Article By: Charles Doswell III Donald V Baker Charles A Liles

Presentation By: Jason Tiffany

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

IntroductionSevere 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

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).

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).

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)

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•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”

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

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

Subsynoptic OverviewAt 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

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

Subsynoptic OverviewIn 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

Subsynoptic OverviewNew 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

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

Subynoptic OverviewScofield 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

Evolution of convectionThe 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

ConclusionThe 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

ConclusionThe 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)

ConclusionAny 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 tornadic supercells

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

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

ReferencesDoswell, 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.