Case #2:  June 29th, 1998 Derecho Over Much of Central Illinois

During the morning of 29 June 1998, a small cluster of severe thunderstorms formed over southeast
South Dakota and moved rapidly east-southeastward across northwest through central Iowa.  Many
of the storms over northwest Iowa exhibited hybrid supercellular structures with strong mid-level
rotation and produced large hail and damaging winds.  The storms evolved into a developing bow
echo complex as they moved into eastern and southern Iowa after 1900 UTC.  Surface-based CAPE
values exceeded 3300 J/Kg from parts of central through southeastern Iowa while the vertical wind 
shear at 1700 UTC from the Slater Iowa profiler revealed a 'deep-layer shear' profile supporting the
development of isolated hybrid supercells.  Magnitudes of 0-6 km (0-3 km) Bulk Shear were
28 m s-1 (15 m/s) respectively (Strong deep-layer shear - moderate shear within the 0-3 km layer).


Fig 1 
Satellite analyses for 2015 UTC 29 June 1998.  Superimposed on the visible imagery is
surface data showing temperature, dewpoint (both in °C) and winds.  Also plotted is the radar
reflectivity field (dBZ) from KDVN and KILX radars, with values of 35 and 50 dBZ contoured and
shaded in black respectively.  Short-dashed lines are state boundaries.  The stationary front,
outflow boundary, and leading edge of moisture surge are also indicated. (Click on image for a 
larger image)

AT 2000 UTC surface analyses showed a moisture boundary extending from the bow echo over
southeast Iowa thorugh central Illinois while a quasi-stationary frontal boundary extended from 
near the northern part of the bow through northeast Illinois.  Surface dewpoints ranged from 76
to 79 south and southwest of the moisture boundary and below 74 degrees north and east of
this feature.  Weak convection formed along the moisture boundary between 1900 and 2000 UTC.


Fig. 2  Skew-T Log-P diagram of the 1800 UTC 29 June 1998 Sounding from Lincoln Illinois (KILX).
 (Click on image for a larger image)


The 1800 UTC KILX sounding showed a weak subsidence inversion - isothermal layer between
910 - 850 mb while the lapse rate was nearly dry adiabatic above 850 to near 700 mb.  Dry air was
present above 650 mb.  The sounding at this time revealed moderate instabilitiy with surface-based
CAPE values were 1630 J/kg.  Magnitudes of 0-3 (0-5) km Bulk Shear at 1800 UTC were 14
(16) m s-1 respectively (moderate shear category).


Fig. 3
  Velocity Wind Profile (VWP) from WFO Lincoln IL (KILX) for the period of
2051 - 2143 UTC. (Click on image for a larger image)

Magnitudes of Bulk Shear were calculated for 0 -3 km layer af 2051UTC and for 0 - 3 km layer
(0 - 6 km) layer at 2120 UTC.  At 2051 UTC the magnitude of Bulk Shear was 13 m s-1 (lower
end of moderate shear) and for 2120 UTC 11 (17) m s-1 respectively (weak shear / moderate
shear 0 - 6 km).  Magnitudes for storm-relative helicity (SRH) were also computed for 0 -1, 0 -2,
and 0 - 3 km for 2102, 2114, 2132 and 2143 UTC respectively from the KILX VWP.  The 
magnitudes of SRH (m2 s-2) are shown in the Table 1 below.


0-1 km
0 - 2 km
0 - 3 km

Table 1  Magnitudes of storm-relative helicity (SRH) taken from the VWP.  Times are in UTC.
Storm motion of 280° 21 m s-1 was considered. Units are in (m2 s-2).

From Table 1, the highest magnitudes of SRH were observed in the 0 - 2 and 0 - 3 km layers
between 2102 and 2132 UTC.  At 2102 UTC the bow echo was located over 100 km northwest
of KILX while at 2132 UTC the bow echo was 75 km northwest of KILX.  KILX was also located
approximately 50 km south of the moisture boundary.

Fig 4  Planview reflectivity (0.5° slice - left) and storm-relative velocity (0.5° slice - right) from
WFO Lincoln (KILX) Illinois at 2103 UTC 29 June 1998.  (Click on image for a larger image)

Mesovortices #1 and #2 during this sequence of bow echo evolution formed near the northern
end of the quasi-linear convective segment in the vicinity of the surface moisture boundary.
Note the nearly linear reflectivity segment embedded within the larger bow echo with strong
low-level reflectivity gradients along the system's downshear flank and several rear inflow
notches (RINS) along the convective system's immediate upshear flank.  The storm-relative velocity
data shows a well defined mesoscale Rear Inflow Jet.

Fig 5
  Planview reflectivity (0.5° slice - left) and storm-relative velocity (0.5° slice - right) from
WFO ILX at 2126 UTC 29 June 1998. (Click on image for a larger image)

By 2126 UTC, mesovortices 3 and 4 formed near and north of the apex (along the cyclonic shear
side of the new bowing segment.  A comma-like reflectivity pattern is seen in the vicinity of 
mesovortex #3 (near the apex of the bow).  Note all of the mesovortices became tornadic and
were located within the strong low-level reflectivity gradient region - along the leading edge of
the new bowing segment.

