Meteorological Analysis

A powerful mid-latitude cyclone coupled with an intense upper level shortwave trough were responsible for the development of the historic Super Outbreak. Very strong heat and moisture advection were ongoing up to the time of the tornadoes. A good parameter to view when examining this process is equivalent potential temperature (theta-e). Theta-e is a parameter that helps forecasters assess the thermodynamical environment of the atmosphere. Since it includes both moisture and heat, examining theta-e can help forecasters determine favored areas for convection in the presence of instability. The sequence of surface maps below really show how the atmosphere quickly destabilized as copious amounts of moisture and heat were advected northward by very strong low-level southerly flow. The 18Z sounding taken at Salem, IL (SLO) really highlights just how unstable the atmosphere was by early afternoon with around 1500 J/kg of CAPE present.

An intense upper level jet streak supported the development of strong wind shear in the presence of this very impressive instability (250 mb and 500 mb maps below). It has been hypothesized by researchers that the lines of thunderstorms that afternoon were forced by large scale gravity waves. The gravity waves provided the enhanced low-level convergence needed to release the ever-growing instability through thunderstorms. The very strong wind shear as noted in the hodograph below set the stage for an impressive tornado outbreak with numerous wind shear parameters being “off the charts”. The following discussion will examine in detail the mesoscale environment of that afternoon.

Synoptic Environment

The 12Z 3Apr74 synoptic analysis featured an impressive negatively tilted upper level shortwave trough located over western NE and KS, embedded in a longwave upper level trough positioned over the western US (Figure 1). This system featured a potent mid-level jet streak of 105+ knots evident over southwest MO with a very strong associated jet streak located at upper levels as indicated by 250 mb wind speed analysis (Figure 2). Very strong low-level flow of 40 to 50 knots at the 850 mb level was advecting copious amounts of heat and moisture northward ahead of the strong upper level shortwave (Figure 3). This increase in low-level moisture was occurring under mid-level cooling and drying in the presence of large-scale ascent, a process that helps increase the potential instability of the environment. This increase in instability can be seen in the 12Z RAOB at Salem, IL (Figure 4). Note the impressive steep mid-level lapse rates associated with the elevated mixed layer (EML) that was being advected off the high plateau of the southwest US. Local research has found that the presence of an EML is one of the key components to violent tornado outbreaks over the Southern Great Lakes. The presence of an EML supports very unstable conditions in the presence of ample low level moisture. This plume of cold-dry air aloft is really accentuated in the 500 mb dewpoint analysis at 00Z 4Apr74 (Figure 5).

The propagation and advection of the aforementioned features into the Southern Great Lakes is evident by the evening of 3Apr74. A potent mid-level jet streak in excess of 100 knots was located across IN into MI (Figure 6). The presence of an intense upper-level jet streak is noted at 250 mb, likely extending farther northeast into IN than indicated in Figure 6. Limited RABO data in this region precludes the determination of the true jet streak length. The location of this strong upper level flow was crucial to the event as: 1) The increased flow aloft supported increased bulk wind shear, leading to an environment conducive to the development of rotating updrafts, and 2) the left exit region of the upper-level jet streak (a position where inferred ageostrophic circulations favoring rising motion is located) was over IN and IL. The added lift associated with this jet streak would help support increased potential instability through mid-level cooling. In addition, a coupled jet structure was in place as the low-level jet present at 850 mb (Figure 7) was located directly under the divergent region of the upper level jet streak. This discussion highlights a few of the important synoptic features present during this event. The reader is encouraged to click here  for a more detailed look at the synoptic environment.

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Figure 1. Analysis of 500 mb heights (black contours) and wind isotachs (dashed red contours and color fill) at 1200 UTC 3 April 1974. A deep mid-level low is evident over western KS with an associated  wind maximum present over southwest MO.	Figure 2. Analysis of 250 mb heights (black contours) and wind isotachs (dashed red contours and color fill) at 1200 UTC 3 Arpil 1974. A shortwave trough was present over eastern CO and western KS. Strong upper-level flow was present extending northeast from Arizona to the Great Lakes region, with a notable 100+ knot jet streak located over OK.
Figure 3. Analysis of 850 mb RAOB plots and dewpoints (dashed green contours and color fill) 1200 UTC 3 April 1974. Strong moisture advection was ongoing across central IL and IN in response to the 40 to 60 knot flow.	Figure 4. 1200 UTC 3 April 1974 sounding taken at Salem, IL. Near adiabatic lapse rates are present from 900 mb to 850 mb. The steep lapse rates continue from 850 mb to 500 mb.
Figure 5. Analysis of 500 mb RAOB plots and dewpoint contours (green dashed lines and color fill) 0000 UTC 4 April 1974. Very dry mid-level air is evident extending from the desert southwest to the Southern Great Lakes.	Figure 6. Same as Figure 1 except at 0000 UTC 4 April 1974. Deep mid-level low pressure had propagated into western IA. Very strong flow in excess of 90 knots was located from AR to MI. An associated 100+ knot mid-level jet streak had pushed into southern IL and central IN, expanding in size.
Figure 7. Same as figure 3 except at 0000 UTC 4 April 1974. Strong low-level flow of 40 to 60 knots continued across much of IN, MI, OH, KY, and TN. Ample low-level moisture was in place across northern IN, southern MI, and northern OH by this time.

