Nevada Supercells Case Study -- Part I I

The synoptic environment was supportive of organized thunderstorms over much of Western Nevada. At the same time, the thermodynamic environment supported the threat of severe thunderstorms, including supercells. Since the reports of the tornadoes/funnel clouds occurred between 2300-0000 UTC, this will be the primary time frame of focus in this study.


The surface chart at 2300 UTC showed a moisture discontinuity across Western Nevada. The dew point at Reno was only 46 F compared to 52 F at Fallon. The dew point at 0000 UTC at Fallon actually spiked to 55 F when storms were in the vicinity. The lower moisture profile across the extreme western portion of the state was probably the result of a dry, downsloping wind out of the Sierra Nevadas. Moisture was able to pool around the Fallon area, and this played a crtical role in storm initiation and intensification.


Given that the area around Fallon is at a high elevation (~4,000 ft ASL) and the high dew points observed, this contributed to theta-e values in excess of 350 K! For comparison purposes, note that this theta-e max in Nevada is higher than the 348 K value observed in Southeast Mississippi. This just hammers the point home for forecasters in higher elevations to pay close attention to theta-e. What may seem like a meager amount of moisture may actually be quite substantial when elevation is considered. That's another discussion for another day, but if you are interested, do some research on the Cheyenne Ridge tornado in 1960.



The environment was moderately unstable given the combination of warm surface temperatures, the pooling of surface moisture in the Fallon area, and the presence of a trough (cold air aloft) just to the northwest. In fact, mixed layer CAPE values were running between 1000-2000 J kg-1 at the time the supercells were ongoing. This degree of instability balanced by moderate deep layer shear (35-40 knots) resulted in an environment supportive of supercells.



The threat of tornadoes appeared quite small given the lack of low-level shear and the high dew point depressions. The 0000 UTC sounding from Reno showed an inverted-V type of sounding which is indicative of strong evaporation potential below 600 hPa. Keep in mind that the environment east of Reno was a bit more moist, but the sounding still would have shown an inverted-V sounding given the dew point depression in Fallon at 2300 UTC was 38 F. The lack of low-level moisture resulted in high downdraft CAPE values between 1000-1200 J kg-1. This is typically too high for tornadoes to develop. The reason this inhibits tornadogenesis is because the cold, dry air descending from the RFD will be ingested back into the mesocyclone. This disrupts the low-level mesocyclone and prevents vortex stretching. Tornadoes need air that is less dense (warm and moist) in order to enhance vortex stretching.




In addition to the high DCAPE, the LCL and LFC were also a bit high. The LCL height was around 1,750 m AGL and the LFC height was around 2,000 m AGL. This is somewhat high, but tornadoes have occurred in environments of high LCL and LFC heights. In addition to the low-level dry air, the lack of low-level wind shear likely played a significant role in inhibiting tornado development. The 0000 UTC sounding from Reno showed minimal speed and directional shear in the lowest 1 km AGL. In fact, the 0-1 SRH was less than 50 m^2 s^-2.

Even with the lack of low-level shear, some mechanism(s) must have been responsible for the funnel clouds and wall clouds observed. It's tough to accurately pinpoint one or more factors with the data deficient observation network in Nevada. The very steep low-level lapse rates may have accelerated parcels fast enough to promote brief low-level rotation and weak vortex stretching (i.e. funnel clouds). It's also possible that the supercells may have traversed a mesoscale or microscale boundary that would have been undetectable in the observation network. This boundary would have enhanced low-level shear and aided in rotation.

This case study has shown that the dynamic and thermodynamic environment certainly supported the threat of severe weather, including supercells. However, a dry boundary layer and weak low-level wind shear precluded the threat for tornadoes. However, it's unclear what exactly generated the rotation observed in the supercells at various times during their lives.