A SNOW EVENT RELATED TO CONDITIONAL SYMMETRIC INSTABILITY (CSI)

SR/SSD 97-12 3-15-97

1. Introduction

During the day and into the evening of January 6, 1997, a major winter storm moved across southern New Mexico and West Texas. The storm dumped from 3 inches (at El Paso) to 2 feet of snow (at Cloudcroft, New Mexico), effectively paralyzing all of southern New Mexico and western Texas. What made the winter storm unique was that much of the snowfall was associated with Conditional Symmetric Instability (CSI), including several reports of thundersnow.

Conditional Symmetric Instability (also called "slantwise convection") is instability that results in slanted mesoscale circulations of saturated air parcels that orient themselves parallel to the thermal wind (Funk and Moore 1994). Those authors went on to say that CSI exists when the atmosphere is weakly stable in the vertical and horizontal but unstable to slanted movement. For example, if a parcel of air were moved vertically or horizontally, it would return to its original position, but a parcel moved slantwise would result in a tilted upward acceleration. CSI results from a combination effect of vertical and horizontal forces. Narrow bands of heavy precipitation can result during CSI, including thundersnows.

Several atmospheric conditions must be met to produce CSI--a nearly saturated atmosphere, near neutral stability (but slightly stable or upright convection would occur), strong vertical speed shear, and large-scale forcing for upward motion to produce parcel displacements (Funk and Moore 1994). In addition, CSI can be evaluated using PC-GRIDDS by comparing the slope of the theta-e surfaces versus the slope of the momentum surfaces. If the theta-e surfaces slope more, or are nearly parallel to the momentum surfaces in regions where the relative humidity is greater than 70 percent, CSI is present (Moore and Lambert 1993). All of these elements came together January 6, 1997, across the El Paso County Warning Area (CWA).

2. Synoptic and Mesoscale Conditions

At 06/1200 UTC a vigorous upper-level low pressure system was intensifying and moving into the southwestern United States (Fig. 1). The upper low, combined with surface high pressure over the central United States, helped to usher cold air and abundant moisture into southern New Mexico and West Texas. Composite reflectivity images from the Santa Teresa, New Mexico (El Paso), Doppler radar showed narrow bands of rain, sleet, and snow had developed during the morning hours (Color Plate, top). With a quick review of PC-GRIDDS, the 06/1200 UTC Eta CSI macro indicated potential for CSI that morning.

A cross section in southern New Mexico and West Texas (Fig. 2) showed the slope of equivalent potential temperature (theta-e) surfaces was greater than that of the momentum in areas where the relative humidity was greater than 70 percent. During the day, precipitation diminished temporarily as the storm's dry slot moved across the area.

By late afternoon the precipitation resumed and increased in intensity. The PC-GRIDDS Eta cross section at 07/0000 UTC (Fig. 3) again showed the potential of CSI development that evening, as the slope of the momentum surfaces was again shallower than the slope of equivalent potential temperature surfaces in areas where the relative humidity was greater than 70 percent. PC-GRIDDS vector wind difference (i.e., thermal wind) between 850 and 500 mb indicated that narrow bands of heavier precipitation would develop in a southwest to northeast direction parallel to the thermal wind (Fig. 4). In addition, isentropic ascent was present, providing the upward motion for parcel displacement (Fig. 5).

The Santa Teresa, New Mexico, sounding at 07/0000 UTC (not shown) revealed a surface-based inversion extending to 700 mb. Above 700 mb the atmosphere was weakly stable and nearly saturated, at least to about 500 mb. The sounding also showed that winds from the top of the inversion to 500 mb increased by over 30 kt.

3. Observations

Reports of moderate snowfall that evening were received from across the El Paso CWA. The official observing station in El Paso reported thundersnow at 08/0101 UTC. Also, Santa Teresa reported thundersnow at 08/0125 UTC. The Color Plate (middle) shows a zoomed portion of the radar composite reflectivity, detailing one of the stronger CSI bands along the Texas-Mexico border at 08/0128 UTC. The high reflectivities associated with this storm were due to ice pellets mixed with the snow. The public reported snowfall at the rate of two inches per hour with this storm as it moved across El Paso. Later that evening, the CSI bands diminished in intensity (Color Plate, bottom), but were still orientated parallel to the thermal wind.

4. Conclusion

On the morning of January 6, 1997, the Santa Teresa (El Paso) Doppler radar showed the banding nature of a wintery mix of precipitation across southern New Mexico and West Texas. With a quick review of PC-GRIDDS data, CSI was identified as a likely cause of the precipitation. The model data also indicated that CSI would regenerate during the evening. When the snowfall reformed during the evening hours, the radar showed even stronger CSI ("slantwise convection"), and thundersnow eventually broke out across parts of the El Paso CWA. With experience gained from the morning CSI occurrence and a more detailed study of PC-GRIDDS data, the evening short-term forecasts were adjusted to include the possibility of isolated heavy snowfall with thunder.

Acknowledgment

The authors would like to thank Mr. Val MacBlain, Science and Operations Officer at NWSO El Paso, for his review and technical advice on this study.

References

Funk, Theodore, and James Moore, 1994: Vertical Motion Forcing Mechanisms Responsible for the Production of a Mesoscale Very Heavy Snow Band Across Northern Kentucky. NOAA Technical Memorandum, NWS CR-112, NWS Central Region Headquarters, Kansas City, MO.

Moore, James T., and Thomas E. Lambert, 1993: Conditional Symmetric Instability. Wea. Forecasting, 8, 301-308.