Predicting the Polar Vortex

After the frigid winter we experienced last year, many are looking anxiously at the upcoming winter months, wondering if the dreaded polar vortex will be there to kindly greet us again. You may have heard about the farmer’s almanac predicting another chilly winter or other stories from media outlets making claims about the polar vortex.

In our last polar vortex post, we took a detailed look at the weather patterns last winter during one of the many intense cold outbreaks. We mentioned key facts about the jet stream and mid latitude cyclones and how the orientation of the jet stream can promote the development of mid latitude cyclones. The jet stream and other upper-air (high in the atmosphere) patterns drive the weather near our surface. In fact, we have the jet stream to thank for the ‘polar vortex’ AND the intense drought ongoing in California.

How can snowstorms and droughts possibly be related? It’s all in the jet stream, yo! Let us take a look at a seasonal forecast from last year (August 2013), showing the forecasted conditions for January 2014. The data displayed is taken from one of our climate models. 

What are we looking for here in this map? The solid black lines are contours of geopotential height. You don’t have to understand what that means, but it is a general rule of thumb that the jet stream flows parallel to these contours from left to right. By looking at these contour lines, we can tell where the jet stream is flowing and where to expect hot/dry vs cold/rainy. The wavy pattern over the west coast where the contour lines turn northward is the semi-permanent ridge at the heart of the west coast drought and also the mid-west polar vortex onslaught. Meteorologists refer to this as a blocking ridge, as this high pressure system has been stuck there for long time, essentially blocking any other weather patterns from coming through. If you recall from part 1 of the polar vortex post, when the jet stream turns north you can expect clear skies and relatively warm, dry weather. When the jet stream turns south there is cloudiness, rain, and other hazardous weather. This particular orientation of the jet stream promotes hot and dry weather to the west, and cool rainy weather here in the midwest.

Now, with all of that background knowledge, we can take a look at the seasonal forecasts for this winter. One thing that many meteorologists (and west coast dwellers) are hoping for is this blocking ridge to finally move out of the way so that California can get some rain! 

This image looks very similar to the previous image, and we do indeed still sense the presence of the blocking high pressure system off the west coast. The deep blue colors to the west of the ridge indicate a building low pressure system, which could (hopefully!) lead to some precipitation for our friends out in California.

Why don’t we take a look at some temperature forecasts to see if we indeed are in store for another bitterly cold winter. First, for verification, we’ll look at a surface temperature forecast from August 2013, valid for January 2014.

As you can see, this particular forecast was very accurate. We have above average temperatures out west, and a big chunk of deep blue polar vortex-y goodness covering the rest of the USA. Notice how the red colors are aligned with the high pressure ridge? Also you can see how the ridge kept the west coast warm, and allowed cool arctic air to creep down south to cover the USA.

Next up is the forecast for the upcoming winter. Here is an image showing the forecasted temperature anomalies for winter 2014/2015.  

Unlike the farmer’s almanac, our climate models are actually predicting above average temperatures this winter! As we have seen, this climate model did a good job of predicting the cold winter last year, we can only hope that these forecasts are just as accurate and that this winter will be more mild.

A blog post written by me wouldn’t be complete without some nerdy Python data visualizations, so lets take a quick look at Christmas Day 2014 around the globe and see what our climate models are predicting…