Sunday, December 20, 2015

Early Signals For Pattern Change in Early January

There are emerging signals for a change to a colder and snowier pattern in the early days of January 2016.

The image above shows the GFS Ensemble mean 500mb height anomaly forecast for hour 384- the infamous fantasy land, end of the run panel. The ensembles are expecting ridging to build up along the west coast of North America, with troughing taking over in the Bering Sea and Arctic Circle. This kind of pattern would likely promote an overall-cooler set-up than the one we're currently in, which will feature well above normal temperatures on Christmas Day.

You all know I wouldn't post a 384-hour graphic without good reason, so let's dive in.

The ensemble forecasts out of the Climate Prediction Center for the Pacific North American (PNA) index show a dip to negative values as we head to Christmas, but then rising to neutral, and even positive values to kick off 2016. The majority of members are in positive territory by the start of the new year. The relative consensus of most ensemble members on a neutral or positive value of the PNA is encouraging for winter weather fans. Positive PNA values typically result in colder and snowier weather for the Central and East US, while negative PNA values result in warmer weather.
Some of you may ask why it's been so warm even though the PNA was positive in December- that can be attributed to the El Nino, producing enhanced convection in a part of the Equatorial Pacific conducive for warm weather here in the U.S., however that's a topic for another post.

A grand overview of four key teleconnections reveals this improving pattern (improving, at least in the eyes of winter weather fans). We see a continued positive EPO through the forecast timeframe- if you're more knowledgeable, you'll recall that the positive phase of the East Pacific Oscillation (EPO) encourages colder weather in the eastern 2/3rds of the country, while the negative phase encourages warmth. The WPO (West Pacific Oscillation) closely follows those guidelines as well.
As noted earlier, we see the forecasted PNA rising to at least neutral values in the long range near the start of 2016.

If all of these forecasted indices and teleconnections verify on an as-is basis (which, I may add, is rather unlikely), we could expect a cooler pattern in the Central and East US. This would be mitigated by the positive phases of the Arctic Oscillation (shown in the first image by negative height anomalies across the Arctic Circle) and North Atlantic Oscillation (shown in the first image by weak troughing over Greenland), both of which tend to discourage persistence of colder weather.

To summarize:

- There are indications that a pattern change may be on the way for the start of 2016.
- Model guidance will change drastically, and this is nowhere near set in stone. However, if this does verify to a certain degree, a cooler pattern may be on the horizon.


Stratospheric Polar Vortex Expected to Weaken in January

The stratospheric polar vortex is expected to exhibit weakening in the near future.

Garfinkel/Hartmann Publication
(Image obtained from Eric Webb)
The above image, from the Garfinkel / Hartmann publication, 'Tropospheric Precursors of Anomalous Northern Hemisphere Stratospheric Polar Vortices', shows precursors in the tropospheric mid-levels to weakening of the stratospheric polar vortex. In layman's terms, the images above (particularly the left panel) give us a glimpse at what the weather pattern should look like a short period of time before the stratospheric polar vortex weakens.

In that left panel, showing geopotential height anomalies at the 500-millibar level, we see strong negative anomalies (troughs / storm systems) in the Bering Sea extending into the Arctic Circle, even a bit into Greenland, and positive anomalies (ridges / high pressure) over Canada, as well as in the north-central Pacific. We also see ridging over Europe. These are indicators that the stratospheric polar vortex could be weakening down the road.

Let's compare these indicators to the long-range forecast from the GFS Ensembles.

Tropical Tidbits
Attached is the 500-millibar height anomaly prognostication for the 11-15 day forecast period (Dec. 30 to Jan. 4). Here, we see negative anomalies across the Bering Sea into Siberia, all the way across the Arctic Circle into Greenland. We also see weak ridging in Canada and the East US, and ridging in the north-central Pacific. Most notable is a strong ridge over Europe.

If you compare this forecast graphic and that left panel earlier in this post, you'll find that they're incredibly similar, almost identical. In sum, long range model guidance is telling us that we should see the stratospheric polar vortex weaken in January. This is good news for snow and cold fans who have been suffering through this first month of 'winter' (more like an extended fall with these warm temperatures)- even though a weakened polar vortex does not necessarily mean cold and snow, it does raise the chances of more cold air intrusions and snow events for North America.

To summarize:

- Long range model guidance is indicating the stratospheric polar vortex will weaken in January.
- Consequentially, the chances of cold and snow may be on the rise for next month.