* Current Affiliation: Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, OK
Corresponding author address: Dr. Charles A. Doswell III, CIMMS, 100 East Boyd St., University of Oklahoma, Sarkey's Energy Center, Room 1110, Norman, OK 73019. E-mail: email@example.com
The case of 7 June 1998 in eastern New Mexico and western Texas is used to illustrate the challenge of recognizing possible negative effects created by a mesoscale region of cloud-covered cool air (that was associated with early thunderstorms) on the tornadic potential of severe convection. Although the tornado potential in the synoptic situation was not highly portentous, supercell storms did eventually form, one of which was persistent for many hours. There were only three relatively brief and weak tornadoes reported from this storm, despite its persistence as a long-lived supercell that produced a long swath of giant hail. In this case, the development of thunderstorms early in the day maintained cloudiness that inhibited the destabilization of the surface-based airmass over which the afternoon thunderstorms moved. The long-lasting supercell formed on the dryline but soon overrode this mesoscale region. It was able to persist as an elevated supercell despite the near-surface airmass stability, but its tornadic potential was apparently reduced by its interaction with this mesoscale feature. Implications for operational forecasting and warnings are discussed.
Possibly one of the greatest challenges facing weather forecasters is the recognition of reduced potential for hazardous weather. Severe thunderstorms are a relatively rare event in any one place, even in the regions where such storms are most common. Thus, by far the majority of forecast days are associated with "non-events" rather than events. However, it is also the case that in parts of the United States, notably the Southern Plains region (including eastern New Mexico, Texas, and Oklahoma), there are many days during the storm season when the possibility of severe thunderstorms with tornadoes is not negligible; see Brooks et al. (2001) for a discussion of the climatology of tornadoes. The challenge to forecasters is to be able to recognize those few days when the potential is going to be realized, versus the majority of days, when it is not. Most of the literature on severe storms and tornadoes necessarily focuses on events, not non-events, even though forecasters mostly have to deal with non-events.
Even though a non-negligible possibility of a tornado is present on many days in the Southern Plains, there is much evidence that mesoscale (and even storm-scale processes) have a major impact on whether or not the tornado potential will come to pass. When considering the role of mesoscale processes related to some severe weather event, most authors have focused on how mesoscale processes contribute to an enhancement of the severe weather potential. There can be no doubt that such enhancements occur (see, e.g., Magor 1959; Doswell 1987; Rockwood and Maddox 1988; Rasmussen et al. 2000), but this is not a complete picture of the role played by mesoscale processes. In fact, within our experience, the most common impact of mesoscale events is to have an inhibiting effect on the chances for severe convection.
Therefore, in the following study, a case example is used to illustrate the apparently negative impacts associated with mesoscale processes on the tornado potential of subsequent deep moist convection. A major dilemma we confront in this work, unfortunately, is that it is not possible to know what would have happened had not the real mesoscale processes been operating. This makes the study of non-events particularly challenging. Since we cannot show conclusive proof that events were indeed prevented on the day of our case, we can only suggest that the observations are at least consistent with the hypothesis that tornado potential was reduced by mesoscale processes. This is not the same as validating the hypothesis, and so studies of non-events are uncommon in the scientific literature; it is only speculation to talk about unrealized potential, even though non-events are by far the most common outcome.
Experienced forecasters know that when convective outflows reach mesoscale proportions, they can act to inhibit further deep, moist convective storms. For this particular case, however, the apparent result of early convection was neither total suppression of convection developing later in the day, nor the inhibition of severe thunderstorm activity, but rather the reduction of the tornado potential. Given that a few brief, weak tornadoes were observed, that tornado potential certainly was not negligible on this day, in part because the storms first developed in a narrow zone of favorable near-surface temperature and moisture.. It is known (Fujita 1960; Agee et al 1975; Forbes 1981) that long-lived supercells are capable of producing a whole family of tornadoes. The strongest tornadoes in such a family are typically not the first produced by the storm. Forecaster experience (Jones et al. 1985, Moller et al. 1994) suggests that if a storm produces at least one tornado, then continuing with tornado warnings until that storm dissipates is a plausible strategy.
