A rather warm powder alert#23


By meteomorris on 3 March 2013 · 0

PowderAlert#23
PowderAlert#23

Warmer air has arrived! The freezing level is between 2500 meters in the western Alps and 2000 meter in the northern Alps and southern Alps. Spring has arrived on the south facing slopes. The snow is turning into slush in the afternoon with a rising chance of wet snow avalnanches in the afternoon.

You still can find some powder on steep faces with a northern exposure. The sun isn’t that strong here and as a result it’s much colder with fresh snow. Same story for faces with trees with a northern, esatern or western exposure. Ergo, if you’ll search, you’ll find powder. Especially in the regions (southern Alps west, southern Alps central, southern Alps east en northern Alps east) where they had some snow the last couple of days. But every ray of sunlight will do what it can to turn it into snow.

The temperature will rise the next couple of days, with a current turning to the southwest.

Current turning to the south(west)
Current turning to the south(west)

South west stau

The first precipitation will fall in the French southern Alps and the Italian Piemonte (southern Alps west) in the night from Tuesday to Wednesday. The snowline will be around 1200 meters, but will rise to 1400-1800 meters on Thursday and Friday, depending on the region and the intensity of the precipitation. I expect 50-70 centimeters of freshies above 2000 meters in some regions. Please keep in mind that this snow is not as dry as we were getting used to the last couple of weeks. The rest of the southern Alps will experience lower temperatures (though temporarily) and some snow. The French northern Alps and the other regions of the northern Alps can expect a southern föhn. Which basically means warm and sunny weather. The snowcover in the föhn valleys will suffer.

You can see the differences between the southern and norhern Alps in this föhn chart. A graph that is published by meteocentrale.ch. If the southern föhn is in the olus, than it will be dry and sunny in the northern Alps, and it will snow (south stau) in the southern Alps.

Southern föhn on Tuesday and Wednesday
Southern föhn on Tuesday and Wednesday

The cause of all of this is a jetstream that’s is turning to the south. The air that’s flowing into the Alps is coming from the south, as you can see on the map below. The jetstream is sending fronts and low pressure areas towards the Alps. The results? Stau in the south and föhn In the north.

Jetstream from the south
Jetstream from the south

PowderAdvice

Freshies in the south on Wednesday and Thursday. I expect the most snow in French southern Alps and theItalian Piemonte. There might fall around 50-70 centimeters of freshies above 2000 meter. No cold smoke blow, but good for shredding.

Long term, after Friday

What will happen after Friday? That’s still a question that can’t be answered. The weather models aren’t quite clear with their output. Maybe the ensembles can help us. Check out the ensembles of four random places in the Alps. The French southern Alps and the Piemonte, the French Northern Alps (Western Alps central en Western Alps north), the northern Alps (Austria/Switzerland) and the southern Alps (Italia).

What is an ensemble? An ensemble (or plume expextation) will give you an idea of the uncertainty of the weather expectations. Every weather model is calculated with little differences over and over again. If the results are sensitive to little changes the expectations can be quite different. If the results are quite similar, the weather is stable and the expectations pretty reliable. However, if the different expectations in the plume are far apart, it shows a great uncertainty. The ensembles below confirm the southern stau and will give you an idea of what will happen from Friday.

Ensemble French southern Alps and Piemonte
Ensemble French southern Alps and Piemonte

Expectation French southern Alps and the Piemonte

Dropping temperatures (temporarily though) with heavy snowfall. A true south western stau. If the intensity of the snowfall is getting less after March 7th, the temperatures will rise. All the models are showing a temperature drop after March 10th. Less chances on heavy snowfall can mean two things. There will be a long period with precipitation, or it will snow in the north and northwest of the Alps. The south will stay dry. If want to ride powder, you should be there between Wednesday and Friday.

Ensemble southern Alps
Ensemble southern Alps

Expectation southern Alps

The other regions of the southern Alps will also experience this southern current. This means dropping temperatures (temporarily though) with heavy snowfall. The arrows show that the temperatures are rising (blue arrow) when the intensity of the precipitation is getting less (red arrow). The temperatures will drop after March 11th and it will be dry.

Ensemble French northern Alps
Ensemble French northern Alps

Expectation French northern Alps

It’s basically too warm in the French northern Alps the next couple of days. The freezing level is around 2100 meters. A weak front will pass later in the week (around March 8th), but when the colder air arrives on March 10th the chances for snow will significantly rise. It’s hard to see what will happen after that. It will be colder (temporarily though) and some models are forecasting snow. More details about that later this week. The conclusion: dry and too warm this week.

Ensemble northern Alps
Ensemble northern Alps

Expectation northern Alps

Too warm and dry till the 10th of March. The freezing level will be around 1900 to 2300 meters today. The models are showing colder temperatures (blue line) and rising chances of snow (red line). More details later this week.

What is the value of ensemles?

Ensembles are great to test the consistency of a model. They also help to put the effects of the jetstream in perspective, but are to rough to forecast the weather for a single village. They’re simple just not that accurate enough. In this case, it works out pretty fine, because I’m forecasting the weather for regions for the long term. The week of March 3-9 will be warm. You can notice that especially in the western and northern Alps. The current is moving to the southwest and it will snow in the southwest (French southern Alps and Piemonte).

Colder temperatures are returning after March the 10th and the chances for significant snowfall will rise in the northern Alps. There are no significant changes in the weather the next two days, so there will be a new update on Tueday.

Enjoy the powder on northern slopes and be prepared for PowderAlert #23!

Stay stoked

Morris


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