Recognition of Negative Mesoscale Factors for Severe-Weather Potential- A Case Study

1. Recognition of Negative Mesoscale Factors for Severe-Weather Potential- A Case Study Article By: Charles Doswell III Donald V Baker Charles A Liles Presentation By: Jason Tiffany

2. Introduction • Severe thunderstorms are a relatively rare event in any one place, even in regions in which storms are most common • Majority of forecast days are associated with “non events” rather then events • Most common impact of mesoscale processes is to reduce the chances for severe convection • This study is completed to illustrate the apparently negative effects associated with mesoscale processes on tornado potential of subsequent deep moist convection

3. Morning Outlook for Severe Thunderstorms • “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 TX and eastern NM. Surface dew points in the upper 50s to low 60s should extend as far north as Amarillo/Tucumcari by late afternoon” (Doswell et al. 2002).

4. Synoptic Discussion • “Active pattern over the central and southern Plains this period as upper low 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 the developing warm front into southern Nebraska later tonight/early Monday”(Doswell et al. 2002).

5. Fig. 1. Analysis of features at 500 hPa at 1200 UTC 7 Jun 1998, showing isohypses (heavy solid lines, contoured at 6-dam intervals) and isotherms (light gray lines, contoured at 2°C intervals)

6. Fig. 2. Map showing the convective outlook product issued at 1523 • UTC • On this basis of the synoptic discussion, the lead SPC forecaster issued • a slight risk for severe thunderstorms (fig 2) for eastern NM and • western TX • This is known as the “threat area” \

7. Fig. 3. The 500-hPa analysis as in Fig. 1 but for 0000 UTC 8 Jun 1998; 12-h height changes (dam) have been added to the station model in the lower right

8. Fig. 4. Visible satellite images on 7 Jun 1998 at (a) 1402, (b) 1702, and (c) 2115 UTC

9. Subsynoptic Overview • At 1800 UTC there were cool temperatures associated with the cold outflows from the thunderstorm activity, which maintained cloudy skies over the affected region • Strong instability gradient was present between the stable surface air mass in the TX Panhandle and the unstable surface air in southeastern NM

10. Fig. 5. (a) Surface analysis for 2100 UTC 7 Jun 1998; (b) 2100 UTC estimated lifted index (°C)

11. Subsynoptic Overview • In the threat area there is considerable vertical wind shear profile including strong veering in the layer just above the surface • By 0015 UTC, two of the three initial thunderstorm areas remained active; One along KS, OK border; one moving through north TX • The TX thunderstorm exhibited an enhanced V signature, but there were no reports for severe weather

12. Subsynoptic Overview • New major thunderstorm area had developed with its active convection at the upstream edge of its largest anvil in eastern NM • New thunderstorms developed near the surface dryline around 2200 UTC on the high plains of eastern NM • At 2302 UTC the thunderstorms were approaching a region were billow clouds could be seen extending from beneath the extensive cloud cover over western TX

13. Fig. 6. False color–enhanced infrared satellite image on 8 Jun 1998 at (a) 0015 and (b) 0202 UTC

14. Subynoptic Overview • Scofield and Purdom (1986) and Weaver et al. (1994) have shown that billow clouds indicate stable low-level air • The western edge of cloud cover might be viewed as an estimate of the location of the boundary between stable and unstable surface air masses • By 0200 UTC, the thunderstorm that developed on the high plains of NM was moving across a region surface air with decreasing instability and increasing CIN • It seems that the persistence of a mesoscale area of cloudiness and outflow inhibited the destabilization at the surface ahead of the persistent supercell

15. Evolution of convection • The storms across NM and Swisher Counties in TX, resulted in several reports of severe weather • Mostly large hail(up to 4.5in) • The tornadoes associated with the supercell occurred in Curry County, NM, and just inside the TX border • The supercell became tornadic during the time when it was interacting with the boundary • Only large hail was produced for several hours after its tornadic phase

16. Conclusion • The supercell that formed, formed near the dryline, near the apex of a ridge of warm moist air approaching the area from the south • The supercell then moved eastward, then became an elevated supercell storm • Maddox et al. (1980) made observations to support the fact that storms moving across a strong boundary are likely to produce tornadoes only during interaction with the boundary • Tornado production will be less likely as the storm moves into a stable surface air mass

17. Conclusion • The development of the southern supercell might have contributed to the demise of the northern supercell by pushing its outflow into the inflow of the northern storm • Once the single supercell moved across the narrow boundary in which conditions were favorable for thunderstorms, it encountered increasingly cool, less unstable air • Tornado production then ceased as described by Maddox et al.(1980)

18. Conclusion • Any strong downdrafts were unable to penetrate to the surface through the strongly stable mesoscale cool pool • It appears that mesoscale cool pool maintained by the early storms limited the potential for tornadicsupercells • The cool pool did not prevent the supercell from continuing to produce large hail • Its behavior is consistent with a negligible tornado potential once the storm moved well beyond the transition zone between the unstable surface air ahead of the dryline

19. Conclusion • The challenge of this storm was being able to recognize the nature of the boundary the storm has crossed and the change in stability associated with it • When it became an elevated supercell, the storms inflow was forced to override the cool air, but the mesoscale cool pool reduced the probability of tornadoes • Storm spotters observed a rear flank downdraft, but there was no wall cloud structure during the passage of the storm • The storms only appeared as a darkening of the cloud base, with a rear flank downdraft being the only visual evidence of the storms supercell character

20. References • Doswell, C.A., III, D. V. Baker, and C. A. Liles, 2002: Recognition of Negative Mesoscale Factors for Severe -Weather Potential: A Case Study. Wea Forecasting, 17, 937-954.