December likely to be mild, Nor’Easter possible next week

Three days 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 month of December and into the first 10 days of January.

Today we discuss the middle of December and look more into pattern between X-mas and New Years.

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 also showing a big -PNA pattern with a deep trough over Western US and Eastern US ridge by second weekend of December; the 12th to 13th.

The 0z/03 Euro Ensemble mean showing big east coast 500mb ridge and deep Western US trough (deep -PNA) for the second weekend of December

The 0z/03 Euro Ensemble mean showing big east coast 500mb ridge and deep Western US trough (deep -PNA) for the second weekend of December

Some index charts may show AO/NAO falling due to some positive height anomalies expanded from Eastern US ridge into the Davis Strait or Greenland. But this is NOT a signal for high-latitude blocking or for a pattern change to for well below temperatures later this month. The index charts are just a crude assessment of the teleconnections. A look at model and ensemble date reveals more about the atmospheric pattern.

The 0z/03 NAEFS (GFS and Canadian ensemble data) showing high probability for above average temperatures during the 8 to 14 day period over the Central and Eastern United States

The 0z/03 NAEFS (NCEP and Canadian ensemble data) showing over 90% probabilities for above normal 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 second half of December, including between around Christmas and New Year’s Day.

The CFS showing a 500mb pattern supporting above average temperatures through for last two weeks of December.

The CFS weeklies showing a 500mb pattern supporting above average temperatures through for middle to late December

Model guidance is also indicating a progressive pattern that looks likely to be dominated by a ridge over the Eastern US. A recurring storm threat has been showing up on medium range model guidance during the middle part of next week — around December 8th. Confidence is currently very low in regards to any impacts in our local area. There are number of disturbances moving behind this storm, embedded in the mid level atmospheric flow, that could force coastal storm to track into open Atlantic and miss our area completely. Even if were to develop and track up the coast, the antecedent airmass appears too warm to support frozen precipitation of any significance. Northern stream phasing appears unlikely as well, with the polar jet running well to our north over Canada.

The 0z ECMWF showing the coastal storm or Nor'easter going out to sea

The 0z ECMWF showing a progressive pattern with over disturbances over the Great Lakes, Northern Rockies and a deep low over gulf of Alaska. This pattern causes next week’s Nor’easter to head out to sea.

Regardless of precipitation impacts, there will be large high pressure system over the Canadian Maritimes that could cause a tight pressure gradient. We will also be heading toward new moon phase during the middle part of next week. Obviously, the potential would exist for coastal flooding should the storm system track close enough to the area.

The aforementioned pattern looks likely to continue through the middle and latter half of December. The CFS weeklies indicate above normal precipitation and temperatures for the Eastern United States continuing from early December through the middle and tail end of the month. Medium and long term model agreement is very strong.

The Dec 2 CFS showing normal to above normal precipitation for Northeast in week 3 and above normal precipitation for week 4

The Dec 2 CFS showing normal to above normal precipitation for Northeast in week 3 and above normal precipitation for week 4

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.

sst.anom

The strong El Nino still appears to be near its peak as of the last CPC update. Official CPC numbers show that Region 1+2 has warm substantially over the last week, while region 3.4 has cooled off slightly. Generally, a basin wide event continues.

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

TAO/Triton SST Data has been indicating the warmest SST anomalies have been shifting into eastern regions:

sst_wind_anom_5day_ps32

Strong 850mb westerly wind anomalies indicate a westerly wind burst propagated over the Eastern Pacific between 120W and 90W during the 11/19-11/23 period. This likely lead to an warm oceanic Kelvin wave to downwell over the eastern regions. Since then, westerly winds anomalies have subsided to weaker trade winds, over that region.

850mb westerly wind anomalies over the Eastern Pacific

850mb westerly wind anomalies over the Eastern Pacific

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. This might be an indication that  Kelvin wave activity is subsiding.

wkxzteq_anm 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 Moving 30-Day SOI value (which changes daily) as of December 2 is -4.80. Which is considered to be neutral territory for ENSO. But there is been slight drop in that value in past week. The Moving 90-Day SOI values–14.40 as of December 1 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.

soi30

The Indian Ocean Dipole (IOD) index is the difference between sea-surface temperatures over the western and eastern equatorial Indian Ocean. The positive phase of the IOD are warmer sea-surface temperatures over western Indian ocean and cooler sea-surface temperatures over the eastern Indian Ocean. The negative phase is the opposite of sea-surface temperatures anomalies in positive phase.

The IOD has been positive in phase since August and has been enhancing El Nino. Latest value is at +0.13, which is now consider neutral. Broader warm temperatures anomalies are now over the equatorial Indian Ocean. This index is forecast to continue to drop through December and January by Australia’s Predictive Ocean Atmosphere Model for Australia (POAMA).

