An Analysis of Five Springtime Derechos

1. Introduction

Five derechos which affected middle Tennessee between 1993-1997 have been analyzed. These derechos, one each year of the study, have been separated according to type. Warm season derecho environments were applied to the Johns (1987) checklist. Transition season derechos are described using Duke parameters (1992). The environments conducive to these two types can be identified before they occur.

A derecho is a widespread convectively induced windstorm that includes a family of downburst clusters produced by an extratropical mesoscale convective system (MCS) (Fujita 1981, Johns 1987). There are two types of derechos. The progressive pattern is characterized by a short, curved squall line oriented nearly perpendicular to the mean wind. The serial pattern is characterized by a squall line oriented nearly parallel to the mean wind.

Since derechos are known to produce widespread wind damage (and tornadoes), forecasting methods which allude to their potential will prove useful. Techniques were developed by Johns (1987) for the warm season. Transition season parameters introduced by Duke (1992) are used to develop a similar technique. This study will focus on the Johns (1987) parameters and outline techniques for recognizing environments conducive to transition season derechos.

Other atmospheric features which are thought to aid derecho generation are described. These include: 1) The gravity wave which sustained the 1993 warm season derecho; 2) The rear inflow jet responsible for enhanced concavity of the bow echo event in 1996; 3) The left exit region of a 500 mb jet maximum over middle Tennessee which helped produce two successive bow echo complexes in 1997.

2. The 4 June 1993 Warm Season Derecho

During the morning and early afternoon of 4 June 1993, a storm complex moved across southern Missouri, southern Illinois, western and central Kentucky, and northern middle Tennessee (Small 1996). An associated high precipitation (HP) supercell evolved into a derecho (Moller 1990, Przybylinski 1993). The derecho moved slightly south of and parallel to a surface boundary along the Ohio River valley. This serial derecho produced widespread wind damage (and F0/F1 tornadoes) exceeding $70 million and injured 79 (Storm Data, June 1993). Przybylinski (1985) showed that bow echoes often contain tornadoes along their leading edge. Funk (1996) and Przybylinski (1996) produced further evidence that short lived, weak, non-supercell tornadoes can form along the leading edge of a gust front.

At 1200 UTC, a stationary front extended west-east across Kentucky and southern Missouri with a low pressure center (1002 mb) over southwest Missouri. A weak 500 mb shortwave extended north-south across the southern plains states. The 1200 UTC Paducah, KY (PAH) sounding indicated a Convective Available Potential Energy (CAPE) of 1838 J kg^-1 and a lifted index (LI) of -6. The low level wind profile indicated little directional shear, but significant speed shear.

The first report of wind damage was at 1345 UTC about 20 miles northwest of Poplar Bluff, MO. Initial storm movement was from the west at 45 kt, although the bow echo complex quickly accelerated as it moved across southeast Missouri (50 kt) and western Kentucky, reaching PAH at 1525 UTC. The derecho reached Hopkinsville, KY at 1615 UTC, speed having increased to 70 kt. The derecho continued across central Kentucky and northern middle Tennessee, maintaining an average speed of 60 kt. The storm reached Bowling Green, KY/Gallatin, TN at 1700 UTC and continued into eastern Kentucky, reaching Somerset at 1810 UTC. The derecho exited Kentucky by 2000 UTC and continued eastward through Ohio and West Virginia. By 0000 UTC, 5 June, the derecho had moved to northeast North Carolina, producing windespread wind damage (Kreis 1996).

Although the derecho evolved from an HP supercell, it was likely sustained by a gravity wave, as suggested by the 60 kt movement of the derecho. The gravity wave developed in an atmosphere exhibiting stability near the surface, with instability aloft. (The 1200 UTC PAH sounding indicated a strong inversion between the surface and 925 mb.) Lewis (1996) states that gravity waves thrive near boundaries. Convergence near boundaries forces air to rise with capping, and the lack of buoyancy will cause the air to sink to its original position. Repetitive upward and downward motion beneath the inversion often induces gravity waves. Areas of high and low pressure, or components of a gravity wave (i.e., crests and troughs), often propagate with and maintain convection above the capped layer (Forbes 1993). Schmidt (1990), in his strong shear numerically simulated squall line, found that gravity waves exert strong control of wind flow and thermodynamic structures of squall lines. Similar conclusions of gravity wave effects on squall line structure were reached by Nicholls (1990).

Storms typically move with the mean wind (the average wind vector between the surface and 500 mb). The 1200 UTC PAH sounding indicated a mean wind of 250^o at 25 kt. When enhanced by the gravity wave, the derecho moved more than twice the speed of the mean wind and 1.5 times the 500 mb wind of 40 kt.

