Severe Weather Climatology for Eastern Kentucky

Karen K. Oudeman & Tony Edwards
National Weather Service Jackson, KY

Introduction:

Just as it is important to the forecaster to have a working knowledge of temperature and precipitation climatology and typical synoptic patterns, the warning forecaster can also gain additional insight from severe weather climatology for their County Warning Area (CWA). This study focuses on the temporal and spatial distribution of these severe weather reports, in combination and separately, for the area encompassed by the National Weather Service (NWS) Jackson, KY current area of responsibility.

Data:

Severe weather reports were collected from the National Climatic Data Center’s (NCDC) online Storm Events database, which is a collection from the StormData publication (June and July 1993 missing), and from the Storm Prediction Center (SPC). Tornado data dates back to 1950, while thunderstorm winds and hail data start in 1955. The three sets of data are current through 12/31/2010.

Severe weather is defined as hail with a diameter of one inch or larger, thunderstorm winds of 50 knots (58 mph) or greater, and any tornado. All times are Local Standard Time.

Counties covered by severe weather study.

The region covered in this study is the portion of eastern Kentucky currently served by NWS Jackson (JKL), which encompasses 33 counties (Figure 1). The total population for all 33 counties is 738,792, which is 17% of Kentucky’s total population.

JKL History:

Prior to 1981, forecast responsibility was held by NWS Louisville. In 1981, forecast and warning responsibility for 17 counties was transferred to the new Jackson office. In 1996, the Jackson office acquired the remaining 16 counties. Since the mid-90s, the National Weather Service has placed more emphasis on public relations, weather spotter training and verification of severe weather warnings. This, along with an overall increase in the public’s weather knowledge, has all led to an increased number of reports over the years.

General Overview:

There have been a total of 3,643 reports during this period of record (Figure 2). Tornadoes have accounted for 3.8%, severe hail reports for 14.4%, and severe wind reports for the majority of the reports, at 81.8%.

Combined Severe Reports by Year (Figure 2)
Combined Severe Reports by Month (Figure 3)

The peak severe weather month is June with 20% of all of the events (Figure 3). The late spring and summer months (April through August) account for 80% of the events and is Jackson’s peak season. Relative minima occur in the fall and winter seasons.

Combined Severe Reports by Hour (Figure 4)

The majority of severe weather occurs in the mid afternoon to early evening (Figure 4). Severe weather events between the hours of 1 PM and 9 PM account for 75% of all events with the peak time between 3 PM and 6 PM (35%). The time between 3 AM and 6 AM has the least amount of severe weather (3%).

Tornadoes:

Tornado Reports by Year (Figure 5)

Tornado Reports by Month (Figure 6)
Tornado Reports by Month MINUS 4/3/1974 Tornadoes (Figure 7)

Tornado Reports by Hour (Figure 8)

There were 139 confirmed tornado occurrences found in this study (Figure 5). While tornado events closely parallel the overall severe event trend of a maximum during the warm season, tornado events quickly ramp up in April and trail off by July (Figure 6). In fact, 68% of tornadoes have occurred in the months of April, May, and June. The largest singular tornado outbreak occurred in the April 3rd, 1974 Super Outbreak, when 18 seperate counties were affected by tornadoes in the 33 counties covered by NWS Jackson, KY. Including all tornado events, the month of April is the peak month for tornado occurrences.  However, removing the events from the 1974 Super Outbreak reveals May to be the most active for tornadoes (Figure 7). Only the month of January had no confirmed tornado occurrences through December 31, 2010. The primary time mode is between 5 PM and 9 PM (42%), a second mode between 12 PM and 3 PM (19%), and a third mode between 4 AM and 6 AM (6%) (Figure 8). A false fourth mode shows up during the 1 O’clock hour (AM), but six of those eight events were spawned on Memorial Day 2004.

Tornado Reports by F Scale (Figure 9)

Figure 9 shows the number of tornado occurrences in relation to their F-scale. Across east Kentucky, F3 and F4 events are relatively rare, with 86% of tornados evaluated at F2 or less. There have been no confirmed F5 tornadoes in east Kentucky during the period of record. F2 and F3 tornadoes have caused the greatest number of deaths (76%) and injuries (86%) in Jackson’s CWA.

On February 1, 2007, the National Weather Service changed the tornado ranking system from the Fujita Scale (F-scale) to the Enhanced Fujita Scale (EF-scale). For the purposes of this study, F-scale and EF-scale classifications are treated interchangeably.

Hail:

Severe Hail Reports by Year (Figure 10)
Severe Hail by Month (Figure 11)

 

Severe Hail by Hour (Figure 12)

This study contained 524 cases of severe hail (Figure 10).  The office can expect to be ready for reports of severe hail especially during the months of April through July, when 77% of these events happened (Figure 11).  Twenty-nine percent of all hail events have occurred in the month of May itself.  Severe hail events are limited in the fall (September through her December) with only 4%.  By far, severe hail is most common from the mid-afternoon through early evening (Figure 12).  Sixty-five percent of hail events have occurred between 1 PM and 7 PM.

Hail Size Distribution (Figure 13)

When looking at size distribution (Figure 13), the vast majority of hailstones are less than one and a half inches in diameter.  Hailstones less than 1.5 inches account for 67% of the total number of severe hail reports.  Very large hail (> 2 inches) is rare in east Kentucky and compromises only 9% of reports.

Damaging Winds:

Damaging Wind Reports by Year (Figure 14)
Damaging Wind Reports by Month (Figure 15)

Damaging Wind Reports by Hour (Figure 16)

Severe wind reports are by far the most common type of severe weather event reported across eastern Kentucky. There were 2,980 severe wind events in this database (Figure 14). Thunderstorm wind events have the longest “season,” with a relative maximum during the months of April through August (79%) (Figure 15). As with other severe events, mid-afternoon through mid-evening is the favored time period (Figure 16). There is even a secondary mode between 5 AM and 7 AM.


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