reports are verified and then archived at NCDC. Storm data archived at NCDC includes Tornado, hail and thunderstorm wind gust reports from 1950 through the present.

Data used for this study includes any tornado, hail, and thunderstorm wind gust reports for each county in the Charleston, WV CWA, between January 1, 1950 and December 31, 2001. Population statistics were obtained from the 2000 Census. The times of severe weather events are all in Eastern Standard Time.

The NWS defines a severe thunderstorm (National Weather Service 1995) as a storm that meets one or more of the following criteria:

• a tornado
• hail three-quarters of an inch in diameter or larger
• wind of at least 50 knots (58 mph) or wind which causes damage, including trees or power lines blown down

One statistical bias that appears in the data and should be noted is population density. Population density likely has an impact on the number of events reported in a county. Counties with a higher population density are more likely to report more events

3. CLIMATOLOGY

A) All Severe Events

1) Monthly Frequency

The monthly distribution of severe weather events across the Charleston, WV CWA represents a "bell-shaped" curve (Figure 3). There have been a total of 2553 severe weather events during the period between January 1, 1950 and December 31, 2000. The peak month for severe weather is June, where over 25% (668) of all severe weather events occurred. 62% (1580) of all severe weather events occur during the typical summer months of May, June and July. The number of events dramatically drops off after August with the transition to fall. There have been only five severe weather events in the month of December.

2) Hourly Frequency

The majority of severe weather events occur during the mid afternoon hours into late evening, which coincides well with the peak heating of the day and highest instabilities. Nearly 75% (1883) of all severe weather events occur between 2 PM and 9 PM (Figure 4). The peak time of day for all events occurs between 5 PM and 7 PM. A secondary mode appears between midnight and 3 AM, which is likely due to Mesoscale Convective Systems (MCS) that develop upstream in the afternoon and push through during the early morning hours. Severe weather is least likely to occur during the morning hours between 6 AM and 10 AM, where less than 2% (44) of all events occurred.

2) Hourly Frequency

B) Tornadoes

1) Monthly Frequency

The frequency of tornadoes across the Charleston, WV CWA is low compared to other areas of the country. Over the 50-year period of this study, there were only 99 confirmed tornadoes.

Over 75% (76) of tornado events occurred during the five month period between April and August, with the peak month being June (Figure 5). The occurrence of tornadoes decreases rapidly on each side of this five month period. Incidentally, there have not been any tornadoes reported in the month of November.

2) Hourly Frequency

Tornadoes typically occur during the afternoon and evening hours, peaking between 6 PM and 7 PM (Figure 6). 65% (65) of tornadoes occur during the hours between 3 PM and 9 PM. A secondary mode appears between 1 AM and 4 AM accounting for 11% of tornado events. Almost no tornadoes occurred (only 1 in the 50 year period) between 6 AM and 1 PM.

3) Magnitude

According to the Fujita Tornado Intensity Scale, F3- F5 tornadoes (very intense tornadoes with winds greater than 158 mph) are not very common across the Charleston, WV CWA.

Table 1. The Fujita Tornado Intensity Scale

Only 10 tornadoes (10%) of F3 intensity or greater have occurred over the 50 year period between 1950 and 2000. F1 intensity tornadoes account for 75% (75) of all tornadoes within the CWA, while 90% (90) of all tornadoes were F2 intensity or lower (Figure 7).

c. Hail

1) Monthly Frequency

Typically, the spring and summer months are the most common for thunderstorms which produce hail (Figure 8). There is a sharp increase in the number of hail reports from March (22) to the peak month of June (224). 80% of the hail events occur during the months between April and July.

There is a significant decrease in thunderstorms that produce hail during the transition from summer to fall (41 events in Sept. compared to 3 events in Oct.). The three month period between October and December is very inactive (7 events), while there are many more events during the transition from winter to spring (53 events occurred between January and March).

