Monday, November 26, 2012

Omega Block in Bering Sea Likely To End Soon

The Omega Block currently ongoing over the Bering Sea, as shown above, looks to be coming to an end in the next few weeks. As the name implies, an Omega Block is characterized by a blocking high pressure system in the atmosphere. In response to such high pressures, persistent troughing (stormy) patterns form to the left and right of this high pressure system, resulting in the Greek Omega shape, and thus the Omega Block.

The top image shows the GFS Ensembles' 500mb height anomaly forecast for Hour 384. Those of you affluent in the weather model forecasting business know that Hour 384 is the end of the GFS run and generally considered worth nothing that far out. However, when you get ensembles that come along, the idea changes, and suddenly you have 20 different forecasts coming together. The GFS Ensembles believe that the Omega block over the Bering Sea will dissipate by Hour 384, thus ending the NE Pacific trough and moderating the continuous warm shots that the nation has seen and will continue to see for another week or two. The bottom image is the GFS model itself, and the model shows this Omega block being pushed to the west- not the best situation, but good enough so that the persistent trough over the West Coast can leave the area and provoke a positive PNA pattern to set up behind the persistent trough, which has now moved west with the Omega Block. What the GFS ensembles actually try to do is push this Omega Block north into the Arctic Circle area to weaken the Polar Vortex and break the floodgates of cold air in Canada, which could then flow south, possibly into the US.

But it's not all about the models- it's also about the teleconnections. This is an image depicting two agencies' forecasts for two teleconnections (atmospheric functions). The top two images are of the EPO (which we don't care about at the moment) and the two bottom images portray the forecast for the WPO, or West Pacific Oscillation. The WPO is in a deep negative phase thanks to this Omega Block. In the positive phase, you typically see a strong low pressure sitting over the Bering Sea.
Anyhow, the two bottom images represent two forecasts. The one on the right is of the NCEP (essentially the GFS Ensembles), and the one on the left is from the ESRL/PSD (they have their own ensembles and thus, in my opinion, are more trustworthy). The ESRL believes the WPO will go down even deeper into negative territory as we approach December, but gradually go back positive as we pass through the first week or two of December. On the other hand the NCEP believes that the WPO will succeed in trying to stay as negative as possible well into December. However, note how the NCEP begins to rise near the end of the forecast. This, combined with the trending-positive ESRL forecast, tells me that there is a consensus for the WPO (thus the Omega Block) to be weaker in a week or two than it is now.

All in all, I feel like I can say that the first two weeks of December will be lost to this Omega Block. However, supportive signs are showing up that tell me the last half of December will be open for more winter-like conditions. Fingers crossed!!


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