Lacking in the snowfall, but we'll be getting the rain

What has happened to our snow this season?  The past several Decembers have produced accumulating snowfall the first week and a half into the month but this year has turned out to be just a little different.

The general track of storms has been, for the most part, southwest to northeast - putting northern Illinois and southern Wisconsin on the warmer side of storm systems.  The one developing in the southwestern U.S. will be no exception.  It's hard to believe that New Mexico has seen accumulating snow before Rockford!  We have entered into a La Nina pattern, but we have to look a little deeper into what's causing this lack of snow and cold.  It wasn't but just a couple months ago signs were pointing to a snowy and cold start to winter, and that may still be the case later in the winter season, but there's one factor that we also have to look at: the North Atlantic Oscillation (NAO).  Discovered by Sir Gilbert Walker back in the 1920's, the NAO is basically the difference in atmospheric pressure patterns in the middle and high latitudes over the North Atlantic Ocean.  Last year we spent a majority of the winter in the 'negative' phase.  This featured a blocking high pressure over Greenland which helped  push the jet stream south across the U.S. allowing for cold, arctic air to surge over a good portion of the country.  For the past several weeks the NAO has been in its 'positive' phase which features an area of low pressure over Greenland and as a result keeps the jet stream further north across the U.S.

Taking us into the present time and the storm that brought rain and snow in the southwest Monday will track to our west from Kansas through north-central Iowa Wednesday before lifting into the upper Great Lakes Thursday.  High pressure over the southeastern U.S. will allow moisture from the Gulf of Mexico to be in full swing and strong southerly winds both at the surface and a few thousand feet above will help transport the moisture northward.  Rainfall totals reaching an inch or more remain possible heading into Wednesday night. 

Once that system has passed the weather should remain quiet until early next week when another low develops and could possibly take a similar track as the one moving in tonight.  The difference between the two is we might not have as strong of high pressure in the southeast pushing the low to our west.  After that passes another looks to develop around the 22nd of December and this one could yield a little snow just in time to make it a white Christmas.  We'll have to wait and see, but climatology tells us the later we get into the winter months without adding on the snow we may be running below our average snowfall.  Keep in mind, though, that all it takes is one storm to put you right back at or above.