Regrettably for forecasters making warning decisions, it is also the case that many supercells produce only a few brief, weak tornadoes, if any at all. The absence of a comprehensive understanding of tornadogenesis precludes making general statements, so this case represents a good example of the challenge to forecasters. Brief, weak tornadoes were observed early in the life cycle of this particular long-lived supercell. Should tornado warnings have been continued for the lifetime of the storm or would it have been possible to recognize when the tornadic phase was over? The example we consider illustrates that an operational diagnosis of atmospheric processes, including those at the mesoscale whenever possible, should not focus exclusively on those aspects favorable to a particular meteorological event. A complete diagnosis should also include due consideration to those mesoscale aspects of a forecast situation that are unfavorable to a particular event's occurrence.
Regardless of our ability to make a convincing argument about what might have happened with this case, we believe it to be useful to consider what might result in a non-event, so that forecasters can have some guidance about what to look for in producing the many non-events they will have do deal with in operations. Therefore, section 2 gives a synoptic-scale overview of the situation on the morning of 7 June 1998 in eastern New Mexico and western Texas, focusing on the tornado potential implied at 1200 UTC. Then, in section 3, a detailed surface analysis is combined with satellite imagery and soundings to describe the characteristics of a mesoscale region of clouds and outflow that developed during the day of 7 June. It is shown that this mesoscale area of clouds and outflow represented a surface-based layer of relatively high static stability. In section 4, a description of convective evolution reveals the apparent influence of that mesoscale region. Finally, some discussion and conclusions are presented in section 5.
A look at the pattern at 1200 UTC shows a strong short wave trough at 500 hPa in the southwestern United States (Fig. 1). Given the upstream 500-hPa winds in excess of 25 m s-1 it was plausible to expect the approaching trough to increase the middle and upper tropospheric winds in eastern New Mexico and western Texas during the afternoon. In the layer from 700 to 500 hPa, a region of high lapse rates (implied by the 700-500 hPa temperature difference) was being advected over the plains in the southwesterly flow ahead of the shortwave trough (Fig. 2). Surface flow at this time was still affected by the recent passage of the surface high pressure, having a large easterly component over Texas, but becoming more southerly in northern Oklahoma and the Texas panhandle (Fig. 3). Surface winds could be expected to increase during the day as pressures decreased with the approach of the trough aloft. Combined with increasing flow aloft, the surface to 500-hPa wind shear could be expected to increase to the point where supercells would bepossible. Although surface dewpoints were not extremely high over the Plains for this late in the spring, the setting seemed primed to begin a rapid moisture return.
Yet another input for the field forecaster was the morning outlook for severe thunderstorms from the National Weather Service's Storm Prediction Center (hereafter, SPC). The outlook issued at 1523 UTC said about eastern New Mexico and far western Texas:
The morning raobs show that rich Gulf moisture ... with boundary layer mixing ratios approaching 15 g kg-1 ... which had been shunted southward into eastern New Mexico during the last 48 h is now rapidly returning northwestward through the Rio Grande valley into far western Texas and eastern New Mexico. Surface dewpoints in the upper 50s to low 60s should extend as far north as Amarillo/Tucumcari by late afternoon.
That outlook's discussion described the synoptic situation in the following terms:
Active pattern over the central and southern Plains this period as upper low and associated speed maximum now over the southern Great Basin shear east-northeastward across the southern/central Rockies. At the surface ... low now over southern Utah should give way to stronger cyclogenesis in the lee of the Colorado Rockies by early this afternoon. The Colorado low should then track east-northeastward along developing warm front into southern Nebraska later tonight/early Monday.
On this basis, the lead SPC forecaster issued an outlook calling for a "slight risk" of severe thunderstorms in the region shown in Fig. 4, which included eastern New Mexico and far western Texas (hereafter, the "threat area"). Moisture, instability, and shear would be increasing during the day, making the threat of supercells and tornadoes more likely.
In the period following that early outlook, the threat area continued under the same "slight risk" threat assessment through several updates by the SPC. By 0000 UTC that evening, the synoptic pattern had evolved more or less as expected. At 500 hPa (Fig. 5), the short wave trough had moved east-northeastward, displacing the ridge over the area at 1200 UTC, and bringing relatively strong (20-25 m s-1) southwesterly winds aloft across the threat area. High mid-tropospheric lapse rates (as shown by the 700-500 hPa temperature difference field in Fig. 6) had not moved very far eastward, however, with the main axis running slightly west of the Continental Divide through New Mexico and into Colorado. At 850 hPa (Fig. 7), a lee cyclone had indeed developed, with a strong southerly current arising over the lower Rio Grande River valley and extending into western Kansas, in response to that cyclogenesis.
