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Cold Front Dropping South Across the Western U.S.; Watching Threat for Tornadoes and Flooding in the South

A cold front will push south across the Western U.S. into Tuesday with mountain snow and areas of gusty to high winds. An area of low pressure will form along this front on Tuesday and bring a potential for severe thunderstorms with tornadoes and excessive rainfall in the lower to mid Mississippi River Valley. To the north, heavy snow is possible in parts of the upper Midwest. Read More >

Lindenhurst Tornado of May 18, 1997


{NOTE: The imagery used here is high resolution Archive 2 data. It is likely that some of the detail would not be seen on the PUP display. One of the intents of this (and later) studies of this sort is to detail events which occurred using conceptual ideas and interpretation of the images. Interpretation of events as they unfold, understanding what is going on and perhaps anticipating later developments is the key to issuing a successful warning.}

During the early evening of May 18, 1997 at about 715PM CDT a F2 rated tornado moved through Lindenhurst in Lake County of northern Illinois. Minor property damage was reported.

The evolution of this storm involved a thunderstorm cell at the western end of a convective line and to a minor extent a much stronger storm just to the west in McHenry County. As you will see, the signature of the McHenry County storm, on the WSR-88D radar was much stronger than the reported Lindenhurst event. No severe weather was reported with this storm despite a ground search by NWS personnel. The following discussion will briefly document the history of these two storms with respect to Doppler reflectivity and storm relative motion (SRM). The entire sequence as shown takes only 46 minutes in real time.

The VAD wind profile at 18/2335 UTC from KLOT exhibits a veering signature near the boundary layer with increasing slightly veering winds through 3km. The corresponding hodograph is strongly suggestive of tornadic activity with helicity values to 3km (10k) of 420m2 s2.

The 18/2336UTC reflectivity image using the 1.5 deg slice shows a line of thunderstorms along the Illinois/Wisconsin border. The focus of this article will be on cells labeled C1 and C2. At this time a secondary line of storms L1-L2 angle northeast across extreme southeast Wisconsin. Further west in Boone County (Il) a cell (X) south of Poplar Grove is moving rapidly northeast (as seen in animation) to merge with cell C2.

One volume scan (VS) later at 2341UTC the 0.5 reflectivity slice [Images will vary between 0.5 and 1.5 scans depending upon the detail to be investigated. Beam elevation along Wisconsin border from KLOT is approx. 5000 ft at 0.5 and 10000 ft at 1.5.] Both primary cells have intensified. There is already a hint of a rear inflow notch (RIN, arrow) associated with C1. Cell X is about to merge with C2. The SRM image at 2341UTC has the primary cell locations noted. The enhanced inbound velocity just west of C1 sort of supports a RIN. The general velocity pattern to the southeast of C1 as well as C2 is convergent with some tendency for rotation with C2 in far northeast Boone County. Even though the beam elevation is probably in excess of 7000 ft east of Lake Geneva (Wis.) there is a distinct inbound (outflow) related to the line of storms L1-L2. The rapid eastward movement of this line, its reorientation more east-west and much slower movement of C1 will soon create a favorable situation for severe weather.

Jumping ahead to 2353UTC a strong further intensification has occurred with C1 and C2. Both are drifting slowly a little south of east. Cell C2 has a very marked inflow reflectivity gradient with developing "notch". It appears to be or be evolving into a classic HP-supercell. Part of the intensification here is likely due to the merging of C2 with storm X in the 2341UTC reflectivity image. To get a sense of the vertical structure a cross section was taken through axis X-X. Here the inflow vault can clearly be seen along with about a 3 mile wide overhang. The core reflectivity is solid below 25000 ft to the lowest elevation sampled.

The true story of this sequence begins to unfold in the SRM imagery. At 2353UTC this 1.5 deg slice reveals an intense gate/gate rotational couplet in northwest McHenry County. Cell C1 continues to become better organized with a convergent-rotational signature seen in the return. The inflow (outbound) is the stronger of the poles and attests to the vigorous southwest flow ahead of these cells. Things are also coming together with the line (L1-L2) further east. The assumed general air flow is noted with the arrows. We are running into an angle problem here with the winds to the rear of the line almost orthogonal to the radar beam. The enhanced inbound just northeast of C1 suggests a more northerly component setting up at the south end of the line. Y-Y is a cross section of SRM . The vertical structure of the apparent rotation within the vertical column is clearly seen [west-east points are in white at the bottom of the image]. Most intense outbound velocity is still quite high at about 21000 ft and is asymmetric in magnitude with the inbound sector. The strength of the circulation, at this point, decreases with decreasing altitude.

Things have now come together for C2. At 2359UTC the 1.5deg reflectivity slice shows a fully formed HP-supercell in northwest McHenry County. A strong well delineated inflow zone continues and the reflectivity core at this elevation has maximized just south of Harvard. Further east, cell C1 is also intensifying and developing an inflow cavity just southwest of Antioch in Lake County. The reflectivity cross section along line X-X depicts a slightly elevated core around 20000 ft. Erosion from the north suggests a RIN may be trying to form. Implications for this sort of development would include an enhancing down rush of rain cooled air and subsequent push of this air, as a flanking line or cold gust front, toward and under the updraft.

The 2359UTC SRM imagery, at about 10000 ft in this area, detects a well formed, although not quite as intense, circulation with C2. The zone just west of the C2 rotation is also strongly convergent and lines right up with the front end of the reflectivity gradient. C1 now has a better rotational couplet although still fairly broad. Northerly winds behind the old convective line just north of the Wisconsin border east of C1 have strengthened in some places especially just north of C1. This suggests a general increase in the convergent tendency of the background flow in this area. Cross section line Y-Y is oriented through the inflow face of this thunderstorm. This vertical profile shows a now enhanced inbound (outflow) stripe both undercutting the updraft core as well as in a position to reflect an rear inflow jet (RIJ) in the vicinity of a RIN. Further aloft an implied divergent signature is seen.

