Long-Range Outlook: Late November and December; December 1 Storm Threat

Welcome to our long-range outlook for the next 30-45 days. We hope to provide these outlooks with more in-depth analysis and discussion about every Monday and Thursday. The discussion is divided into several topics to cover all aspects of the model guidance and climate indicators.

For today, we discuss the overall pattern and what to expect going into late November and through much of December.

1. Model Guidance Discussion

The 500mb pattern this week will feature a North Atlantic ridge being suppressed by a deep polar vortex developing over Greenland and Iceland. This is a classic +AO/NAO pattern that typically supports a strong ridge for the East Coast. A -EPO ridge forming over the the Northeast Pacific and Alaska will cause a trough to dig into the Western US. In response, a ridge will build over the Central and Eastern US.

At the surface, high pressure over the Ohio or Tennessee Valleys will shift into the Canadian Maritimes over the next few days. This will cause a light northwesterly flow to turn more southeasterly during the middle of this week. At the end of this week, this high pressure will begin to break down and more southerly flow will develop.

As a result of all these changes, temperatures will slowly moderate each day of this week. By Friday, an unseasonably mild airmass may be in-place over the Northeast US, with temperatures possibly running between 10 to 15 degrees above normal or higher.

The GEFS showing a deep Polar vortex over Greenland and a ridge over the Eastern US on Friday

The model guidance shows the -EPO ridge deamplfying and a portion of Western US trough shifting quickly east next weekend. Some energy from the trough gets left behind over the West Coast. This results in the cold airmass coming that weekend to be transient or modified in nature, once again.

However, during the first 5 to 10 days of December, ensemble guidance are showing the -EPO ridge amplifying and then deamplifying over Alaska or Western Canada. The AO/NAO will continue to remain largely positive with the polar vortex still near Greenland or the north pole. This pattern will support more troughs with transient or modified cold airmass over the Northeast US.

The NCEP/GFS and ECMWF ensembles are also showing an increasing active pacific jet for early with multiple disturbance moving across the CONUS. The pattern will be progressive, with temperatures overall ranging from near normal to well above normal. The timing of each disturbance coming through the sub-tropical jet and into cold airmasses will be crucial in seeing any wintry precipitation. Boundary level temperatures may also be an issue for any frozen precipitation or snow/ice accumulation; especially for the coastal plain, this early in the season.

That being said, models and ensembles are starting to key in on a significant east coast storm for the beginning of December. However — not surprisingly at this juncture — there are a wide range of solutions amongst the operational models and ensemble suites, with every run. This analysis is based on the 500mb pattern the models are currently showing as of Sunday night’s overnight runs. But this could vary a lot in the upcoming days, as it’s not definite that a storm will occur in our region, as well.

Basically, the models track a closed 500mb low eastward underneath the -EPO ridge over Alaska/Western Canada. The 0z/23 ECMWF maintain a stout -EPO ridge and confluence zone over Southeast Canada. This allows more cold into the Northeast US and forces the low to our south.  If the Euro is correct with 500mb pattern in Canada, then the chances for a winter storm with snow or mixed precipitation increase for the entire region.

On the other hand, the GFS is not as stout with the -EPO ridge over Alaska/Western and has no confluence zone over Southeast Canada. Thus less cold air is over the Northeast US and the storm can track further north. This solution simply favors more rain for the coastal plain and perhaps some mixed precipitation for the interior.

The NCEP/GFS and ECMWF ensembles show the development of a ridge over the Barents-Kara Sea that causes a perturbation the of polar vortex near the the North Pole. This causes the AO to fall somewhat on the NCEP ensemble forecast. This is a good sign that the Eurasian snowcover is causing a high or ridge to build in that region. But 11-15 day forecast shows this ridge weakening. So the impact here appears to be temporary.

After this period in early December, we see a typical El Nino 500mb pattern showing up on the CFS and ECMWF 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. These are showing a more active subtropical jet for the second and third weeks of December.

CFS Weeklies showing a +EPO/AO/NAO pattern with a ridge and warmer than normal temperatures over parts of the Central and Eastern US during the middle of December

2. PDO and ENSO Discussion

Sea-surface temperatures continue to be warmer than normal over the Northeast Pacific. But over past week, these anomalies have cooled off. El Nino still appears to be strengthening. 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):

11NOV2015 23.5 2.0 27.9 3.0 29.7 3.0 30.3 1.7
18NOV2015 23.8 2.1 28.0 3.0 29.7 3.1 30.4 1.8

 SST anomalies over Pacific. El Nino becoming more basin-wide. Slightly cooling waters in Northeast Pacific

Latest indications show El Nino continues to strengthen and will likely peak in another few weeks or so. From there 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 Gulf of Alaska, allowing pacific air to flow into the CONUS for December on average.

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.

50 hPA temps are forecast to be well below average in the next several days. (left image). EP flux vectors pointed equatorward (west) in the last several days (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 pointed equatorward (westward). Thus, no significant change in temperatures are expected between 1 hPA and 30 hPA.

12z/22 ECMWF model forecast zonal winds and fluxes (left image). Temperatures in the upper-levels of stratosphere (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. This wave propagates onward in 8-10 days, thus allowing the polar vortex to reconsolidate. But we anticipate more wave disruptions with Eurasian snow cover still increasing.  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.

12z/22 ECMWF model geopotential wave forecast in 6 days (left image) and 10 days (right image).

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 3 weeks. 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 anomlaies (tropical forcing) from near the dateline to the Eastern Pacific

Latest long-range forecasts from the dynamical models have the MJO spinning between phases 2 and 3. Then the ECMWF forecasts show the MJO going into the COD (circle-of-death, meaning no discernible signal) in early to middle December. The CFS 200mb VP anomalies indicate this as well for Dec. 2-11th. If this is the case, then the MJO will have less influence on the atmospheric pattern going in early-mid December, at least. After that, the CFS 200mb VP  indicate perhaps another MJO wave developing near the Dateline.

CFS 200mb Velocity Potential Anomalies on left. ECMWF weekly MJO forecast

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 on some model guidance. This will have to be monitored for  perturbations with the stratospheric polar vortex in the next few weeks.

Illustrations on how mountain torques work form ESRL

Last year around this time, we were watching Super Typhooon Nuri recurve over the Western Pacific. Then it phased with an upper-level trough and became a very deep non-tropical storm near the Aleutians. This year we have another recurving typhoon over the Western Pacific named Typhoon In-fa (Marylin in English) . However this doesn’t appear to have same level of potential to enhance a -EPO ridge and delivering an massive arctic air intrusion into the CONUS as Nuri did last November, though it will still have to be monitored for any influences to the Northeast Pacific.

Typhoon In-fa is already weakening due to strong upper-level shear. The GFS and ECMWF shows Typhoon In-fa weakening further and its energy 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 weakening to a tropical depression well south of Japan by Nov. 27th. There will be a very deep non-tropical low near the Aleutians enhancing the -EPO ridge later 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 late December or January.

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. This will support +EPO returning and more Pacific air to flow into much CONUS by the middle of December.