Stratospheric Warming Event Taking Place

A stratospheric warming event is taking place as we round out the month of August. You may be wondering how the stratosphere can be experiencing a warming event because we are in summer and not winter. Truth be told, the stratosphere can warm at any time- the reason it warms so significantly in the winter and barely at all in the summer is because of just how dynamic the atmosphere is in the winter with the polar vortex, Arctic Oscillation and all that jazz.

Back to the point, the arrows on the images on the chart above show where this warming event is occurring. At the moment, we see the strongest activity placed between the 40N and 70N latitude areas. This corresponds roughly to the area between the southern Great Lakes and Mid-Atlantic to northern Canada when looking at the North American continent. Because this is a stratospheric warming event, I do anticipate a body of colder air to transition from the upper atmosphere down to the surface, just as what happens in a winter stratospheric warming event.

The question is, where will the cold air end up? Remember that the 40N - 70N area includes the rest of the world, and the effects of stratospheric warmings are not limited to North America. A quick glance at ensemble projections of the mid-level atmospheric height anomalies beyond the 10 day period shows a troughing pattern over the Great Lakes. This could clear the way for not only cooler weather created by the troughing pattern, but also, if the atmosphere cooperates, colder weather being shifted to the surface from this stratospheric warming event. But at of right now, this is all educated speculation.

Andrew

Official 2013-2014 Winter Forecast Comes Out Tomorrow

The Official 2013-2014 Winter Forecast comes out TOMORROW at 12:00 PM Central Time.

Also coming out tomorrow at noon are the 20 pre-selected cities' winter forecasts, and the winter forecasts for the North Atlantic Oscillation and Arctic Oscillation. 

Winter is quickly approaching!

Andrew

2013-2014 Winter Update: August 2013

With just 5 days left until the release of my Official 2013-2014 Winter Forecast PLUS 20 selected cities' winter forecasts at noon central time, I'll shed a little light on where we currently stand in the atmosphere, and what implications it could have for this winter.

Let's start off with sea surface temperatures over the world's oceans. Sea surface temperatures are significant indicators to high and low pressure formation over certain areas, so we want to be paying close attention to these SST charts as we move on through fall and into winter. There are a few areas I want to point out for this SST chart. The first region of interest is the body of above normal sea surface temperatures across the northeast Pacific. Per recent developments around the Pacific, the negative PDO regime that has been holding a grip over synoptic weather in North America for years now, dissipated. In its place, we have strong +SST anomalies. I'm feeling pretty good about the placement of those +SST anomalies, in the sense that such warm temperature anomalies could provoke persistent high pressure formation over the west coast of North America, which would go hand in hand with my analog set for this winter, which also shows high pressure straddling the west coast of North America. This persistent high pressure along the west coast of North America could very well then provoke a positive PNA pattern, which would add to cold weather prospects for the Central and East US. But of course, this is all educated speculation.
Moving on to the temperature anomalies along the Atlantic waters stretching from the Canadian Maritimes directly west to Europe, we see +SST anomalies in this area. Just like the Pacific, these +SST anomalies could provoke high pressure anomalies over the Atlantic, and this is something I have seen in another analog set I published on this blog, and do not discount the idea of such a scenario unfolding. If we look north into Greenland and the bodies of water in Canada, we see a continued trend of positive SST anomalies. If these positive SST anomalies continue into the winter, we could see the chances of a negative NAO increase, which could then lead to increased instances of coastal storms along the Eastern Seaboard. However, these coastal storms could go to the Midwest if that Atlantic ridging pattern I discussed earlier works out and expands into the East Coast. Again, it's all educated speculation.

Next, we will take a look at stratospheric temperature anomalies. The graph above comes from the Japan Meteorological Agency, and shows 30 millibar temperatures (black line) superimposed on the average 30mb temperatures for that time of year (gray line). The trend this summer has been above normal stratospheric temperatures, and the 30mb level is included in this trend. The upper stratosphere is not really included in this above normal temperature trend, but the mid and lower stratospheric levels are, and it is these two regions of the stratosphere that hold the strongest influence on the weather down here in the troposphere. This is all good news for winter weather lovers, especially if the warm stratosphere trend can continue through the fall and into winter.

