THE CLOCK IS TICKING....
The Madden Julian Oscillation (MJO) has now reached phase 7. That's a key benchmark as it means the Midwest is entering a prolonged period where the MJO will remain in cold phases that correlate strongly to below normal temperatures. In fact, this is likely to last much of the next 15 to perhaps 20 days once the cold kicks in this weekend.
If you follow the dotted green lines you can see the MJO's path through 7, 8, and 1 between now and January 1st. To the right you can see the corresponding temperature anomalies for each phase.
I looked at the CFSv2 mean temperature departures the next 15 days and it agrees much of the nation and Canada will be consumed by the chill. The Midwest is right in the thick of it.
All the models agree some of the most intense and widespread cold will be in the period December 25th-30th. Look at these departures.
This is in line with what a weak La Nina typically delivers for winter temperatures.
Here's data from NOAA showing the current weak La Nina in progress.
Essentially, the clock is ticking on this recent stretch of mild weather. The past 14 days show this for a departure. We are in for a rude awakening by Christmas.
Here's the 500mb jet forecast for December 29th that will change the look and feel of things.
With the cold should come some snow the next 10 days. Unfortunately there is very little model agreement when and where. Here's what the EURO has for total snow between now and Christmas. This comes from 2 minor disturbances. One Thursday night the other around Christmas.
The GFS has this for the same period.
After that the EURO and GFS go entirely different directions. The EURO has a 10 day snowfall forecast that looks like this.
The GFS is more phased and less progressive and produces a series of snow events that bury my area. I would be thrilled to see this! However, I think this is an extreme solution that needs to be backed away from. It would be quite a shock to see such a radical change to snowy weather in a year that has yet to produce more than a dusting here in Cedar Rapids.