How a storm comes to MB
On Thursday 24 August, 2006, 21 climbers of various nationalities decided, despite a clear meteorological warning, and apparently against the advice of some guide, to attempt the climb of Mt. Blanc via the Gouter route. As predicted, after an unsettled morning, a big storm passed over the area in the early afternoon, reducing visibility to zero even at comparatively low altitudes, with violent winds and snowfall down to 2600m.
The 21 climbers tried to go back to the Gouter hut, but got stuck on the south side of the Dome de Gouter, a very dangerous place in poor weather, as the lack of any point of reference makes it very easy to lose your bearings in a whiteout. In such bad weather the PGHM couldn't attempt a helicopter mission to rescue the climbers, so they had to resort to survival tactics (like digging snowcaves) to spend the night, and wait for morning (and rescue). All survived without significant consequences, but had the bad weather lasted another day, a tragedy would have been highly likely.
[ Note 2007: in the first week of July 2007, with a very similar setup (unfavourable meteo, general instability, "no go" warning from local guides), a group of five, very young Polish climbers got first avalanched then stranded on the Bionassay glacier of the Miage (Italian) side of MB, after unsuccessfully trying the Italian Normal route, and having turned back near the Dome (not far away to the place of the 2006 epic). Sadly, this time the storm lasted three full days; despite a valiant, massive and well coordinated effort from the Aosta Mountain Rescue to reach the missing climbers, three of them (including a girl) died of exposure. ]
All meteo forecast available in the area gave high probability for stormy weather on the 24th, but even without a formal warning, the signs were all those of big trouble coming - and coming fast. The weather had been unsettled all the morning, with uneven visibility, and strong winds. Around noon, the winds turned decidedly from WSW, and clouds started to mass beyond the Col De La Seigne.
On the same Thursday 24th I was returning from a walk up the trail that brings to the Monzino hut, and I made this sequence of pictures. I hope it will be useful to all people climbing in the area to understand (and hopefully, make good use) of the local weather's signs.
At 14:39, the clouds are massing above the Col De La Seigne, on WSW, from where a strong wind blows. To the right there are visible patches of blue sky. The glacial snout in the foreground is the Italian Miage.
At 14:50, the clouds burst in.
Again at 14:50, this is the sky more or less in the opposite direction. This shows nicely how in this area the bad weather almost always comes from West. The peak in the foreground is Mt. Chetif, the mountain that dominates Courmayeur.
At 15:09, the clouds have invaded the higher Val Veny, covering the entire Trelatete sub-sector (not shown, on the right of this picture). The wind now is very strong, and I heard the first thunder while taking this picture.
At 15:20,the Jorasses are engulfed by the storm. It's raining in buckets now, and the wind is so strong that I had my difficulties to take this picture. Just below the cloud ceiling the wind is now at 150 kph, if not stronger.
Now it's 15:30. The visibility in the valley has become very poor, and it's raining sideways. The peak visible through the fog is the little Aiguille du Chatelet, near the Monzino hut, at the base of the south face of MB, so Mt. Blanc proper is more or less 1km behind there. I took this picture at the car park where the Monzino trail starts - the first picture of this sequence was taken below the rock barrier barely visible above the tree line.
Two hours later, at 17:37, the worst of the storm had passed, and over Courmayeur there was a bit of sun. The Mt. Blanc was, however, still completely covered by clouds, and only the Aiguille Noire was barely visible behind the Chetif. By late evening the sky was clearing rapidly over the Italian side, but high on Mt. Blanc the storm continued unabated.
I'll had all relevant link to MB meteo soon here - a bit of patience!