Friday, March 21, 2014

Seasonal to Above-Normal Temperatures Just Days Away

Though one of the harshest winters to hit this nation in recent memory is still leaving its mark to this day, with below normal temperatures continuing to persist across the country, I'm finding new, concrete data indicating that warmer weather is as little as 10 days away - and it could be here to stay.

Tropical Tidbits
There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. In this sense, we can use the long-term weather pattern over Japan to predict when some seasonal, warmer weather will arrive in the US. Take a look at the graphic above. We see 500mb height anomalies from the GFS Ensembles over the Western Pacific. I use the ensembles rather than the GFS model itself, as I find the ensembles to be more useful in determining the synoptic (large-scale) pattern than the one model (remember the GFS Ensembles have multiple 'members' that add credibility and accuracy to its forecast). The image here shows 500mb height anomalies valid for March 24th, and looking towards Japan, we see oranges over Japan, indicating the presence of high pressure and warmer weather. Using the 6-10 day correlation, we can thus expect warmer, quieter weather in as soon as March 30th. We're about 9 days away from that today, and it might seem like one of those passing warm shots that then leads to another cold blast. The truth of the matter is, this warm-up looks to be sustained.

Tropical Tidbits
Fast-forward to the GFS Ensemble forecast on March 27th. Once again, we're looking at 500mb height anomalies over the Western Pacific. Looking at Japan, we see sustained high pressure and warmer than normal weather still over the area. At this point, if we go out to the end range of our 6-10 day gap from March 27, we might expect this warm-up to go as far out as April 6th. Bear in mind that this would be a relatively uninterrupted warm-up, as ridging would stick over Japan during this entire timeframe. No big storm systems, only quiet, warm weather. So, using this tool, we've established a warm-up timeframe of March 30th to April 6th. That's a full week of potentially non-stop seasonal to above-average temperatures!

But the warmth just keeps coming.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies over the Western Pacific, now valid on April 1st. We are still seeing strong ridging over Japan, meaning our warm-up here in the United States could extend as far out as April 11th, the end range of our 6-10 day correlation tool. Beyond this timeframe, the ensembles get too spread out and we begin to see less and less confident forecasts in terms of projecting 500mb height anomalies in above or below normal categories. This is natural, as the ensemble members typically diverge in more noticeable features as the forecast period goes on. Despite that, the agreement of ensembles on continuous ridging over Japan is remarkable, and would give us at least 2 weeks of sustained warmth.

That's not the only item arguing for a end to winter in the next several days- the stratosphere is also doing its part.

The image above shows two panels of observed temperatures over the North Pole. The left image gives you observed temperatures at the 10 millibar level of the stratosphere, with the average temperature line in gray. The right panel depicts observed temperatures at the 30 millibar level of the stratosphere, with the average temperatures for that level shown in gray. The 10mb and 30mb levels are pretty high up in the atmosphere, when you consider that we live at the surface level of 1000 millibars. They seem even higher up when you consider most airplanes only fly up to the 250 millibar level or so, which is around that 35,000 foot mark. You can see on both panels how temperatures have been really bouncing around in recent months, until the last several days, when temperatures have skyrocketed to well above normal levels. After a relatively calm November through January in the stratosphere, what's going on up there?

The polar vortex is collapsing.

In my post the other day, I discussed how we were seeing a reversal of zonal winds in the upper levels of the stratosphere, indicating the collapse of the upper stratospheric polar vortex, and the significant weakening of the overall polar vortex. I talked about how this phenomenon was likely the result of the Final Warming, which is a massive stratospheric warming event in the end of winter that dethrones low pressure and instates high pressure in the stratosphere, a natural occurrence that happens each year. These Final Warmings usually signal the end of the winter in the stratosphere, and typically result in the end of winter 2-4 weeks later here at the surface.

The image above shows a six-panel graphic, each marking a day's observation of zonal wind anomalies by latitude, as the legend on the bottom shows, and by height in millibars, as the legend on the left shows. Going from the top row of panels to the bottom row, from least to most recent, we see a notable and rather sudden drop in positive zonal wind anomalies in the upper right-hand corner. Looking closer, we find this drop in anomalies over the 1 to 10 millibar level, in the highest reaches of the stratosphere (remember that we are based at roughly the 1000 millibar level, so 1 millibar is far higher than even what planes cruise at). Now, this transition from orange colors to blue colors in the top right hand corner of the most recent observation, marked under March 19 2014, means that the winds have reversed. In the Northern Hemisphere, positive zonal winds are also known as 'westerlies', as they move to the east in a counterclockwise formation. That is why we look for areas of positive zonal winds to identify the polar vortex in the stratosphere, because the polar vortex is essentially just one big low pressure system. In the same sense, negative zonal wind anomalies define 'easterly' winds, as they blow towards the west. Recall that high pressure winds spin to the west in a clockwise motion, providing the reason why we look for negative zonal winds to tell if the polar vortex has weakened. In this case, rather than the positive zonal winds just weakening a bit, it looks like they completely reversed in the far upper stratosphere, marking a collapse of the polar vortex at that level. We saw that reversal of winds in the upper stratosphere a few days ago, but it's only now that we see a rapid decline in oranges and reds across the middle and lower stratosphere, as well as the staying power of the negative zonal winds in the upper stratosphere. This all tells me that we are indeed in the process of the Stratospheric Final Warming, and we should see the end of wintry weather in the next 2-4 weeks, when the effects of occurrences in the stratosphere usually impact the surface. There may be a chill in that timeframe, as this is still, to some degree, a stratospheric warming event, which can result in chillier than normal weather. However, this would likely be short-lived, as our East Asian tool tells us warmer than normal weather is the new norm in coming days and weeks.

To summarize:
• There are increasing amounts of data that suggests we will see sustained warmth to start off April.
• The stratosphere indicates we'll be seeing an end to wintry weather in just 2-4 weeks.
• Other indications suggest sustained warmth may be just 10 days or so away.