8.Juni 2012 - in english

Under the spell of the supercell :  When the hunter became hunted

translated in April 2013

1. Weather situation and development

Days before the 8 June 2012 became apparent as potential severe storms day. A deep low lying over Great Britain led to a vigorous southwesterly UL flow with 50 Kts in 500 mb. Initially, the cold front ran flow-parallel north of the Alps from Switzerland to Bavaria, and pushed on with the developing squall line which raced eastward to the Vienna Basin. A weak vorticity maximum during noon favoured convective developments but propagated northeastward across the Waldviertel (extreme NW Lower Austria).

During nighttime a pressure wave (outflow boundary) developing out of strong thunderstorms over S Germany marched eastward and remnant clouds inhibited solar radiation north of the Alps. However, due to the pressure wave and resulting BL mixing, morning temperatures have been comparatively high, with widespread 17-20 degrees east of the Alps. Daytime heating led to high temperatures after clouds dissolving. In Upper Austria, the wind already turned back early to Ely/Nly winds, warming has been delayed.

A classic weather situation should develop: Prefrontal south foehn is responsible for dissolving clouds and will manage to suppress widespread deep-moist convection. Prerunning thunderstorms will possess enhanced supercell potential due to strong direction and speed shear (backing surface winds) while the passage of the vorticity maximum linked to the cold front will clear the unstable air mass in the shape of a squall line.

The spectacle began with the onset of two deep thunderstorms between 12 and 12.30 UTC on the Northern Rim of the Alps (Allgäu and Ammergebirge). The eastern storm persisted and increasingly formed a line east of the Estergebirge (part of the Bavarian Alps) while the western storm became a right mover into the Karwendel mountains. By 15 UTC, the line became larger and arrived with high radar echo reflectivity the Austrian border. Ahead of the line, two distinct (supercell) storms evolved in the Mühl- and Waldviertel and in Ctrl Upper Austria.

The southern supercell storm moved with the basic flow (only slightly right moving) ahead of the line becoming a bow echo close to St.Pölten (capital city of Lower Austria) and displacing the southern part of the squall line (fading away thereafter). The northern supercell storm became a strong right mover and pushed into the accelerating squall line. Both features merged into a mesoscale convective system. Arriving the eastern rim of the Alps, the southern line gradually dissolved resulting in a weakening multicell cluster. As we drove back from our chasing we could still see several CC lightnings in the Pannonian Basin of the still active cluster.

We focused on the prerunning single cell in Ctrl Austria developing at 12.40 UTC in the Hagengebirge south of Salzburg. It became a left mover over the Tennengebirge. Then, the storm passed an unfavourable wind field and temporarily weakened to the lowest level of the radar. The reflectivity minimum has been reached at 14.30 UTC north of Gmunden. Short time later, the low-level convergence became more pronounced with the approaching squall line, and the storm intensified rapidly becoming a right mover and moving towards the easterly wind regime.

Temperature differences ahead and behind the squall line were impressive, with 15 degrees in Munich and 27 degrees in Vienna at 14 UTC but pressure differences did not seem to be conducive to generate a pronounced pressure wave: 1012 mb in Munich, 1007 mb in Salzburg and 1009 mb in Vienna.

Just after the merging supercell storm with the squall line, the differences increased. At 17 UTC, 1012 mb in Haag, 1010 mb in Amstetten and 1005 mb in St. Pölten and Vienna have been measured. Besides of local downbursts (90-100 km/h west of the Strudengau) and gap flow effects on the eastern rim of the Alps (Jubiläumswarte, Vienna: 108 km/h), only sub-severe wind gusts (70-80 km/h) were observed. For a long time, the thunderstorm line remained updraft-dominant and mainly produced large hail and downpours. These hazards could have been favoured by enhanced BL moisture.

