Increasing signals for a January pattern change

Meteorological winter began about three weeks ago. Over the next 10 days, it appears the pattern will be un-supportive of winter weather — and may further support record breaking warmth. But some changes maybe underway for the New Year. Our long range outlook will discuss the meteorological pattern for the upcoming 30 to 45 days. With an in depth look into meteorological patterns and the developing global circulations, we’ll explain the processes occurring that will drive our weather pattern for late this month and January.

1.Model Guidance Discussion

This past weekend we had a taste of winter weather, with some temperatures actually near normal for December. Not shockingly, the pattern that brought us that cold air is already changing quickly. Another trough will become established over Western North America and cause a ridge to build into the Eastern US. This will transport another unseasonably mild airmass into the area this week. Several disturbances and fronts will also bring chances for  showers, and possibly even a few thunderstorms, this week. Christmas Eve (Thursday) looks exceedingly mild with 850mb temperatures at 12°C or higher, possibly supporting record highs in the upper 60’s or lower 70’s.

With all of that being said, some changes are beginning to — for the first time this cool season — show up in long-range ensemble guidance. The GFS and ECMWF ensembles are showing a +PNA pattern with a ridge building over Western America and a deepening trough over the Eastern US shortly after New Year’s Day. If correct, this could bring more seasonable cold temperatures for early January.

The 18z GEFS showing +PNA pattern over North America after New Year’s

2.PDO and ENSO Discussion

Sea-surface temperatures continue to be warmer than normal over the Northeast Pacific. But over past the few weeks, these anomalies have cooled off somewhat — at least compared to where they were early in Fall. The updated PDO number for Nov, was +0.86, which is a drop from Oct at +1.47. We have seen a steady decline in PDO values since September, which is typical for a strengthening El Nino.

This El Nino has reached “Super Nino” status officially with a September-October- November ONI value at 2.03. This value beats the SON ONI between 1982 and 1997. The Oct-Nov MEI value is now 2.308. To put this in perspective, only 1982 and 1997 have had higher Oct-Nov MEI value.

The Southern Oscillation Index (SOI) measures the difference of between air pressures over Tahiti and Darwin, Australia. A lower negative value indicates below normal pressures over Tahiti and above normal air pressure at Darwin, thus supporting stronger El Nino conditions. The SOI indicates how much influence El Nino has on the atmospheric pattern over the Pacific. But there is no relationship between when a transition to a colder or warmer pattern over North America will occur.

As of December 20th, the Moving 30-Day SOI value (which changes daily) is -10.92. The Moving 90-Day SOI value-13.58 . Both indices now indicate modest El Nino conditions in the atmosphere for this month, so far.

Moving 30-Day SOI values

Currently, El Nino still remains strong. However, the El Nino itself is forecast — by many forecast models — to steadily weaken over the next few months. A faster decline in region is 1+2 is also indicating that the El Nino will become more central based during the months of  January and February.

Dynamical and Statistical model forecasts showing El Nino weakening for the rest winter

At this time, El Nino appears to be near or past a secondary, larger long term peak, as we discussed in our previous long-range discussion. Some slight weakening and an east shift of the warmest SST anomalies have appeared to have occurred since November. How quickly the warm SST anomalies weaken in regions 3.4 and 1+2 will have an influence on how much the deep low/trough over the Gulf of Alaska region will shift west on average, later this winter.

Generally, the earlier “double peak” of El Nino this year seems to be supported — and that continues to support our idea of a colder/snowier second half of the winter.

3. Stratosphere Discussion

The GFS is suggesting that a Scandinavian Ridge will develop at 500mb, which may cause a split of the 50hpa vortex in the stratopshere. The Euro is also showing a similar Scandanvian ridge, but keeps the ridge itself a more transient feature. The impact on the stratospheric polar vortex is, accordingly, less.

GFS forecasting polar vortex at 50mb to split at the end of this month

The CFSv2 continues to show a 10hpa PV displacement starting at the beginning of the month. If correct, this could further support the GOA trough/low retrograding and allow more ridging over NE Pacific or on the West Coast (-EPO/+PNA) during the month. As stated, more of model guidance is suggesting this development for early January. Often times, on forecast models, these changes are rushed. But the overall pattern progression suggests the displacement of the stratospheric vortex should begin in early January and roll forward to a mid or late January displacement. This is backed by the high Eurasian snowcover, varient tropical forcing, and mountain torques through the period. This will eventually lead to high-latitude blocking developing by late January, and a change to a colder and snowier pattern by February.

CFSv2 showing polar vortex displacement during the first half of January

The 30 hPA QBO also decreased slightly from +13.38 in October to +12.79 in November. The 50 hpa QBO for November was +9.13 , a slight rise from October . The graph below indicates weaker westerly zonal wind anomalies have descended from 10mb, since October. 30 hPA QBO values higher than +10.00 tend to support a stronger stratospheric polar vortex. The ECMWF continues to show some weakening of the QBO winds at 30mb and 50mb over next 10 days, as weaker/easterly winds start to descend down from the upper-stratosphere.

ECMWF zonal winds circled near equator, represent the QBO between 30mb and 50mb and winds dropping from the upper-stratosphere

If a decreasing trend in the 30 hPA QBO continues this month, this could support more weakening or displacement of the stratospheric polar vortex in January.

4. MJO/Tropical forcing

Over the past week MJO activity has become more organized over Indonesia. This has begun to influence the atmospheric pattern over Western Pacific Ocean. Convection elongated near and east of  the dateline, has been more associated with the El Nino tropical forcing and enhancing a troughiness over Gulf of Alaska and Northeast Pacific.

200mb Velocitiy Potential anomalies, 200mb divergence vectors and OLR anomalies (Image Credit: JMA)

There is some uncertainty on the future of the MJO. Some GFS and ECMWF ensembles show the MJO weaken or head into circle of death over next 15 day, while other ensemble members show the MJO staying stronger and propagating into phase 7/8. This could result in more major changes to the pattern next month. A weakening trend would suggest less influence on the atmospheric pattern.

ECMWF and GFS ensembles with differences with MJO in next 11-15 days.

Conclusion and Summary

It appears we may be starting to see a break or end of the atmospheric pattern that has supported numerous unseasonably mild days and record high temperatures.

For the rest of December, a deep low/trough over the Gulf of Alaska as well the polar vortex will continue persist near the north pole, Alaska, and Siberia. This will allow more Pacific air to flow into the United States and for a stronger ridge to build over Eastern US from time to time.

In terms of sensible weather, we expect temperatures to mostly run above average until around New Year’s. The pattern is likely to remain active with several storm systems and precipitation events. But the aforementioned pattern will not likely support any significant snowfall events for the region during this period.

The development of the Scandinavian Ridge and the displacement of stratospheric polar vortex may come together to transition to a pattern featuring more seasonable cold weather just after New Years Day. At the present time, it remains uncertain whether or not this pattern change will be transient or permanent. The propogation of the MJO and the evolution of the stratosphere will have major impacts on the changing pattern and will have to be carefully monitored over the next week.

Over time, we expect the pattern will continue to step-down over time, accelerating a pattern change into the middle and end of January. By February, we expect a colder pattern with chances for wintry weather.