The seeds to a warm, wet, zonal December have been sown. We have an on going cooling of the polar stratosphere at all levels which will bundle the coldest air within the arctic circle through much of this month. This draws warmer air from subtropics into mid-latitudes and powers a stronger than normal jet stream across Pacific, North America, Atlantic and Europe.
Any chill we do see in the next couple of weeks will be thanks to cold being pulled in on the backside of Atlantic lows from Greenland.
[s2If current_user_can(access_s2member_level1)]
Polar stratosphere clearly strengthens over the pole through the next 10 days.
00 hrs
168 hrs
240hrs
A core of -80 to -85C at 50hpa shows a very strong PV.
There are two things that could happen as we go forward. 1) We get a winter that never truly comes, similar to 2011-12 and indeed 2013-14 or we get a complete flip like we saw in the week of Christmas 2009. One of the most extreme examples of a pattern flip from warm to cold was 1984-85.
Here’s what can happen when the polar stratosphere tightens and cools dramatically…
Here’s a really cool 3D view of the SSW event back in 2009.
Following a major strengthening of the polar vortex early on when there’s abundant hemisphere wide snow cover and waters in the far north Pacific, Atlantic and even Arctic are warmer-than-normal.
Note in the above 2008 into 2009 example (top video) how the vortex tightens and cools as warming circles the vortex and penetrates from both sides in a spectacular way.
As we know, the above doesn’t always occur. Sometimes the vortex is weak, sometimes it’s strong to a point where it’s literally impenetrable.
For a full scale sudden stratospheric warming event to occur, there’s two important aspects. 1) an above normal snow cover surrounding the arctic, especially Eurasia and 2) warmer-than-normal SST’s in the north. We have both this year.
NH snow and ice coverage for Nov 28 is more extensive this year compared to last.
Eurasia as of Dec 1.
All the above being said, nature tends to do what she wants to do and often it doesn’t come. TWO things, sign of a stronger PV through this upcoming winter or a ‘building before break’ scenario in which heralds a mild December and turn to much colder January or February.
Canadian 500mb height anomaly for January and February. Note the northward progression of positive heights! A good sign.
CFSv2 shows less positive up into the Arctic and Greenland for January but shows it for February.
You may ask, if there’s going to be a flip, when will it arrive? Tough question but what continues to catch my eye is the NE migration of high pressure from Canada into western Greenland in the day 10-15 off BOTH GFS ensemble and EPS control. This would be around the 17-18th. At this point in time, this potential large-scale shift ISN’T in my forecast yet because I’ve seen this time and time again and it simply keeps getting pushing back or doesn’t happen at all. This is interesting because we’ve many similarities to 2009 including the timing. Remember we had a majorly wet November with cold end and then more warm and very wet weather through the first half of December before we enjoyed a cold, white Christmas.
GFS ensemble. Check out the strength of Greenland negative and twin positives over Eastern US, Western Europe in 0-5 days then weakening 5-10 days and flip come 10-15 days.
EPS Control
See this morning’s video for today’s discussion. Exciting times to come I believe…
[/s2If][s2If current_user_cannot(access_s2member_level1)][magicactionbox id=”18716″][/s2If]
Recent Comments