Tuesday, July 15, 2014

2014-2015 Winter Forecast Update; Colder Outlook Now Favored

This post will examine the factors being analyzed for the upcoming winter, their impacts on the cold season, and anything that may have changed from the issuance of the Preliminary 2014-2015 Winter Forecast to today.

After going over the factors presented in our Preliminary winter forecast, and examining new data presented to me, I’m electing to favor a colder outlook to this winter, over the original warmer projection earlier this summer.

The above image shows observed sea surface temperature anomalies across the globe from July 1, 2014, to July 8, 2014. In this image, we notice three particularly interesting features. One of these features is the pool of well above normal water temperatures in the Northeast Pacific into the Gulf of Alaska. This particular mechanism is one of substantial importance, given it also played a significant role in our last winter.

In the winter of 2013-2014, we had this warm pool present throughout the entire season, and it was quite possibly the most influential factor in making that winter as cold as it was. The warm water enabled strong high pressure to form over the Northeast Pacific, something that is continuing even to this day. See the examples below.
The image above shows mid-level geopotential height anomalies in early January 2014. Cold colors indicate the presence of strong low pressure, hence below normal temperatures. The yellows and reds indicate high pressure, and warmer than normal weather. This image was recorded in the middle of the extreme cold spell that kicked off the new year, also spawning the ‘polar vortex’ hype. In this graphic, we can see the polar vortex located somewhere in south-central Canada, bringing that extreme cold to much of the United States. We also saw strong high pressure centered over Nova Scotia, and quite extensive high pressure from the Southwest United States to Alaska. It was this second body of high pressure that formed as a result of those aforementioned warm waters in the northeast Pacific, and we saw this ridge stay in place consistently throughout the winter. Now, with the warm waters in the Gulf of Alaska still in place a good year later, I am significantly concerned with the idea that we could see this extensive ridge forming yet again this coming winter.

Building off of this idea, in addition to the water temperature pattern in the Pacific going relatively unchanged since last winter, the upper air flow has remained relatively unchanged as a result.

Tropical Tidbits
This forecast image, valid on Monday, July 14, shows the same 500mb geopotential height concept that we saw earlier in this post, with blues indicating low pressure and reds indicating high pressure. In this forecast image, once again, we see strong high pressure stretching along the western coast of North America, with another body of high pressure being observed over Nova Scotia and far eastern Canada. And in a situation just like the polar vortex plunge last winter, we see anomalous low pressure crashing into the United States. This upper air pattern is almost (keyword almost) exactly like the one we saw last winter, bringing into question if we could see this pattern persist into next winter as well. In my opinion, if the sea surface temperature anomalies remain the same into next winter as they are now, I see no reason to discount the idea that the coming winter could be like last winter; a cold, potentially brutal one.

Adding to the pieces of this puzzle is the body of significantly above normal temperature anomalies near Greenland. This is a change in what we saw from last winter. This past cold season, we observed primarily below-normal water temperatures near Greenland, resulting in some very stormy periods in the northern Atlantic. However, now that we’ve entered July, we now find ourselves with a formidable body of positive water temperature anomalies, something likely to change the game for the coming winter.

When warmer than normal water temperatures exist near Greenland, it provides a similar opportunity to that in the northeast Pacific, where high pressure can form. In this instance, high pressure can form over Greenland, and actually force the jet stream to buckle in the East US, resulting in an influx of colder than normal air to that area, as the image below shows.

 We didn’t have that warm water in place last year, meaning such a sequence of events couldn’t happen often, if at all. However, now that the anomalies have reversed, we’re in a much better position to see the jet stream buckle more frequently over the East US, resulting in another cold winter.

On a side note, we also see persistent above normal water temperatures near Nova Scotia, which could provide the mechanism needed to create high pressure in that area. This could then combine with the warm SSTs in the northeast Pacific to create an environment very similar to the one observed with the polar vortex incident.

We now continue on with a look at temperatures in the Arctic Circle. The graph above shows observed temperatures in Kelvin over the Arctic Circle (colored red), from the first day of 2014 to present day. We also see a green line, indicating the average Arctic Circle temperature for that given day, allowing us to compare with the observed temperatures to detect any anomalies. The consistent blue line highlights the freezing temperature mark. On this image, we see Arctic temperatures have been steadily below normal this summer, something that does not bode well for us in North America.

In order for winter to even be cold to begin with, one needs cold air. It seems simple to understand, but now we need to understand where this cold air will come from. By using this graph above, we can get an early glimpse at how much cold air the upper latitudes may be storing up for this winter. In this instance, below-normal Arctic temperatures in the summer might tell us to prepare for a cooler than normal winter ahead, while a warm Arctic summer could indicate a warmer winter ahead. Going by this guideline, we might be best off preparing for a chilly winter.

Now, last but definitely not least, we will go over the evolving situation in the Equatorial Pacific.
The image above shows equatorial upper-ocean heat anomalies, in Celsius. This means that the temperature anomaly for the upper portion of the Equatorial Pacific is calculated, giving a helpful indicator to the state of the El Nino-Southern Oscillation (ENSO) phenomenon. In this case, positive anomalies indicate the presence of warmer than normal waters, and potentially an El Nino. Similarly, below normal upper ocean heat anomalies highlight below normal water temperatures, potentially also the presence of a La Nina.

Looking at this graph over the past year, particularly since February 2014, we see that the upper ocean heat anomalies have skyrocketed, up to abut 2.0ยบ Celsius above normal in late March 2014. It was at this time that we were expecting a strong El Nino to evolve for the coming winter. Things have certainly changed since then. The warm waters began to hit the surface this past spring, but couldn’t hold their ground, per se. As a result, we have seen the positive anomalies drop off to neutral territory, indicating that any possible El Nino no longer theoretically exists. Time will tell if we see another body of warmer than normal water temperatures pushing to the surface, possibly creating an El Nino. If that does not happen, we can expect the winter pattern to be dominated by more mesoscale (smaller-scale) features, such as the bodies of warm water temperatures in the northeast Pacific and around Greenland. This could then lead us into - you guessed it - a cold winter.

All of this is expected to change as we progress through summer and into fall; this is merely an update to the situation, since many things have changed since the initial issuance of our Preliminary 2014-2015 Winter Forecast.



Josh Herman said...

Gary Lezak would suggest this is many months too early to tell as the new pattern doesn't set up until Oct-Nov. :)

Anonymous said...

Hi Andrew, I love reading your blog and I have a question. Given your forecast for an abnormally cold winter, is there a chance that this can come true?: http://theweathercentre.blogspot.com/2013/12/ecmwf-weeklies-send-polar-vortex.html

Anonymous said...

Sorry I can't read the blue writing, doesn't show up good on my phone, so I will just follow your post