As a result of National Weather Service (NWS) Modernization and Associated Restructuring, the Midland-Odessa NEXRAD Weather Service Office (hereafter, NWSO Midland) has seen its County Warning Area (CWA) increase to include a large portion of southwest Texas between the Pecos and Rio Grande Rivers, as well as two additional counties to the east (Reagan and Scurry), and one additional county (Eddy) in New Mexico (Figs. 1 and 2). This represents an 80 percent increase in the size of the Midland CWA from the WSO's pre-modernization CWA. Encompassing over 50,000 mi2, the Midland CWA is now the second largest in the NWS Southern Region. A variety of topographic features and climatic regimes are contained within this CWA, which likely ranks as among the most diverse found in the Southern Region.
Providing severe thunderstorm, tornado, and flash flood warnings continues to be the primary task for the NWSO staff. A severe thunderstorm is defined as a storm that produces wind damage or wind gusts of at least 50 kt, tornadoes, or hail with a diameter of at least 3/4 in. A flash flood is defined as a flood caused by heavy or excessive rainfall within a short period of time, generally less than 6 hours.
Forecasters should be able to recognize climatologically favored times and areas for severe weather within their CWA. For this reason, a local study of NWSO Midland's severe weather climatology was initiated at the same time the new CWA was acquired in October, 1995.
2. Preliminary considerations
a) Population distribution
Population distribution varies considerably among the counties within the
CWA (Fig. 2). The United States Census Bureau reported 1990 county populations
ranging from 123,620 persons in
With three national parks and one national forest located in the extreme
southern and western portions of the CWA, the number of people in these areas
can vary substantially throughout the year. For example, the
The new Midland CWA contains especially diverse topography. Mountains in the
western CWA result in dramatic elevation changes with distance.
Another notable topographic feature in the CWA is the Caprock Escarpment
(Fig. 1). The escarpment is marked by an abrupt rise in elevation that
separates the High Plains of the northwestern CWA from the Low Rolling Plains
of the extreme eastern parts of the CWA. It is likely that this escarpment
helps create a favorable environment for severe weather (Doswell 1982). Terrain
is generally flat on the High Plains, but it becomes more varied toward the
canyons near the
Most data for this study were extracted from the database maintained by the
NCEP Storm Prediction Center (SPC) - formerly the
As noted in other severe weather climatology studies, there are many
non-meteorological factors that can skew severe weather data (e.g., Hales and Kelly
1985, Hales 1993). Figures 3 and 4 illustrate how the density of significant
severe weather reports (i.e., hail at least 1.75 inches in diameter and
tornadoes of at least F1 intensity on the Fujita scale) is related to the
population densities seen in Fig. 2. Notice the clustering of reports generally
north of the
4. Severe weather climatology
a) Yearly trends in severe weather reports
An inspection of the annual distribution of severe weather reports (Figs. 5-8) reveals a general upward trend in the number of severe weather reports through the 1980s into the mid 1990s. This trend can be explained partially by the fact that much of the CWA had a significant increase in population during the 1970s and early 1980s, due to a boom in the local oil industry. As population density increased, public severe weather awareness likely improved due to an increase in NWS outreach programs. Also, reports of severe weather have been more aggressively pursued in recent years thanks to the development of a SKYWARN Spotter program and a severe weather warning verification system.
It is also hypothesized that the data were affected by unusually active severe weather seasons in the El Nino years of the early 1990s (Trenberth and Hoar 1996). In fact, 47 percent of all severe hail reports, 33 percent of all damaging wind reports, and 18 percent of all tornado reports were received from 1990 to 1994.
Other noteworthy trends are apparent in the annual tornado data. Although the number of reported tornadoes has dramatically increased in recent years, a close inspection of Fig. 6 reveals that the proportion of strong or violent tornadoes (F2 or greater on the Fujita scale) has decreased since the 1960s. During the first 15 years of the period (1950-1964), the proportion of all reported tornadoes that were strong or violent was 22 percent, but for the entire period, only 12 percent of the documented tornadoes were given at least a strong rating. This is likely another reflection of increasing population and improved tornado awareness and reporting with time; that is, only the more destructive tornadoes were likely observed and reported in earlier years (Ostby 1993).