Back to the right mover: By 15.20 UTC, it intensified to the highest radar reflectivity and revealed a distinct reflectivity gradient at 15.40 UTC (figure 1) from the highest to the lowest level (see pictures 3-20), and has been situated near Steyr (Upper Austria). The heavy echo signal persisted while the radar echo temporarily indicated a tip to the south (hook echo?), e.g. at 16.30 UTC (figure 2) as we photographed and filmed a suspicious lowering. At 17.20 UTC, the storm merged into a bow echo between Melk and St. Pölten revealing book-end vortices at 17.55 UTC (figure 3), the northern vortex has been most clearly visible.

The following radar sequences will show the course of the supercell:

Figure 1: Radar image from 8.6.12, 15.40 UTC

Gewitterlinie um 17.40 MESZ

The strong radar reflectivity gradient (see picture 3) just west of Steyr. You can easily see the anvil blown towards northeast. As a result, the environment of the updraft region has not been shaded by clouds and could be heated by the sun (establishing SBCAPE).

Figure 2: Radar image from 8.6.12 at 16.30 UTC

Radarbild um 18.30 MESZ

AT 16.30 (picture 9-12), there is a little tip on the southern edge, possibly a hook echo.

Figure 3: Radar image at 17.55 UTC

Radarbild um 19.55 MESZ

After merging into a bow echo, the storm became a right mover with developing ''book-end-vortices'' where the northern vortex was better pronounced (rotation and translation adding themselves). Tornadoes are occasional seen with these vortices.

You can find more detailed descriptions of the mesoscale ingredients and impressive radar and satellite imagery in the Blog of UBIMET chief Manfred Spatzierer.

2. Pictorial documentation

On the lines of 3rd June 2012, the models suggested convective initation ahead of the thunderstorm line between Wechsel and Schneeberg (eastern rim of the Alps). So we met at 9 UTC at ESSL (European Severe Storm Laboratory) in Wiener Neustadt. At this time, the pressure wave had passed through entire N Austria and remnant clouds were persistent. However, satellite imagery promised clearing skies from the west.

Initially, we expected a low-level convergence on the eastern rim of the Alps and confidence in this scenario increased looking at satellite imagery revealing first towering cumulus. So we drove to the south to the rest stop Natschbach along the Semmering highway. However, nothing happened and towering cumulus dissolved again. Thereafter, we drove close to the runway of Wiener Neustadt's (local) airport but didn't find the viewpoint of 3rd June 2012. Radar imagery displayed first thunderstorms in the Allgäu and further chasers suggested LL convergences in the foothills of the Alps. Finally, we headed towards the west and met with further chasers at the rest stop St.Pölten-Süd.

Now we were equipped with mobile weather station, hail lattice, radio (Gerald) and another car with mobile lightning detection (Michael).

Picture 1: At the rest stop, I observed this "horse-shoe vortex" , a sign of enhanced vertical wind shear in the lower atmosphere (horizontal vorticity).

1

After observing the situation and discovering the left mover over the Tennengebirge (suddenly intensifying as a right mover) we decided to drive to the west and head for the viewpoint of 3rd June close to the highway's exit with the sounding name Oed (english: bleak). Later we were very happy about this decision as the viewpoint offered perfect view to the developing supercell storm.

On a level with Amstetten we already saw that the huge anvil threw a giant shadow. In radar imagery the storm appeared to propagate only slowly eastward, so we headed exactly for storm's center.

North of the Greutlgraben we recovered the viewpoint - with Biesen- and Franzensberg to the north (for your orientation).

First glance:

Picture 2: Laminar cloud base and huge anvil

2

Precipitation developed at the cloud base, above a huge anvil cloud screwed into the sky, with distinctly sharped edges. The anvil covered our location soon and reached far northeastward. Low clouds startet to flow towards the storm in the vicinity forming wavy clouds (lenticularis), possibly also Kelvin-Helmholtz waves.

The storm slowed down while approaching our location. The FFD became visible at first while the RFD dwelled for a long time behind an upstream ridge.

