As we all know, it's been a terrific start to the mainland snow season and indeed, the best since 1968 in NSW.

But last Wednesday night, a light snowfall turned to rain, as a surge of slightly warmer air moved through the mountains.

The result?

Look at the pictures below. You can clearly see what are known as "rills" or "runnels" running down a snowy slope near Guthega, NSW.

The incisions in the snowpack are caused by rain running down the snow, much as running water would cut into loose soil on a hill slope.

Interestingly, the top half of the high rounded mountain in the second pic (below) is rill-free. That's because it was just cold enough for snow to fall at the very highest altitudes Australia has to offer, above about 2000 metres.

The mountain pictured is Mt Tywnam, Australia’s third-highest peak at 2196 m (Kosciuszko is 2228m), and its upper slopes show a smooth surface unaffected by rain runoff. The lower slopes in both images are at about 1650m.

Is rain common in our highest mountains in winter?

Though snow enthusiasts hate the word rain and often write it as "r**n" because it is after all a four-letter word, winter rain does fall at times, most typically in the warmer northwesterly airstream ahead of a cold front.

Indeed we could well see exactly that situation in the mountains overnight and early tomorrow. The Weatherzone snow page predicts a mix of rain and snow, and it's likely to happen in that order.

Whether it snows enough later tomorrow to cover the rills remains to be seen A decent snowfall of at least 10 centimetres is required to fill in the ditches and smooth out the cover. Last week there was no follow-up snow after the rain, hence the patterns.

Speaking of follow-up, Tuesday's front looks like a bit of a dress rehearsal for a much stronger system due Friday and into the weekend. There's the potential for a solid dump of snow in that one, and we'll tell you more later in the week as it approaches.