Surface patterns in the threat area were rather complex, with surface cyclones in Colorado and New Mexico developing in the lee of the Rocky Mountains, apparently in response to the approaching short wave trough. We will be elaborating on the details of this evolution in section 3, but a dryline had formed in eastern New Mexico, not far west of the Texas border by this time (Fig. 8). Surface winds increased to 10-15 m s-1 from the east-southeast, thereby producing the anticipated surface to 500-hPa vector wind shears in excess of 30 m s-1(corresponding to an estimated value for within that layer on the order of 7.5 x 10-3 s-1), generally considered sufficient for supercells (Weisman and Klemp 1986). While surface dewpoint values in the southern part of the threat area had increased in the 12 h since 1200 UTC (cf. Fig. 3), temperatures in western Texas had not increased as much as those in southeastern New Mexico.
At synoptic scales, the situation certainly contained the potential for development of tornadic supercells, even though that potential was well short of that associated with a major outbreak of tornadoes (e.g., as described in Doswell et al. 1993). Simply by looking at the analyzed weather charts for this case, we believe that tornadic supercells had to be considered a credible possibility in the threat area. Even for "synoptically evident" cases (Doswell et al. 1993) that produce tornado outbreaks, however, the actual evolution of events can be strongly dependent on the mesoscale details. Figure 8 indicates some important mesoscale complexity, so we will consider this in more detail in the next section.
Justifiable "mesoscale" analysis requires a data density that is not always available; hence, we consider this analysis to be "subsynoptic". Given that the operational surface data have an average distance to their nearest neighbor on the order of 100 km, it is possible at least to do a "subsynoptic" scale analysis of them, especially when considering the hour-by-hour changes, to infer the mesoscale structure with some limited confidence. Confidence in the resulting analysis can be substantially improved, however, by combining satellite imagery with the surface data.
Extensive mixed low and high clouds were present over the threat area at 1402 UTC (Fig. 9a), extending southeastward to near the Gulf of Mexico. Relatively clear skies prevailed that morning over north Texas, most of Oklahoma, and the eastern Texas Panhandle. By late morning (1702 UTC; Fig. 9b), however, thunderstorms developed in far western Texas, northward from near Midland into the western Panhandle.
Relatively cool temperatures were centered in the northwestern Texas Panhandle, extending eastward to near the Oklahoma border and southward to south of Lubbock, Texas. As suggested by the surface data at 1800 UTC (Fig. 10), this is likely due to the outflows from the aforementioned thunderstorm activity, which maintained cloudy skies over the affected region. Observe that the storms were developing within relatively cool temperatures, with warmer temperatures and higher dewpoints upwind, to the south, where skies were clearing and the flow was carrying in more moisture.
Keeping in mind the potential impact of this continued cloudiness, by 2115 UTC, the thunderstorm activity had consolidated into three complexes: one moving out of the extreme northeastern Texas Panhandle, a second area of thunderstorms along the southeastern portion of the Texas Panhandle, and a third moving into north Texas (Fig. 9c). Considerable cloudiness persisted over most of the far west Texas part of the threat area, with only scattered high and low clouds in most of eastern New Mexico. The associated 2100 UTC surface analysis (Fig. 11) reveals the persistent cool temperatures under the cloud cover in far west Texas. Continuing cloudiness there was reducing the insolation available to warm the near-surface temperatures, in comparison to those in southeastern New Mexico. Figure 11 indicates that the southernmost supercell apparently developed near the intersection of the dryline with the tongue of warm temperatures associated with the clearing skies in southeastern New Mexico.
As shown in the WSR-88D vertical wind profile from the Cannon Air Force Base, New Mexico radar (Fig 12), at this time, the threat area had a wind profile with considerable vertical wind shear, including notable veering in the layer just above the surface. It is well-known that such a wind profile is favorable for supercells and possible tornadoes (Moller et al. 1994).
