Stratosphere Analysis - July 27, 2013

The 70mb temperatures are, for the first time in many years, well above normal, and have been for some time now. My question is, will this encourage a weaker polar vortex this winter? This brings about the battle between an abnormally warm lower stratosphere and the now-mature positive QBO, which is detrimental to the weakening of the polar vortex. I brought about the very hostile environment this +QBO is currently in a little while ago, but it is managing to survive in this environment. Thus, we must consider the possibility that there will be a +QBO battling this warmer-than-normal stratosphere. If this does happen, I do not believe these two items would cancel each other out- such an occurrence in the synoptic-scale is rather uncommon. 

Which side do I believe will win? The 'good guy' (above average strat temps), or the 'bad guy' (+QBO)? The answer is not nearly as simple as you may want it to be. I think the positive QBO will have undergone serious damage by the time late fall comes around, while the above normal strat temps (barring any significant cooling for the rest of the summer and into the fall) should persist into the winter. While the temps may or may not persist into the winter, I firmly believe the effects of these above normal strat temps we have already seen will come into play for this winter. 

I will have more details on the stratosphere in the near future. Also, this blog will begin to see more heavy-duty weather information like the things above.  I will try to maintain as easy a reading level as possible, but as we enter fall, be prepared to see some hardcore weather posts.

Andrew

Positive QBO May Not Survive into Winter

The positive QBO we have seen evolve in the past few months appears unlikely to survive into this winter.

The graph above shows the evolution of the Quasi-Biennial Oscillation (QBO) over the past few years, up until the present day. The QBO involves a wind pattern in the stratosphere- in positive QBO winters, the polar vortex is strengthened, and opportunities for cold weather in North America are lowered. In a negative QBO, the wind pattern opposes the polar vortex, leading to additional chances for colder weather. We see more bouts of negative pulses than positive ones on this graph, and the positive QBO waves that do come about are not nearly as strong as the negative pulses. Looking to the far right side of the image, we see a new positive QBO pulse has evolved in recent months. Looking at the far, far right part of the image, we see that the positive QBO has been weakening recently. Could this be a sign that the +QBO pulse is weakening? Will it survive the winter? The first question is a yes (at the moment), while the second question could be a no.

I managed to string together several other atmospheric factors that can help determine the future of this +QBO wave. The first one involves a rather complicated pattern known as the AAM.

This image shows the observed AAM values from August 2012 to the present day. If you look closely, you can see that the AAM trend matches up with the QBO trend. We see negative AAM values from August to roughly November; a quick look at the QBO chart shows negative values were present at that time. Towards the winter months of 2012-2013, we see a definitive positive trend in the AAM, and this occurs around the time the QBO begins its ascent to neutral, and eventually positive territory. Now, with two big negative dips in June and July on the AAM chart, we are starting to see the QBO decrease. Is this correlation a coincidence, or an actual correlation between the two? It's an actual correlation- There is a positive correlation on the order of 0.391 between the AAM and QBO. This means that whatever the AAM does, the QBO tends to do as well, as we showed in this analysis of the two charts. The QBO's trends are also reflected onto the AAM. A value of 0.391 is pretty substantial, compared to other correlations I have seen in different atmospheric patterns, so this should be closely watched.

There's an even more significant correlation that is aiding in the positive QBO dwindling down, and the atmospheric pattern causing this new correlation is the Southern Oscillation Index, or SOI.

This screenshot displays the average SOI values over the past 30 and 90 days- the third box is something we will not discuss here. In the last 30 days, the SOI has averaged out to 8.9, and the last 90 days have brought the SOI to a 7.6 value. A look at any possible correlation between the SOI and QBO reveals a significant negative correlation of -0.521. This negative correlation means that whatever the SOI does, the QBO tends to do the opposite- in this case, the presence of positive SOI anomalies could very well be helping along the weakening of this positive QBO pulse. As I said in the previous correlation, 0.391 is a pretty noticeable correlation; -0.521 is an even bigger correlation, and one that, in my opinion, ought to be driving this fading +QBO pulse. If we want to see the +QBO weaken (which would be good for cold prospects (a negative QBO would be great for cold weather)), I would want to see the positive SOI strengthen to double digits for a prolonged period of time- that would seriously hurt the +QBO pulse.

Our last correlation is not nearly as significant as the previous two, but is another factor assisting in the weakening of the current +QBO phase.

