AVALANCHE CATASTROPHE 05 - 25 FEBRUARY 1999 - THICKNESS TROUGH

by ZAMG

The typical synoptic situation can be summarized as:

One typical example is the 08 February/00.00 UTC ("First case"), a second one the 23 February/06.00 UTC ("Second case"). Both cases show intensive Stau Cloud which extends northward into Germany. But there are also differences in the various model fields between these two cases concerning, in particular, the orientation of flow in relation to the Alpine mountain chain. The interesting point is that despite the regional discrepancies, very similar cloud configurations are produced which can, therefore, be regarded as typical of a thickness trough situation.

08 February 1999/00.00 UTC - Meteosat IR image; green: equivalent thickness 500/1000 hPa, red solid: temperature advection - WA 700 hPa, red dashed: temperature advection - CA 700 hPa, blue: Thermal Front Parameter (TFP)
13 February 1999/06.00 UTC - Meteosat IR image; green: equivalent thickness 500/1000 hPa, red solid: temperature advection - WA 700 hPa, red dashed: temperature advection - CA 700 hPa, blue: Thermal Front Parameter (TFP)
In both cases, the thickness trough is dominant, but there are differences in the relationship between the orientation of the Alpine mountain chain and trough axis: In the first case the thickness trough axis cuts the W. Alps with a NE-SW orientation, in the second case it cuts the Central Alps with a NW-SE orientation. Both cases show CA over the mountain area but some WA to the NW. There is no TFP in or close to the Alpine region.
08 February 1999/00.00 UTC - Meteosat IR image; red: height contours 1000 hPa, green: height contours 500 hPa
13 February 1999/06.00 UTC - Meteosat IR image; red: height contours 1000 hPa, green: height contours 500 hPa
The different orientation, in relation to the thickness trough, is also reflected in the different behaviour of the stream fields. There is an upper level trough over the W. Alps in the first case, but a NW stream over the whole Alpine area in the second case. Consequently in the first case, Stau Cloud only develops in the W. Alps. Stau cloud exists over the E. Alps (Austria) in the second case. The surface stream also differs between northern and north-western directions.
08 February 1999/00.00 UTC - Meteosat IR image; cyan: wind vectors 850 hPa
13 February 1999/06.00 UTC - Meteosat IR image; cyan: wind vectors 850 hPa
08 February 1999/00.00 UTC - Meteosat IR image; magenta: wind vectors 500 hPa
13 February 1999/06.00 UTC - Meteosat IR image; magenta: wind vectors 500 hPa
The low and upper level wind fields for both cases show the situation very clearly. The main difference between the two cases is that in the first case, there are only western directions east of Tyrol in a rather thick layer but there are north-western direction at the upper level in the "Second case". Consequently, Stau Cloud is restricted to the W. Alps and Tyrol in the "First case" but is very distinct also over the E. Alps in the second case.
08 February 1999/00.00 UTC - Meteosat IR image; yellow: positive vorticity advection (PVA) 500 hPa; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation)
In the region of the Stau Cloud, there is much snow in both cases. Additionally, in both cases PVA seems to play a role. In the first case there is a PVA maximum immediately north of the Alps over Bavaria coincident with Stau Cloudiness, which extends far northward into Germany.

For the complete sequence of satellite images during this phase see Overview - Phase 3 .

13 February 1999/06.00 UTC - Meteosat IR image; yellow: positive vorticity advection (PVA) 500 hPa; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation)
13 February 1999/12.00 UTC - Meteosat IR image; yellow: positive vorticity advection (PVA) 500 hPa; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation)
13 February 1999/18.00 UTC - Meteosat IR image; yellow: positive vorticity advection (PVA) 500 hPa; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation)
In the second case, a PVA maximum is associated with a small scale cloud spiral over North Germany and The Netherlands. This cloud system, and the connected PVA maximum, cross the Alps during the next 12 hours. At 12.00 UTC, the PVA maximum is immediately north of the Alps and very intense. The cloud spiral can be found over central Germany. At 18 UTC, both cloud spiral and PVA maximum have already crossed the Alps and decreased in intensity and structure.

For the complete sequence of satellite images during this phase see Overview - Phase 10 . The most interesting image in this regard is from 23/15.00 UTC, where the eastern part of the Comma tail extends southward, with a fibrous structure, from the Czech Republic across the Stau Cloud over E. Austria, whilst the Comma head appears over South Germany. A cloud free stripe has developed between the Comma head and the northern boundary of the Stau Cloud. The satellite loop reveals that the Stau Cloud has intensified under the influence of the southern part of the Comma cloud.
Close to this time the catastrophic avalanche of Galtuer in Tyrol occurred.

SUB-MENU OF AVALANCHE CATASTROPHE 05 - 25 FEBRUARY 1999
THICKNESS GRADIENT FORMING A TROUGH
COMMA FEATURE (COLD AIR DEVELOPMENT CAD)