Long Range Outlook: Active Pattern in December. But Will it Snow?

Meteorological winter is just one day away, and while snow lovers are beginning to get excited, 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 occuring that will drive our weather pattern through the month of December.

Today we discuss more what may be in store stormwise in December and look more into pattern late December and early January

1. Model Guidance Discussion

The 500mb mean pattern for next two weeks at least will feature a trough over portions Northeast Pacific and Alaska (+EPO), A polar vortex between Baffin Island and Greenland (+NAO), and an active subtropical jet. The outcome is a split flow pattern over North America. The northern branch of the jet will be running mainly over Canada. The southern branch further south with large disturbances coming out the Pacific and tracking across the CONUS. This means the pattern will continue be progressive. But possibly become more active. Temperatures are likely to run near normal to well above normal, for much of the period. A few transient cold shots are possible.

The models show these disturbances becoming storm threats for the east coast. We discuss a couple of threats that have been showing up consistently, in a little more detail.

The storm threat around the Dec. 1-2 now appears to be mostly rain for the region. The -EPO ridge over Western Canada will collapse east quickly and the confluence zone over SE Canada will diminish. This will allow the antecedent cold airmass to modify quickly and also the storm to track into Great Lakes.

The GFS for next Tuesday, showing low pressure in Great Lakes region

The GFS for next Tuesday, showing low pressure in Great Lakes region

Models have been suggest another east coast storm threat between December 4th and 6th.  The models show a sub-tropical disturbance coming ejecting out of the Southwest US. This threat has some support the GFS and ECMWF ensembles as well.

The 0z/26 GFS overnight had this as a coastal storm phasing and intensifying near the 40/70 benchmark. This could some snow or mixed precipitation in our region, if correct. However, the GFS also shows a trough coming into the west coast and no high-latitude blocking in the Atlantic. The GFS is likely to amplified with it’s solution here. The overnight CMC and ECMWF models don’t support this solution. The 6z/26 GFS has come in with less amplified solution, as well.

0z/26 solution for Dec 4th-5th

0z/26 solution for Dec 4th-5th. Deep low near the 40/70 Benchamark. But progressive pattern with trough over Western US.

The track and evolution of each disturbance in December will likely depend on PNA pattern supporting a phase of the northern and southern branches at the right time. It will be very difficult to main a stout +PNA ridge with a large trough over the Gulf of Alaska and lack of high-latitude blocking on the Atlantic side–as shown on GFS, CMC, and ECMWF ensemble means. Chances are higher the pattern remains more progressive. Therefore supporting storms to bring rain versus snow in our region or even miss our region area completely to the south. But these storm threats will still be carefully monitored.

gfs-ens_z500a_namer_34

The GFS ensemble mean pattern supports a progressive solution for Dec. 4th-6th storm threat.

We continue to see a typical El Nino 500mb pattern showing up on the long-range ensembles and weeklies. A large, deep trough over the Gulf of Alaska and another deep trough over Greenland, which is a classic +EPO/NAO/AO pattern that allows for mild air off the Pacific Ocean to flow into much of the CONUS. The subtropical jet is also becoming increasingly active.

Nov 25th CFS weeklies showing +EPO/+NAO patterns with above average temperatures for much of the CONUS

Nov 25th CFS weeklies showing +EPO/+NAO patterns with above average temperatures for much of the CONUS

2. PDO and ENSO Discussion

PDO number for Oct 2015, was 1.47. Sea-surface temperatures continue to be warmer than normal over the Northeast Pacific. But over past few week, these anomalies have cooled off.

El Nino still appears to be strengthening, as of the last CPC update. Official numbers from the CPC show that ENSO region 3.4 has reached 3.1 in the weekly OSSI values as of Nov 18th, which breaks the record set in 3.4 region with the 1997-98 El Nino. Other ENSO regions saw slight rises from this last week (Region 1+2, 3, 3.4, and 4 listed in order with absolute temperatures (degrees C) and anomalies):

Nov 11 2015: Region 1+2 = 2.0 Region 3 = 3.0, Region 3.4 = 3.0, Region 4 = 1.7

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

sst.daily.anom

There are few indicators that show that this El Nino may be nearing peak, however. According the Australia Weather Bureau, the 30-Day Moving Southern Oscillation Index (SOI) values have risen sharply over the past two weeks. The latest for November 26 is -3.49.  Which is considered to be neutral territory for ENSO.  But the Moving 90-Day SOI values are still well in El Nino territory.

30-day Southern Oscillation Index Values

Moving 30-day Southern Oscillation Index showing a sharp rise in recently

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 continues will likely peak in the next couple weeks (late November or early December). 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 Heat 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 to begin to shift west in January.

