Long Range: Warmth continues, when will the pattern change?

Meteorological winter began two weeks ago — but winter itself still seems to be on a hiatus. Over the next few weeks, it appears the pattern will continue to be unsupportive of winter weather — and may further support record breaking warmth. 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

Unseasonably mild conditions will continue into early this week. Low pressure tracking into Great Lakes is likely to bring a period of steady rain from Monday Night into Tuesday, as a cold frontal boundary approaches the area. Directly behind it, a slightly cooler airmass will attempt to settle in to the area. But heights build again later in the week as another low tracks into Western Great Lakes. A cold front associated with this storm system will bring the return of unsettled weather on Thursday and Thursday Night.

Model guidance shows a progressive ridge developing over Western US later this coming week, perhaps related to very deep non-tropical storm developing near the Aleutian Islands. This will cause a deeper trough to bring a blast of cold air over Eastern US next weekend. High temperatures will be in the lower to middle 40s — which will feel brutally cold compared to what we are experiencing now, but actually only settles near average for this time of year. This trough will lift out of the region next week and temperatures will begin to moderate (in the 7 to 10 day time frame).

The GEFS showing temps near average for the region on Christmas

If you were hoping for Santa’s sleigh to land on your snowy roof, you may be out of luck. A White Christmas seems extremely unlike this year.  We discussed in our previous long-range discussions that the period from Christmas to New Year’s Day would likely feature above average temperatures. The GFS and ECMWF ensemble means are showing another big -PNA pattern with a deep trough over Western US and ridge over the Eastern US for Christmas. The -PNA supports a large trough along the west coast, and ridging in the Eastern United States. This period could be similar to the unseasonably mild weather we’ve seen over the past several days. Record warmth is possible again, with temperatures possibly reaching the 60s or lower 70s.

GFS superensmeble analog composite support ridge over the Eastern US around Christmas.

Moving further along into very late December and early January, the CFSv2, JMA, and ECMWF weeklies continue to show +EPO/-PNA and positive AO/NAO patterns. These patterns will support more above average temperatures for the Eastern US for the rest of December and into first 10 days of January. Average high temperatures in the NYC metro for late December and January are in upper 30s or lower 40s.
A look at CFS monthlies show above normal temperatures +1 and +2 Kelvin degrees on average for January for the region. This matches our temperature forecast on average for the month.

The CFSv2 show +EPO/AO/NAO patterns bringing above average temperatures for the late December and early January in Northeast

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.

Current sea-surface temperature anomalies over the Pacific Ocean

This El Nino has reached “Super Nino” status officially with 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 13th, the Moving 30-Day SOI value (which changes daily) is -10.22. The Moving 90-Day SOI value-14.38 . Both indices now indicate modest El Nino conditions in the atmosphere for this month, so far.

Moving 30-Day SOI values

We believe this El Nino might have had first peak sometime between September and October. TAO/Triton data shows SST anomalies rising from August in September, then falling in October. Thus far, the August/September MEI value is also highest value.

This is a very important factor, because the 1982-83 and 1997-98 ninos had a secondary peaks in January to March period. For the NYC metro, the 1997-98 winter remained mild and snowless. The 1982-83 winter featured temperatures above average for each month and slightly above every month. The pattern became more conducive for a blizzard a February 12-13 that produced around 20″ for NYC.

The CFSv2 has the El Nino weakening by January in regions 1.2 and 3.4. Values drop below 2.0 further in January and February. POAMA forecast from Australia are showing slower decline in January and February.

Sea-Surface temperatures model forecasts from CFSv2 and the Ponama’s forecast

At this time, El Nino appears to be near or past a secondary, bigger peak. Some slight weakening and east shift of warmest SST anomalies have appeared to have occurred since November. How quickly warm SST anomalies weaken in regions 3.4 and 1+2 will have 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

Current observations show temperatures in the stratosphere are running colder than average. Temperatures are forecast to continue to drop close to record levels over the next week. These records go back only to 1978-79. EP flux vectors have been mostly pointed equatorward over the last few days.

70 hPA temperatures on left image. Zonal winds at 60N and 10 hPA

A significant rise in temperatures, easterly zonal winds, and more poleward EP flux would indicate the beginning of a sudden stratospheric warming event. The ECMWF forecasts westerly zonal winds to continue. The geopotential and heat flux are forecast to drop than rise late into the period. EP flux vectors at 10 hPA are also mostly pointed equatorward (westward). EP vectors start to shift int poleward direction 10 hPA and 30 hPA temperatures between 90°N – 60°N are forecast to remain steady or slowly drop .

The CFSv2 continues to show a PV displacement at the beginning of next month. If correct, this could support the GOA trough/low retrograding and allow more ridging over NE Pacific or on the West Coast (-EPO/+PNA) within 2 or 3 weeks thereafter. These changes are often rushed on model guidance. But we have been anticipating some weakening or displacement of stratospheric polar vortex by middle January, due to high Eurasian snowcover, varient tropical forcing, and mountain torques through the period. This will lead to high-latitude blocking developing in late January, and thus a change to colder and snowier conditions by February.

The CFSv2 showing a Siberian warm wave displacing the 10 hPA polar vortex to Europe, during the first week 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 was 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 shows some weakening of the QBO winds at 30mb and 50mb over next 10 days.

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 Discussion

The ECMWF ensembles forecast a MJO signal to propagate through phases 4/5/6 for about the next two weeks. This supports a ridge south of Aleutian Islands, a deep trough over Western US, and a strong ridge over the Eastern US (-PNA). The MJO may continue to propagate into phases 7/8 at the end of the month or early January. This would support the trough shifting west into the Northeast Pacific, allowing a ridge to build over the West Coast.

The ECMWF ensembles based on climo for the MJO

5. Conclusion and Summary

All of this pretty much still falls right into line with our winter forecast for December and early January. Most indications from pattern observations and longer-range guidance support a deep low/trough over the Gulf of Alaska as well the polar vortex remaining strong 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 for late December and early January. The pattern is likely to remain active with several storm systems and precipitation events. But the aformentioned pattern will not likely support any significant snowfall events for the region during this period.

Finally, we continue to anticipate a negative trend in the EPO, AO, and NAO that would support a colder pattern by late January. We will continue to monitor any changes with El Nino, the MJO and the stratosphere.