A comparison between the 0000 UTC soundings at Amarillo (AMA) and Midland (MAF), Texas is also revealing (Fig. 13). Unfortunately, the absence of a sounding in eastern New Mexico precludes knowing what the surface-based instability was ahead of the thunderstorms that developed there on that afternoon. The Amarillo sounding was within the cool, cloudy airmass left behind in the wake of the early-developing thunderstorms, whereas the Midland sounding was within a relatively warm, moist airmass (cf. Fig. 8), although it was not as warm as in southeastern New Mexico. The AMA sounding was about 5C colder than the MAF sounding from the surface to about 600 hPa. From the surface values of temperature and dewpoint reported in each sounding, the 0000 UTC surface wet-bulb potential temperature () at AMA was about 20C, whereas that at MAF was roughly 23.5C. In southeastern New Mexico, in the narrow band of warm, moist air, surface values of were as high as 25C at 2300 UTC (not shown). Although there apparently is a small amount of convective available potential energy (CAPE) associated with the surface parcel in the AMA sounding (~500 J kg-1), there is considerable convective inhibition (CIN) associated with that parcel's ascent curve (~200 J kg-1). On the other hand, considerable CAPE (~2500 J kg-1) is found in the surface parcel for the MAF sounding, with little or no CIN. Thus, an analysis of soundings based on the surface parcels from these two locations gives very different indications of the surface-based instability, as confirmed in Fig. 13.
Three hours later, at 0015 UTC, two of the three initial thunderstorm complexes remained active; one along the Kansas-Oklahoma border and the other moving through north Texas (Fig. 14a). The latter exhibited an "enhanced-V" infrared satellite image signature (McCann 1983), but there was no reported severe weather with this storm. However, another major thunderstorm complex had developed, with its active convection at the upstream end of its large anvil in eastern New Mexico, just west of the Texas border. An "enhanced-V" signature was also evident with the southernmost storm in this complex. These thunderstorms had developed around 2200 UTC (not shown) in the High Plains of eastern New Mexico west of Clovis, when the surface dryline was still in the vicinity. Recall the surface analysis (cf. Fig. 8) shows that the thunderstorms at 0000 UTC were located near the surface position of the eastward-advancing dryline. Thus, they were in the narrowing gap between the dryline and the pool of cold, cloudy air left in the wake of the thunderstorms that developed in the late morning. Although the presence of this cool, cloudy air was not completely preventing a return of surface-based moisture, as evidenced by rising dewpoints in the threat area, it was still relatively cool in west Texas. At 0000 UTC it is relatively late in the day, limiting the chances for further destabilization by insolation.
By 0200 UTC, the thunderstorm that developed on the High Plains of New Mexico was moving across western Texas. Infrared satellite imagery (at 0202 UTC on 8 June; Fig. 14b) shows that this storm continued to exhibit an "enhanced-V" signature. Note that the storm in northern Texas that earlier had also shown a similar signature was apparently decreasing in intensity; the signature was no longer present. It can be seen from the 0300 UTC surface analysis (Fig. 15) that the surface-based airmass over which the storm in the Texas Panhandle was moving was still relatively cool.
Therefore, it appears the thunderstorms that developed during the late morning in western Texas caused the surface-based airmass in the threat area to remain relatively cool, cloudy, and with much reduced CAPE and ehanced CIN over what might have been anticipated from the synoptic evolution. By maintaining this subsynoptic region of cool, relatively stable air at the surface, the thunderstorms and their associated outflow apparently created an inhibiting mesoscale structure. That is, the persistence of a mesoscale area of cloudiness and outflow inhibited the evolution of unstable conditions at the surface. Next, we need to consider just what happened on the storm-scale events, to see how this mesoscale feature influenced the weather.
The National Weather Service's operational network of WSR-88D Doppler radars provides a reasonably detailed picture of the character of the convection on this day. As shown in Fig 16, the nearest WSR-88D radar (at Cannon Air Force Base, NM [KFDX]) is relatively close to the events in Clovis, New Mexico, although that in Lubbock, TX (KLBB) is rather distant from the storms. At 2205 UTC (Fig. 17a), there were two strong storms in eastern New Mexico, as depicted by KFDX radar, the northernmost of which was more mature and displayed an operational algorithm-detectable mesocyclone [see Zrnic´ et al (1985)]. By 2232 UTC (not shown), both storms were showing mesocyclone signatures and at 2323 UTC (Fig. 17b), the northernmost storm was beginning to decay, while the southernmost storm was approaching Clovis. The presence of persistent WSR-88D algorithm-detectable mesocyclones and the low-level radar morphology make it pretty clear that both storms had become supercells; see Doswell and Burgess (1993) for a discussion of the observable characteristics associated with supercells. The WSR-88D image of the southern storm at 0017 UTC is notable for its obvious supercellular character (Fig. 17c).
