2011 Summer Outlook

[weatherwiz]

The 2010-2011 winter is behind is and were well into the 2011 spring season which only means one thing, summer is right around the corner. The summer of 2010 was pretty much above-normal for the majority of the United States and especially in the Northeast United States where many locations saw a summer which was in the top 5 for hottest summers on record. At this current point in time I don't believe we will see a summer anywhere near what last summer was as far as heat goes, in fact when compared to normal this summer may indeed be slightly cooler for most. Besides the outlook on temperatures for the nation what will precipitation look like? What are we looking at for severe weather potential and which areas may be more active than others? These questions and more looked at and answered below.

Before we get into the basis behind the outlook it is important to remember seasonal forecasting is still a relatively new science and there really is no exact way to go about composing a seasonal forecast nor is there any one correct way to go about so. Season forecasting is a very challenging task and involves a major understanding in knowing or gauging how the atmosphere works and how it may evolve from it's current state. There are many professionals out there in the field of meteorology who dedicate lots of time and effort into long-range forecasting and even do it for a career, there are some who happen to be pretty good at it and have had some decent success.

As for myself, I've done seasonal outlooks in the past and have had horrible success, in fact I have had no success at all. The only way to become better and have a better understanding at seasonal forecasting and how the atmosphere works is to just keep working at it and learn from mistakes. With this said let's rock and roll!

When composing a seasonal outlook there are several key things to look at; the current ENSO state (whether we are in an EL Nino, La Nina, or Neutral state), the potential state ENSO may be trending as you go towards and through for the season you are composing the outlook for and some of the global atmospheric circulations such as the PDO (Pacific Decadal Oscillation), AMO (Atlantic Multi-decadal Oscillation), QBO (Quasi-biennial Oscillation), NAO/AO (North Atlantic/Arctic Oscillations), and the current state of solar activity. These are only some of the global circular oscillations that exist and are known, however, research shows that many of these seem to have a much higher impact on the potential weather patterns more so than they do on the potential summer pattern.

ENSO state

One of the most important things to asses when looking ahead at the potential long-term pattern is the current ENSO state. Since about early summer we have been in a La Nina state. La Nina, as defined by the Climate Prediction Center (CPC) is when sea surface temperature anomalies throughout the equatorial Pacific are below 0.5°C for a minimum of 5 consecutive trimonthly periods. The equatorial Pacific is broken down into four separate regions referred to as Nino regions. They are Nino 1.2, Nino 3, Nino 4, and Nino 3.4. Across the equatorial Pacific each of these Nino regions cover a portion of the Pacific. Nino 1.2 covers the area from the South American coast to 90°W in longitude and 10°S to the equator. Nino 3 covers 90°W to 120°W and 5°S to 5°N. Nino 4 covers 150°W to about 160°E and 5°S to 5°N. Nino 3.4 covers 120°W to about 160°W and 5°S to 5°N. Here is a visual interpretation:

Of these four Nino regions it is Nino region 3.4 which holds the highest regard when assessing the ENSO state. It is in this region where if the SSTA are at or below 0.5°C for a minimum of five consecutive trimonthly periods. The trimonthly periods are as follows; DJF (December/January/February), JFM (January/February/March), FMA (February/March/April), MAM (March/April/May), AMJ (April/May/June), MJJ (May/June/July), JJA (June/July/August), JAS (July/August/September), ASO (August/September/October), SON (September/October/November), OND (October/November/December), and NDJ (November/December/January).

Besides looking at Region 3.4 sea surface temperature anomalies to determine ENSO state you can also use it to determine the strength of the ENSO state. By definition of the CPC an ENSO event is classified as weak if the SSTA's in region 3.4 are between +/- 0.5°C and +/- 0.9°, if the SSTA's are between +/- 1.0°C and +/- 1.4°C the ENSO event is classified as moderate and if the SSTA's are +/- 1.5°C or greater the even is classified as strong.

Going back to the JJA period of 2011 we have been in a La Nina state, the La Nina peaked at -1.4°C during both the OND and NDJ periods making this La Nina event a borderline moderate/strong one. Currently the Nina has weakened to a weak to moderate La Nina. This moderate/strong La Nina was on the heels of what was a strong EL Nino which occurred during the 2009-2010 winter and by the time late winter/early spring rolled around the EL Nino quickly weakened and transitioned to the La Nina we are currently in.

