Powerful thunderstorms roared across western Nevada on Monday, July 21st which produced large hail, damaging winds, heavy rain, funnel clouds, and even possible tornadoes. This outbreak of severe weather was exceptionally rare for western Nevada . A rough estimate would rate this as a “once in ten year” event for the area. These strong storms were the result of a low pressure system over northern California tapping into monsoon moisture over the Great Basin . Monsoon moisture had gradually increased over the area during the previous two days due to southerly winds rotating around a high pressure system anchored over the Desert Southwest. Figure 1 below is a water vapor satellite image which shows the location of the low pressure system hours before storms exploded over western Nevada

Figure 1: The primary weather features that produced the severe weather outbreak on Monday, July 21, 2008.

Many ingredients for severe weather came together during the afternoon. The low pressure system, along with daytime heating and resulting instability, produced significant lift in the atmosphere. Wind shear, or the change of wind direction and speed with height, was also increasing across the area. Wind shear is a critical ingredient necessary for producing organized thunderstorms, which is a term meteorologists use to describe storms that are long lasting and tend to rotate. These ingredients, combined with abundant low level moisture, produced a rare day of widespread severe weather and supercell thunderstorms, the strongest types of thunderstorms on Earth. One of our meteorologists, Kyle Mozley, took this picture (Figure 2) of a well developed mesocyclone associated with one of the supercells passing over Fallon, NV. Mesocyclones often precede tornado development.

Figure 2: Supercell over Fallon, NV.

See Figure 3 below to view some of the severe thunderstorms, along with their storm tracks, that occurred over the area.

Figure 3: Severe thunderstorm tracks on July 21, 2008 in western Nevada. The numbers on the image represent each individual storm with each arrow representing the tracks every 30 minutes. Further down in the summary, Storms 1 and 3 will be decribed in greater detail.

Radar Imagery from the National Weather Service’s Doppler Radar sampled numerous severe thunderstorms that were producing large hail. Figure 4 below is a radar image from July 21, 2008 at 4:01pm PDT, indicating a “hail spike” in the radar. Hail spikes in the radar imagery are actually false echoes generated by very large hail within a thunderstorm and are very good signatures that show a storm is likely severe. The NWS definition of a severe storm is one where hail is 3/4” of an inch or greater and/or wind gusts of 58 mph or greater. Figure 5 below is a reflectivity cross section of one of these severe storms. The most intense rain and largest hail within this storm is represented by the pink, purple, and red colors. Of importance is the notch of lower radar reflectivities below much higher reflectivities, which is annotated in the image. This notch represents a very strong updraft which is required to allow hailstones to remain suspended in the thunderstorm longer, allowing them to grow into larger hailstones without falling out. The stronger the updraft, the larger a hailstone can become.

Figure 4: Radar Imagery at 4:01 pm PDT indicating hail spikes from strong thunderstorms (circled), which is a signature of very large hail.

Figure 5: Cross section indicating a strong updraft into a severe thunderstorm.

Due to the widespread severe weather, many severe thunderstorm, tornado and flash flood warnings were issued on July 21st. There were a total of 20 severe thunderstorm warnings, 6 tornado warnings, and 2 flash flood warnings issued for western Nevada. This is an exceptional number of severe weather warnings issued for western Nevada for a single day. Figure 6 shows all the statements we issued and related paperwork from the event.

Figure 6: An impressive stack of statement printouts from this event.

Meteorologists from the National Weather Service in Reno surveyed the areas of reported damage and funnel cloud sightings along U.S. Highway 50 from near Lahontan Reservoir through the Fallon area, and eastward to near Sand Mountain . The storm survey is available here

There was evidence of severe hail, flash flooding and wind damage, however a tornado touchdown could not be confirmed. Tornadoes are easiest to confirm when they cause damage to property or trees. Nevada tornadoes typically are weak and often move through remote areas, which make confirming their existence extremely difficult. We did receive several pictures of possible tornadoes and funnel clouds, but they are difficult to verify unless we can see video to see if they are rotating. Cloud features can sometimes be misleading by even trained weather spotters and can be misinterpreted as a tornado or funnel cloud. See Figures 7-9 below of hail and rain shafts that could potentially be mistaken for tornados. These photos were taken by Sean Lewandowski, a weather enthusiast in the Fernley, NV area. The key in identifying a tornado or funnel cloud is that the downward facing tube of air is rapidly rotating.