Picture 3: Well-defined downdraft area and wet microburst

Niederschlagsbereich

Within the white rain curtain illuminated by the sun to the rear side, a darker curtain evolved producing a short-time baggy excrescence at 15.50 UTC - a typical microburst due to evaporative cooling and downdraft acceleration.

The darker curtain recovered over and over, the process repeated until a wall cloud formed by 16.09 UTC.

Picture 4: Development of a wall cloud

Wallcloud

On the left side of the background, the updraft plate became apparent, with first virgae on the right side. In between, there is still a well-defined precipitation area and the small crumbs of the clouds below the updraft base will soon form the wall cloud

Picture 5: Wall Cloud has formed

Wall Cloud Bildung

Just a minute later, the appendix offered characteristic shape of a wall cloud, with downpours behind.

Picture 6: Rain-free (clear slot) to the left, much rain (and/or hail) in the middle and heavy rain to the right

gestuft

By 16.13 UTC, in quick succession the wall cloud grew significantly and the downdraft area is clearly graded. The darker part has been approaching our location and hurried us along.

Being very happy about this coincidence we took the tripods and dashed off. Looking at the driving mirror we had to stop the next hill to throw another glance at the nicely shaped thunderstorm.

Picture 7: The storm accelerated

Das Gewitter intensiviert sich

We passed the highway's entrance but stayed on the state road (as it is impossible to stop on the highway besides rest stop). The storm approached clearly and became significantly stronger.

We jumped into the car and continued our way to the east, across the highway and stopped the next hill.

Picture 8: Karli retrieved quickly another lens.

Bild 8

Driving through Oed a daunting positive lightning struck besides us - in the rain-free updraft region! Gerald, our driver, enjoined us to hurry up as we move in exposed terrain. Karli dashed around to fetch up tripod and lens - he was successful as you will see at the end of this case study. At 16.23 UTC, we still deal with a well-defined rain curtain.

We decided to take the highway to have sufficient distance to the rain curtain (if you're in it, you can hardly photograph anything), and stopped after Boxhofen, just ahead of the side road to Zeillern. At this time, in the border region of updraft and downdraft, suspiciously lowering caught our attention. To our shame, we have to admit that we did not note any touchdown because we had to hurry up to avoid an early core punch.

Bild 9: Sagging, quite turbulent wall cloud with suspiciously cone-shaped lowering, possibly a tornado

Bild 9

At 16.29 UTC, south of the rain curtain, RFD and subsequent dry-adiabatic subsidence caused a clear slot.

Editing the pictures I discovered the cone-shaped lowering beneath the wall cloud. Compare the following sketch taken from the english wikipedia entry "supercell" ...

hp-supercell

... with subsequent, contrast enhanced extract of picture 9 :

Picture 10: Enhanced contrast: The wall cloud is frayed which is typical before tornadogenesis. Below the wall cloud, a cone-shaped gudgeon continues.

Bild 10

Unfortunately, the possible touchdown of the funnel cloud (not necessarily of the vortex which could have reached the ground without condensation) is vanished behind the trees. Moreover, we were at the altitude of 380 m while the region of the possible touchdown is situated about 90 m lower in a narrow rift.

The lowering - about two thirds of the distance cloud - ground argues for a touchdown. However, so far (April 2013) we do not have any clue or leads for tornadic damage or clear, independet eyewitness reports / footage, and this case will remain a suspicious case.

Picture 11 and 12 show the scenario few seconds later behind the bus stop shelter. The frayed wall cloud and lowering are visible even without enhanced contrast.

Bild 11

Bild 12: Kontrastverstärkt

With respect to the position we have no doubt as the suspicious feature is situated in the updraft region, with a broad rain-free cloud base. Eyewitness reports appear to difficult as we deal with a rain-wrapped tornadic vortex.

In any case, we stayed only for four minutes at this place and dashed off to the highway. During the drive, the leading edge of the supercell storm changed rapidly: 

Picture 13: Arcus cloud with Inflow Tail, Wall Cloud and Clear Slot

13

The wall cloud persisted, while an arcus cloud with inflow tail formed.

