May 27, 2000 MCS event across Northern and Central Missouri
During the evening of 27 May 2000 a large
mature linear convective line extended from
far northern Missouri through central sections of the state. The convective line
already
produced isolated wind damage over parts of northwest through parts of west-central
Missouri. We tested the utility of the MARC velocity signature to
identify or further
fine-tune potential areas where damaging winds may occur along the larger
convective
line echo pattern. .
(A) Brief Synoptic Overview
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Fig 1. 0200 UTC 27 May 2000
surface analysis.
The convective squall line
traveled along and north of a warm frontal boundary which
extended from an area of low pressure near St. Joseph Missouri to just south of
St. Louis and eastward into western Kentucky. Surface dewpoints of 73°F pooled
near and south of the warm front. The squall line at 0200 UTC (shown in purple)
extended from 30 km west of Kirksville MO (IRK) to Sedalia MO (SZL). The
squall line traveled along and north of the warm frontal boundary throughout much
of its lifetime.
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Fig. 2. 0000 UTC 27 May 2000 Skew-T
Log-P from Springfield MO (KSGF). Magnitude of
Surface-based Convective Available Potential Energy (CAPE) was 2512 J/Kg. Magnitude
of 0-3 km Bulk Shear was 14 m/s (moderate shear). Surface - 500 mb delta
theta-e
was
(B) WSR-88D Imagery from
KLSX
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Fig. 2. WSR-88D plan view
reflectivity imagery (0.5° slice)
for 0217 UTC from KLSX. Reflectivity values are in dBZ.
At 0217 UTC, a large squall line extended
from near Kirksville Missouri (IRK) to 10 km
west of Sedalia Missouri. Several small poorly defined bowing segments, embedded
within the larger squall line, were identified from Kirksville to Macon Missouri.
The warm
frontal boundary at this time stretched from just north of Jefferson City (JEF) to just
south
of St. Louis. Note the development of new convection from Columbia Missouri
northeast
through Mexico Missouri just north of the warm frontal boundary.
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from KLSX. Range rings are 50 nm. Light blue color arrows point to the location of MARC at this time.
The WSR-88D reflectivity image at 0247 UTC (0.5° slice) from KLSX shows two small
embedded bowing segments within the larger squall line. The northern most segment was moving across
northern Shelby county while the second segment traveled across northern and central parts of Monroe
county. The MARC velocity signature is identified at three locations (white arrows);
(a) along the leading edge of the northern segment (central Shelby county - 12 Kft AGL), (b) along
the northern part of the second segment (northern Monroe county - 11 Kft AGL) and (c) with the
convective cluster near Harrisburg Missouri or 15 to 20 km northwest of Columbia (northwest Boone county 11 Kft AGL). Two critical points
must be raised with the two northern MARC signatures; 1) the viewing angle is becoming more critical
with the far northern MARC velocity signature where the magnitude of MARC is likely underestimated, and 2) an enhanced
local area of MARC is identified over the northern part of the second bowing segment. With an
increasing number of cases, our studies have shown that these enhanced areas may be linked to more intense
surface wind damage.
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Fig. 4. May 27, 2000, 0303 UTC;
Four panel presentation reflectivity / Storm-relative velocity
(SRM) images. Top reflectivity / SRM velocity images (1.5° slice); Bottom reflectivity /
SRM
velocity images (0.5° slice).
At 0303 UTC, the far northern
bowing segment moved into northern Marion and southern Lewis counties in
in northeast Missouri while the second bowing segment entered southeast Shelby - northeast
Monroe
counties. Strong reflectivity gradients along the storm's forward flank suggested
the presence of strong
updrafts with this storm. Further southwest, a third linear segment entered western
Audrain county. Similar
to the Monroe county storm, strong reflectivity gradients along the storm's forward flank
was suggestive
of strong updrafts. The SRM velocity image at 0.5°slice (lower right) showed a
nearly continuous narrow
ribbon of outbound velocities along the convective line's leading flank (light blue
arrows). This narrow
ribbon is a reflection of the squall line's updraft current. During my participation
in the North Dakota
Cloud Modification project (late 1970s), pilots I directed along the leading edge of
convective lines near
cloud base generally reported nearly a smooth - continuous current of rising air (region
of updraft /
800 - 1000 ft/min). However, in some cases, pilots reported local pockets (e.g. 2 -
4 miles long) of
stronger updrafts (e.g. 1500 - 2000 ft/min rise) embedded within the larger rising
current. Some of these
local pockets of stronger updrafts were directly downshear from the storm's high
reflectivity core region.
