|Large Hail Climatology for the United States with an Emphasis on Southwestern Kansas
National Weather Service, Dodge City, KS
June 14, 2011
There are 27 counties served by the Dodge City National Weather Service office. This area encompasses Scott City, Syracuse, Elkhart and Liberal in the west, Hays, St. John, Pratt and Medicine Lodge in the east, and Dodge City, Ness City and Ashland in between.
1. Purpose and Methods
A climatology of large hail events for the United States is presented with a special emphasis on southwestern Kansas. Different approaches were taken for southwestern Kansas versus the United States as a whole. In order to create large and giant hail climatologies for southwestern Kansas, the number of occurrences from 1955 to 2010 in which there was at least one report of hail 1.75” (golfball size) or larger and 2.75” (baseball size) or larger were counted and then placed into 1/3 month periods. For example, for January 1955, the number of days with at least one report of large and giant hail were counted for January 1-10, January 11-20 and January 21-31. The number of hail events for each 1/3 month period throughout the year were added over all the years from 1955 to 2010. Hail events occurring 12 hours or more apart were considered as separate events. So the climatology created for southwestern Kansas is really a hail event climatology since each day in which hail occurred is counted as one event regardless of how many or how few hail reports occurred on that day (again, unless the hail reports were separated by 12 or more hours). In some events that occur in populated areas, many reports are often received for the same storm in the same area at nearly the same time. Thus, events that occur in populated areas often dominate the database in terms of the number of hail occurrences. For example, one hailstorm occurring in Dodge City might result in ten hail reports while the same event that occurs in open country may only result in one report. After a severe thunderstorm warning is issued by the national weather service, an attempt is made to verify that warning. If a warning is issued for a rural area, meteorologists call trained storm spotters and other residences in the affected area until a report of 1” or larger hail is received. Occasionally, after this verifying report is received, calling is ceased, even though larger hail may have occurred. In contrast, when storms affect the towns of Garden City, Liberal, Dodge City, Hays, Pratt and so on, many reports of large hail are often received without even the need for phone calls to the affected area. And there is a much greater chance that the very largest sized hail will be documented in the major towns. On days with tornadoes, there is a tendency for less hail reporting for obvious reasons. It is also important to note that the largest sized hail is sometimes not documented since intense hail cores are often small and located between residences. This is mitigated somewhat by the practice employed by the National Weather Service in Dodge City of calling people in the rural directory books in cases where spotters and cooperative observers are too widely separated.
Next, a more general climatology was created for most of the contiguous United States by plotting and counting the number of 1.75” and greater hail reports for the six 2-month periods starting with January-February. Keep in mind that the methodology for the United States climatology is different in that it uses the number of reports instead of the number of events. The hail reports for the United States are plotted for 1955 to 2006 using John Hart’s SeverePlot program.
After the seasonal hail climatologies for southwestern Kansas and the United States as a whole are presented, the basic scientific underpinning behind these results is briefly presented in as simple terms as possible.
Additionally, the number of large hail (1.75” or larger) events were counted for each year from 1955 to 2010. Aside from giving some indication of the interannual variability in the frequency of severe hailstorms, the changes in National Weather Service warning procedures and storm reporting are reflected in these results.
Finally, there is quite a bit of variability in hail climatology across southwestern Kansas from Hays and Pratt in the east to Elkhart and Syracuse in the west. So the 27 counties in the NWS Dodge City area of responsibility were divided into 5 regions and the same technique used for the 27 counties as a whole was utilized for these subdivisions.
