Monday, January 15, 2018

Stratospheric Warming Event Projected To End January

Model guidance continues to indicate that the stratospheric polar vortex will come under further stress as we head toward the end of the month, with a wavenumber-1 event potentially unfolding in the stratosphere and opening the chance for February to be colder than normal.

Shown above is the 240-hour forecast of geopotential heights (contours) and temperatures (colored fill) from the ECMWF model, analyzing the 70-millibar slice of the stratosphere. This is around the middle or lower-middle part of the stratosphere, and as a rule of thumb stratospheric warming events tend to be stronger and more able to dislodge the polar vortex when the warming event is seen throughout multiple layers of the stratosphere. While I'm only showing the 70-milliber layer, model guidance has latched on to this warming potential throughout the stratosphere.

In this image, we see the stratospheric polar vortex still pretty much in control, centered almost over the North Pole. The piece to analyze, however, is the swath of warmer air trying to push into the Arctic Circle, with the warmest temperatures located over the Bering Sea. The placement of these warmer anomalies is important, because when stratospheric warming events push into the Arctic Circle from the Bering Sea region, they tend to be more successful at disrupting the polar vortex. I'm sure there's some academic literature on exactly why this happens, but in the absence of reading such literature I take that as a rule of thumb.

We can get a better look at the polar vortex through forecasted potential vorticity charts.

In a nutshell, these charts show the orientation of the polar vortex at different levels of the stratosphere. This map shows potential vorticity values at the 550 K level, which is somewhere around the 20-millibar level. Warmer colors depict areas of higher potential vorticity - in this case, the location of the polar vortex. Colder colors indicate where the polar vortex is not, in essence. At hour 240, we see the polar vortex in a rather elongated shape, dislodged from the Arctic Circle and positioned from Greenland across northern Eurasia. This shape of the polar vortex signals that it will be under duress at this point in time, as the polar vortex is 'typically' seen in a more circular shape. This elongated shape suggests pressure being applied from the Bering Sea area in the form of a ridge, which is confirmed by the deeper blue area stretching from the Gulf of Alaska to the Bering Sea. Similar to how higher levels of potential vorticity indicate the polar vortex, lower values (especially in this case) can be indicative of a ridge of high pressure.

So far, we've gathered that model guidance projects the stratospheric polar vortex to come under substantial pressure in the next several days, likely continuing through the end of January. This pressure looks to be applied across much of the stratosphere, boosting the chances that any disruption in the polar vortex could then show up in the troposphere in the form of colder than normal weather. Let's sum it all up with a few graphs.

The top panel shows observed zonal wind speeds at the 1-millibar level of the stratosphere - in other words, the strength of the far-upper stratospheric polar vortex. The 1-millibar level isn't as important as the 30-millibar or 50-millibar levels, as we discussed with the rule of thumb earlier in this post. However, it can still be valuable to look at, as a reversal in wind direction at the 1-millibar level could then filter down to lower levels. In this forecast image, the 1-millibar wind speed is forecast to continue declining slightly from a recent peak, before strengthening again and then once more weakening. This would take it back to roughly the same level we're at right now, if this forecast verified exactly as shown. Let's see if any signals can be identified in other panels.

The second panel shows the 10-millibar zonal wind speed (blue line) and the 30-millibar zonal wind speed (red line), where both slices of the stratosphere look to see the strength of the polar vortex gradually decline as wind speeds slow down. While the deceleration here appears modest, there are signs that it will continue through the end of January. Both the geopotential flux and heat flux indicators in the third panel are set to significantly increase during the forecast period, which will lead to a slight deceleration across the 1-millibar, 10-millibar and 30-millibar areas. This will then revert back to near-zero, but at the end of the forecast period the ECMWF sees these fluxes strengthening again, leading to further wind speed deceleration in the aforementioned levels of the stratosphere.  Perhaps most significantly, for this second acceleration in the two fluxes, you'll note how the EP-flux on the bottom panel is pointing straight up, when looking at it two-dimensionally. Stratospheric warming events are most probable when the geopotential and heat fluxes are high (which is forecasted) and the EP-flux arrows are pointed directly up (which is also forecasted for the end of the forecast period). As such, while the ongoing increase in fluxes may not do much damage to the polar vortex, the re-configuration of the EP-flux in time for the second projected warming event in about ten days could do far more damage, especially if it is a sustained warming event.

As such, I expect a broadly seasonal to warmer-than-normal temperature pattern for the eastern 2/3 of the U.S. through the end of January, but the chances of colder-than-normal weather increasing, particularly after the first few days of February.

To Summarize:

- Model guidance continues to indicate a potentially-disruptive warming event in the stratosphere towards the end of January.
- As this warming event is expected to affect nearly all sections of the stratosphere, effects in the troposphere are likely in the first and second full weeks of February.
- Colder than normal conditions are expected to increase in likelihood after the first few days of February.


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