Knock, knock. Who's there? It's winter!

The winter of 2015 is about to get into full swing.

The atmosphere is setting up for a very cold and stormy two-week cycle. The first event is on deck Wednesday.

A quick-moving, clipper type of storm will produce a roughly 50-mile-wide band of accumulating snow across parts of the Delaware Valley on Wednesday afternoon and evening.

Models are currently keeping a southern shift on where that snow band sets up. Right now, it looks like the best chance of seeing one to two inches of snow would be in Philadelphia and across southern New Jersey and northern Delaware. It appears the city's north and west suburbs should see only a coating to an inch. It's a real fine line between getting an inch or two or just flurries.

Here's the expected timing of the snow Wednesday:

Noon-2 p.m.: Light wintry mix and snow starts to spread west to east across the area, again especially from the city and points south.

4-7 p.m.: Snow falling. Sticking on grassy surfaces first, then roadways as temperatures drop and the sun sets.

8-10 p.m.: Light snow to flurries.

It may not be long before another storm arrives. There is now a nor'easter threat rising rapidly for later this weekend.

For more than 10 days now, I have been watching the date of Jan. 24 closely. Computer models have been all over the place trying to pinpoint whether a coastal storm would impact the region on that day.

As recently as three days ago, all models had the storm about 150 miles off the coast and no threat.

Well, today, that has dramatically changed.

Models are rapidly coming into agreement that a major storm will start to develop along the North Carolina coast first thing Saturday morning and then head up the Atlantic seaboard with increasing strength.

At the start, cold air will be lacking, so look for a cold rain to arrive Saturday morning. As the storm starts to intensify off the Delmarva coastline on Saturday evening, it could pull enough cold air back into its circulation to change the rain to a wintry mix, and eventually all snow. As it stands right now — and is is subject to significant changes — heavy, accumulating snow cannot be ruled out across our northern and western suburbs.

This is a developing situation that bears very close watching as dramatic swings like these in computer models tend to give me a low amount of certainty.