Two potentially severe thunderstorm cells, C1 and C2, are now in progress in far northern Illinois at 19/0005UTC. Both have marked reflectivity inflow notches denoting a very strong updraft. These two cells are now at the west end of a now east-west line of storms which extend along the Illinois/Wisconsin border. Note the orientation of this line was SW-NE a few volume scans ago. The eastern-most cell, C1, is bearing down on Lindenhurst. If you look closely one can see an arc outflow just west of Woodstock which appears to be the beginning of a surge of rain cooled air emanating from C2.

That can be confirmed, to a point, by looking at the 19/0005UTC SRM for 1.5deg. While there is still some rotational signature with C2, the couplet is weaker than the previous volume scan. The outbound core (red) and broader inbound zone just to its west are offset to suggest cyclonic divergence. Some convergence in seen in a small area just west of Wonder Lake while a more defined somewhat cyclonic convergent area is seen northwest of Woodstock. The arrow denotes the area with an apparent tendency for increasing outflow. To the east with C1 a strong inbound zone is coupled with a broad inbound region. Additional inbound pockets northeast into Wisconsin reflect the northerly components to the rear of the extended convective line.

According to Storm Data the Lindenhurst event occurred about 0017UTC. By the volume scan at 0011UTC the reflectivity image at 0.5deg displays the full glory of C1 near Lindenhurst. At this lower elevation (4800ft) the reflectivity is aligned to reflect a growing outflow (flanking line) from C1 west of Lindenhurst (compare SRM). Comparison with the corresponding SRM image shows two outflow zones in eastern McHenry County. The furthest southeast is probably associated with the recent decay of the rotational couplet while the western most is related to the still strong core just west of Wonder Lake.

If the beam is raised to 1.5 deg a bit clearer evidence of a RIJ can be seen to the rear of C2 (see arrow just west of Hebron) bearing down on Woodstock. The intensity of the couplet with C1 just west of Lindenhurst can also be better seen. It is likely the F2 tornado was occurring or imminent at this time. Note the broad zone of outbound (red) in northeast McHenry and northwest Lake Counties. At 0.5 deg much of this area is inbound, suggesting the outflow surge from C1 is shallow and being undercut by the broader southerly component ahead of C2.

A closer look at the 1.5 deg reflectivity for this time clearly shows the inflow/updraft notch on Lindenhurst. The small circle is the same location as the tight SRM couplet for this same time. The rotational core is embedded in heavy rain on the southeast flank of the main cell, right where it should be for a HP-supercell! The small black area is called a "drop-out" zone. Here the velocity in undetermined by the radar perhaps because the in-out rotation is too strong. It can be directly caused by a tornado vortex. While such a feature is occasionally seen on Archive 2 data, it would be much more rare, if possible at all, on the PUP display due to the gate averaging done in the processing.

The SRM cross section along Z-Z in from these zoomed images shows the vertical extent of the rotation and maximized velocity around 9000 ft. The whole circulation extends to about 18000 ft and is undercut at this point by outbound (inflow, red) below about 5000 ft.

In the following VS at 0017UTC the 0.5deg reflectivity is showing the effects of gust front surges from both cells. Whatever occurred at Lindenhurst is ongoing now! The appendage of return southwest of Lindenhurst (denoted as a cold front) is the flanking line structure initiated by the cold outflow and subsequent interface lift. The inflow notch is still very evident and the whole cellular structure has the classic "S" configuration of a HP-supercell. Further west one can see a RIN in the reflectivity just north of Wonder Lake as the bowing from C2 continues.

Comparison of the SRM at 0.5deg for 0017UTC confirms the gust front structure. At Lindenhurst itself a rotational couplet still exists, indicative of the events in that area. Remember, the movement of this activity is toward the east and thrust of the gust front is toward the east..essentially orthogonal to the radar beam. That, of course, means the detection of in/out component wind is somewhat minimized. It is remarkable that such clear signatures are present at all.

Elevating the SRM beam again at this time (0017UTC) to 1.5 deg where the elevation is just under 10000 ft the variation in vertical composition can be inferred. The rotational couplet near Lindenhurst is better defined and the offset centroids suggest cyclonic convergence. The trailing inbound (green) zone associated with the flanking line gust front is much smaller and the outbound (red) at the southern end indicates the shallow wedge is being over run by southerlies just ahead of C2. Even C2 has a much smaller inbound zone to its west attesting to the shallow nature of the gust front.

At the lowest elevation scan once again the final reflectivity image at 0022UTC shows the dual gust fronts swinging out and away from the original parent cells. Cell C1 still has quite good HP-supercell structure with a RIN evident and noted by "R". The tornado has probably faded away by now but a defined inflow notch can still be detected between Lindenhurst and Zion. Wind damage was reported from this cell up to about 0023UTC at Gurnee which would be about mid way between Lindenhurst and Waukegan. Cell C2 is now collapsing into a straight line wind event although no damage was directly attributed to it at this point. RIN zones can clearly be seen at McHenry and northwest of Fox Lake.

The gust front nature of the situation can now be clearly seen with the SRM image from 0022UTC. Any rotation left at this height, about 4000 ft, left over northeast Lake County is broad and disorganized. Maximum inbound velocity in southeast McHenry County associated with C2 is about 30kts but is likely well understated due to the incident beam angle with eastward moving system.