Picture 15: The hunter became hunted

14

There is a huge, rain-free updraft base in the background, with a forming gust front and approaching virgae

Don't try this with your motor bike! To calm down: We drove at slow speed when the rain started.

Picture 15: Inflowing low clouds and wall cloud are visible in the driving mirror.

15 

At 16.37 UTC, wall cloud is still visible as well as the low clouds ingested into the updraft region. We took the next exit (Amstetten-Ost) drive along the state road luckily running straight eastward.

Between 16.41 and 16.46 UTC, we are situated opposed to Hubertendorf on the other side of the railway. Some sand hills promise good viewpoints, with somewhat queasy feeling when you know that this storm already produced lightnings under the rain-free base.

Picture 16: The storm became too huge for my Canon Powershot G9

16

Turbulently decreasing cloud base is visible on the left side, with the clear slot behind and the precipitation on the right side. On the right side, the gust front continues to lower and two elongate cloud frazzles pranced towards the surface.

Picture 17: Sadly, the picture is a little bit out of focus, it shows the northern part with inflow tail.

17

Looks like sort of a beaver tail in the background (laminar structure above the lowered base), which would be another characteristic (besides clear slot) for a strong supercell storm.

Picture 18: Just awestruck with a powerful supercell storm approaching

18

The gust front approached and remained turbulent. New clouds formed in the updraft area being sucked upward.

Here is a video with the entire degree of the giant supercell, especially with an impressive rain-free base as well as the depth of the thunderstorm cloud.

 

 

Picture 19: Turbulent updraft region

19

Vertical motion increased in the updraft area while the downdraft area is still dominated by RFD and clear slot.

Picture 20: Strong condensation ahead of the main precipitation area

20

Shortly before we left, another lowering cloud formed, with virgae (hail) on the left side reaching into the clear slot. We hurried up but get stopped by speed limitations in the village of Neumarkt an der Ybbs. Finally, we decided for a core punch but looking for a safe place for our motorbike chaser. So we looked for a place where we could get him into the car when the storm would become too dangerous.

Unfortunately, we missed the sideroad to Wieselburg where hailstones with 6 cm in diameter have been observed. So we turned too late and got caught in a roadway ending in a blind alley (Thalling). It was too late to return as the storm passed through soon. We still managed to capture some impressions from the storm:

Picture 21: squall line and heavy precipitation

21

The villages in the background disappeared in the rain curtain.

Picture 22: A large shelf cloud formed

22

The spray of the downdraft area approached and a huge shelf cloud became visible.

Thereafter, we jumped into the cars and became overrun by the storm. For the first time, Gerald was happy to use his extendable hail lattice. Thus we could survive the storm even in the case of large hail without damage. Gale-force winds were only observed at the very beginning of the storm, without rain. As the rain started, wind decreased. We were situated too close to the updraft region to experience severe wind gusts but also too far to observe very large hail. Most of the stones reached only 3 cm, isolated up to 4 cm. 20 minutes later, the core of the supercell had passed and the rain lessened.

Video tape of the core punch (93 seconds.):

 

The video tape shows the jump up of the hailstones while the storm itself was comparatively tame, except for the rain, but nothing special.

As the storm was through we knew that we would never get close enough for pictures or video footage. Then we heard about the very large hail in Wieselburg und headed for this location. Numberous disrupted branches lied in the roads, covering partly every square centimeter. NE of Wieselburg, in a soccer field near Petzenkirchen, we found hailstones and passed a puddle dunking our car completely in mud water. We found hailstones, hidden in the bush, with notable size:

Picture 23: Between Edichenthal and Petzenkirchen

23

Being so excited I forgot to put a coin beside the hailstone but I think 4 cm is close to reality. The hailstones were entirely freezed with much clear ice around, very solid and not composed by smaller hailstones.

