**A Severe Weather Climatology for the WFO Raleigh, NC County Warning
Area**

**Brandon Locklear**

**National Weather Service Raleigh, NC**

**Introduction:**

The National Weather Service\'s (NWS) primary responsibility is the
"protection of life and property". NWS Weather Forecast Offices (WFO)
are tasked with issuing severe weather watches and warnings for their
areas of responsibility or County Warning Area (CWA). For Central North
Carolina, the combination of abundant low-level moisture from both the
Atlantic Ocean and Gulf of Mexico along with frontal boundaries that
interact with this moisture often set the stage for strong to severe
thunderstorm development. As a result, WFO Raleigh's CWA experiences a
wide variety of weather phenomena, including severe thunderstorms that
produce tornadoes, large hail, and damaging wind gusts. A local severe
weather climatology is essential for determining severe weather risk.
This study provides a severe weather climatology for Central North
Carolina. Frequency and magnitude of severe weather events along with
seasonal, time of day, topographical and demographic influences are
highlighted.

**Data and Methodology**

Data used for this study includes tornado, hail, and thunderstorm wind
gust reports for each county in the Raleigh CWA, between January 1, 1950
and December 31, 2002. NOAA's NWS Storm Prediction Center (SPC) in
Norman, OK and NOAA's National Data Center (NCDC) in Asheville, NC
provide online access to documented severe weather events. All times are
referenced to Eastern Standard Time (EST).

As defined by the NWS, a severe local storm is one that is sufficiently
intense to threaten life and/or property, including thunderstorms with
large hail, damaging wind, or tornadoes. More specifically, severe
thunderstorms (National Weather Service 1995) are further defined 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

**County Warning Area Topography and Demographics**

**Topography**

WFO Raleigh's CWA (Figure 1) is comprised of 31 counties including 10
counties in the northern piedmont, 5 counties in the central piedmont, 4
counties in the southern piedmont, 4 counties in the northern coastal
plain, 3 counties in the central coastal plain and 5 counties in the
southern coastal plain. Raleigh's CWA covers 16,459 square miles and
includes the metropolitan areas of Raleigh/Durham and Chapel Hill
(Triangle), Greensboro/Winston-Salem and High Point (Piedmont Triad),
Rocky Mount/Wilson and Fayetteville/Fort Bragg.

![](media/image1.png){width="5.115277777777778in"
height="3.7006944444444443in"}

Figure 1: Raleigh\'s CWA.

Topography is a contributing factor in the initial development of
convective storms. The two principal topographic regions that encompass
Raleigh's CWA are the piedmont and coastal plain regions. The Piedmont
is characterized by rolling hills and soils that are mainly loam and
clay-loam. A notable increase in elevation occurs west of the "fall
line" which runs roughly along Interstate I-95. The "fall line" is the
dividing line between the piedmont and coastal plain.

The elevations generally range from 250 feet along the "fall line" to
elevations

of approximately 1100 feet in the extreme western part of the CWA .
Conversely the coastal plain is characterized by flat land with a
variety of soil types that are soft sediment in nature. The elevation
generally ranges from 50 feet over the extreme eastern part of the CWA
to approximately 250 feet over the western boundary or near the "fall
line". It is here along the "fall line", where the variety of soil types
of the coastal plain, are bounded by the piedmont clay-loam soil of the
piedmont. Within the coastal plain is a smaller topographic area called
the "Sandhills", where sand is the primary soil type. On sunny days, as
the sun heats the earth's surface, the airmass over the Sandhills, heats
up more quickly than the air over the surrounding area. It is the
differences in these soil types that create differential heating of the
surface. While this phenomena alone, does not determine the severity of
thunderstorms, it does have a significant impact in defining the areas
that are more or less likely to experience convective storms.

![](media/image2.png){width="6.0in" height="4.55625in"}

Figure 2: Topography map of Raleigh's CWA. Elevation: thousands of feet
(kft)

**Demographics**

One statistical bias that appears in the data and should be noted is
population density. Population statistics were obtained from the 2000
Census.

The population of the WFO Raleigh's CWA is roughly 3.7 million people
(Census 2000). WFO Raleigh's CWA is home to 7 of the 10 largest
municipalities in the state. Although the CWA contains these large
population centers, the CWA has a low population density. Outside of
these population centers, the CWA is mainly rural farmland or heavily
forested, and contains sparse population. This uneven distribution of
people across the CWA (Figure 3) can lead to skewing of observed severe
weather events toward the more heavily populated locations (e.g., Wake
County).

![](media/image3.png){width="5.995833333333334in"
height="3.2006944444444443in"}

Figure 3: Comparison of county population to number of severe events
reported from 1950-2002.