2. Southwestern Kansas Hail Climatologies
The large hail and giant hail climatologies are shown in Figure 1a and 1b below. Large hail is very rare in southwestern Kansas from late-October through February. There have been no giant hail events (2.75” and larger) from late-September through early-March. The only large hail (1.75” or larger) events during this time of year occurred on February 24, 2000, November 15, 1996 and October 21, 2008. During the cool season, large hail tends to occur in the southern plains, Deep South and lower Midwest where low-level moisture from the Gulf of Mexico is occasionally available. Large hail is still rare in early to mid-March across southwestern Kansas, with only 4 documented events from March 1 to March 20 over the 56 year period from 1955 to 2010. This number increases to 10 by late-March and 15 by mid-April. The peak months for large hail events across southwestern Kansas are May and June. Severe weather in southwestern Kansas typically begins in earnest in May. The number of golf ball or larger hail events shoots up to 37 by early-May, 41 by mid-May and 60 by late-May. The number of large and giant hail events seems to plateau from late-May into early-June, with the number of events dropping slightly from 60 to 57. However, notice the spike in large and giant hail events in mid-June. Mid-June is the most favorable time of the year for large hail and giant hail events for the NWS Dodge City area of responsibility as there have been 87 events with at least 1 report of golf ball or larger sized hail since 1955 and 28 events with giant hail. Most giant hail events (81%) occur from early-May through late-July. Mid-June is by far the most favored time of year for giant hail, with 20% of events occurring between June 11 and June 20. June can have multiple active days in a row. The same is generally not true of April and early-May when severe weather tends to progress across the region followed by a cold front that sweeps the moisture away. Although not typically as stormy as mid-June, late-June is still fairly active, with 46 events with at least 1 report of golf ball or larger sized hail. Interestingly, there are more days with golf ball or larger hail events in late-June (46) than early-May (37) or mid-May (41). Even in the “dog days” of summer, severe storms are no stranger to southwestern Kansas. In July and August, there have been 85 and 53 events respectively with at least one report of golf ball or larger sized hail. It may come as a surprise to some that large hail is more common in August (53 events) than in March (14 events) or April (40 events).
3. Hail Climatology for the United States
The locations of all of the large hail (again, defined in this study as 1.75” or larger) reports across the country for January-February, 1955 to 2006 are plotted below in Figure 2a. There has only been one documented golf ball or larger hail event during January and February in southwestern Kansas from 1955 to 2006 and this occurred on February 24, 2000 in Comanche county. Data for 2006-2010 from the NCDC database indicates no additional reports. Nationwide, there were 1180 reports of 1.75” or greater diameter hail from 1955 to 2006. The highest incidence of large hail during these winter months is from eastern Texas into Arkansas, northern Louisiana and parts of Mississippi. Only 2.1% of the annual reports have occurred in January and February. Large hail is uncommon across the northern United States during the winter since warm and moist air typically do not make it that far north.
The locations of all of the golf ball or larger hail reports across the country for March-April, 1955 to 2006 are plotted below in Figure 2b. The highest incidence of large hail in March and April is across central and eastern Kansas and southward into Oklahoma and Texas and eastward into the Deep South and Midwest. Nationwide there were 13,202 reports of 1.75” or greater diameter hail from 1955 to 2006, and this represents 23.3% of the annual reports.
For Kansas, the highest frequency of hailstorms in March and April is across the eastern half of the state.
The locations of all of the golf ball or larger hail reports across the country for May and June, 1955 to 2006 are plotted below in Figure 2c. Nationwide, there were 27,504 reports of 1.75” or greater diameter hail from 1955 to 2006, and this represents 48.6% of all the annual reports. The 2-month period from May to June is by far the most active period of the year in general. The highest concentration of large hail during this time is from central and northern Texas northward into Nebraska, and northeastward into southern Minnesota. There is also a relatively high concentration of reports in far eastern New Mexico and from northeastern Colorado into southeastern Wyoming.
The locations of all of the golf ball or larger hail reports across the country for July and August, 1955 to 2006 are plotted below in Figure 2d. Nationwide, there were 11,485 reports of 1.75” or greater diameter hail from 1955 to 2006, and this represents 20.2% of all the annual reports. The highest incidence of large hail is from Nebraska and adjacent areas of northeastern Colorado northward and northeastward into the Dakotas, Minnesota and Iowa. During summer the jet stream migrates northward into the northern tier of states. However, large hail can occasionally occur in the southern states in summer despite the weak high-level winds. These occur with intense but short-lived “pulse-type” storms.
The locations of all of the golf ball or larger hail reports across the country for September and October, 1955 to 2006 are plotted below in Figure 2e. The frequency of large hail tends to drop off even more by September and October. However, there is still the risk of large hail from the northern Plains and upper Midwest southward into the southern Plains. Nationwide, there were 3225 reports of 1.75” or greater diameter hail from 1955 to 2006, and this represents 5.7% of all the annual reports.