This screenshot shows sea surface temperatures (SST's) in the Nino 1+2 region from now all the way back to August 2012. If you recall, the Nino 1+2 region is located in the waters immediately offshore of western Ecuador. Recently, we have seen very strong below normal SST anomalies in this region, and I decided to see if there was a match between the Nino 1+2 temperatures and the QBO phase. There is a positive correlation of 0.144 between the two. Again, it is not nearly as significant as the other two. However, as the Nino 1+2 region has been so anomalously below normal recently, there must be at least a slight helping by this factor to help weaken the +QBO.

If you're interested, there is a correlation two times stronger than the Nino 1+2/QBO correlation between the Nino 1+2 and AAM. The correlation is a positive one, coming in at 0.282. If the Nino 1+2 region remains negative, it means the AAM will tend to go negative, and that means the QBO will tend to go negative. Likewise, if the AAM goes negative, the Nino 1+2 region's sea surface temperatures will also tend to stay in negative territory.

If these correlations work out, the AAM continues to go negative and the SOI strengthens in its positive phase, the chances of the QBO at least weakening further are rather high.

Andrew

Tropical Storm Dorian Threatening United States

Tropical Storm Dorian has increased the threat of making a landfall on the United States overnight, as computer models suppress the storm to the south in an effort that could draw the system into the Gulf of Mexico.

[Image Removed]

Latest model guidance has a fairly straightforward solution this morning. TS Dorian is favored to continue on a west-northwest path, staying just north of the Caribbean in coming days. As it approaches the waters north of Cuba, some models start to curve the system northward in the beginning stages of what would have Dorian make a u-turn and move back out to sea. However, as many GFS Ensemble members are showing now, some other models hold off on this u-turn plan and continue on a WNW path. This is a new development, and one we need to discuss.

This is the current atmospheric flow chart for TS Dorian. The system is seen just below that big anticyclone of white circles as a small red area in the bottom right corner of the image. If we follow the white lines in and around TS Dorian, we see a WNW track is indeed favored over the next couple of days, as the massive anticyclone to Dorian's north suppresses the system and pushes it along.

This is where things get interesting. On the GFDL model's forecast for the same atmospheric flow level in the image above (valid for July 29), we see Dorian ENE of Cuba, shown as a green and blue swath in this image. But look where the arrows directly west of the green area are pointing: west. Some arrows on the northern fringes of Dorian are going for that u-turn pattern, but the arrows (streamlines) in the center of Dorian are the most influential on its path, and call for a continued WNW movement. If we were to go into the future another 24 hours, we would see that this westward trend continues, and Dorian would be headed for not only Florida, but possibly the Gulf of Mexico.

This all depends on just how the major anticyclone in the second image operates. This continued WNW path would depend on if more anticyclones are able to suppress the storm into taking a Florida or Gulf-bound route, as the above forecast is currently showing.

The two most prevalent tracks I see involve the U-turn idea, where Dorian would curve north and out to sea before hitting Florida, although greatly impacting the Bahamas, and a track where Florida is hit, but not by a direct landfall. Landfall then occurs on any land from Mississippi to the Florida Panhandle, as a u-turn curve pattern takes place. Regardless of the track, the Bahamas will experience tropical activity if the latest forecasts hold up (which I think they will).

Andrew

Winter Analog Set Prospers in Summer; Cold Winter Chances Rising

My winter analog set I presented in my preliminary winter forecast a few months back is doing very well at this point in time after success in the summer month of June, and indications are its success will continue into the winter.

I bring up the analogs' success because of recent developments in the past couple of months. Data shows that the Pacific Decadal Oscillation (PDO), a major driver in the seasonal climate around the world (including North America), spiked to positive levels for the first time this year. Previously, we had been in a solid negative PDO state. I did a little research to find other years that had a quick spike to positive PDO values before dropping back to negative territory, like we had in May and June, and I only found one year- 1951. If you recall, the winter of 1951-1952 is one of my two analog years, the other being 1962-1963. To have one of my analog years already accounting for this quick spike to a positive PDO greatly enhances its chances of working out come winter.

Since the analog set has had recent success this summer, let's see what it is telling for this winter.

The mid-level atmospheric pattern for the winters of 1951-1952 and 1962-1963 included low pressure across nearly all of Canada into much of the United States. This depression in heights was extended into the Northeast by the presence of a negative North Atlantic Oscillation, depicted as above normal height anomalies in and just south of Greenland. This negative NAO also assisted in below normal temperatures, which I will show a bit later in this post. Anomalous high pressure was also observed in the Bering Sea, which then enhanced the probability of lower heights in North America. You'll notice these two areas of enhanced above normal heights are positioned in key areas. I already discussed why the Greenland height anomaly was important, but the Bering Sea anomaly is also key to the winter. If we see persistent above normal heights in that region for the winter, it greatly enhances the chances of sudden stratospheric warmings, which can then lead to a dismantling of the polar vortex, and that delivers us into our next subject.

