1 Diagnosis of the Fallon Hail Event 21 July 2008: A Rarity in Nevada By Dana M Williamson Atmospheric Sciences 453 Case Study Abstract The hail event that occurred on 21 July 2008 at 22:45Z in Fallon and Silver Springs, Nevada was a rare event due to its production of heavy rains, a large amount of hail and two tornadoes. The event occurred as a result of the coupling of favorable synoptic and mesoscale conditions. The synoptic environment prior to the storm helped create favorable conditions for severe weather at both the 300 MB and 500 MB levels. At the 300 MB level, the curvature of the upper level flow created a weak region of upper divergence, which acted as a weak dynamical lifting mechanism. At the 500 MB level, a cut-off low located west of the storm site helped to decrease temperatures and destabilize the middle layers. The cut-off low also created a southerly flow that feed moisture into the Nevada region at mid and low levels. The 700 MB and 850 MB set-up aided the influx of moisture. Going further into the mesoscale set-up, daytime radiation heating caused thermals in the atmosphere to warm the lower levels, overcome the convective inhibition present earlier in the day, and destabilize the lower atmosphere. With a destabilized atmosphere, orographic lifting over the mountains provided enough lifting to place air parcels into a region of free convection. A favorable wind profile, CAPE, low wet bulb freezing height and abundant moisture enhanced the convection and formed a rare, Nevada supercell.2 I. Introduction On the 21st of July, 2008 a devastating hail event struck western Nevada, devastating the Silver Springs and Fallon areas. The event began mid-afternoon and quickly developed into a severe supercell event. The first reports of severe weather were called in at 22:45 Z, reporting localized flooding with nickel sized hail. Then, at 22:49 Z, another report was called in saying an accumulation of three to four inches of hail was blanketing the ground. The following reports, only minutes later, called in the sightings of rain-wrapped tornados 5 miles west and north of Fallon. Storm reports throughout Nevada continued to for nearly an hour and a half, each claiming heavy rain and hail. The hail event that occurred on July 21, 2008 was a unique and rare event to the state of Nevada due to its mountainous and dry location. However, the atmospheric conditions of 21 July 2008 took advantage of and manipulated these persistent conditions to develop such a rare system. Figure 1 illustrates a schematic of the manipulations that were prevalent during the supercell hail event. Represented in the figure are two often unique and separate processes that where combined: orographic lifting and the resulting developing supercell. In the first process, daytime heating of the earth’s surface cause the atmosphere to destabilize and a slight pressure gradient to form along the mountain slope. The small pressure gradient along the mountain side causes a weak upslope flow and convergence atop the mountain. Here, convection enhanced by the destabilization of daytime temperatures occurs. As the time progresses, the mean flow carries the system off the mountain top and convection continues to occur; this schematic shows the mean wind travelling upslope from the left side of the image to the right side. As the system increases in size and strength, processes two represented in Figure 1 occurs. In process two, a strong updraft begins to carry cloud droplets and rain droplets past the freezing level, supercooling them as they are lifted. The supercooled droplets then become accreted onto a nuclei and begin to form a hail stone. As the hail stone continually smashes into supercooled droplets in the updraft, the droplets glaciate and cause the hail stone to grow. Once the hail stone is either too heavy to be sustained by the updraft or is shot out of the updraft, it falls to the ground, melting on its way down. Often times, the hail stone will once again re-enter the updraft column and continue to grow. The cyclic process of formation and decay can occur many times throughout the life-time of a hail stone and can produce a various sizes and amounts of hail. Figure 1: A Conceptual Model of the Nevada Hail Event. This case study of 21 July 2008 will analyze the key synoptic and mesoscale features present to cultivate this supercell hail event. It will argue that multiple dynamic mechanisms were present to3 destabilize the atmosphere above Nevada and that orographic lifting initiated the upwards vertical motions carried out by free convection. The paper will also argue that the combination of wind shear, low CAPE levels, a low wet bulb freezing height and abundant moisture caused the large hail amounts and heavy rains that occurred on 21 July 2008. II. Data This report utilized many different sources of information. The primary source used was the Storm Prediction Center: Severe Storm Archive; this source provided data concerning the Nevada Hail Event in the form of 300 MB, 500 MB 700 MB and 850 MB level flow charts, surface observations, radiosonde and index data, and storm reports received during the event. More data concerning the 300 MB level, 850 MB level, the LI indice and radiosonde data was utilized using the Unysis Archives and the University of Wyoming Archives. Additional sources of information concerning hail formation, geography, and environmental characteristics commonly associated with severe weather were used to supplement my own knowledge and verify details. In addition to internet and lecture sources, computer programs were used to assess a visual progression of the storm. The program known as GARP was used to create a radar and cross sectional analysis of the system throughout its development and decay. Similarly, McIDAS-V was used to create a visible imagery of the storm. With the data from these two animations, an understanding of the development and progression of the hail event was found and was used in combination with dynamics to discover the synoptic and mesoscale mechanisms that forced the event’s production. III. Synoptic Overview The synoptic conditions leading to the hail event, in no way, were the classical text book examples commonly depicted. Despite the non-classical set-up however, the conditions did provide an atmosphere suitable for breeding such a hail event. a. 300 MB Analysis Analyzing the upper levels on 21