Pictures 24 and 25: Double rainbow after the severe storm

24

25

The atmosphere was stunning. Silent, the ground steamed, the storm still moved on, with more large hail and merged into a bow echo. We stood here, with a wonderful double rainbow with strong colours, and large hail lied in the meadow like Easter eggs (here: 4,5 cm)

26

Thereafter, we headed for Vienna, seeing much water in the fields. Shortly before sunset we arrived the hills of the Wienerwald (A21) close to the highway exit Brunn am Gebirge where you have a wide view to the Vienna Basin. In the distance, over N Burgenland, Hungary and Slovakia, the dying multicell cluster flashed and illuminated the entire horizon intermittently. A dignified final of the chase.

Following map shows the individual locations where we photographed the storm:

verlauf

 

The wall cloud existed from 16.10 to 16.46 UTC with intervisibility before we were overrun by the storm itself. The tornado suspicion is present between the first sight at 16.29 UTC until about 16.40 UTC (+/- 10 min). Subsequently, the storm became increasingly outflow dominant which was also documentated by other chaser collegues of Skywarn Austria.

3. Documentations by others

3.1 Friedrich Föst (train ride, in german)

Bereits kurz hinter Wien konnte man schon die riesigen Eisschirme der Zellen sehen. Etwa auf Höhe St.Pölten wurde der westliche Horizont bedrohlich dunkel. Nachdem die Sonne endlich hinter dem Amboss verschwunden war, konnte man erste Konturen entdecken. Wir näherten uns mehr und mehr Amstetten und die Strukturen der Zelle wurden immer deutlicher. Als ich rechts (=nördlich) aus dem Fenster sah, konnte ich den FFD erkennen, links (=südlich) von meinem Platz sah ich den scharf abgegrenzten Eisschirm. Da erahnte ich, dass der Zug möglicherweise ideal an der Südkante der Zelle vorbeifahren könnte. Wenig später erfüllte sich meine Hoffnung, denn der Updraft wurde eindrucksvoll sichtbar und der Zug fuhr knapp südlich von Osten kommend in die Meso hinein. Ich schnappte mir meine kleine Digicam, rannte aus dem Abteil über den Gang zu den Ausgangstüren, wo ich nun beide Seiten besser im Blick hatte. Rechts von mir rauschte eine Shelf Cloud vorbei, dahinter war der grüne Hagelkern sichtbar, dort schien die Welt gerade unterzugehen. Links von mir sah ich nun einen Vorhang von Niederschlag auf den Zug zukommen, ich vermute, dass es der RFD war. Es setzte wenig später Hagelschlag und Sturm ein. Wisst ihr, wie es sich anhört, wenn ein Zug mit 120 km/h in 2-3cm dicken Hagel fährt? Das Geräusch war unglaublich und mehrere Passagiere wurden unruhig. Doch sehr rasch ließ der Niederschlag wieder nach, während nördlich des Zuges immer noch dicker Hagel fiel. Plötzlich wurde die Sicht klarer und der Blick wurde frei auf die Wallcloud, aus der schon deutlich sichtbar ein Funnel hervorging. Der Funnel pflanzte sich weiter Richtung Boden fort, wobei ich nicht weiß, ob Bodenkontakt da war, weil eine bewaldete Hügelkette mir die Sicht versperrte. Allerdings wird auch auf den Bildern sichtbar, dass bereits 2/3 der Säule auskondensiert war und es sich somit mit großer Wahrscheinlichkeit um einen Tornado handelt. Hinsichtlich der Dauer des Tornados kann ich nur sagen, dass ich ihn rund 30 Sekunden beobachten konnte, ehe er aus meinem Blickfeld verschwand. Ich habe mich nicht getraut,  die Notbremse zu ziehen. ;-) Der Tornado trat um 18.50 MESZ bzw. 16.50 UTC nur wenige Kilometer nordöstlich des Bahnhofes Amstetten auf.
  [...] Ich bin mir nicht sicher, aber ich habe den Eindruck, dass der Tornado „rain wrapped“ Charakter hatte. Dafür spricht, dass es erst hagelte als wir von Osten her in die Zelle reinfuhren und der Tornado nördlich dieses Hagelstreifens auftrat. [...]  Auf dem weiteren Weg in Richtung Salzburg standen noch eine Stunde später (19.50 MESZ bzw. 17.50 UTC) ca. 10 km östlich von Attnang-Puchheim in Oberösterreich Unterführungen, Straßen und Felder unter Wasser. Hier sollte ein Flash Flood-Ereignis in die ESWD eingetragen werden.
Alles in allem ein beeindruckendes Ereignis, zumal wir wenige Stunden vorher beim Testbed ein Level 2 für die Region herausgegeben haben. Es war der krönende Abschluss einer unglaublich tollen Woche im ESSL. Auf diesem Wege nochmals herzlichen Dank dafür!