Gravity waves weaken considerably when capping is eliminated. Lindzen (1976) states that without trapping, the amplitudes of most gravity waves decrease rapidly. Accordingly, this derecho weakened considerably as it entered Ohio and West Virginia during the late afternoon after the capping had been eradicated by afternoon heating.

Applying atmospheric parameters to the Johns (1987) checklist showed that forecasters could easily have been alerted to the derecho potential. Environmental parameters satisfied the criteria.

3. The 15 April 1994 Transition Season Derecho

During the evening of April 14, 1994, a squall line formed over west central Missouri (Przybylinski 1996). This long lived squall line evolved into a serial derecho and produced widespread damage across portions of Missouri, Kentucky, and extreme north central Tennessee. The derecho preceded a cold front which, at 0000 UTC, 15 April, extended southwest from eastern Nebraska into the Texas/Oklahoma panhandles. By 1200 UTC, the cold front had moved into Missouri and northwest Arkansas. The 500 mb analysis indicated a deep shortwave extending southwest from the Dakotas into New Mexico.

At 1200 UTC, a 50 kt 850 mb jet was located over the southern Ohio valley (Funk 1996). This low level jet, coupled with a 300 mb jet streak, enhanced the derecho over southern Kentucky and northern middle Tennessee. The atmosphere across this region was very unstable. Using the SHARP workstation (Hart 1991), the 1200 UTC soundings taken at PAH and Nashville, TN (OHX) indicated LI's of -4 to -8 with CAPE between 2000 and 2500 J kg^{- 1}. Wind profiles taken from the soundings and WSR-88D at Louisville, KY indicated approximately 50 kt of wind shear (mostly speed shear) in the 0-2 km layer. Furthermore, helicities were 250-400 m²s^-2, with a Bulk Richardson Number (BRN) below 50. This profile is indicative of environments conducive to bow echoes (Johns 1993).

A comparison of the parameters of the 15 April environment with those of two additional transition season derechos, the 9 April 1991 event described by Duke (1992) and the 20 April 1996 event described in section 5 of this paper, is outlined in the conclusion.

4. The 14 May 1995 Warm Season Derecho

During the early morning of 14 May 1995, a line of severe thunderstorms formed over extreme southeast Missouri and western and central Kentucky. These thunderstorms evolved into a bow echo complex which produced wind damage and large hail across middle Tennessee. This derecho was classified as serial, its orientation being parallel to the mean wind direction.

The 0000 UTC OHX sounding indicated a Showalter index of -6, LI of -8, and CAPE of 3995 J kg^-1. Helicity within the 0-3 km layer was 330 m²s^{- 2} with BRN of 45, indicating the possibility of bow echo formation.

The line of severe thunderstorms moved southeast from southern Illinois at 0400 UTC to along the KY-TN border at 0800 UTC. The Fort Campbell Army Base, 50 miles northwest of OHX, measured a 70 kt wind gust. By 1000 UTC, extensive wind damage had occurred in and to the west of the Nashville metropolitan area. The southern extent of the squall line slowed as the line evolved into a bow echo northeast of OHX.

By 1200 UTC, a cold front extending from the Illinois-Kentucky border west-southwest into central Oklahoma had become nearly stationary. The front was supported by a 500 mb shortwave stacked vertically above the surface front.

Widespread wind damage continued along the derecho path until it weakened while moving across the higher terrain of the Cumberland Plateau east of OHX.

Again applying atmospheric parameters to the Johns (1987) checklist shows that forecasters could easily have been alerted to the derecho potential. Environmental parameters satisfied the criteria.

5. The 20 April 1996 Transition Season Derecho

During the early morning of 20 April 1996, a line of severe thunderstorms formed in northwest middle Tennessee. This line evolved into a bow echo complex as it moved rapidly east across middle and east Tennessee, causing wind damage in excess of $6 million. In middle Tennessee, fifty-five incidents of wind damage, fifteen cases of hail at least 0.75 inches in diameter, three at least 1.75 inches, three incidents of flash flooding, and two tornadoes were reported. The tornadoes, F0/F2, formed along the leading edge of this serial derecho. Moller (1990) and Johns (1992) showed that stronger tornadoes such as the F2 which formed in southern middle Tennessee have been observed at other points along a bow echo.

The environment across middle Tennessee at 0000 UTC was conducive for derecho development. A cold front associated with low pressure (994 mb) over southern Wisconsin was approaching middle Tennessee. The cold front extended southwest through the Mississippi River valley and into Texas. A strong 500 mb shortwave lying west of the Mississippi River valley defined the mid level pattern. The 500 mb wind was 65 knots over OHX. A strong southerly low level flow advected significant moisture into middle Tennessee. A warm air advection maximum existed just east of the derecho genesis area. Surface relative humidity (RH) surrounding the genesis area was greater than 80%, and highest RH at 850 mb was also near the genesis area. Conversely, mid level moisture (700 and 500 mb), were low (T- T_d>10^o C).