Figure 7: White hail shaft in center of image. Figure 8: Rain shaft in right half of image.

Figure 9: Rain/Hail shaft in lower right of image.

Severe Thunderstorm #1

One of several significant supercell thunderstorms that affected the area includes one which tracked east of Fallon, near Sand Mountain. This storm produced a swath of hail pock marks/indentations in the mud along Highway 50, approximately 20 to 22 miles east of downtown Fallon. These marks indicated that numerous hail stones of pea to marble size occurred with this cell, with a number of marks suggesting hail up to quarter size (see Figures 10-13 below). The largest estimated hailstones within this storm occurred well away from the area we were able to easily access along Highway 50.

Figure 10: Hail swath across U.S. Highway 50 just west of Sand Mountain. Yellow colors indicate hail up to 2” in diameter (size of a golf ball, while the large area in light/dark blue depicts hail between 3/4" (size of a penny) and 1.25" (size of a half dollar).

Figure 11: Hail pock marks in the mud along Highway 50, west of Sand Mountain.

Figure 11: Hail pock marks in the mud along Highway 50, west of Sand Mountain, as compared to a penny (3/4" in diameter).

Figure 13: Hail pock mark compared to a quarter (1.00").

Several trained weather spotters in Fallon reported funnel clouds with this storm between 3:30-4:30 pm PDT, but it is uncertain whether these touched down. See Figures 14-15 below of the reported funnel cloud, which were taken by Henry Miller, a trained weather spotter in Fallon.

Figure 14 and 15: Reported funnel cloud in Fallon, NV.

There was also damage to a power pole along US Highway 50 east of Fallon, which appeared to be caused by strong straight line winds or even possibly a weak tornado (see Figures 16-17 below). However, a specific wind speed cannot be estimated solely based on this evidence. A trained weather spotter in the Fallon area did report estimated wind gusts of 50-60 mph, along with power outages around 4:15 pm PDT.

Figures 16-17. Note the broken off pieces of wood next to the pole.

A Department of Transportation employee who happened by the site mentioned flooding had occurred across Dixie Valley Road. The ditches along U.S. Highway 50 were still full of water the morning of the survey (July 22nd), indicating torrential rainfall fell with these storms. See Figure 18 below for a picture of the flooding. These photos were taken by Sean Lewandowski, a weather enthusiast in the Fernley, NV area.

Figure 18: Flooding in Fallon.

Severe Thunderstorm #2

Another significant supercell thunderstorm affected the area in the vicinity of Fort Churchill State Park in Lyon County, south of Silver Springs. Park rangers indicated a brief period of small hail, but a more prolonged period of heavy rain and strong winds associated with the initial stages of this storm. Only minor tree damage was observed in this area, although park rangers mentioned that cleanup had already occurred and that a portable toilet had been blown over in the storm. Based on this information, wind speeds were estimated at approximately 60 to 70 mph.

Trained weather spotters in the area also reported heavy rain producing localized flooding east of Lahontan Reservoir. A trained spotter in Silver Springs reported up to quarter size hail, with hail covering the ground. See Figure 19-21 below of the hail swath image highlighting the track of the storm and hail size, and pictures of hail in Silver Springs, taken by trained weather spotter Ken Barnes.

Figure 19: Hail Swath Image depicting hail size. Light blue colors indicate hail around 3/4" in diameter (size of a penny), and the dark blue colors indicate hail up to 1.25" in diameter (size of a half dollar).

Figures 20 and 21: Hailstone near 1.00” in Silver Springs, NV and Hail covering the ground in Silver Springs, NV.

The strong thunderstorms that occurred in western Nevada on July 21st were a rare event for this area. If you have any pictures of these storms or damage, the National Weather Service in Reno would love to see them. You can email them to our webmaster at mailto:w-rev.webmsater@noaa.gov .