The brighter green inbound SRM velocities (37 kts) over northern Boone and western Audrain
counties
reflect the southern storm's mesoscale rear inflow. The SRM velocity image
at 1.5° slice showed two
regions of MARC over northeast Monroe and northeast Boone counties. The magnitude of
MARC
increased to 43 m/s (northeast Boone - 5.18 km/ 17 Kft), and 30 m/s (northeast Monroe -
5.2 km/17.2 Kft)
respectively from 0247 UTC image. Based on MARC values from the preceding two volume
scans and
the current 0303 UTC images, severe thunderstorm warnings were issued for northern Monroe,
southern
Shelby and the western half of Audrain counties.
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Fig. 4. May 27, 2000, 0313 UTC;
Four panel presentation of reflectivity / storm-relative velocity (SRM)
image from KLSX. Top reflectivity / SRM velocity images (1.5°slice); lower
reflectivity / SRM velocity
images (0.5°slice).
The four panel presentation at 0313 UTC in
Fig. 4 shows the second of the first two original small bowing
segments embedded with the larger squall line moving into southern Marion and northern
Ralls counties
in northeast Missouri. A third linear segment began to show a bowing structure at
this time as it traveled
across was moving across west-central Audrain county. Reflectivity values exceeding
60 dBZ were
common with each convective segment. The viewing angle of the northern bowing
segment (Ralls county
storm) became increasingly more critical in viewing the MARC velocity signature. A
relatively strong
magnitude of MARC (32 m/s - 1.5° slice) is observed along the southern flank of this
bowing segment.
The third convective segment over western Audrain county continued to reveal a strong
low-level reflectivity
gradient along the leading edge of the segment suggesting the presence of a strong updraft
/ downdraft
structure. Strong radial convergence was detected at the 1.5° slice (45 m/s (near
location of the blue
arrow). The blue arrow on the 0.5° SRM slice shows the storm's advancing cold
pool. The region of
inbounds extending from the location of MARC upwind (northwest) to the Audrain - Monroe
county line
continues to reflect the presence of a mesoscale rear inflow. Damaging winds occurred 3 -
5 minutes
after 0313 UTC over parts of Mexico Missouri.
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Fig 5. WSR-88D reflectivity (left)
and storm-relative velocity (right) cross-sections along the 300° radial from 79 nm
to 56 nm away from KLSX at 0313 UTC 27
May 2000. The cross-section is taken nearly orthogonal to the convective
line over central Audrain county.
Reflectivity and storm-relative velocity
cross-sections were taken along the 300° radial at 0313 UTC over
central Audrain county. The reflectivity cross-section revealed multicell evolution
with a new convective
tower between 10 and 12 nm and Weak Echo Region (WER) along the storm's forward flank.
The
later stages of a mature storm is noted between 7 and 10 nm with a descending high
reflectivity core
below 14 Kft. Storm-relative velocity cross-section shows the MARC velocity
signature at an altitiude
between 10 to 15 Kft (12 nm) just below the developing new convective tower and in the
vicinity of the
WER. The strongest convective outflows (medium green color) appeared to be
associated with the
descending high reflectivity core (7 - 10 nm). Two other interesting velocity
features were also
identified with this cross-section. First, the initial surge of the convective system's
cold pool accelerated
downwind (eastward) to near 14 nm (or 3 nm) downwind from the mature storm (descending
high
reflectivity core. Convective outflows associated with MCSs can be as deep as 5 to 7
kft
(Burgess 1999). Secondly, local outbound velocity maximums embedded with the MCS's
front-to-rear flow are detected at an altitude of 21 - 25 kft (10 - 11 nm) and
again near 21 - 23 kft
(8 nm). Smull and Houze (1985) have documented these local velocity maximums with
MCS events
across south-central Kansas during the Pre-Storm Project.
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Fig. 6. Tracks of MARC identified
with the May 27, 2000 MCS event.
