Mild December Likely to Continue…Pattern Change for January?

About one week into Meteorological Winter, much of the meteorological community is aware that the coming weeks are not likely to offer many wintry precipitation prospects. 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 through the rest of this month and into January.

1. Model Guidance Discussion

Model guidance continues to agree that the mid level atmospheric pattern will mirror the historic analogs for December patterns during strong El Nino events. A large, deep trough over the Gulf of Alaska and another deep trough over Greenland, will allow for mild air off the Pacific Ocean to flow into much of the United States. The GFS and ECMWF ensemble means are still showing a big -PNA pattern with a deep trough over Western US and Eastern US ridge by this upcoming weekend. Temperatures could be unseasonably mild this weekend or early next week. With a southwest flow and higher than normal heights supporting highs well in the 50s and 60s over much of the Northeast and Mid-Atlantic regions.

The 0z/07 NAEFS (GFS and Canadian ensemble data) shows a high probability for above average temperatures during the 8 to 14 day period over the Central and Eastern United States.
Moving further along, the CFS and ECMWF weeklies continue to show +EPO/-PNA and negative AO/NAO patterns. These patterns will support more above average temperatures for Eastern US for the later half of December and into early January, including Christmas and New Year’s Day.

The sub-tropical jet appears to be getting more active as anticipated over the next week. Most of the storm systems this week appear to be weaker and passing off to our south. Ensemble data is in good agreement that a storm system will eject from the Southwestern United States next week. The strong ridge over the Eastern US will force this system to take an inland track toward the Great Lakes.

With a mild antecedent airmass forecast, only rain seems likely from this system in our region. A colder airmass may come behind this system, but it will likely be short-lived as most have been this year. This pattern will likely repeat in some fashion over the next few weeks, with a lack of high-latitude blocking keeping us in progressive pattern with the east coast ridge building from time to time. Temperatures will run mostly near average to well above average.

Further along this month, the CFS weeklies continue to show a low over the Gulf of Alaska and positive AO/NAO. These patterns will support more above average temperatures for Eastern US, for late this month and into early January, including Christmas and New Year’s Day.

2. PDO and ENSO Discussion

The updated PDO number for Oct 2015, was 1.47. 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 from early in fall.

This El Nino has reached “Super Nino” status officially with September-October- November ONI value at 2.03. This value beats the SON ONI is between 1982 and 1997 SON ONI values.
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 strong El Nino still appears to be near its peak as of the last CPC update. Official CPC numbers show that ENSO regions 3, 3.4 and 4 has cooled off slightly 0.1. While region 1+2 has remained the same from last week. The latest trend in numbers over the past weeks, possibly indicate a slow west to east collapse of this El Nino is beginning.

Nov 18, 2015: Region 1+2 = 2.1, Region 3 = 3.0, Region 3.4 = 3.1, Region 4 = 1.8

Nov 24, 2015: Region 1+2 = 2.4, Region 3 = 3.0, Region 3.4 = 3.0, Region 4 = 1.8

Dec 2, 2015: Region 1+2 = 2.4, Region 3 = 2.9, Region 3.4= 2.9, Region 4 = 1.7

Strong 850mb westerly anomalies decreased over an average period of 30 days over the Tropical Pacific. This also indicates that El Nino might be in final phase.

In November, the warm oceanic Kelvin wave continued to downwell between 140W to 100W. But later in the month, a sub-surface cold SST anomaly began to move east, from the Western Pacific. This might be an indication that Kelvin wave activity is gradually subsiding

However, there some indications new oceanic Kelvin wave with a westerly wind burst may be for near dateline. At this time, this thermodecline is only between 100m and 200m. So significant warming of eastern regions doesn’t appear likely. But it will continue to be monitored.

The Southern Oscillation Index 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 monthly for November was -3.23 up from -21.30 in Oct. The highest monthly average value since April.

The Moving 30-Day SOI value (which changes daily) as of December 6 is -6.98. Which is considered to be neutral territory for ENSO. But that value been dropping in past week. The Moving 90-Day SOI value-15.24 as of December 6 are still well in El Nino territory. This index 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.

Latest indications show El Nino is near peak. From here it could plateau for another few weeks, before finally beginning to weaken. All in all, this further supports a deep low/trough further east over the Gulf of Alaska, allowing pacific air to flow into the CONUS for December on average. Then the Gulf of Alaska low/trough to begin to shift west sometime in January.

3. Stratosphere Discussion

Current observations show temperatures in the stratosphere are running colder than average. Temperatures forecast to continue to drop close to record levels over the next week. These records go back only to 1978-79.  Zonal winds are also westerly, at near record levels:

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 over the next days, for 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 shift poleward than back equatorward with EP flux weak. 10 hPA and 30 hPA temperatures between 90°N – 60°N are forecast to remain steady or slowly drop .

Thanks to Tropical Tidbits we can now see 10mb temperatures and heights on the CFSv2 weeklies and monthlies. This model is showing a polar vortex displacment beginning sometime between the end of this month or early January. 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 thereafter. These pattern changes are often rushed on the model guidance. But we are anticipating some weakening or displacement of stratospheric polar vortex by middle January, due to high Eurasian snow cover ,varient tropical forcing, mountain torques, etc 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 30 hPA QBO also decreased slightly from +13.38 in October to +12.79 in November. The 50 hpa QBO also for November was +9.13 was a small rise from October at 9.32. 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. If a decreasing trend in the 30 hPA QBO continues this month, this could support more weakening of the polar vortex by January.

4. MJO/Tropical Forcing Discussion

Last week saw deep convection near the dateline and some convection over Indian Ocean. However, the past weekend we saw convection near the dateline gradually diminish.
The ECMWF ensemble forecasts show the MJO/tropical forcing signal in phases 3 or 4 over the next 11-15 days. If correct, either phase 3 or phase 4 would support a trough over Gulf of Alaska region and strong ridge over Central and Eastern US for middle to late December. The UKMET ensembles also agree with MJO/tropical forcing signal in phases 3 and 4. in this week. Euro weeklies have the MJO forecast in phase 4 than perhaps propagating through phase 5/6 into early January.

 

The Constructed analog forecasts and GEFS indicate more enhanced convection over the Eastern Indian Ocean and Indonesia, that support phases 3 and 4.

The CFSv2  shows more convection and tropical forcing focused over Indian Ocean in the middle of December. Then convection reorganizes back near the dateline for Dec. 27 to Jan 2. At this point we could see some support for the GOA trough to retrograde west, at least temporarily.

 

There is some uncertainty as to whether or not this tropical forcing is being induced by El Nino or actually MJO wave activity. Either way, the forcing will support trough/low over the Gulf of Alaska with Pacific air flowing into CONUS through Christmas. This trough could start pulling back west closer to New Year’s Day.

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 the climate pattern observations and longer-range guidance support a deep low/trough over the Gulf of Alaska for much of the month, as well the polar vortex strong near the north pole near Alaska/Siberia.

The pattern will support more Pacific air to flow into much CONUS and result in mostly above average temperatures for in this region. Some storminess is still possible. But the progressive pattern will support more rainfall than wintry precipitation. Especially for the coastal areas.

At this time, we continue to anticipate a negative trend EPO/AO/NAO that would support a colder pattern during middle to late January. We will continue to monitor any changes with El Nino and stratosphere.