IOD

Latest indications show El Nino as either peaked as of last week or will in the next couple weeks. From there it could plateau for another few weeks, before finally beginning to weaken. While weekly OSSI record highs have been set, we don’t believe this El Nino will reach the magnitude of the 1997-98 El Nino in the Oceanic Nino Index (ONI) values. 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.  EP flux vectors have been mostly pointed equatorward over the last few days.

70hpa temps EP flux

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 (from Nov. 30th) model 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 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.

Epflux tempsThe ECMWF model showed a wave-2 in geopotential heights downwelling into the lower stratosphere — that caused a significant disruption to the stratospheric polar vortex — which has been quite impressive. However, this wave has subsided this week. This has allowed the polar vortex to reconsolidate over the highest latitudes.

gfs_z70_nh_f00

The stratospheric polar vortex at 70 hPA near the north pole on Tuesday, Dec. 2

We anticipate more wave disruptions with Eurasian snow cover still increasing, varient tropical forcing, mountain torques, etc through December. From Judah Cohen’s research, this will enhance poleward heat flux and vertical wave propagation, eventually leading to a further weakening of the stratospheric polar vortex in late December, then a sudden stratospheric warming event occurring in January. 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 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. Special thanks to Graupel a USAWx forum member for this information.

Zonal winds at levels in the stratosphere and troposphere.

Zonal wind anomalies at levels in the stratosphere and troposphere. Image Credit: CPC

4. MJO/Tropical Forcing Discussion

Enhanced convection and tropical forcing as indicated by the OLR anomalies became elongated from around the Dateline east into the Eastern Pacific, during November 2-21. There has also been another area of convection over the Indian Ocean. This likely has resulted in more variant forcing than what is typically seen with a strong El Nino. However, from November 22-December 1, enhanced convection grown just east of dateline.

OLR Anomalies

Outgoing Longwave Radiation (OLR) Anomalies for past 30 days. Image credit: CPC

This week’s ECMWF ensemble forecasts show the MJO/tropical forcing signal going into the COD (circle-of-death, meaning no discernible signal) over next few days. But then back out and stronger in phases 3 or 4 by next week. 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 Euro weeklies have the MJO forecast in phase 4 than perhaps propagating through phase 5/6 through Jan 1st. These phases all support a Gulf of Alaska trough/low as well.

ECMWFMJO

ECMWF ensemble MJO forecast for Dec. 2nd to Dec. 16th

The CFS model shows area of 200mb and 850mb velocity potential anomalies elongated over Eastern Pacific, particularly near 120W. This area of forcing supports a trough further east over Gulf of Alaska region and more pacific air to flow in the CONUS. This forcing weakens between Dec. 12 an Dec. 21. Then strengthens again in the same region late in December and early January.

CFS 200mb and 850mb VP Circles

CFS observed and forecast velocity potential anomalies at 200mb (left image) and 850mb (right image) form Oct. 23, 2014 to Jan. 10, 2015

Based on these model forecasts, tropical forcing  to be more related to El Nino conditions rather than MJO wave activity. Either way, it will support trough/low over the Gulf of Alaska with Pacific air flowing into CONUS for the next 30-45 days.

5. Miscellaneous Discussion

We look back here at moderate-strong El nino with moderate +QBO years for the period around X-mas to New Year’s. 1957-58 had a Gulf of Alaska low (+EPO) and allowed Pacific air flow in the CONUS. 1982-83 had a very positive +NAO, that support a ridge over the Eastern US. 1987-88 had a -EPO with a -PNA, that supported a strong SE ridge and near average temperatures for Northeast US. 2006-07 had +EPO with deep negative height anomaly over Alaska and support Pacific air to flow into CONUS. 1963-64 had deep low near the Aleutian Islands. Which support a strong +PNA pattern for the CONUS and below normal temperatures for the Eastern US.

X-mas to New Year's 1957-58, 63-64, 82-83, 87-88,06-07So most of these analogs years would support above average temperatures for period between Christmas and New Year’s, as current ensemble guidance and climate pattern observations all suggest. These years also had the Gulf of Alaska region low/trough shift further west and a transition to colder weather pattern sometime in January.

6. Conclusion and Summary

All of this pretty much still falls right into line with our winter forecast for December. 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.  It will not necessarily feel “warm”. But it won’t feel much like winter here. Some storminess is possible. But the progressive pattern will support more rainfall than wintry precipitation. Especially for the coastal areas. We don’t believe there will be major changes that would result a negative trend in the EPO/AO/NAO until sometime in January. The period around Christmas and New Years is likely to very mild given, the CFS and ECMWF weeklies forecast as well as the  Nino and +QBO analogs.

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