Surface theta-e values were greater than 330K, with a theta-e ridge lying along the genesis area. The derecho formed along this theta-e ridge with its axis parallel to the ridge. Movement was parallel to the theta-e gradient. The CAPE at OHX was 2640 J kg^-1, and the BRN of 32 supported bow echo formation. Vertical directional wind shear was 70^o in the lowest 2500 m. High buoyancy and significant low level vertical wind shear favored long lived (steady state) squall line thunderstorms (Weisman 1984).

WSR-88D reflectivity imagery indicated bowing of the derecho as it moved across middle Tennessee. Such concavity gave strong evidence of a locally stronger current within the rear inflow jet as described by Smull (1987). Weisman (1990, 1992, 1993) described the importance of the rear inflow jet in maintaining a long lived convective system and intense damage paths.

A comparison of these parameters with those of two additional transition season derechos, the 9 April 1991 event described by Duke (1992) and the 15 April 1994 event, is outlined in the conclusion.

6. The 13 June 1997 Warm Season Derecho

During the afternoon of 13 June 1997, a line of thunderstorms moved into middle Tennessee. This line was spawned by a mesoscale convective complex (MCC) which had moved across portions of southeast Missouri, southern Illinois, and western Kentucky. The MCC, defined by Maddox (1980), had developed near an area of low pressure centered near Kansas City, MO and an associated east-west surface trough.

The 1200 UTC OHX sounding was modified using the SHARP program (Hart 1991), giving an LI of -8 and CAPE approximately 3000 J kg^-1, indicating deep instability. Unlike most derecho producing environments, though, wind shear indices were low with negative helicity and BRN of 119.

As widespread wind damage occurred across middle Tennessee, the line developed into a bow echo complex which continued to produce widespread wind damage. Following this derecho and subsequent rapid destabilization, a second derecho occurred during the evening. As before, a line of thunderstorms moved into middle Tennessee and central Kentucky. This second bow echo complex was marked by a large comma head in south central Kentucky, and produced extensive wind damage. The derechos were classified as progressive.

Interestingly, the 0000 UTC, 14 June 500 mb analysis indicated a 50 kt wind maximum over southwest Missouri, with middle Tennessee in the left exit region. This, according to Uccellini (1975) is a favorable location for upward vertical motion.

Environmental parameters during the morning of 13 June satisfied the Johns (1987) criteria. A PCGRIDDS macro written by the authors was used to apply the criteria to forecast data, alerting forecasters to the potential for derecho genesis across middle Tennessee.

7. Conclusions

Several conclusions regarding transition and warm season derechos have been made. The Johns checklist (1987) works well in recognizing warm season derecho producing environments. The only discrepancy involves 500 mb 60 m height falls listed in Part B of the checklist. With all three warm season derechos described here, no such height falls occurred. While important, such height falls did not occur with these events in middle Tennessee. All other criteria listed by Johns were satisfied in all three warm season events.

The most notable difference between transition and warm season environments is surface boundary characteristics. An identifying warm season feature is the absence of a progressive cold front. In each event noted, and in accordance with the Johns (1987) parameters, a quasi stationary surface boundary existed north of the initiation region. A strong, east moving cold front was west of the initiation region in each transition season event.

While precursors of warm season derechos have been thoroughly described by Johns (1987), the authors have made conclusions regarding transition season derechos in order to establish recognition techniques. Using the two transition season events listed in this paper and the 9 April 1991 derecho described by Duke (1992), similarities have been noted. 1) Transition season derechos are marked by approaching cold fronts accompanied by 500 mb troughs of at least moderate strength. 2) Derechos tend to form along surface and 850 mb moisture ridges where dry air intrusion is also occurring at 700 and 500 mb. This area is also characterized by a surface theta-e ridge. 3) All three transition season derechos mentioned originated in regions where CAPE was greater than 2200 J kg^-1 and BRN indicated support for bow echo formation. Weisman (1993) showed that generation of long lived bow echo complexes requires CAPE at least 2000 J kg^-1 and strong low level wind speed shear. 4) Derecho movement tends to be toward the 850 and 700 mb thermal ridges.

Parameters for both transition and warm season derechos have been incorporated into PCGRIDDS macros, providing a simple method for recognizing derecho potential.

Acknowledgements

The authors thank Darrell Massie and Scott Sharp, meteorologists, Chris Darden, meteorologist intern, and Henry Steigerwaldt, Science-and-Operations-Officer, National Weather Service Office Nashville, TN, for their reviews of this paper.

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