3. Stratosphere Discussion
Current observations and forecasts show temperatures in the stratosphere are running much colder than normal and zonal winds are also running higher than normal from a westerly direction. EP flux has been mostly pointed equatorward over the last few days.

70 hPA temperatures on (left image) and EP flux for 11/18 - 11/24 (right image)

70 hPA temperatures on (left image) and EP flux for 11/18 – 11/24 (right image)

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 model forecasts westerly zonal winds to continue. The geopotential and heat flux are forecast to rise and then drop within 5-10 days. EP flux vectors at 10 hPA are also mostly pointed equatorward (westward). EP vectors shift poleward towards the end of the forecast. But EP flux appears very weak. Thus, no significant change in temperatures are expected between 1 hPA and 30 hPA.

The ECMWF model winds & fluxes (left image) and upper-stratosphere temperatures (right image)

The ECMWF model winds & fluxes (left image) and upper-stratosphere temperatures (right image)

The ECMWF model shows a wave-2 in geopotential heights downwelling into the lower stratosphere — causing a significant disruption to the stratospheric polar vortex in 5-6 days. Some decent warming at 10 hPa showing up over Eurasia now at 6 days. Alongside geopotential and heat flux, momentum flux (UV) is also very high in the upper-stratosphere. Which is quite impressive now. We will have to monitor these type wave from over Eurasia to cause some kind PV perturbation, that could lead a weak, transient ridge developing over the Artic region or near Alaska.

UVflux 10hpa temps

Otherwise, this wave is forecast to subside in 6-10 days, thus allowing the polar vortex to reconsolidate. But 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 could lead to high-latitude blocking developing in January, and thus a change to colder and snowier conditions.

4. MJO/Tropical Forcing Discussion
Tropical forcing, as indicated by the OLR anomalies has become elongated from around the Dateline east into the Eastern Pacific over the past 30 days It also shows another area of tropical forcing over the Indian Ocean. This likely has resulted in more variant forcing than what is typically seen with a strong El Nino.

olr.anom.30day

Then the ECMWF forecasts show the MJO/tropical forcing signal going into the COD (circle-of-death, meaning no discernible signal) in early to middle December. The CFS 200mb and 850mb VP anomalies indicate this as well for Dec. 2-11th. If this is the case, then the MJO/tropical forcing will presumably have less influence on the atmospheric pattern going in early-mid December, at least.

The ECMWF ensembles for 11/24 to 12/09

The ECMWF ensembles for 11/24 to 12/09

After this period, the models indicate the MJO Euro weeklies indicate may enter phase 3 and 4 near Xmas. The CFS MJO/tropical forcing signal is more mixed between Christmas and New Years. 850mb Velocity Potential anomalies (VP) seem to be stronger than forecast at 200mb and seem to support phases 3/4/5. The tropical forcing in these phase could mean more warmer than average temperatures for late December and into early January.

CFS 850mb VP

CFS 850mb VP Anomalies (left image). Phase with 850mb VP anomalies (right image), courtesy of CPC.

5. Miscellaneous Discussion

Rossby waves often descend downstream in East Asia. The mountain range over East Asia can enhance the amplitude of Rossby waves over North America. This is called mountain torque. This may be partly responsible for -EPO ridge occurring late this week. This will have to be monitored for perturbations with the stratospheric polar vortex in the next few weeks.

Illustrations on how mountain torques work

Illustrations on how mountain torques work

Typhoon In-fa has weakened to 35kt tropical storm due to strong upper-level shear. The GFS and ECMWF shows In-fa remnants either being absorbed by a non-tropical low developing over Western Pacific, or remaining a separate feature and dissipating entirely. The Joint Typhoon Warning Center has this typhoon becoming extratropical in a few days. There is a very deep non-tropical low near the Aleutians enhancing the -EPO ridge late this week.

6. Conclusion and Summary

All of this pretty much falls right into line with our winter forecast for December. Given what we are seeing so far with the climate signals, there is little reason to go against the ensemble and weekly guidance at this point. We don’t believe there will be major changes occurring to the stratospheric polar vortex that would result a negative trend in the AO/NAO until very late December or more likely sometime in January. Which is still in line with our winter forecast.

That being said, we did not quite anticipate the -EPO ridge at this magnitude shown this early in the guidance. Therefore the threat for storms with more widespread wintry precipitation is a little higher than usual early in the month. But all indications from the climate pattern observations and longer-range guidance support a deep low/trough over the Gulf of Alaska for the body of month. Latest model and ensemble suggest this will now happen later next week. This will support +EPO returning and more Pacific air to flow into much CONUS for much of December.