The northern storm continued its decline, but the southern storm maintained its supercell characteristics across the northern parts of Clovis, New Mexico and on into Texas. At 0037 UTC, the KLBB radar revealed a bounded weak echo region (Fig. 17d), indicative of continuing supercell characteristics for the storm. Further evidence for the continuing supercellular character can be found in the low-level reflectivity (Fig. 18a) and radial velocity structure (Fig. 18b). An hour later, at 0136 UTC, the storm maintained its supercell characteristics (not shown), and this was still so at 0237 UTC (Figs. 18c,d). After 0237, the storm began gradually to decay.
The path of this storm across Curry County in New Mexico, as well as Parmer, Castro, and Swisher Counties in Texas included several reports of severe weather (Fig. 19), mostly large hail (up to 4.5 inches in diameter in Castro County, TX). Notably, the tornadoes associated with this supercell occurred in Curry County, New Mexico and just inside the Texas border, in extreme western Parmer county. In spite of having supercell characteristics for many hours, the storm became tornadic only briefly, but went on to produce large hail for several hours thereafter.
The surface analyses show that after forming on the dryline but during its tornadic phase, the supercell was near the apex of a ridge of warm, moist air approaching the area from the south. As the supercell produced its tornadoes, however, it was moving eastward away from the axis of this ridge of warm and moist air, and deeper into the cloudy, relatively cool air left behind in the wake of the deep convection that had developed early in Texas. Shortly after moving east of the warm ridge, the storm became an "elevated" supercell storm. Rasmussen et al. (2000) document that storms encountering old outflow boundaries sometimes may have enhanced tornadic potential. However, unlike the 02 June 1995 case presented by Rasmussen et al., the pool of cloudy air the supercell encountered on 07 June 1998 was cool and stable, rather than showing high values as a result of modification by solar heating. The 0000 UTC soundings at AMA and MAF confirm the relative stability of the two airmasses, although neither sounding samples the extremes suggested by the surface data, a typical situation. If the surface air at MAF (with a of ~23.5C) is lifted over the cool pool, of course, it exhibits considerable CAPE (~2500 J kg-1), depending somewhat on the sounding to which the parcel ascent curve is compared.
The early deep convection produced outflow and continuing cloudiness that maintained the cool temperatures all afternoon. Therefore, only a relatively narrow zone of surface-based moist, unstable air was present, in between the approaching dryline to the west and the mesoscale cold pool to the east. It was within this narrow zone that the supercell became tornadic. The development of the southern supercell storm probably contributed to the relatively early demise of the northern supercell, by intercepting the northward flow of low-level high- air. Once the single remaining supercell moved across the narrow region where conditions were most favorable for tornadoes, it encountered increasingly cool, stable air at the surface and the tornadic potential fell dramatically, as discussed by Maddox et al. (1980). However, it appears that the environment continued to be favorable for supercells above the surface-based, mesoscale cool pool, so the now-elevated supercell continued to produce copious amounts of hail (including giant hail). Note that the surface-based cool air also apparently limited the strong surface wind gust potential for this storm; the only reported severe wind gust occurred in southeastern Curry County in New Mexico. Any strong downdrafts created by the supercell were apparently unable to penetrate to the surface through the strongly stable mesoscale cool pool.
Although it is not possible to know what might have happened had the early storms not developed in the Texas Panhandle, it certainly appears that a situation with at least some potential for significantly tornadic supercells was modified substantially by the mesoscale cool pool maintained by the early storms. In this case, the cool pool did not prevent the supercell (which became elevated after encountering the cool pool) from continuing to produce large hail for several hours, but its behavior is consistent with a negligible tornado potential once the storm moved east of the narrow zone between the dryline and the cool pool.
Forecasters certainly need to maintain vigilant diagnostic procedures to be aware of the changing mesoscale circumstances. In this case, even at its lowest elevation angle (0.5°), the KLBB radar beam overshot the cool pool for most, if not all, of the storm's passage across it. Therefore, the appearance of the storm on the WSR-88D continued to look potentially tornadic even though the actual tornadic potential was apparently diminished. One of us (DVB) was working in the Lubbock, Texas, National Weather Service office that evening. Tornado warnings for the storm were maintained as it moved across all of Parmer county, even though spotters did not report any tornadoes. At that point, however, the office staff realized the elevated character of the storm and dropped the tornado warnings while continuing severe thunderstorm warnings. The challenge during the evening had been to recognize the nature of the boundary the storm had crossed, and the change in stability associated with it. The precise boundaries of the mesoscale cool pool were never very clear and they did not exhibit much of a windshift, which would have been an obvious clue to the presence of a boundary.