Mesoscale Environment

The following analysis will take a closer look at the local storm environment associated with the Monticello storm. Impressive theta-e advection was ongoing at 12Z 3Apr74. Surface observations across extreme southern IL and southeast MO indicated dewpoints in the lower 60s F with a developing theta-e ridge across southern IL and western KY (Figure 8). A slow moving warm front is evident by 15Z extending across central IL and IN (Figure 9). Rapid destabilization was ongoing across southern IL with surface temperatures already reaching the lower 70s F and dewpoints well into the 60s F. The 18Z surface analysis reveals just how unstable the environment was becoming (Figure 10). Temperatures soared into the mid and upper 70s F across southern IL/IN, with dewpoints in the mid and upper 60s F. Sustained wind speeds of 20 to 30 mph with gusts to 50 mph were common across much of southern IL, continuing to support intense heat and moisture advection north. The 21Z analysis was astounding for April (Figure 11). Temperatures were now well into the 70s F across central/southern IL/IN with lower 80s F present in southern IL. Dewpoints had reached summer levels with mid and upper 60s F widespread across the aforementioned region. An impressive theta-e ridge was present extending from western KY into west central IN. The apex of this ridge axis pointed directly into the path of the advancing Monticello supercell (indicated by the superimposed radar picture). The location of this storm in relation to the slowly advancing warm front, surface theta-e axis, and highlighted upper level features was crucial to its longevity and intensity. The 22Z surface analysis reveals that the storm continued to propagate in conjunction with the advancing theta-e ridge (Figure 12). This analysis (5:00 pm local time) was 17 minutes prior to the destruction of Monticello. Features of note at this time were the strong backed surface winds at Lafayette (just southeast of the advancing thunderstorm), the location of this storm to the surface warm front (which can be approximated to have extended from Fort Wayne to just north of the Monticello storm), and again surface theta-e ridging into the thunderstorm. The thunderstorm continued to follow the theta-e ridge axis across northern Indiana, before finally dissipating around 00Z (Figure 13).

An investigation to the local storm environment was preformed utilizing the 18Z SLO sounding data. A hypothetical sounding for Lafayette, IN was created imputing the 22Z surface data into the SLO sounding data set (Figure 14). Even though this is outside of the 100 mile/3 hour window preferred by most researchers when utilizing sounding data, the author of this study feels that the hypothetical sounding still provides a relatively good approximation of the environment that day given data limitations. The sounding and hodograph (Figure 15) both reveal some impressive parameters (Table 1). Perhaps the most alarming parameters are the 0-6 km and 0-1 km bulk shear values of 97 knots and 39 knots respectively, “considered off the charts” by typically accepted sufficient values of 35 knots and 15 knots (35 knots of bulk 0-6 km shear for supercell thunderstorms and 15 knots of 0-1 km shear for tornado production).

Figure 8. Analysis of surface theta-e (red contours and color fill) 1200 UTC 3 April 1974. Note the plume of very warm and moist air pushing into extreme southern IL	Figure 9. Analysis of surface theta-e (red contours and color fill) 1500 UTC 3 April 1974. Theta-e ridging continues across southern IL and IN. A warm front continues to slowly push north across central IL and IN, associated with showers and thunderstorms.
Figure 10. Analysis of surface theta-e (red contours and color fill) 1800 UTC 3 April 1974. Impressive theta-e advection continues across much of IL and IN. A pronounced theta-e ridge is evident across southwest IL.	Figure 11. Analysis of surface theta-e (red contours and color fill) 2100 UTC 3 April 1974. The theta-e ridge has now shifted east into western IN with strong low-level flow present. The position of the Monticello storm is indicated in east central IL by the superimposed radar image. Note how the theta-e ridge axis is in line with the storm's direction of movement, which is northeast.
Figure 12. Analysis of surface theta-e (red contours and color fill) 2200 UTC 3 April 1974. The Monticello storm is now located on the theta-e ridge axis. Strong backed low-level flow is present just southeast of the storm.	Figure 13. Analysis of surface theta-e (red contours and color fill) 0000 UTC 4 April 1974. The thunderstorm continued northeast across northern IN, continuing to propagte with the surface theta-e ridge axis.

Figure 14. Hypothetical 2200 UTC Lafayette, IN sounding. The positive CAPE area is indicated by red shading. Note the impressive amount of positive CAPE for early April.	Figure15. Hypothetical 2200 UTC Lafayette, IN hodograph. Note the impressive low-level turning and the extreme amount of deep-layer shear.

The radar data was obtained and analyzed, revealing many impressive signatures and features evident in tornadic thunderstorms. Loops of the radar data are included below in addition to satellite data. The CHILL radar (located at Champaign, IL) was running in tilt sequence during the event and we were very fortunate to obtain this data from Dr. Ernest Agee of Purdue University. A probable rear-inflow notch can be seen in Figure 16, with an impressive inflow notch (weak echo region or WER) present in Figure 17. The presence of these two features reveals that this thunderstorm was well organized in structure and confirms the intensity of this particular thunderstorm. The rear-inflow notch revealed the likely presence of storm induced rear-flank downdraft (RFD). The interaction of the RFD with the enhanced storm inflow evident by the WER was likely a contributing factor to this thunderstorm’s tornado production. Loops of the CHILL radar and National Weather Service radar at Marseilles, IL (MMO) follow.

Figure 16. Radar image from the CHILL radar at CMI. Arrow indicates a likley rear inflow notch.	Figure 17. Radar image from the CHILL radar at CMI. The circled area indicates an inflow notch. This image was taken one half hour before the storm hit Monticello.

CHILL radar loop.	MMO radar loop.

Satellite loop.

NOAA Technical Report