I translated the most important findings (red):

Friedrich Föst observed a wall cloud with a funnel below. The funnel continued its path to the ground. However, he does not know whether a touchdown took place as a forested ridge blocked his view. However, his pictures (not shown) indicate that 2/3 of the rotating column have been condensated and a tornado is quite possible then. He observed the funnel cloud for about 30 seconds, at 16.50 UTC a few kilometres northeast of the railway station Amstetten. He is not sure but he had the impression that the tornado had a rain wrapped character.

3.2 Georg Pistotnik (Chasing)

Preview:

georgpistotnik

http://www.wetterturnier.de/phorum5/read.php?2,8928,8940#msg-8940

3.3 Thomas Rinderer (a collegue of mine at UBIMET)

Accidently, Thomas managed flying with the airplane at the time of the possible touchdown in the vicinity of the supercell storm. I thank Thomas and the airline for the clean window glasses.

The pictures originate from 16.25 and 16.26, respectively.

trinderer1

trinderer2

4. Summary:

For the very first time, i succeeded to capture the whole development of a supercell storm with aid of my collegues of the stormchaser club "Skywarn Austria". Radar analysis and visual details prove the existence of a classic supercell. The rear flank downdraft managed to form a persisting clear slot which is another sign for a tornadic supercell.

To our shame, we did not recognize the possible tornado during the time of observation. We were prompted to give a closer look to the footage as we saw the report of Friedrich Föst, meteorologist and participant at ESSL testbed in Wiener Neustadt (training center for deep-moist convection) who accidently took a train ride to Ebensee (Upper Austria) and documentated, according to the ESWD report at 16.50 UTC northeast of Amstetten station, a lowered funnel cloud.

Pictures 9-12 reveal a cone-shaped lowering below the wall cloud which can be seen as typical for supercells. The rotation of the funnel cloud is confirmed by time lapse we made via video cam as well as by the cam on the car. Touchdown appears to be very likely as the distance between cloud base and surface is very short, but we were not able to confirm tornadic presence due to the lack of damage and/or eyewitness reports.

We will finally deal with a suspicious case.

The storm kept its single updraft from 12.50 UTC (Hagengebirge, district of Salzburg) until 17.20 UTC as it faded behind St. Pölten (Lower Austria). The covered distance added up to 200 kilometres. The storm manifested itself as a weak left mover between 12.50 and 14.30 UTC, and a strong right mover between 14.30 and 17.20 UTC. By 17.20 UTC, the storm merged into a bow echo forming temporarily book-end vortices at 17.55 UTC.

The bow echo has been separated on the eastern rim of the Alps and died in the southern Vienna Basin, the southern part still survived until 18 UTC dying in N Burgenland.

Independent of whether a touchdown has been present or not, I experienced the most exciting weather event since I am interested in weather at all (since 1997). 

5. More chasing reports and documentations

More reports of the day can be also found in the  subforum "Chasingberichte" of the "Skywarn-Austria" forum

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