Transcript Mesoscale and Synoptic Scale Interactions Leading to

Anthony A. Rockwood
Robert A. Maddox
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An unusually intense MCS produced large hail
and wind damage in northeast Kansas and
northern Missouri during the predawn hours
of June 7th, 1982.
Takes a look at preconvective period and the
interactions between mesoscale processes and
the synoptic scale environment that lead to
convective development
Some scenarios as to why this development
occurred are proposed and examined.
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The first storms formed over southwest Nebraska
shortly after sunset.
Many forms of severe weather were produced by
this MCS:
60-90 kt wind gusts
 Large hail
 Funnel clouds
 At least one tornado
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At dawn, winds at about 90 kts
(~103 mph) came through downtown Kansas City
knocking down trees and damaging buildings as
well at creating widespread power outages.
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This event is a prime example of the
interactions between mesoscale and synoptic
scale processes
Forecasting convective weather can be
described to have two phases
Identification of large areas (on the order of 100,000
km2) where there is/may develop, a high potential
for sever storms
 Efforts to predict when and where, within these
regions, the convection will actually develop and/or
move.
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Over NE Colorado and most of Nebraska a mild,
dry air moved SE along a weak high plains trough.
Surface moisture persisted through afternoon
heating in Kansas helping form a weak dry line
between this area and the dry air behind.
In the lower troposphere, SSW flow ahead of a
cold front brought moist air across eastern Kansas
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The 0000 UTC sounding
for Topeka, Kansas
resembles a “Loaded
Gun” sounding, but the
inversion over the moist
layer was an obstacle to
thunderstorm
development. If any
storms were to develop,
the wet bulb potential
temperature being about
13° C cooler than the
surface would have
provided strong winds
with the storms.
Even though large scale
ascent in lower layers
could have weakened the
inversion by the time the
storm moved through
Topeka, it would not have
been enough to eliminate
the cap all together.
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Very high θe air (350-360 K)
flowed toward the shallow
front at lower levels with much
lower θe above.
Such a decrease in θe with
height ahead of the front is a
key characteristic of high
instability.
Behind the front, low values
(310 k) within the dry High
Plains air mass show that there
was no support for moist
convection.
RH can also be seen to be low
behind the front, but ahead of
the front, values are between
80%-90% showing high most
convective potential that is very
near saturation.
Even with all of these settings,
this air would have to be lifted
to the depth of the inversion
(about 6500-8000 ft) to reach the
LFC.
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Due to the amount of thunderstorm potential in
this area and the rapidly changing environment,
this situation was obviously difficult to forecast.
The National Severe Storms Forecast Center/
Severe local Storms Unit (NSSFC/SELS)
convective outlook included a moderate risk of
severe storms in NE Kansas.
By 1500 UTC the threat had been downgraded
because the storms in the morning were
approaching an area that was still unstable but
unexpected rapid movement of the entire synoptic
system was to shift the entire system and severe
threat more east.
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The local environment clearly changed between
0000 UTC and 0300 UTC due to large scale
processes.
A southern Colorado 850 mb low moving NE
resulted in increased moisture advection, upward
vertical motion, and convective destabilization.
This could have caused the replacement of the
large scale subsidence at 0000 UTC with upward
motion by 0300 UTC.
Midlevel moisture was shown through satellite
imagery which showed mid level cloudiness
developing in the area. This thermodynamic
profile could have supported the convective
development seen in the actual surface
observations.
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A concept proposed by Doswell et al (1985) could
explain the change in local environment.
In this concept regional areas of unstable air in the
midtroposphere are advected over low level moist
air which would create a deep convectively
unstable environment.
In this specific case of storms forming over the
western High Plains, the origin of unstable air is
usually the Rocky Mountains. If the large scale
setting is right and daytime heating combines with
dynamic lift then it can produce unstable air in the
midtroposphere which can be carried over the
Plains to the east.
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Composit of some of the key
convection parameters including
lapse rates in the 700-500 mb layer.
These high lapse rates over central
and southeast Colorado are the
combined effects of surface heating
of elevated terrain, differential
temperature advection, and
upward vertical motion.
This figure also shows the
combination of factors that came
together to form this strong MCS
It has also been suggested that
diurnal changes in the low level
wind field may have increased the
advection of warm air and moisture
into SW Nebraska, and that ascent
of air could have led to saturation
and thunderstorm development.
This may not be the case because of
such large scale interactions that
were probably underway well
before sunset.
