1 P3.1 AN ANALYSIS OF A PROLIFIC TORNADO PRODUCING CYCLIC SUPERCELL THUNDERSTORM IN NUCKOLLS COUNTY NEBRASKA, MAY 24, 2004 John P. Monteverdi*, Elizabeth Polito, Matthew Gough, Rebecca Bethke and Terrance Seddon San Francisco State University, San Francisco, CA 1. INTRODUCTION On 24 May 2004, a supercell thunderstorm (hereafter referred to as the “Thayer County storm”) near Hastings, Nebraska and moved through Adams, Clay, Nuckolls and Thayer Counties, Nebraska and Republic County, Kansas, spawning 11 tornadoes in Nebraska alone (Fig. 1and Table 1). Figure 1 Locations and tracks of 11 tornadoes associated with the Nuckolls County storm in Nebraska (source SPC, SeverePlot). Figure 2 Tornado I (right, near Deshler) at 2115 UTC. View towards the southwest. (Photo: John Monteverdi). *Corresponding author address: Prof. John P. Monteverdi, Dept of Geosciences, San Francisco State University, 1600 Holloway Avenue, San Francisco, 94132; e-mail: [email protected] Table 1. Nebraska tornado statistics, May 24, 2004. Letters refer to locations on Fig. 1 (Source: SPC) Tornado Locations (Nuckolls and Thayer Counties) Time UTC Path Length (km) /Width (m) Mag Lat Lon A Nelson 2005 0/50 F0 40 13 98 06 B Nelson 2020 0/25 F0 40 12 98 03 C Nelson 2028 0/25 F0 40 16 98 07 D Oak 2035 0/15 F0 40 12 97 54 E Nora 2041 0/15 F0 40 11 97 59 F Ruskin 2052 0/15 F0 40 11 97 52 G Ruskin 2058 3/100 F1 40 04 97 52 H Deshler 2102 3/75 F0 40 04 97 49 I Deshler 2106 3 /450 F1 40 03 97 46 J Hardy 2116 0/25 F0 40 01 97 55 K Byron 2129 0/25 F0 40 00 97 41 The lead author witnessed more tornadoes than were recorded in SPC’s final tally. For example, there were actually three tornadoes west of Hebron, Nebraska, shortly after 2115 UTC. One of the secondary, or satellite tornadoes, can be seen at the left of the main funnel, on Fig. 1. The Nuckolls County storm then moved into Kansas and produced another 5 tornadoes (Fig. 3). Figure 3 Belleville, KS tornado at 2200 UTC. View towards the northwest. (Photo: Thom Trimble).2 Figure 3 Visible satellite imagery at 1932, 2002, 2012 and 2032 UTC showing storm initiation near Hastings. Analysis of the radar data shows that the Nuckolls County storm was a cyclic classic supercell. However, unlike other cyclic supercells, its long life cycle and repetitive tornado production were not related to interaction with subsynoptic scale boundaries, but to the favorable synoptic scale shear and buoyancy environment into which the storm moved. The purpose of this paper is to present a brief examination of the synoptic and thermodynamic controls of this event. We also will show how the cyclic nature of the Nuckolls Couny storm’s tornado production was consistent with the high values of low level shear found in the storm environment. 2. STORM EVOLUTION AND HISTORY The Nuckolls County storm initiated near Hastings, Nebraska at around 1930 UTC. The storm quickly became severe and was responsible for many severe hail and wind reports in Adams County. At the time of convective initiation, the area of south-central Nebraska lay in the northeast portions of a developing wave cyclone, centered in northwest Kansas (Fig 3). Backed surface flow (relative to winds in the mid-troposphere) ahead of a rapidly developing dry line bulge occurred northeast of this center. Storm initiation appeared to take place near the triple point intersection of cold front, quasi-stationary front and the dry line in the flow of air with high dew points into south central Nebraska. Analyses of the radar information show that initially the storm moved northeastward with the mid-tropospheric flow (motion vector of ~220o, 10 m s-1) and split several times. Southwest of Edgar, Nebraska, the right moving storm became a supercell, developed strongly deviate motion (motion vector of ~330o, 15 m s-1). This deviate motion combined with strongly looped hodograph, created excessive values of storm relative helicity in the storm proximity (discussed below). A bounded weak echo region (BWER) developed as the storm entered central Nuckolls County at around 2000 UTC and was prominent in each storm cycle. For example, at arount 2051 UTC , the 0.5o base reflectivity from the Weather Surveillance Radar 88 Doppler (WSR-88D) at Hastings, Nebraska (KUEX) showed a well-developed hook echo (Fig. 4a) with a prominent velocity couplet (storm relative) (Fig. 4b). The 3.4o base reflectivity (Fig 4c) depicted a circular echo free region over the inflow notch shown in Fig. 4a. The first tornado with a long track (Fig. 5) formed on the extreme northeast (forward) portion of the lower level “horseshoe” lowered base, just ahead of the rear flank downdraft (RFD) cut (not shown) near Ruskin at around the time of the radar plots shown in Fig. 4. The tornado persisted for around 8 minutes until it dwindled through its “rope out” stage. At that time, the lead author was located near Ruskin, Nebraska, approximately 10 km away from the tornado to the southwest. During the 3 hours the storm traversed Nuckolls and Thayer Counties in Nebraska and Republic County, Kansas, the radar evolution of the storm was complex. Several times the low level hook became disorganized to3 be replaced by a new mesocyclone-generated hook further southwest. During the second of these cycles, the first tornadoes developed near Nelson. (Fig. 6). In the 20 min following 2100 UTC, the storm’s lowered base showed spectacular evolution, with as many as three tornadoes visible simultaneously (Fig. 2). The storm continued to move southeastward over the next several hours. As it did so, it underwent at least two more cycles and produced tornadoes H, J and K (see Fig. 1 and Table 1). In the storm’s final cycle, reorganization essentially produced a new supercell on the southwest flank of the storm, This final phase was associated with five tornadoes in Republic County, Kansas. Figure 3 Subjective analysis of (a) 1513 and (b) 1743 UTC 24 May 2004 surface data. Storm initiation area shown as red box on b. Region