Late January Temperature Roller Coaster with Periodic Snow Chances
By Justin Poublon
January 18, 2019 - 9:30 AM CST | 75 0
The big story early this week was the colder air coming into Wisconsin this weekend and beyond. That hasn't changed. It is the coldest time of the year and the days ahead will certainly be colder relative to the first half of January.
We are still expecting below average temperatures in late January beginning this weekend...it's mainly the degree and persistence of the cold which has decreased. Initially it looked much more severe.
Late January Temperature Roller Coaster?
The cold for late January is looking less formidable. Instead of the polar vortex becoming stationary for days across Wisconsin...the cold now looks to come in shots interrupted by warmer days. Those warmer days corresponding to storm systems working south into Wisconsin. I think late January will be a temperature roller coaster with periodic clipper light snow events in northwest flow. Before we get there though...we are tracking a storm system for Tuesday that could bring several inches of snow.
In regards to the weather pattern overall we seem to be stuck in transition. To explain I'll need to get into some technical details so please hang in there:
Madden Julian Oscillation Reverting Back to Phase 4
In the tropical Pacific one of our major weather pattern drivers is the Madden Julian Oscillation (MJO). We visualize this "relationship of surface temperature anomalies" in eight phases. It rotates around these phases counter-clockwise in a generally predictable pattern. Though occasionally it will do loops and cross back over. Each phase has corresponding impact on our weather pattern (wetter, colder, warm, drier) and it changes depending on time of the year. It gets complicated but I think of it like a clock.
Right now we are in Phase 4. We were all set to push into phase 1 - the coldest phase - and models were picking up on that (there is a lag in our actual observed weather). In these phases we tend to build a warm ridge in the western US and cold trough in the east which pulls the polar vortex south into the Great Lakes (in January). We stopped heading that direction and instead took a sharp left back across the unit circle into Phase 4. So tropically we are back to early December while the polar vortex continues to become increasingly negative with chucks coming off. One potential reason for the delay could have to do with a fast moving pacific jet stream preventing the pattern flip. In conclusion the MJO is in conflict with colder forecasts for late January which leads me to less severe cold.
The first reason I think February will be colder than average relates to the progression of the MJO. If we are in Phase 4 today, assuming we don't spend too much time here: I assume by February we should be rolling back into 8,1,2 which are the coldest phases for the JFM time-frame. Obviously we could repeat our current pattern and never reach 1, 2 in any concrete manner. I have been wrong about this. But that is my expectation today.
Arctic Oscillation Taking Temporary Positive Break
The arctic Oscillation is a measure of how much the polar vortex spreads south into the mid-latitudes. Generally Positive = more contained, consistent, warmer temps. Negative = more amplified pattern with cold shots.
With the Arctic Oscillation (AO) we appear to be on a cyclical pattern with the corresponding time frame of October-Nov-early December 2018. Oct 16 = Dec 8. This pattern starts positive/warmer than average and decreases gradually for two months into negative/colder weather. If we look back for the corresponding point in time during the last cycle we would be in early November when the AO went briefly positive in the otherwise long term downward trend.
November turned out to be the 12th coldest on record in Green Bay. Since cold air tends to flow south in -AO, I'm expect a colder than average February. This would coincide with an MJO in phases favorable for cold. If we can build a snow cover this would help the February forecast even more.
I still can't say on the degree of cold ahead. I believe February will be more persistently cold. There is still time left in winter and even late January for severe, record breaking cold in Wisconsin. I'm just not seeing that signal YET consistently.
In the meantime it opens up the opportunity for snow cover to finally build across Wisconsin. Beyond the early next week system, snow will have to come in northwest flow clipper like systems for the foreseeable future as gulf moisture gets absolutely pummeled south. Perhaps with the subtropical jet in a wet phase and AO heading towards cold we can get some interesting phasing stuff to happen. I guess we will find out!