W represents locations of wind damage. A signifies location of
Hail and dashed lines indicate location of tornadic damage tracks.
The map above shows wind
damage reports and traces of where a persistent MARC velocity signature was
identified; traces A - G (Fig 6). We will take a closer look at traces D and E
and their respective time-height
MARC traces. It is important to note that the surface warm frontal boundary during this
period extended
from near St. Joseph to just south of Columbia and southeast to just south of St. Louis
Missouri at this time.
It is difficult to determine the depth of the stable layer, however MARC traces (B, E, and
G) located further
north and away from the warm frontal boundary are likely over a region of a deeper
low-level stable layer.
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Fig. 7. (a) represents
Trace D time-height cross-section of MARC across northern Boone through central Audrain
counties
in central Missouri. (b) represents Trace E time-height cross-section of MARC across
northern Monroe through northern
Ralls counties in northeast Missouri. Magnitudes of MARC are in (m/s). W represents
time of wind damage.
Characteristics of the magnitudes of MARC for
traces D and E are shown above (Fig. 7a and 7b respectively).
Trace D initially showed that MARC magnitudes were near or just below 25 m/s at the lowest
two elevation
slices for the period 0242 and 0247 UTC. At the 1.5° elevation slice, values of
MARC increased to 31 m/s
at 5.5 km (0253 UTC) suggesting a increased potential for damaging winds. A second
and significant
increase in MARC magnitudes on the 1.5° slice is observed after 0258 UTC where velocity
differentials
increased from 29 to 43 m/s (5.2 and 4.9 km (17.2 - 16.0 Kft) respectively). The
trend of significantly higher
values further add credence for a higher damaging winds threat. Wind damage occurred
just after 0313 UTC
in the city of Mexico Missouri or approximately 21 minutes after values exceeded 25
m/s.
Trace E, north of Trace D, is located in a region where a deeper low-level stable layer of
air is likely present.
At the initial starting point of Trace E over northwest Monroe county, MARC values of 40
m/s were observed
near 3 km suggesting strong radial convergence in the vicinity of the storm's WER
(northern flank of the
storm). Such magnitudes would suggest a high probability of damaging winds.
However, in this case, one
needs to be aware of a stable layer near the surface. Can convective-scale
downdrafts penetrate through
this stable layer? At the next volume scan, magnitudes of MARC fell to 30 m/s, still
suggesting the probability
of damaging winds. Isolated wind damage did occur between Traces E and C over
north-central Monroe
county at approximately 0255 UTC. Between 0258 and 0308 UTC, MARC values of 30 to 35
m/s at and
just below 5 km were detected and continued to suggest the potential for damaging winds.
The storm
during this period (not shown) showed signs of bowing with a rear inflow notch along the
trailing flank.
At approximately 0305 UTC, a second isolated report of wind damage occurred over northeast
Monroe
county. Beyond 0308 UTC, MARC magnitudes continued to 30 m/s between altitudes of 4
and 5 km along
the southern part of the bowing structure, even with an increasingly poorer viewing angle
with time. From
many of our earlier observations, we found the strongest magnitudes of MARC at or just
below 5 km (18 Kft).
This case further supports our earlier findings. The consistent strong values of
MARC between 4 and 5 km
(0308 to 0323 UTC) continue to suggest the potential for damaging winds.
Isolated reports of wind damage
occurred for a third time with this storm at and after 0323 UTC over northern Ralls county
Missouri.
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Fig. 8 Map of wind damage
reports (W), and tornado tracks from central Missouri
through west-central Illinois. Squall
Line positions are denoted approximately
every 30 minutes.
Fig. 8 is a map showing, squall line positions every 30 minutes, reports of damaging winds
(W), and hail (A).
Dashed line signify location of tornadic damage. The squall line positions show a
broad bowing structure
entering northeast through central Missouri around 0300 UTC. One hour later (0400
UTC) two smaller
bowing segments are observed over Pike county Missouri and Pike county Illinois.
Smaller bands of
scattered smaller convection extended orthogonal to the bowing segments. More
information on
the issue of tornadogenesis with this case will be forth-coming in Group 2 "Intensifying
Stage of MCS
Evolution."