The locations of all of the golf ball or larger hail reports across the country for November and December, 1955 to 2006 are plotted below In Figure 2f. The frequency of large hail drops off even more by November and December. Due to strong cold frontal passages, the time interval between hail events tends to be much larger than during the late-spring and summer. Large hail events are typically absent from southwestern Kansas during this period. Nationwide, there were 821 reports of 1.75” or greater diameter hail from 1955 to 2006, and this represents 1.4% of all the annual reports. The spatial distribution of hail events in November-December is similar to January-February (Figure 2a).
4. Science Behind Hail Climatology
So why are May and June the most active months for large hail in southwestern Kansas and for the nation as a whole? During these months, the sun is getting much higher in the sky so that the air near the ground becomes much warmer. However, the air in the upper troposphere (where airplanes fly) is still fairly cold since this part of the troposphere warms up last (there is a lag time). As a result, there is a greater difference in temperature from the surface of the earth up to the jet stream level. This difference in temperature is called the “lapse rate”. When lapse rates are large, the chances for severe weather increase provided there is sufficient moisture from the Gulf of Mexico at low levels (surface to 4,000 feet above ground level). Deep, rich moisture return from the Gulf of Mexico tends to occur at about the same time that the lapse rates are becoming steep. Finally, although the jet stream weakens some by late spring, it is typically still strong enough to promote severe storm development. So high lapse rates, rich low-level moisture, and the jet stream overlap to the greatest extent in May and June. In the absence of the passage of a strong upper-level storm system and associated cold front, these ingredients can be present over the same areas for several days in a row, especially by late-May and June. The same is generally not true of April and early-May when severe weather tends to progress across the region on a particular day, followed by a cold front that sweeps the moisture away for an extended period. Also, by May and especially June, there is a larger variety of jet stream patterns that can result in severe weather. Unlike March and April when strong upper-level disturbances are typically needed for large hail, large upper-level troughs are typically not needed for severe weather in June since small-scale disturbances can lead to localized areas of severe storms even when the large scale pattern is not ideal.
When high-level winds weaken in mid-summer across Kansas, thunderstorm updrafts tend to develop, produce a burst of heavy rain and small hail, and then collapse on themselves. Storms that behave this way are often referred to as “pulse-type”. Pulse-type storms are typically short-lived and can produce smaller hail and damaging winds. But giant hail typically falls from longer-lived, supercell thunderstorms in the presence of stronger vertical wind shear. Deep into summer, warm temperatures and moisture are often present at lower levels, but the jet stream has typically retreated into the northern United States and Canada by this time. Also, lapse rates tend to decrease in mid-summer due to the delayed warming of the upper troposphere. In addition, low-level moisture often “mixes out” through a deep layer with strong daytime heating on the high plains of western Kansas in mid-summer. However, large and giant hail events can still occur in mid-summer if the high-level winds increase to above climatological levels and if low-level moisture can “pool” along and immediately north of a surface front or outflow boundary. Outflow boundaries can develop when nocturnal and morning clusters of thunderstorms generate a pocket of cooler air, leading to small-scale temperature gradients that can persist into the afternoon. Then, in the afternoon, new thunderstorms can develop along the edge of these small scale outflow boundaries.
During autumn, because of the lower sun angle, the lower levels of the troposphere (near the earth’s surface) begin to cool off while temperatures at high levels cool off more slowly (there is a lag). Therefore, the temperature typically does not cool off as rapidly with height above the earth’s surface (lower lapse rates). Therefore, hailstorms can occur in the fall, but the frequency is much less compared to the spring.
In the winter, the jet stream is strong but the other ingredients are usually lacking, especially across southwestern Kansas. However, in the Deep South, the close proximity to the warm Gulf of Mexico allows for the occasional return of rich moisture ahead of intense upper-level disturbances. This can lead to large hail events and even tornadoes during some winter seasons in the Deep South.
5. Large Hail by Year
Figure 3 below shows the number of events in a given year in which there was at least one report of golf ball or larger sized hail in NWS Dodge City’s county warning area.