In becoming an elevated supercell, the storm's inflow was forced to override the cool air, but the mesoscale cool pool reduced the likelihood that any vortex would penetrate to the surface. There were numerous storm chasers following this storm for most of its life, making it unlikely that any tornadoes would have gone unobserved. One of us (CAD) was among those who chased this storm, and observed a classic rear-flank downdraft "clear slot" (Moller et al. 1974), but there was no visual evidence for surface-based inflow producing a "wall cloud" structure during the passage of the storm across Parmer and Castro Counties in Texas. The convective towers of the storm were not visible owing to stratiform overcast, and the storm appeared only as a darkening of the stratiform cloud base, with the "clear slot" being the only visual evidence of the storm's supercell character.
Given the problems with recognizing the character of the situation from radar alone, forecasters having the responsibility for issuing warnings would need to be aware of the changing environment for this storm as it moved into Texas. The physical limitations of the WSR-88D radars (notably the "horizon problem" associated with the beam overshooting low-level features) to detect the important near-surface characteristics of this storm make this an especially challenging situation. Moreover, surface data alone do not necessarily mean that the cool pool is both deep and notably stable. The 0000 UTC soundings at MAF and AMA do reveal the character of the airmasses rather well, but 0000 UTC sounding data arrived well into what had been a challenging situation for issuing warnings. A well-trained storm spotter (see Doswell et al. 1999) might have recognized the elevated nature of the storm and been able to inform the forecasters of the situation. Storm spotter input is most helpful when (a) the forecaster has maintained "situation awareness" through continuous mesoscale diagnosis, (b) forecasters are knowledgeable about the potential for severe weather revealed by storm (in this case, the elevated character of the supercell), and (c) storm spotters are well-trained in recognizing salient features of the storms they are seeing.
In spite of the difficulties in knowing definitely what might have happened in the absence of the early convective storms, the observed evolution of the storm is at least consistent with the proposed inhibiting character of this particular mesoscale process. The tornadoes produced by the storm were not significant, either in intensity, duration, or impact. Had the storm gone on to produce a family of tornadoes, it is possible that at least one community in the storm's path could have been struck by a stronger, longer-lasting tornado than those that occurred. On days like 3 May 1999, when tornado outbreaks occur, it seems that something quite different is happening on the meoscale than on days like the one presented herein, in which tornadoes are isolated, and mostly brief and weak. It appears that the mesoscale factors that are an important component of hazardous weather events might well be most effective at inhibiting hazardous weather (at least for rare events, like tornadoes). Forecasters strive constantly to reduce false alarms associated with non-events wherein it appears that the situation has some potential to produce an event, but that potential may not be realized. Given the relatively low "penalty" associated with a false alarm compared with that for a tornado that devastates a community without a tornado warning, it takes particular care and some courage not to continue with tornado warnings for a persistently severe supercell storm. The science and the observations generally do not make this an easy decision. We believe that when scientific studies focus mainly on events, rather than non-events, this creates an asymmetry in the scientific literature that makes forecasting using available scientific concepts quite challenging. In effect, science has to address the "failure modes" in situations that have some hazardous weather potential, if it is to help solve the forecasters' challenge to know when not to be so concerned about an event. If scientific studies continue to ignore non-events, they necessarily omit consideration of the most frequent type of weather evolution.
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Figure 1. Analysis of features at 500 hPa, 1200 UTC on 07 June 1998, showing isohypses (heavy solid lines, contoured at 6 dam intervals) and isotherms (light gray lines, contoured at 2C intervals). Station plot shows temperature (ºC) in the upper left, dewpoint depression (ºC) in the lower left, and geopotential height (dam) in the upper right.
Figure 2. Analysis of features at 700 hPa, 1200 UTC on 07 June 1998, showing isohypses (heavy solid lines, contoured at 3 dam intervals), 700-500 hPa temperature difference (heavy gray lines, contoured at 2C intervals, beginning at 18C), and isotherms (light gray lines, contoured at 2C intervals, beginning at 0C). Station model is same as Fig. 1, except the 700-500 hPa temperature difference (ºC) is plotted in the lower right.