The 50mb level, usually acknowledged as the middle of the stratosphere, is shown to be overtaken by massive above normal height anomalies across the upper latitudes. Typically, one would expect to see the polar vortex over the Arctic Circle, but it has been drastically weakened and forced south into the western North America region. These two large domes of above normal heights tell me the polar vortex did not have an easy time between these two analog years, and if all goes as planned, this winter's polar vortex will not have an easy time either. As I told of in the 500mb paragraph, the Bering Sea high pressure anomaly very well could have played a role, as it typically does with breaking down the polar vortex.

The temperatures for the combined years averaged out below normal across much of the country. One year had above normal temperatures, while the other had very below normal temperatures. When they come together, though, the average is clearly in favor of the below normal. After my recent post about the consistency of the CFS in its winter forecast this summer, I do not doubt the idea of below normal temperatures in the Great Lakes, Plains, Midwest, Ohio Valley, and Northeast. With drought conditions still looming over the Southern Plains, above normal temperatures do not seem that far off of the spectrum of reason. All in all, this analog set is making a reasonable case for this winter at the moment.

Precipitation wise, the average of the two winters was rather dry across the Southern Plains into the Midwest, especially hitting the Pacific Northwest hard. Areas including the Mid-Atlantic and New England areas had limited success in the above normal precipitation department, but the Southwest ended up as the wettest in the nation. An analysis of the two years contrasted greatly, with a very wet Ohio Valley and southern Midwest in 1951-1952, but a very dry Central US for 1962-1963. It is for this reason that I am skeptical about the precipitation idea for this winter. I feel it will not be as dry as this image indicates, but it could end up a tad below normal in portions of the Plains and Midwest. I'll reveal the whole story in my official winter forecast in September.

Overall, I'm feeling very good about the analog set I have going right now. No problems appear to be arising, and I plan on using these same years for my official winter forecast.

Andrew

Tropical Storm Dorian Forms

Tropical Storm Dorian has formed overnight in the eastern Atlantic as a result of the vigorous tropical wave I discussed a few days ago getting its act together at sea.

[Image Removed]

The latest model suite for Dorian has the system placed directly east of St. Lucia in the Caribbean, moving at a generally west-northwest path over coming days. The model guidance above does appear to have a consensus with taking Dorian along the open Atlantic waters towards Hispaniola and Cuba. This is where things get iffy. Some models begin to curve Dorian to the north to allow the system to make a u-turn and move away from the US Mainland and out to sea again. Some other models attempt to push Dorian towards Florida, possibly as a bid for a landfall on the Sunshine State. Still other models (not shown on this image) would appear to want to push Dorian through Cuba and into the Gulf of Mexico, while our last category of models would try to do a re-curve out to sea, but would curve north too late and pose a real landfalling threat to the Carolinas.

The latest depiction of the atmospheric flow for a system of Dorian's strength (1000 millibars or higher) is rather straightforward, but has no model support. Going by this image, Dorian (shown as the 'L' on the bottom right corner of the picture) would go with the flow in a west-northwest pattern, in similar fashion as the model suite. From there, it falls apart. While the atmospheric flow will certainly change in 48 hours, Dorian would likely encounter two options of this overall pattern in the above image were to remain intact:

1) It sees the gap between the two anticyclones in the open Atlantic and pushes through, recurving well out to sea and posing no threat to land.

2) The gap between the anticyclones closes and Dorian is forced south, which could lead to a threat to some northern Caribbean regions, including Hispaniola, Cuba and some other land masses in that area.

As of now, after analyzing the full model suite and some other factors, I would be more inclined to believe the second option, mainly out of both model agreement on a more WNW track over a NW track, and also the tendency for weaker systems to maintain a more westward track.

I bring up the topic of weaker systems because we will see Dorian go through more ups than downs in the next 120 hours. Shown above is a chart of current wind shear values. Dorian is not shown here, but you can see approximately where the system is, shown by a sudden drop in shear values in the bottom right corner of the image.

If we look ahead on this chart and follow the track proposed by the model guidance at the top of this post, we see Dorian heading directly into 40-50 knots of wind shear. That's a lot of wind shear, folks. Dorian will have a very rough time battling this, and we will likely see the system struggle. No matter the track, Dorian is likely to enter that large swath of 40 knots of wind shear extending from the northern fringes of South America well out to the eastern Atlantic.

As of now, I prefer the current model suite in the top image to the extent of when it begins to approach the waters north of Hispaniola. Beyond that time frame, the models become too muddled and uncertainty grows to a point I believe a forecast will be unsuccessful.

Andrew