This image shows zonal-averaged winds at the 30 millibar level- this is the same level we discussed in the temperature chart above. However, this zonal wind graphic has a much different bearing on winter weather. Take a look at the blue line stretching along the far left side of this chart. See those blue colors? That is a trademark sign of a westerly (positive) QBO. The QBO involves the stratospheric zonal wind pattern, and can have strong impacts on winter weather. The positive phase of the QBO means the stratosphere is less inclined to experience sudden stratospheric warming (SSW) events. If you recall from this past January, SSW events can lead to Arctic outbreaks of cold air, if the pattern is right. So, if you like winter weather, you don't want to see a +QBO for the winter. But let's look a little closer at this +QBO. As you can see, the blue strip of positive zonal wind anomalies signifying the positive QBO has been present since roughly May, and has strengthened since mid July. But look at the zonal wind chart since then. If you look closely, you can see the blue strip slowly narrowing along the 10S line on the bottom legend of this chart. Looking even closer along the EQ line right at the end of the left side of the graphic, you can see the darkest blue color is starting to become raggedy, and not as strong as it was earlier in June and July. If you didn't really see what I'm talking about, this is the point: there are small signs that the positive QBO is weakening. This would be great news for trying to break out of the positive QBO pattern and getting into at least a neutral QBO, which would enhance chances for sudden stratospheric warming events that would have otherwise been hampered by the +QBO.

All in all, there's still a lot of time until winter comes around, but current trends are rather favorable for this winter in terms of cold weather for the Plains, Midwest and portions of the East US.

Remember: The Official 2013-2014 Winter Forecast comes out at 12:00 PM Central Time this Saturday, August 31.

Andrew

Tropical Activity About to Spike; Landfall Threat Low

Tropical activity across the Atlantic basin is about to spike, with the GFS model predicting the development of two tropical cyclones in just over a week's time.

This spike in tropical cyclone development comes as a Madden-Julian Oscillation pulse creates a favorable tropical cyclone development environment for the Atlantic. Those more familiar with the MJO may recognize this as Phase 8 and 1. As the MJO pulse pushes over the Atlantic, it is expected tropical waves shifting west from Africa will have a favorable environment to develop. When they move out to sea, it appears probable that the northern Caribbean may be scraped by the first tropical cyclone, as the GFS forecast above shows. Beyond that, the dome of high pressure stretched across the Atlantic should erode to a point where both cyclones would be able to curve north and east, away from the US Mainland.

Beyond this first spike in activity, ridging in the north Atlantic should rebuild, and any tropical cyclones that develop in the next 14+ days after these two cyclones could move west enough to threaten the Gulf and western Caribbean. Of course, this is all speculation, but we are rapidly approaching peak hurricane season, and these spikes of tropical cyclone development are to be expected as we move through the rest of August, September and even into a bit of October.

Andrew

Significant Heat Wave Next Week May Become Prolonged

The anticipated heat wave that will begin to push east over much of the nation over the coming workweek will eventually draw back west, but will be able to sustain itself in that position.

The GFS model forecast above projects a very strong dome of high pressure over the Plains, extending its influence across much of the nation and into the Eastern Seaboard. The entrance of lower heights into the West Coast provides reason as to why the ridge of high pressure has decided to pulse east, but as soon as those lower heights shift east, it is likely portions of the East US will see a reprieve from the expected hot temperatures.

This shift of hot weather into the east US coincides with the development of a moderate to strong MJO wave that will be entering Phase 8 or 1 in coming days. Temperature composites on the left-hand side of the graphic show that Phase 1 of the MJO sees warm temperatures across much of the nation, and this will certainly be shown by this ridge of high pressure that will encompass much of the nation to kick off the workweek. Models then diverge, but the European and American ensemble suites seem to be insistent on keeping the MJO wave around Phase 1 territory. This could easily pressure the dome of high pressure into shifting east again at any time after it retreats west towards the end of the upcoming week. Basically, this could be a prolonged warm weather event. This does all depend on if model projections are correct, and we all know that high confidence in medium/long range models is playing with fire. Very long range forecasts from the European weather offices see the MJO shifting towards Phase 2, which would likely alter the weather pattern into one that sees portions of the East and Central US getting a break from the warm weather. But if the MJO stays in Phase 1, millions of people in the United States may be dealing with several days of anomalously hot temperatures.

Andrew