This graph shows clearly that there are many more days after 1993 in which golf ball size or larger hail was reported. This is likely due to National Weather Service modernization and the inception of the WSR-88d Doppler radar. Additionally, the increase in the number of storm spotters, the increased use of rural directory phone books during warning verification and heightened severe weather awareness likely all contributed to the increased number of hail reports. In the modern weather service there is a much greater effort made to seek out ground truth reports. In fact, 51% of the large hail events in southwestern Kansas have occurred in the 16 years from 1995 to 2010. The other 49% occurred over the 40 year period from 1955 to 1994. Also, 74% of the large hail events occurred in the 29 year period from 1981 to 2010, but only 26% in the 26 year period from 1955 to 1980.
Despite the differences in reporting practices, some years clearly stand out from the others, with some being more active and some less active. For example, 1958 and 1959 were active compared to the years before and after. 1967 and 1968 were active years as well. It would be very interesting to see how these years would compare to recent years using modern verification methods and technology. 1960, 1972, 1976, 1978 and 1988 and 1990 were inactive years in terms of large hail, while 1977 and 1986 were very active. Further research would probably show that the storm track was north of our area during the spring and summer of 1990 since this was an active severe weather year from northeast Colorado into Nebraska. 2000, 2005 and 2006 were less active than adjacent years. The most active years were 1996, 1998, 2001, 2004, 2007 and 2008 as there were 23 to 25 events with at least 1 report of golf ball or larger sized hail across the Dodge City county warning area.
6. Localized climatologies
In Section 2, hail climatologies for the 27 central and southwestern Kansas counties served by NWS Dodge City were shown. However, these climatologies do not show the variability that exists inside of this 27 county area. Therefore, as mentioned in the introduction, the Dodge City CWA (county warning area) was divided into 5 regions and the same methodology used in section 2 was employed to investigate the smaller scale hail climatologies. Region 1 covers far western Kansas including Morton, Stevens, Grant, Kearny, Stanton and Hamilton counties. Region 2 covers the counties adjacent to the east of Region 1 including Scott, Lane, Finney, Gray, Haskell and Seward counties. Region 3 includes Trego, Ellis, Ness, Rush and Pawnee counties. Region 4 includes Edwards, Ford, Hodgeman, Clark and Meade counties. Finally, Region 5 includes Stafford, Pratt, Kiowa, Comanche and Barber counties. These regions are shown below.
6.1 Region 1
Across far western Kansas including the towns of Syracuse, Johnson, Elkhart, Lakin, Ulysses and Hugoton, there have been no large hail events from early-October through mid-March. There is a distinct maximum in 1.75” or larger hail events in mid-June (June 11-20). Interestingly, almost 3 times as many of these hailstorms have occurred in mid-June than early-June and twice as many have occurred in mid-June than late-June. There is actually a relative minimum in early-June as the number drops off to 11.
6.2 Region 2
For Region 2 including the towns of Scott City, Dighton, Garden City, Sublette, Cimarron and Liberal, large hail is rare from early-November through mid-March. There is a general maximum in golf ball or larger hail events from early-May through early-July, with a peak concentration from late-May through mid-June. Mid-June has the highest number of events at 24, with late-May a close second at 22. There is a small drop-off in early-June. Hailstorms can occur in the summer as well, but at a much lower frequency. There is a very small autumn peak in hail occurrence during mid to late-October.
6.3 Region 3
For Region 3 including the towns of Wakeeney, Hays, Ness City, La Crosse and Larned, there is a general maximum in large hail events from early-May to late-June. However, like Region 2, there is a dual peak in late-May and mid-June, with a relative minimum in early-June.
This relative minimum in early-June is much more pronounced in Region 3. There is a mid-summer minimum in hailstorms in early-August followed by a relative maximum in hail events in mid to late-August. Like Region 2, there is a very small autumn peak in hailstorms in mid-October. But in general, large hail is very rare from late-September through mid-April.
6.4 Region 4
Like Regions 2 and 3, there is a pronounced peak in large hail events from late-May into mid-June, with a relative minimum in early-June. Mid-June has the greatest number with 21.
6.5 Region 5
The large hail season begins a little earlier and ends earlier in Region 5 which includes St. John, Greensburg, Pratt, Coldwater and Medicine Lodge. Like Regions 2-4, there is a peak in hail events from late-May through mid-June. After mid-June, there is a sharp decline in the frequency of hailstorms.
7. Final Remarks
The hail climatologies for southwestern Kansas will be updated each year. It will be interesting to see if the above findings hold for years to come.