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While the large scale was primarily
responsible for providing the
destabilization to the area,
mesoscale processes can be shown
to have provided the low level
lifting necessary to initiate
convection.
At 0300 UTC satellite imagery
shows the development of clouds
along the intersection between the
dryline and approaching surface
trough. This convergence could
have combined with outflows from
a larger area of convective clouds
moving from NE Colorado into SE
Nebraska.
This could be seen as the lift that
created the initial convection.
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Another mechanism that could initiate convection can
be called “underrunning” (Carlson et al (1983))
Moist boundary layer air flowing northward ahead of a
front is overrun by a southwesterly flow of hot, dry
continental tropical air that originated over elevated
terrain to the SE. While enhancing convective
instability, the dry air creates a kind of “lid” that
suppresses deep convection.
The moist low level air flows out from beneath the
“lid” or “underruns” the lid along its northern
boundary.
Case studies show that along this “lid” thunderstorms
can form, and as in this case, cold advection aloft
would decrease stability.
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This cross section shows that there
is a slight increase in low level
southerly flow toward the front,
but most of the flow aloft is
southwesterly, or parallel to the
front. This would make
underrunning have a hard time
being a significant process in this
case.
In the region of initial storms the
low level moisture was overrun by
the cool dry High Plains air rather
than by the warm air.
The Northern boundary of the “lid”
is extended from SW Kansas to NE
Kansas which is pretty far south of
the initial storm area. If the
convection developed where the
moist air was underrunning the lid
then the storms would have been
more in central Kansas.
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Satellite imagery shows that the storm system was slow to intensify.
The θe cross section shows that early development started in areas of
320-330 K air with slight convective instability.
Very moist and unstable air was just to the east where large scale
ascent was occurring. Rapid expansion of the cloud shield began by
0730 UTC and by 0830 UTC the satellite view of the system met the
initiation criteria of an MCC and many reports of sever weather
began to come in.
This explosive intensification came about when the systems main
outflow pushed southeastward through the frontal zone which
caused lifting and released the moist potential energy of the
prefrontal environment that we saw in the Topeka sounding. Low
level air with θe values of 350-360 K and RH of over 90% were fed into
the system while very low θe above the inflow gave the downdrafts
even more acceleration downward.
With all of this intensification, outflow winds of almost 90 kt were
reported in parts of NE Kansas as the MCC reached its maximum
extent just before dawn.
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Synoptic scale data over the area of initial storms indicated
slight convective potential and quasi-geostrophically forced
subsidence.
Advection of moist, unstable, ascending air, associated with
the movement of an upper level shortwave trough was
increasing the potential.
In the destabilizing environment, mesoscale convergence
was enhanced by outflows from high convective clouds
which initiated thunderstorms along the surface dryline.
These weak thunderstorms moved eastward into an
environment that had much higher potential for intense
convection.
When the storm was organizing, the system crossed the
frontal zone into an area of unstable, ascending air, and it’s
outflow and inflow were strong enough to overcome the
weakening inversion, which lead to the storms dramatic
intensification.
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This storm system provides an example of how synoptic
scale processes can effect a local environment and make it
more capable of moderate convection. Combining this with
low level mesoscale lifting during dirunal changes can
create convective initiation.
Synoptic scale influences play a larger role in evolving or
intensifying convective storms rather than the cause of their
initiation.
If a synoptic scale setting is unfavorable for thunderstorms,
mesoscale influences may still create convection but without
the synoptic influence the storms may not grow into a large
storm system.
In this specific case, storms formed in an environment with
a small amount of convection potential and moved into a
highly favorable synoptic scale environment.
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Many thunderstorms can form in areas that are downplayed
because of their preliminary evaluation of unfavorable
synoptic scale vertical motion and limited storm potential.
If calculations are made regarding vertical motion,
forecasters can focus on how marginally favorable areas
could change based on the effect of the mesoscale processes.
If the mesoscale processes can create convection then the
forecaster should try to figure out if the storms could move
into a more favorable synoptic environment.
A mesoanalysis of the preconvective environment could
give clues as to what initiation processes could occur.
The challenge to operational meteorologists is to recognize
these subtle clues and respond correctly to the multiple
scenarios that could play out and lead to thunderstorm
development.
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Rockwood, Anthony A., and Robert A.
Maddox. "Mesoscale and Synoptic Scale
Interactions Leading to Intense Convection:
The Case of 7 June 1982." Weather and
Forecasting 3 (1987): 51-68.
Much of the wording in this presentation is from the article itself to describe the figures and overall
information from the article above.