AVALANCHE CATASTROPHE 05 - 25 FEBRUARY 1999 - CLASSICAL WARM FRONT SHIELDS

by ZAMG

The typical synoptic situation can be summarized as:

Two main cases have been selected: the 19 February 12.00 UTC and the 21 February 12.00 UTC. For the complete sequence of satellite images during this phase see Overview - Phase 6 and Overview - Phase 8 .

The cloud features differ quite distinctly from those of the Detached Warm Front. There are more extended Warm Front cloud shields which are linked to a Cold Front system. During this catastrophic phase, double structures of Warm Front Shields could very often be observed and which developed from the merging of several systems. Cloudiness is fibrous and Stau Cloud features are not so apparent in the IR images. Both situations are observed: The ending of the fibres at the highest mountain chain as well as an overflow of cloud into the southern regions of the Alps.

19 February 1999/12.00 UTC - Meteosat IR image; red: height contours 1000 hPa, green: height contours 500 hPa
21 February 1999/12.30 UTC - Meteosat IR image; red: height contours 1000 hPa, green: height contours 500 hPa
The Warm Front passages in these cases are accompanied by a broad western upper level stream (at least much more western than for the Detached Warm Fronts) and by one or (in case of two systems) two surface troughs. These surface troughs are classical indications of surface fronts which are missing in case of Detached Warm Fronts.
19 February 1999/12.00 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)
21 February 1999/12.30 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)
Frontal conditions are rather distinct but vary between the two cases presented here. As there are frontal conditions, pronounced thickness gradients exist in the region of the Alps with the cloud shields within and behind the gradient. The TFP is not well developed in both cases but it can easily be detected and/or continued along the warm boundary of the thickness gradients. In both cases, it crosses the Alps but the orientation is completely different. The WA maxima superimposed on the thickness gradient are very distinct and intensive. Both cases show a double structure which is indicative of several systems merging in the Alpine region.
19 February 1999/12.00 UTC - Meteosat IR image; cyan: wind vectors 850 hPa
21 February 1999/12.30 UTC - Meteosat IR image; cyan: wind vectors 850 hPa
19 February 1999/12.00 UTC - Meteosat IR image; magenta: wind vectors 500 hPa
21 February 1999/12.30 UTC - Meteosat IR image; magenta: wind vectors 500 hPa
The wind fields at 850 hPa clearly show western components while a more north-western component is evident in the 500 hPa field. As a consequence, Stau Cloud features can be observed mostly at the West Alps.
19 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)
21 February 1999/12.30 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)
Precipitation is widespread but it (mostly) ends at the southern border of the Alpine mountain chains. Snow is restricted to the highest levels whilst, according to the character of a Warm Front, rain dominates north of the mountains. Further northward, over Germany, rain changes more and more to drizzle. Distinct PVA maxima are associated with the Cold Front and Comma area to the rear of the Warm Front cloud shields.
19 February 1999/12.00 UTC - Meteosat IR image; position of vertical cross section indicated
Relevant parameters shown in the vertical cross section and relative streams are also presented for the Warm Front Shields using 19 February 12.00 UTC as an example. The vertical cross section line F is perpendicular to the Warm Front Shield over France and the W. Alps as well as the cloud shield north of the Alps which has developed from a former Warm Front Shield.
19 February 1999/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values
The isentropes in the vertical cross section show two upward inclined frontal zones: the north-eastern one from 49.5N/12E at the surface up to 400 hPa is accompanied by a tighter gradient than the western one, which is from about 48N/00 up to 300 hPa.
19 February 1999/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thin: temperature advection - CA, red thick: temperature advection - WA, orange thin: IR pixel values, orange thick: WV pixel values
19 February 1999/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thin: divergence, magenta thick: convergence, orange thin: IR pixel values, orange thick: WV pixel values
19 February 1999/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values
The double structure might be recognised more easily with the help of temperature, divergence and vertical motion (omega). All three parameters indicate the double structure with two maxima within and above the two frontal surfaces which can be regarded as well developed Warm Front systems.

Relative streams are computed for 300K and 306K. The lower surface is a transition between the two frontal surfaces, the higher one belongs to the western Warm Front Shield.

19 February 1999/12.00 UTC - Meteosat IR image; magenta: relative streams 300K - system velocity 317° 13 m/s; yellow: isobars
19 February 1999/12.00 UTC - Meteosat IR image; magenta: relative streams 306K - system velocity 317° 13 m/s; yellow: isobars
On both surfaces, a rising Warm Conveyor Belt is the dominant relative stream associated with the Warm Front cloudiness. This is completely different to the cases of Detached Warm Front, where an upper relative stream from the colder air to the north is the dominant feature. In the case presented here the Warm Conveyor Belt is the more to the east the lower the isentropic surface is. Consequently the two different Warm Front systems can also be detected in the relative streams.
SUB-MENU OF AVALANCHE CATASTROPHE 05 - 25 FEBRUARY 1999
DETACHED WARM FRONT AND SUBTYPES