**Severe Weather Climatology**

**Tornado Climatology**

**Monthly Distribution**

The monthly distribution of tornadoes (Fig. 4) shows the Raleigh CWA can
experience tornadoes any time of the year. However, tornadoes are most
likely to occur during the spring, with the peak frequency in May. A
total of 231 tornadoes occurred in the Raleigh CWA, of which, 51
tornadoes (22% of the total) were in May. Spring is the peak tornado
season because the Raleigh CWA is still vulnerable to strong synoptic
scale systems with jet stream influence. The occurrences of tornadoes
decrease dramatically during the summer months (June-August), as the jet
stream migrates north. There is a pronounced, secondary peak of
tornadoes in the fall. The secondary peak tornado season is explained by
land-falling tropical systems and the southward extension of the jet
stream to the southeast United States.

![](media/image4.png){width="5.635416666666667in" height="3.3125in"}

Figure 4: The monthly distribution of tornadoes (1950-2002) for WFO
Raleigh CWA.

**Hourly Distribution**

Diurnal trends indicate an increase in tornadoes after the noon hour
(Fig. 5).

Over half of all tornadoes (56%) occur between the afternoon hours of 3
PM to 8 PM (EST). Tornado activity peaks in the late afternoon between 5
and 7 PM. Sixty-two tornadoes (27% of the total) occurred during the 5
PM to 7 PM time frame. The data shows a gradual decrease in the
occurrence of tornadoes during the evening hours, and that tornadoes
occur infrequently during the late night through early-to mid morning
hours. Atmospheric instability is a key ingredient in the generation of
tornadic storms and is usually maximized during the mid- to
late-afternoon hours. Of interest, during the second peak season
(September-November), the tornado events are more evenly distributed
throughout the day.

![](media/image5.png){width="5.9944444444444445in"
height="3.6444444444444444in"}

Figure 5: The hourly distribution of tornadoes (1950-2002) for WFO
Raleigh CWA.

**Intensity (Fujita Scale)**

Tornado intensity was rated using the Fujita Scale (Table 1), which is
based on the extent of the wind damage. Of the 231 tornadoes that were
reported to have occurred in Raleigh's CWA, nearly three-quarters (165
or 71% of the total) were classified as weak F0 or F1 tornadoes (Fig.
6). Sixty-one tornadoes or 26% were rated strong (F2 or F3) and only 5
(2%) were rated as violent F4 tornadoes.

Of the five F4 tornadoes, three of the F4 tornadoes occurred in the
March 28, 1984 Carolina Tornado Outbreak. There were no documented F5
tornadoes.

![](media/image6.png){width="6.250694444444444in"
height="2.904861111111111in"}

Table 1: Fujita Scale.

![](media/image7.png){width="5.354166666666667in"
height="3.4895833333333335in"}

Figure 6: The tornado distribution by Fujita Scale (1950-2002) for WFO
Raleigh CWA.

**Hail Climatology**

**Monthly Distribution**

The monthly distribution of severe hail (3/4 inch diameter or greater)
indicates a strong inclination toward the spring season (Fig. 7). There
is a sharp increase in the number of hail reports from March (131) to
the peak month of May (325). The largest majority (78%) 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 (26 events in Sept. compared to 9 events
in Oct.). The four month period between November and February is very
inactive (14 events). The peak occurrence of hail in spring is largely
due to the combination of relatively warm near surface temperatures with
freezing temperatures in the mid levels of the atmosphere.

![](media/image8.png){width="5.635416666666667in" height="3.3125in"}

Figure 7: The monthly hail distribution (1950-2002) for WFO Raleigh CWA.

**Hourly Distribution**

Similar to the hourly tornado distribution, there is a dramatic increase
in severe

hail after the noon hour (Fig. 8). 783 severe hail events of the 1211
total occurrences (65%) were during the hours of 3 PM to 8 PM. A steady
decline of severe hail occurrences was indicated during the late
afternoon and evening hours. Severe hail was rare overnight and during
the morning. The peak occurrence of hail frequency during the
early-to-mid afternoons can be attributed to several factors. The
natural evolution of thunderstorms in which updrafts of the storm are
strongest is during the formative or initial stage. Strong updrafts are
more favorable for hail formation. Secondly, as was the case for
tornadoes, is atmospheric instability. Atmospheric instability is
maximized during the afternoon.

![](media/image9.png){width="5.997916666666667in"
height="3.892361111111111in"}

Figure 8: The hourly hail distribution (1950-2002) for WFO Raleigh CWA.