Figure 3. Regional surface analysis at 1200 UTC on 07 June 1998, showing isobars (heavy solid lines, contoured at 2 hPa intervals), isotherms (thin gray lines, contoured at 5F intervals), and isodrosotherms (heavy gray lines, contoured at 10F intervals, beginning at 40ºF). The station plot model is conventional.
Figure 4. Map showing the Storm Prediction Center (SPC) Convective Outlook product issued at 1523 UTC.
Figure 5. As in Fig. 1, except for 0000 UTC on 08 June 1998; 12-h height changes (dam) have been added to the station model, in the lower right.
Figure 6. As in Fig. 2, except for 0000 UTC on 08 June 1998.
Figure 7. Analysis of features at 850 hPa, 0000 UTC on 08 June 1998, showing isohypses (heavy solid lines, contoured at 3 dam intervals). The scalloped line denotes the dryline, the dashed line denotes a geopotential trough, and frontal symbols are conventional. A heavy gray line with an arrowhead denotes the low-level jetstream axis. Station plots show temperature (ºC) in the upper left, dewpoint depression (ºC) in the lower left, geopotential height (dam) in the upper right.
Figure 8. Regional surface features at 0000 UTC on 08 June 1998, showing surface isobars (solid lines, contoured at 2 hPa interval, labels only show the last two digits); the scalloped line denotes the dryline, stippled lines denote thermal boundaries, dashed lines denote troughs.
Figure 9. Visible satellite images on 07 June 1998 at (a) 1402 UTC, (b) 1702 UTC, and (c) 2115 UTC.
Figure 10. Analysis of surface temperature (stippled lines, ºF) and dewpoint (solid lines, ºF) for 1800 UTC on 7 June 1998. The approximate location where the southernmost supercell developed (between 2100 and 2200 UTC) is indicated by the bold "X".
Figure 11. As in Fig. 10, except for 2100 UTC.
Figure 12. Time section of vertical wind profile from the Cannon Air Force Base, NM WSR-88D radar, with UTC time indicated at the bottom, increasing to the right. Heights in thousands of feet are shown on the left. Wind barbs follow the National Weather Service operational convention, in knots; "ND" indicates no data.
Figure 13. Skew-T, log p plots of Amarillo, Texas (AMA; in blue) and Midland, Texas (MAF; in red) soundings at 0000 UTC on 08 June 1998; lifted parcel ascent curves shown include the virtual temperature correction. Isobars (thin solid lines) are labeled in hPa, and isotherms (thin dashed lines) and isentropes (curved thin solid lines) are both labeled in deg C.
Figure 14. False color-enhanced infrared satellite image on 08 June 1998, at (a) 0015 UTC and (b) 0202 UTC. Dark greens and grays indicate the coldest cloud top infrared temperatures.
Figure 15. As in Fig. 10, except for 0300 UTC, and the "X' marks the approximate location of the supercell storm.
Figure 16. Map showing the location of Albuquerque, NM (KABQ), Cannon Air Force Base, NM (KFDX), and Lubbock, TX (KLBB) WSR-88D radars, as well as Clovis, NM, superimposed on the county boundaries (and names) in gray.
Figure 17. WSR-88D radar depiction from KFDX, showing 0.5º elevation scan reflectivity (dBz) at (a) 2205 UTC, and (b) 2332 UTC, 07 June 1998. Yellow circles depict algorithm-detected mesocyclone signatures [see Zrnic´ et al (1985) for details about the algorithm]., Shown in (c) is a close-up view of the southern supercell at 0017 UTC, 08 June 1998, and in (d) is a WSR-88D cross section of the supercell at 0037 UTC, as seen on the Lubbock, TX (KLBB) radar, showing reflectivity structure.
Figure 18. KLBB view of (a) reflectivity (dBz) and (b) storm-relative radial velocity at 0036 UTC on 08 June 1998, at 0.5º tilt; (c) reflectivity (dBz) and (d) storm-relative radial velocity at 0237 UTC on 08 June 1998, at 0.5º tilt.
Figure 19. Map of the reported severe weather, with "H" denoting a hail report (diameter given as reported, in inches), "W" denoting a wind report (speed given as reported, in knots), and "T" denoting a tornado (F-scale given). A third, brief tornado (not included in the official log of severe weather) was observed by some storm chasers (P. Markowski 2001; personal communication) just east of Farwell, TX at 0042 UTC. This location is shown by an asterisk.