ON THE NEED FOR VOLUMETRIC RADAR DATA WHEN ISSUING SEVERE THUNDERSTORM AND TORNADO WARNINGS David A. Imy* NOANNEXRAO/OSF Norman, OK Kevin J. Pence NOANNWS Forecast Office Jackson, MS Charles A. Doswell III NOANERLlNational Severe Storms Laboratory Norman, OK Abstract a tornado warning was necessary for this storm. No other warnings were issued by JAN in the three Mississippi count­ On 28 February 1987, a violent tornado (F4) struck Jones ies which the storm had moved through prior to this time. COl/nty in southeast Mississippi. The radar echo associated This paper briefly investigates the environment that pro­ with this storm exhibited low-level features that only can be duced the event and then describes the evolution of the storm speculated upon, because a volumetric radar scan was not at low levels (0.5") as observed by the WSR-57 radar at done. This paper focuses on reasons why examining the JAN. Following the examination of the radar echo, a short three-dimensional structure of a storm is necessary for issu­ summary delineating the significant storm-structural features ing effective warnings. The volumetric or "tilt" scan also indicative of severe and tornadic storms is included. Lastly, (~ids meteorologists in understanding the origins of low -level a brief description of some tilt techniques for use with con­ features, such as those that will be illustrated in this manu­ ventional radars and the Weather Surveillance Radar-1988 script. This strategy will be a routine, automatic part of .Qoppler (WSR-88D) system (Alberty et al. 1991) will be pre­ warning operations with the Weather Surveillance Radar- sented. 1988 !2oppler (WSR-88D) system, so it behooves fol:ecasters to become familiar with the basic idea. Use of the WSR- 88D system to evaluate the three-dimensional structure (~f 2. The Synoptic and Mesoscale Environment storms also will be discussed. We take it as axiomatic that effective warnings for severe weather begin with a careful examination of the synoptic 1. Introduction and mesoscale structure prior to convection. Anticipating Between 1500 and 1600 (all times UTC) on 28 February potential severe convective events during a forecast shift is 1987, an intense thunderstorm spawned a devastating tor­ a critical part of detection and recognition; one must know nado in southeast Mississippi (Fig. O. The tornado, rated what one is looking for in order to find it. Analysis of such F4 on the Fujita intensity scale (Fujita, 1971), was on the parameters as shear and buoyancy is extremely critical, but ground for 40 min with an average path width of 400 yards it is most important to assess how changes in the synoptic (370 m), resulting in 6 deaths and 350 injuries. The event and mesoscale environment will affect these values prior to occurred on a Saturday, which was fortunate, since the the development of the convection. Glade, Mississippi elementary school was demolished. An The synoptic situation on 28 February 1987 was typical of estimated 28.5 million dollars in damage occurred, with 383 potentially severe convective situations in the southeastern structures destroyed and an additional 270 damaged. As bad United States during the cool months (Davies 1989; Davies­ as the damage was, the number of fatalities and property Jones et al. 1990). As we shall see, even though the atmo­ damage likely would have been significantly worse had the sphere was only slightly unstable (as measured by, e.g., tornado tracked through a major urban area on a weekday. Convective Available Potential Energy (CAPE)), the vertical This tornadic storm occurred about 50 n mi (93 km) south­ wind shear was strong, with impressive helicity values, a west of the Meridian, Mississippi (MEl) National Weather combination favorable for severe weather during the cool Service Office (NWSO) and occurred in their area of warning season (Johns et al. 1990). responsibility. The National Weather Service Forecast A persistent long-wave trough had been quasi-stationary Office (NWSFO) in Jackson, Mississippi (JAN) also was over the western United States during late February 1987. monitoring the storm during the time it spawned the tornado However, in response to the amplification ofa trough moving since the storm had moved through their warning area earlier: southeastward from the Gulf of Alaska, the western United Although a tilt scan was not pelformed at JAN, a tornado States trough began to move slowly eastward on 24 February. warning was issued approximately IO min before the tornado As is generally true of all such systems, several short-wave initially touched down. The warning was issued because a troughs were rotating through the long-wave trough. One of brief reflectivity hook was noted by an attentive radar opera­ these short-wave troughs moved into the base of the long­ tor; otherwise, JAN echo might not have alerted MEl that wave trough on the 27th (not shown), and into southeast Texas on the 28th (Fig. 2). This short-wave trough became increasingly negatively tilted as it moved northeastward from "'Present Affiliation: NOAA/NWSFO, Denver, CO. the base of the long-wave trough. 2 Volume 17 Number 4 November, 1992 3 JONES COUNTY TORNADO (F4) February 28, 1987 o I I I 0 1 '/ I 1 1 US-6-4 Northeast Jones High School Warehouse (PIB) l::'Pine Belt F Scaler 5 10 15 mi ! ! ! Fig. 1. Map of Jones county tornado track (adapted from Storm Data). 4 National Weather Digest -2 -15------ -10 576' ---------- Fig. 2. 500-mb wind data and analysis at 1200 UTC, 28 February 1987. Solid lines are geopotential height contours (interval 60 dm), thin dashed lines are isotherms (interval 5°C), and thick dashed lines indicate locations of short-wave troughs. At 120028 February, a surface cold front extended south­ The Lifted Index value for the 1200 JAN sounding was ward into central Louisiana from a complex surface low - 3 (Fig. 5), while at Boothville, Louisiana (BVE), the value pressure system in eastern Oklahoma (Fig. 3). Although it was only - I. (BVE is located at the site plotted in southeast is difficult to determine from the 1200 sUlface data alone, a Louisiana on Fig. 2.) A more comprehensive measure of prefrontal pressure trough was located approximately 50 n buoyant energy is the CAPE, which is a measure of the mi (93 km) east of the front and was distinguished by an positive area on a sounding located between the Level of accompanying line of thunderstorms. These thunderstorms Free Convection (LFC) and Equilibrium Level (EL). Using had developed the previous evening along the cold front in the 1500 surface temperture of 66°F (l9.4°C) and dewpoint central Texas, and had moved ahead of the front during of 65°F (18.8°C) at Pine Belt, Mississippi (PIB), the CAPEs the night. Three-hourly surface pressure changes (Fig. 4) at JAN and BVE were 685 m~ s -2 and 70 m~ s -2, respectively. depicted a fall/rise couplet that strengthened as it moved Considering a sUlface parcel from further south, at New from southwest Louisiana into southern Mississippi. This Orleans, Louisiana with a temperature of 70°F (21.I OC) and couplet appeared to be associated with a northeastward mov­ a dewpoint of 69°F (20.6°C), the CAPE values at JAN and ing mesolow that developed along the prefrontal trough in BVE increased to 1148 m2 s - ~ and 532 m2 s-~, respectively. southern Louisiana prior to 1200 . A slow moving warm front CAPE values from 1500 to 2500 m2 s - ~ represent significantly extended from the low pressure system in Oklahoma south­ unstable soundings, so it can be seen that this amount of eastward into Mississippi. This warm front was oriented from buoyancy was relatively modest. the northwest to southeast across southern Mississippi, per­ The storm-relative helicity in the lowest 2-3 km of the pendicular to the track of the meso low . Higher dewpoints atmosphere, defined and discussed in Doswell (1991), has were surging northward to the east of the surface trough and become accepted as a relevant parameter in assessing the south of the warm front , which was increasing the instability potential for supercell development. 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