Fig 6
  Vertical cross-sections through the storm-reflectivity line and the mesoscale Rear
Inflow Jet from near WFO ILX through the apex of the new bowing segment and into the
trailing flank of the bowing segment.  This image was taken at 2149 UTC.
 (Click on image for a larger image)

Note the classic multicell evolution along the leading edge of the line with the new cells developing 
aloft (right side of the image) and weaker older cells near the center of the image.  The bright green 
region in the storm-relative velocity cross-section represents the gradually descending mesoscale
RIJ while the small red region above the nose of the RIJ signifies the updraft region of the line
(portion of the Front-To- Rear flow).

During the period of 2155 and 2215 UTC four mesovortices were examined with the 
29 June 1998 bow echo over central Illinois. 
Three of the four mesovortices were tornadic
while the first circulation did not spawn a tornado.  Mesovortices 1 and 2 formed in the vicinity of the
surface moisture boundary and the northern end of the developing bowing segment.  These 
mesovortices did merge with the larger northern line- end vortex.  In contrast, Mesovortices 3 and 4 
moved in the direction of the bow echo and did not merge with the larger line-end vortex.  Through 
numerical simulations, Trapp and Weisman (2003) have shown that mesovortices to decrease in 
number and grow upscale with time.  Eventually some of these mesovortices grow in size 
comparable to and even evolve into the bookend vortices.  Our observations showed that particularly 
Mesovortices #3 and #4 did not grow upscale (core diameter increasing with time) and more 
importantly do not merge with the larger line-end vortex.  In contrast Mesovortex #2 did merge with 
the larger line-end vortex.  A summary of the characteristics of these circulations are presented

Fig 7
  Rotational velocity (Vr) time-height cross-section of Mesovortex #1.
Magnitudes are in knots.  (Click on image for a larger image)

Mesovortex #1 appeared to show descending characteristics and upscale growth as the 
mesovortex evolved with time.  The strongest rotation occurred after 2108 UTC as the vortex
reached core diameter greater than 15 km.

Fig. 8
  Rotational velocity time-height cross-section of Mesovortex #2.
Values are in m s-1.

Mesovortex #2 formed near the intersection of the surface moisture boundary and the northern end of
the convective line.  This mesovortex initially formed at low-levels (below 3 km) and rapidly deepened
and intensified with time.  The strongest rotation remained below 3 km as
the meso-vortex 
deepened.  Tornado touchdown (F1) occurred just preceding the mesocirculation's greatest depth 
and during the period of strongest low-level rotation.

Fig. 9 
Rotational velocity time-height trace for Mesovortex #3 - near the apex
of the bowing segment.  Values are in m s-1.

Mesovortex #3 also showed non-descending characteristics during its early stages and was 
associated with a Tornadic Vortex Signature (TVS) during the first ten minutes of its lifetime.  A first 
tornado occurred during this early period of non-descending and prior to the vortex reaching its 
greatest depth.  No tornadic activity was reported when the vortex height dropped between 2126 and
2132 UTC.  Mesovortex #3 increased in strenght and depth a second time after 2132 and peaked
at 2144 with an overall height of 5 km.  Magnitudes of Vr reached 25 m s-1 at 2 km at 2144 (time of 
second peak).  The second tornado occurred just after 2138 UTC.  Simultaneously Mesovortex #3's 
core diameter fell from 3.7 (2138) to 2.1 km (2144) (0.5° slice).

Fig. 10 
Rotational velocity time-height trace for Mesovortex 4 - north of the apex of
the bowing segment. Values are in m s-1.

Mesovortex #4 was located north of the apex of the bow and Circ 3 and showed nearly classic non-
descending - upscale growth during the period of 2121 through 2207 UTC.  Similar to Mesovortex
#3, this circulation spawned a brief tornado during the very early stages of mesovotex evolution and
again a longer lasting tornado just preceding and during Circulation 4's greatest height and during
the period of strongest rotation within the lowest 3 km.  The core diameter was initially at or below 3
km and slightly increased in size (3.5 to 4 km) during the period of the second tornado touchdown.

Fig 11
  Composite map showing the path of radar detected circulations (solid
 black lines) and larger line-end vortex (large dashed circles) for the
 period of 2050 to 2210 UTC.  The squall line positions are marked
 by the thin solid-dash lines every 30 minutes.

Figure 11 depicts the pathways of the tornadic mesovortices that moved with the large bow echo 
during the period of 2050 to 2215 UTC.  Mesovortex #2 did merge with the larger line-end vortex 
after 2135 UTC.  However, Circulations 3 and 4 moved in the direction of the bow echo and did not 
merge with the larger line-end vortex.  Estimated surface wind gusts of 80 to nearly 100 mph 
reported along the southern periphery of the larger line-end vortex.  More information about the June 
29, 1998 Line-end vortex can be viewed in the following section. 

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