GFS Hinting At Long Range Storm System

The past 4 model runs of the GFS had some form of storm system and subsequent cold weather outbreak hit the Eastern US in the long range. This event usually happened between the December 10 - December 12 timeframe, with only the 12z GFS disagreeing from a unanimous December 10th storm date.

The 0z GFS had a monster storm system form along the East Coast in an event that would have decimated the region with more than 2 feet of snow and a monster cold outbreak just behind the system that would have brought freezing temperatures down to the Gulf Coast.

The 6z GFS had a similar set-up, but took the storm into Canada while leaving the cold outbreak to sink south just like the 0z GFS. Not as far south, but still down into Georgia and Alabama, among others.

The 12z GFS is the anomaly to these 4 model runs, as it took the storm to New England and pulled the cold air with it (out to sea) in unusual fashion. Additionally, unlike the 0z and 6z model runs, this forecast had the event happen 2 days later, on December 12th.

Luckily, the 18z GFS erased any hopes of a 12z GFS occurrence, but changed the storm track as it brought a strong system into the Midwest and Great Lakes, with snow hitting those same areas. Cold air did not have time to flow south, as another storm system was right on its heels.

Now, the GFS model alone in the long range is equivalent to playing with fire- you must be VERY careful with it, or you will get burned (i.e. disappointments after a storm does not happen). However, considering this has now been at least 4 model runs in a row that have showed some form of storm system and subsequent cold outbreak in the Lower 48, this does catch my eye. The GFS has been known to catch trends when certain events come back in forecasts over and over again, like the 4 examples shown above.

I will watch this system and provide updates as necessary, but this is a wait-and-see moment for now.

Andrew

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!!

Andrew

CFS Model Grossly Over-Exaggerates December Forecast

The Climate Prediction center's Coupled Forecast System model, or the CFS model (version 2) has put out a new December forecast that crosses the line between unrealistic and just plain absurd.

The top image is the anomaly forecast for the month of December, and the bottom image is the skill forecast (areas where the model is very confident in its forecast). As you can see, the CFS has temperature anomalies well over 1.5 degrees above normal spreading from the West Coast, through the Rockies and well into the Plains. Temperature anomalies exceeding 1 degree above normal exist in just about every state in the Lower 48, with the possible exception of eastern Florida. The skill map is confident in the East US' warmth and the Plains, but not so much for the Rockies.

This forecast is just plain ridiculous. The model is going off the idea that the persistent Omega Block will continue through the rest of the month, which, as shown in the GFS forecasts below, is likely not going to happen.

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.

Now, there is definitely potential for this blocking to stick around in the Bering Sea and make December warm. However, there is one more reason why you should not take the CFS for what it's saying.

This is the CFSv2 model's RMS Error for 2m temperatures in forecasts made in November. What this shows is the gap between forecasted situations by the CFSv2 and observed conditions for that same forecasted timeframe. Basically, this is the difference of those two factors. The DJF period (December, January, February) is shown above, and it appears that forecasts made in November typically verify extremely warm in the DJF period. This not only includes this December forecast, but the other two winter months that we don't particularly care for at the moment.
The CFS' track record is definitely a warm-biased one, meaning it has a tendency to forecast conditions too warm according to this map.

Not only that, but this chart of multiple models' forecasts for the month of December show that the CFSv2 is the ONLY MODEL to show a torch for the first month of winter. The only reason the NMME and IMME models are showing warmth is because of the CFSv2 is averaged into their forecasts.

So please, winter weather lovers, do not get so fired up about the CFS' forecast. In my eyes, it's the least desirable model (next to the Japanese and Canadian models) to use for a forecast. Do you REALLY think that December will bring extreme warmth to the nation??