1) Excessive
January–February Snow
Much of the Alps region of central and eastern Europe recorded well
above-average wintertime precipitation during late-January–February, which led to a
series of deadly avalanches in France, Switzerland, Austria, and Italy. In many areas,
this precipitation led to snowfall totals which were the highest in 50 years. This
excessive snowfall, combined with alternating warm and cold spells, led to unstable snow
packs at higher elevations as the top layers of snow thawed and refroze, thus becoming
more compact and dense than the underlying powdery snow. These conditions ultimately
culminated in the deadly avalanches.
The excessive snowfall resulted from a series of four major storms that
impacted central and eastern Europe during the period, as highlighted by the time series
of liquid equivalent precipitation at St. Anton, Austria (Fig. 58).
During each of these storms the precipitation was associated with extremely heavy
snowfall, with substantial snowfall occurring every day between 16–26 February in
association with the last two storms.
St. Anton is located less than 20 km from the mountainous ski resort of
Galtür, which was the site of several landslides during February. One of these
landslides, which devastated the resort on 24 February, resulted from the buildup of an
exceptionally large and unstable slab of snow above the town. This slab was estimated to
have weighed 170,000 tonnes, and to have sped downhill at 83 m s-1, taking less
than one minute to reach the valley floor (Leslie Malone, WMO, personal communication).
The 700-hPa heights and wind speeds (Fig. 59)
associated with each of the four storms on the day of heaviest precipitation at St. Anton
(28 January, 7 February, 17 February and 24 February) indicate that the precipitation was
related primarily to very strong upslope flow in the region upstream of a major amplifying
trough. The first two of these storms (Figs. 59a, b) eventually
became classic Genoa cyclogenesis events (Mattocks and Bleck 1986; Bell and Bosart 1994),
featuring a closed cyclonic circulation at upper levels over central/ southern Italy. The
last two storms (Figs. 59c, d) were centered over eastern Europe
and also brought heavy precipitation to that region.
These four storms occurred within a mean flow characterized by a
diffluent trough extending southward across central Europe and a large-amplitude ridge
over the eastern North Atlantic (Fig. 60). The jet stream winds
were highly asymmetric with respect to the trough axis, with the strongest meridional
(northwesterly) flow located between the ridge and downstream trough axes. This
circulation favored a situation whereby transient short-wave trough/jet streak systems
propagated through the ridge axis and subsequently amplified as they approached the
downstream diffluent trough. In the first two storms, cyclogenesis was also aided in the
southern lee of the Alps by low static stability air at lower levels of the atmosphere,
which is typically observed in that region due to the proximity of the Mediterranean Sea
and the confinement of deep, cold air masses to the region north of the Alps.
2) December wind storms
Extremely strong (hurricane force) winds were observed over large parts
of Europe during December 1999 in association with a series of severe storms. These storms
caused more than one hundred deaths and billions of dollars ($US) in damage. Also,
numerous buildings and vast areas of forest were destroyed by the winds, and transport and
power outages were common and lasted for days. In perhaps one of the most unfortunate
instances of tree damage, several hundred extremely old trees were destroyed at the Palace
of Versailles, France in late December by the extremely strong winds.
The first major wind storm hit Europe on 3 December and affected
Denmark, northern Germany, southern Sweden and the Baltic states. Locally, Denmark was hit
with wind speeds reaching 37 m s-1 and the Island of Romo recorded gusts of 51
m s-1. A second major wind storm on 26 December brought 42 m s-1
sustained winds to northern France (Fig. 61b), southern Germany
and Switzerland. In the mountains, many stations recorded sustained wind speeds exceeding
55 m s-1. Extreme winds and heavy rains also ravaged southern France on
27–28 December (F.ig 61b), with speeds exceeding 33 m s-1
throughout the region.
These strong wind storms were associated with a very powerful Atlantic jet stream that
extended across the eastern North Atlantic and into western Europe during the month (Fig. 61a). This enhanced jet stream was associated with a
large-scale pattern of below-average heights over the eastern North Atlantic and
above-average heights over the central North Atlantic (not shown). This overall anomaly
pattern reflected the positive phase of the North Atlantic Oscillation (NAO), but with the
negative anomaly center at high-latitudes shifted from Iceland toward Scandinavia.