Ironically, despite a decreasing proportion of strong or violent tornadoes
with time, the only documented violent tornadoes (F4 or F5) during this period
occurred in the later years. The most deadly was an F4 tornado that struck
b) Monthly distributions of severe hail, damaging winds and tornadoes
All severe weather, including tornadoes, is most common during May and June (Fig. 10). With the CWA at a subtropical latitude, the Bermuda High usually develops far enough west to influence the CWA by mid-spring and hold the surface dryline to the lee of the mountains. This maintains deep Gulf moisture through the afternoon and into the evening across most of the CWA. In fact, 66 percent of all tornado events, 66 percent of severe hail events, and 52 percent of damaging wind events occur during May and June (Figs. 11-13).
The most common severe weather event in the Midland CWA is hail, with 1229 events reported during the months of May and June, compared to only 580 damaging wind events during this time.
May is the most active month for severe hail reports (Fig. 12), while June is the most active month for severe wind reports (Fig. 13). This may reflect stronger upper-level dynamics and cooler mid-level temperatures in May, which would aid hail growth. By the end of June, mid-level temperatures increase and the atmosphere becomes dry adiabatic in the lower levels, which sets the stage for more damaging wind events.
It is interesting to note in Fig. 10 that the late summer-early fall "tail" in the severe weather distribution is more pronounced than the late winter-early spring "tail." It is in late summer and early fall that the atmosphere over the CWA loses its strong mid-level capping inversion, homogenous surface features, and weak westerly mid-level flow, and westerly mid-level troughs and associated Pacific cold fronts return to the area, maintaining the ability to generate severe weather.
c) Hourly distributions of severe hail, damaging winds and tornadoes
When all severe weather reports are combined, the hourly distribution (Fig. 14) is remarkably uni-modal. Peak occurrence is centered around 1800 CST, with only a hint of a secondary maximum around sunrise. When the hourly distributions are separated into warm and cool seasons (March through August, and September through February, respectively) some common features are noted. Figures 15-20 all indicate relative maxima in severe weather occurrences near the time of maximum diurnal heating, with most events occurring between 1700 and 2000 CST.
By comparing the vertical scales of the warm and cool season figures, one can see that far more severe weather occurs during the warm season than the cool season. There is a very strong correlation between severe weather occurrence and time of day in the warm season, which leads to the unsurprising conclusion that atmospheric destabilization caused by diurnal heating is a very important ingredient for severe weather in the spring and summer months. Severe weather is most common around 1800 to 1900 CST in the warm season. Cool season data also indicate an afternoon and early evening maximum, but the distribution of severe weather events is somewhat more spread out over the day. This likely reflects more of a dependence on dynamically driven systems for cool season severe weather. For example, the cool season tornado events seen in Fig. 16 support this idea by showing a nearly uniform frequency of tornado occurrence between 1200 and 2200 CST.
5. Severe weather phenomena unique to the
a) Dry microburst wind events
A special type of damaging convective wind found in the High Plains, and consequently part of the CWA, is the dry microburst. A subtropical jet stream usually located near the CWA during the warm season is often able to provide a layer of moisture to the mid or upper levels of the atmosphere. This moist layer aloft, combined with a deep dry adiabatic layer above the surface, sets the stage for high-based thunderstorms. A recent dry microburst study for the CWA using data from 1985 to 1995 (Murdoch 1997) indicates dry microburst storms are most common in the warm season during the late afternoon and early evening hours. This supports the finding in this study that diurnal heating plays a very significant role in warm season severe weather events.
b) Giant hail events
Another notable aspect of the severe weather found in the High Plains of the
c) Enhanced storm severity due to local terrain effects
It is noteworthy that the two previously mentioned violent tornadoes in the Midland CWA occurred in roughly the same geographical area. This may be more that just mere coincidence. Szoke and Augustine (1990) and more recently Bosart, et al. (1996) have documented cases where it is believed that complex terrain features may have helped create more favorable environments for severe storm evolution and maintenance.
