Take a look at this image, forecasting 500mb geopotential height values in colored shadings, as well as mean sea level pressure (MSLP) values in the contoured lines. Check out that sagging of contour lines just south of Japan, right where the arrow is pointing. In this image, valid just six hours prior to the time period in the first image we analyzed, we see a strong storm system in the Sea of Japan, correlated to the strong upper level low. In this graphic, we see a negatively-tilted upper level low pushing east into Japan, as the elevated vorticity values indicate. We can’t tell for sure just yet, but the storm shooting northward to the east of Japan could mean a few possibilities. Other model guidance confirms this idea of a second body of low pressure forming south of Japan and skirting the nation to the east before phasing with the very strong storm in the Sea of Japan.

For the GFS forecast in the middle, we see a much stronger negative Arctic Oscillation signature, with high pressure forming a bridge across the North Pole. This has been the trend through much of the winter, until recently. Looking at the observed Arctic Oscillation from December 14th onwards, the trend has been for the observed AO to be more negative than forecasts believed it would be. 8-10 day 500mb height anomaly forecasts from the ECMWF (left), GFS (middle) and CMC models. The image above shows the forecasted 500mb vorticity values on the morning of December 16th, valid over the West Pacific. This image shows the Pacific North American index (PNA) forecast over the same timeframe as the AO forecast above. This four-panel image shows the 500 millibar height anomaly forecast over the Northern Hemisphere from the GFS model (top left), the GFS Ensemble Control (top right), and the GFS Ensemble mean (bottom left). The bottom-right image shows the ensemble spread across the Northern Hemisphere.

In this forecast, the polar vortex appears to be broken into two pieces, with one lobe located in northern Eurasia and the more significant piece of the vortex displaced over Greenland, producing a strong positive North Atlantic Oscillation (NAO). In its place ought to be a warm Southeast as some ridging takes hold thanks to that negative PNA, but also some cold weather in the Northern US as the negative Arctic Oscillation persists. We can observe the negative tilt by the height contours seeming to “dig” in a southeast-ward direction. While most holidaymakers can enjoy a spot of bird-watching simply while relaxing in their Menorca villa, some of the more passionate twitchers may want to go out of their way to seek rare sightings. I do agree with the negative AO projection, though I think we may see high pressure near the Gulf of Alaska expanded a bit in the future.

We see the ECMWF model on the far left prefers to hold a negative Arctic Oscillation (AO) regime in place, as is shown by stagnant high pressure in the Arctic Circle. It almost looks like another low pressure system! It looks like things will dry out this weekend as the trough moves eastward. We also see those superimposed panels like we saw in the Arctic Oscillation image. This image shows an ensemble projection from the Climate Prediction Center for 5 days out, unlike the week-plus forecasts we discussed above. All four forecasts are more or less similar to the GFS image we discussed at the beginning of this post, but take a look at the ensemble mean. We can look at the errors previous forecasts made. Well, we can probably expect a break from the intense cold we will see to round out February. This splits the vortex fully into two pieces; one part ends up in northeast Asia, while the other part is weakened and stretched out between Greenland and the United States.