2) Hourly Frequency

The frequency of hail events is highest in the afternoon and evening hours, similar to tornado events, but can occur anytime (Figure 9). Nearly 75% (589) of all events occurred between 2 PM and 8 PM, with the peak occurring between 3 PM and 4 PM. As in the tornado frequency, there is also a secondary mode of hail events during the early morning hours between 2 AM and 3 AM. Hail events are least frequent between 9 AM and 10 AM.

3. MAGNITDUE

The National Weather Service has deemed that hail three-quarters of an inch or greater is the minimum criteria to verify a severe thunderstorm. Therefore, only hail events with this size or greater were included for the study.

Very large hail (2 inches or greater) is extremely rare in the Charleston, WV CWA, where only 34 events (4%) have occurred (Figure 10). Hail less than 1 inch accounted for more than half of the events (52%), while nearly 80% (631) of all hail events contained hail less than 1.50 inches.

d. Thunderstorm Winds

1) Monthly Frequency

Strong, damaging winds resulting from intense thunderstorms, fast moving squall lines or bow echoes are the most frequent event across the Charleston, WV CWA. Over the 50-year period between 1950 and 2000, there were 1622 thunderstorm wind events (64% of all severe events).

There is a sharp increase in thunderstorm wind events from March to April (31 events to 175 events, respectively) and a continual increase in events to the peak month of June (Figure 11). The month of June represents 25% (408) of the annual thunderstorm wind events. The months between April and August contain 88% (1421) of the thunderstorm wind events. An equally sharp decline in events occurs between August and September (272 events to 66 events, respectively). The least active month for thunderstorm wind events is December where only 4 events have been documented.

2) Hourly Frequency

Thunderstorm wind damage is most common during the mid-afternoon through evening hours (Figure 12). Although thunderstorm wind events peak between 6 PM and 7 PM, they do not decline significantly until after 9 PM, when daytime heating is lost. 71 % (1155) of all thunderstorm wind events occur during the 2 PM to 9 PM time frame. Thunderstorm wind events level off to around 25 events during the midnight to 3 AM time period, and then drop off significantly between 3 AM and 8 AM. However, thunderstorm wind events may occur during any hour of the day.

4. SUMMARY AND CONCLUSIONS

• Tornado events are not very common in the CWA with an average of around 2 events per year.
• More than 75% of all tornado events occur between April and July and peak during the month of June.
• The peak hour of tornado events is between 6 pm and 7 pm, while 65% of all events occur during the late afternoon to early evening hours between 3 pm and 9 pm.
• F1 intensity tornadoes account for 75% of all tornadoes in the CWA.
• 90 % of all tornadoes are F2 intensity or lower.
• Only 10 tornadoes during the 50-year period were F4/F5 intensity.
• April through July are the peak months for hail events.
• The peak time of day for hail is between 3 pm and 4 pm.
• Hail less than 1.50 inch in diameter accounts for 80% of all hail events.
• Thunderstorm wind events account for 64% of all severe weather events.
• Severe thunderstorm wind events are most likely to occur during the spring and summer months.
• Thunderstorm wind damage is most common during the afternoon and evening hours, peaking between 6 pm and 7 pm.

The advantage of establishing a local severe weather climatology is to provide a historical database and resource for the staff at WFO Charleston, WV, to use in efforts of improving severe weather warning service. Becoming familiar with the peak times of year and day that severe weather occurs, the staff will have an increased awareness and will be able to make more informed decisions regarding warnings and staffing needs.

5. ACKKNOWLEDGEMENTS

The author would like to thank WFO Charleston, WV Science Operations Officer (SOO), Jeff Hovis, for his efforts in obtaining the data necessary for this study and for insightful input and review.

REFERENCES

Fujita, T.T., 1981: Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci. 38, 1511-34.

National Weather Service, 1995: National Weather Service Operations Manual, Part C, Chapter 40.

National Data Climatic Center. Storm Events. Online. Internet, May 2002.

United States Census Bureau. 2000 Census Statistics. Online. Internet, June 2002.