Figure 1 shows some of the more prominent terrain features in southwest
Many times in late May or June when synoptic scale ridging begins to develop
over the mountainous terrain of the western
6. Other hazardous weather phenomena
Various topographic features contribute to additional weather hazards within the Midland CWA. The mountainous terrain of the western CWA enhances flash flood potential and allows locally strong channelled winds.
a) Flash flooding
Beyond the mostly flat high plains of the northern CWA, increasingly rocky
and sloped terrain is found south of the
A survey of Storm Data from the 1980s into the mid-1990s reveals that flash
flood fatalities have occurred mostly in the rugged terrain of the southwest
CWA. Recent deaths occurred along the
Within the Midland CWA some of the most rugged terrain around the
Rocky terrain and numerous small tributaries also make flash flooding
possible along the
b) Channelling of winds through the
A frequent hazardous phenomenon in the
Severe weather within the county warning area of NWSO Midland shows several distinct trends. Most severe weather occurs during the months of May and June, with a small secondary maximum in hail occurrence noted in September and October. Severe hail events are most common in May, while severe wind events are most common in June. Severe weather showed a strong tendency to occur during the afternoon and early evening hours of the warm season, while the cool season had a more uniform distribution of severe weather occurrence among most hours of the day.
Annual data showed a marked increase in severe weather reports over the past decade. During these later years, a greater proportion of tornado and hail reports were minimally severe (i.e., F0-F1, and 0.75-1.75 in, respectively). Since severe wind events were not similarly categorized, it is not known if a similar trend exists with severe convective wind gusts.
Although it is acknowledged that terrain plays a significant role in the
general evolution of weather across the CWA, it also appears that certain
geographic areas such as the Caprock, the eastern range of the
Variations in terrain also appear to play a role in concentrating flash flood hazards in the southern and western portions of the CWA, where rockier soils and greater variations in topography exist. Relatively flat terrain in the high plains makes flash flooding a less serious problem in the northern CWA.
By studying severe weather and flash flood climatologies, as well as topographic features, for the Midland CWA, one can become better acquainted with the unique forecast problems found within this diverse area. It is hoped that this severe weather climatology provides a general representation of severe weather trends within the local CWA, so that forecast and warning accuracy can continue to improve.
The authors appreciate the review and assistance of Brian Francis and Loren
Phillips (SOOs at NWSO Midland and NWSFO Lubbock, respectively). Thanks also to
Michael Vescio at the Storm Prediction Center (SPC) in
Bomar, G. W., 1995:
Bosart, L.F., Seimon, A., and Bracken, W.E., 1996: Supercells over complex
terrain: the Great Barrington tornado of 29 May '95. Preprints,
Conf. on Weather Analysis and Forecasting,
Carlson, T. N., S. G. Benjamin, G. S. Forbes, and Y.-F. Li, 1983: Elevated mixed layers in the regional severe storm environment: Conceptual model and case studies. Mon. Wea. Rev., 111, 1453-1473.
Doswell, C. A. III, 1982: The Operational Meteorology of Convective Weather
Volume I: Operational Mesoanalysis. NOAA Tech. Memo. NWS
Hales, John E. Jr. and Donald L. Kelly, 1985: The relationship
between the collection of severe thunderstorm reports and warning verification.
Preprints, 14th Conf. on Severe Local Storms,
Hales, John E. Jr., 1993: Biases in the severe thunderstorm data base: ramifications
and solutions. Preprints, 13th Conf. Wea. Forecasting and Analysis,
Hart, J. A., 1993: SVRPLOT: A new method of accessing and manipulating the
NSSFC severe weather data base. Preprints, 17th Conf. on
Severe Local Storms,
Murdoch, Greg P., 1997: Forecasting Dry Microburst Potential Using the WINDEX. Southern Region Topics. SR/SSD 97-6.
National Oceanic and Atmospheric Administration, 1986: Storm Data. 28, No. 10, National Climatic
National Oceanic and Atmospheric Administration, 1996: Storm Data. 38, No. 1, National Climatic
Ostby, F.P., 1993: The changing nature of tornado climatology. Preprints, 17th Conf. on Severe Local Storms,
Rhea, J.O., 1966: A study of thunderstorm formation along drylines. J. Appl. Meteor., 5 58-63.
Szoke, E.J., and J.A. Augustine, 1990: A decade of tornado occurrence
associated with a mesoscale flow feature: The Denver Cyclone. Preprints, 16th Conf. on Severe Local Storms,
Trenberth, K.E. and Hoar, T.J., 1996: The 1990-1995 El Nino-Southern Oscillation event: Longest on record. Geophysical Research Letters, Vol. 23, No. 1, pp 57-60.
Vescio, M.D., 1995: CLIMO: Software to generate severe weather statistics
for NWS County Warning Areas. NCEP,
Figure 21 Area hypothesized to have terrain features that enhance low level storm inflow.