Monday, December 1, 2014

Long Range Outlook: Cold, Stormy Period Expected in Mid-December

It's looking like a cold and stormy pattern will overtake the US for the mid-December timeframe.

Tropical Tidbits
The image above shows 500mb height anomalies over the Western Pacific on the evening of December 1st. In this image, we see what is very likely a lobe of the polar vortex passing just north of Japan, occluding and cutting north as it does so. Minimum values here at the 500mb level in this image, off the GFS ensembles, appear to drop close to 490 dam, an indication of either an incredibly strong upper level low, or the more-likely polar vortex lobe.
This situation appears similar to the case we had last winter, where we saw a very strong upper level low crash into Japan to round out the year 2013, which then led to incredibly cold weather in the first week of the new year.

December 27, 2013
If we recall the Typhoon Rule, which states weather phenomena occurring over East Asia is reciprocated in the United States about 6-10 days later, we could expect a bout of  substantial cold here at home in the December 7-11 timeframe. Should this verify, which is becoming increasingly likely given model consistency, the Central and parts of the East US may expect a bout of intense Arctic air to hit in the middle of December.

But it doesn't end there...

Tropical Tidbits
The image above once again shows 500mb height anomalies from the GFS ensembles, now valid on December 6th. Notice that we still see below-normal heights over Japan. This means Japan has been under a strong upper level low's influence for nearly a week, and still going. Building off of the first graphic, it's quite possible that the United States may also undergo a harsh beating from cold weather for nearly a week straight. The degree of this cold air is still under examination, but if it's anything like the upper level low expected to trek north of Japan to open December, watch out. 

The stratosphere is also working in conjunction with this strong upper level low over Japan. The image above shows the last week or so of temperature anomalies over the North Hemisphere at the 10 millibar level.

When we look at the final days of November, we find ourselves on the tail end of a stratospheric warming event, as the pink colors show in the boxes labeled November 24th through 26th. When we factor in that stratospheric warming events typically reflect in the troposphere about 2-4 weeks after the warming event, evidence for a synoptic cold weather outbreak strengthens for the early-mid stages of December, only enhanced by this Typhoon Rule component.

From here, the waters get a little murky.

The image above shows the projected phase space for the Madden-Julian Oscillation (MJO) off the GFS Ensembles. Each triangle-esque shape represents a specific phase of the MJO, with the phases number 1 through 8 on the exterior of each shape. Typically, in the winter, cold weather is favored in the Central/East US when the MJO is in Phases 7, 8, 1 and 2, while warmer weather prevails in the remaining stages. The GFS Ensembles predict the MJO to translate eastward with time, shown well by the quick movement through Phases 4, 5, 6, and 7, arriving in Phase 8 around the mid-December timeframe. Hey, isn't that when we're supposed to get a cold blast? Yep, and the MJO moving into Phase 8 only re-assures this outlook.

But this time, things aren't as clear.

The graphic above shows the same Madden-Julian Oscillation forecast as the GFS Ensemble one above, but this one now shows the forecast from the prestigious European model, abbreviated as the ECMWF ensembles. These ensembles take the MJO through Phases 4, 5, and 6 rather quickly, but then the MJO event weakens, as ensembles minimize strength into that circle in the center, which indicates the MJO event is too weak to be put into a certain phase.
Big difference from the GFS ensembles, right? If this ECMWF forecast verified, the chances of a chilly mid-December would likely be on the decline. Of course, that East Asian component would play a role, but this ECMWF forecast could very well dampen the cold in areas.

So who do we believe? For now, we see the United Kingdom's UKMET and Japan's JMA model guidance supporting a GFS ensemble-like solution, where the MJO would move into those latter phases supportive of a cold weather event. I'll take a middle ground and stick with a weaker version of the GFS ensembles, but still keeping an eye on a full-on GFS ensembles solution.

Kyle Griffin
Now this is where the fun begins...

The image above shows the GFS model's forecasted jet stream wind speeds (colored regions), mean sea level pressure (black contours), and 1000-500mb thickness values (dashed red and blue lines).
The forecast s valid for the morning of December 7th, and tells us a lot about the coming weather pattern. On the right (if I'm aligning this post correctly), you'll see the current jet stream pattern laid out across the Northern Hemisphere. Notice the ragged jet stream in the Pacific, but there's a very strong ribbon of the jet crossing Eurasia and currently pushing into Japan. The model guidance above indicates this strong jet will shove off mainland Asia in a matter of days, overpowering the Pacific basin and shunting moisture towards North America. As this happens, and we see extratropical cyclone formation on the surface (as indicated by the tight black SLP contours in the image above), pressures will lower just west of the Gulf of Alaska, even into the basin itself. In response, high pressure will form along the West Coast, which then in turn creates a trough in the East US, and suddenly you've got yourself a cold start to the second full week of December. This brings the Typhoon Rule, stratospheric, and MJO concepts full-circle to favor a cold weather pattern for the December 7-17 period.

The 'fun' part is that moisture I mentioned earlier in that paragraph. As the jet forces that moisture east, it will dive into North America on the heels of a powerful jet stream. With aforementioned high pressure in the West and low pressure in the East, conditions look to be ripe for storm formation in the Central US in this December 7-17 timeframe. Depending on how the atmosphere unfolds closer to this event, it could end up being a decent winter storm.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA and NAO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.
 The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

Looking at the forecast at hand, the PNA is expected to spike positive and stay positive throughout the extended range. This comes as that jet stream makes its way into the Pacific, and resultant stormy weather in the Gulf of Alaska forces high pressure to form in the West US. The NAO, however, will remain positive. This could be a bit problematic. If we see a positive NAO and positive Arctic Oscillation (AO; +AO supports warm weather, -AO supports cold weather), as forecasts currently indicate, we may also see this cold weather set-up slip away. I'm not sold on that happening yet, and in addition to all the content above, here's why.

The image to the right shows the Climate Prediction Center's verification of Arctic Oscillation forecasts. From the long-long range forecasts on the bottom panel, to observed AO values for the same time on the top panel, we can see what, if any, biases models are retaining with the AO. If you can't read this sort of chart well (I don't blame you, this one's a bit tricky to maneuver), it can be summarized by saying model guidances tends to forecast the AO to be positive too often; it ends up negative more often than not in those cases. That's not to say this time around will see the positive forecasts drop, but it must be noted that model guidance is too "trigger-happy" about forecasting a positive Arctic Oscillation event. If the same holds true for this mid-December time period, we'd likely see a handful of colder outlooks.

To Summarize:

- A warm start to December is expected (December 1-7).
- A predominantly-cold weather event is anticipated in the middle portion of December (December 7-17).
- A storm system (possibly more than one) could be seen in this December 7-17 time period.
- Following a brief warm spell in the mid-late December period, things could turn stormy again in time for Christmas.