Nevada Supercells Case Study -- Part I

Two tornadoes were reported in Nevada on Monday, and it appeared that the state's four-year tornado "drought" had ended. Not so fast, said the National Weather Service in Reno. After surveying three suspicious damage areas in Western Nevada, the NWS could not confirm the damage caused by the storms was tornado-related. Regardless, it was quite amazing to watch supercells moving through Western Nevada, an area that typically only experiences 13-15 thunderstorm days per year!


The first part of this case study will break down the synoptic levels. The 300 hPa pattern was chracterized by a trough over Northern California and a moderately strong jet streak (65 knots) moving into Northwest Nevada. The speed divergence and diffluence associated with the jet streak entering Northwest Nevada resulted in an environment supportive of rising air. Make note of the SSW flow at this level and compare it to the wind direction in the lower levels.



The 500 hPa pattern was somewhat similar to the 300 hPa and showed the base of the trough just offshore of California. There were relatively strong winds (45 knots) at this level, and the mid-level jet streak provided the deep layer shear needed for supercells. A strong lobe of vorticity was also associated with this trough. Given the nearly southerly flow at 500 hPa, positive vorticity advection occurred across Northwest Nevada and also enhanced rising air across the area.


The flow at 700 hPa was weak and from the west. There was some mid-level moisture that was pooling along the eastern slopes of the Sierra Nevadas. The key ingridient on this chart was probably the warm tempearture aloft. The warm temperatures at 700 hPa acted to cap the lower level energy that was building throughout the day. When it was released (due to the upper level dynamics -- jet streak and vorticity advection), strong to severe thunderstorms developed quickly.


Extremely warm conditions were prevalent at 850 hPa across a large chunk of the Western U.S. Granted, many locations in the west begin near 850 hPa, but there was ample warmth (26-28 C) at this level. A very weak convergent flow can also be seen across a broad area of Nevada, California, and Oregon. One thing to note on this chart is the lack of low-level moisture in Northwest Nevada, which may have inhibited the tornadogenesis process. We'll discuss this in more detail in Part II.

It's quite evident that the synoptic environment was supportive of organized thunderstorms. In Part II of this case study, we'll look at the thermodynamic environment in order to better assess the potential for supercells and tornadoes. I'll probably post it early next week.