**Magnitude (Hail Size)**

Nearly half of severe hail reported (595 events or 49% of the total) in
the Raleigh CWA was less than one-inch diameter (Fig. 9). Occurrences of
hailstones ranging from one to two inches accounted for 47% of the
reports. Severe hail of over 2 inch diameter accounted for only a small
percentage (\~4%). The largest hailstone measured in the Raleigh CWA
during the period was 4.5-inch diameter. The softball size hail occurred
in Montgomery County on May 27, 1998

at 625 pm EST with a non-tornadic supercell thunderstorm.

.

![](media/image10.png){width="5.634722222222222in"
height="3.3118055555555554in"}

Figure 9: The size distribution of hail events (1950-2002) for WFO
Raleigh CWA.

**Severe Thunderstorm Damaging Wind Climatology**

**Monthly Distribution**

Strong, damaging winds resulting from severe thunderstorms, fast moving
squall lines or bow echoes are the most frequent severe weather event
across the Raleigh CWA. Over the 52-year period between 1950 and 2002,
there were 2119 severe thunderstorm wind events (60% of all severe
events). Severe thunderstorm damaging wind events from convective storms
show a steady increase during the spring and peak in June (Fig. 10).
Nearly one quarter of the severe thunderstorm damaging wind events (23%
or 488 events of the total) occurred in June. During the late spring and
early summer months of May, June and July, 1268 events (60%) occurred.

![](media/image11.png){width="5.99375in" height="3.3819444444444446in"}

Figure 10: The monthly distribution of severe thunderstorm damaging wind
events (1950-2002) for WFO Raleigh CWA.

**Hourly Distribution**

Thunderstorm wind damage is most common during the mid-afternoon through
evening hours (Fig. 11). Although thunderstorm wind events peak between
4 PM and 7 PM, they do not decline significantly until after 9 PM, when
the impact of daytime heating on atmospheric instability is lost. The
majority (75 %, 1597) of all thunderstorm wind events occur during the 2
PM to 9 PM time frame. Severe thunderstorm damaging wind events drop off
significantly between 3 AM and 8 AM. However, thunderstorm wind events
may occur during any hour of the day.

![](media/image12.png){width="6.0in" height="4.09375in"}

Figure 11: The hourly distribution of severe thunderstorm damaging wind
events (1950-2002) for WFO Raleigh CWA.

**Conclusion**

The severe weather climatology for WFO Raleigh CWA provides a historical
database that is a resource for both forecasters and weather users.
Knowing the frequency of severe weather events relative to the
topographic, demographics, time of year and time of day considerations
heightens an awareness of the relative risk of severe weather events for
the general public and is a key consideration for meteorologists making
severe weather warning decisions. A summary of some of the study's more
notable findings include the following:

-   While tornadoes can and do develop at any time of the year, the
    majority of tornadoes (60%) occur between March and June, peaking
    during the month of May.

-   A secondary peak for tornadoes occur in the fall associated with the
    inland effects from tropical cyclones and from the influence of
    synoptic scale storm systems associated with seasonal migratory
    upper level jet stream.

-   Over half (56%) of the tornadoes occur between the afternoon hours
    of 3 PM to 8 PM (EST) with the activity peaking between 5 and 7 PM.

-   Nearly 75% of all tornado events were classified as weak tornadoes
    (F0 or F1) and only 5 tornadoes (2%) of F4 intensity have been
    reported. There have been no reports of tornadoes rated as F5
    intensity.

-   78% of severe hail events occurred between April and July with 65%
    of severe hail events observed during the hours of 3 PM to 8 PM.

-   Almost half (49%) of all severe hail reports are less than one-inch
    diameter (quarter size).

-   Severe thunderstorm damaging winds are the most frequent severe
    event (60%) across the Raleigh CWA.

-   Over half (60%) of all thunderstorm damaging wind events occur in
    the late spring and early summer months of May, June and July,
    peaking in June.

-   A large majority (75 %) all thunderstorm damaging wind events occur
    during the 2 PM to 9 PM time frame, peaking between 4 and 7 PM.

-   Counties with a higher population density are more likely to report
    more events. Low population density counties are more likely to have
    events that are not witnessed firsthand and therefore go unreported.

**Acknowledgements**

The author would like to thank Jeff Orrock, Warning Coordinator
Meteorologist, and Scott Sharp, Lead Forecaster, at WFO Raleigh for
assistance in validating data. The author would particularly like to
thank Kermit Keeter, Science Operations Officer, at WFO Raleigh for his
careful review of the paper and offering many helpful suggestions for
improving this paper.

**REFERENCES**

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

National Weather Service, 2002: *National Weather Service Instruction
10-511.*

National Data Climatic Center. Storm Events. Online. Internet, February
2004.

North Carolina State Climate Office. Carolina Sandhills. Online.
Internet, February 2004

United States Census Bureau. 2000 Census Statistics. Online. Internet,
February 2004.