Since we know where we stand ENSO wise right now the next thing to asses is where are we heading ENSO wise? This will have an important role in the upcoming summer pattern and will play an important role in how the pattern transitions over the upcoming weeks and months. In order to do this several things are looked at; trends within the SSTA's over the past few weeks and months, the strength of the trade winds across the equatorial Pacific and the trends in the strength of the trade winds over the past several weeks and the recent trends/state of the Southern Oscillation Index (SOI). The SOI as defined by the CPC is the differences in the air pressure anomaly between Tahiti and Darwin, Australia. The negative phase of the SOI represents below-normal air pressures over Tahiti and above-normal air pressure over Darwin, Australia. This is significant as prolonged periods of a positive SOI correlate to La Nina episodes.

• Trends of the SSTA's throughout the Pacific.

Below is an animation of the SSTA's throughout the Pacific Ocean ranging from February 2nd, 2011 to April 20th, 2011. This image is obtained from the CPC website:

As you can see over the past several weeks the cool anomalies have actually began to warm some thus showing that the once moderate-to-strong La Nina has been in a weakening state. In fact, we have actually seen a quite a bit of warming occurring across Nino region 1.2, enough warming to where there actually positive anomalies across that region right now.

The questions here now are; how long does this warming last? How quickly will things continue to warm, if they do? And finally, where will we be heading ENSO wise and end up ENSO wise in the summer?

Well while we have seen a great deal of warming over the past several weeks with the surface temperatures when we take a look a little bit deeper depth wise into the ocean we see that any warming should be somewhat limited. As you can see while there virtually no negative anomalies deep down in the waters of the Pacific there are really no positive anomalies seen either.

• Strength of the trade winds over the past severe weeks/months

When looking at ENSO state and what may happen with the ENSO state down the road on good thing to look at are the trade winds (both the strength and the direction). The trade winds are the winds that circulate the globe around the equator just above the Earth's surface. The trade winds are the easterly winds that circulate the Earth over the tropics. During La Nina phases these easterly trade winds strengthen, or are stronger than normal. During EL Nino phases these easterly winds weaken and become weaker than normal and actually begin blowing in a more westerly direction. Here is a map showing the trade winds and the strength of the trade winds since November of 2010. For reference the "cool colors are negative anomalies which are the westerly winds and the "warm" colors are positive anomalies or westerly winds.

As you can see as the weeks and months have moved on across the eastern Pacific the trade winds have actually become weaker than normal and have started blowing in a more westerly direction, this has helped lead to some the warming we have seen occurring across Nino region 1.2

However, referring back to the above map showing the trade wind anomalies as you can see much of the western and central Pacific is still experiencing a good deal of easterly winds, this also indicates the Pacific is still in a La Nina regime and any warming over the upcoming weeks will be on the slow side, if any warming occurs at all.

• State of the SOI

Finally one of the last things to look at is the SOI. The SOI is heavily correlated to ENSO as there is a major connection between the index and the state of the Pacific Ocean. As mentioned a few paragraphs ago prolonged positive values of the SOI are associated with La Nina conditions and prolonged negative values are associated with EL Nino conditions. Since April of 2010 the SOI has been in a positive phase, and not once has the monthly number dropped into negative territory. Even through this month and up to this day the SOI continues to be in positive territory and hasn't really shown any signs of plunging downward anytime soon.

Well with all this said what can we expect this La Nina to do and where will we be by the summer? Given the latest trends over the past several weeks and some of the forecasts by a series of long-range computer models I believe that we will continue to be in a La Nina state through the rest of the spring and through most of the summer with the La Nina gradually weakening to a weak La Nina and quite possibly a neutral state by the end of summer.

Now that we just did an overview of ENSO and where things may trend over the next several months let's take a look into some of the other global indices that also have an influence on the upcoming pattern and what significance they may have.

AMO

The AMO (Atlantic Multidecadal Oscillation) is a mode of variability that occurs in the North Atlantic Ocean. It is defined by sea surface temperatures (SST'S) and sea surface temperature anomalies (SSTA's) in the Northern Atlantic. The AMO alternates between positive and negative phases. Unlike that of ENSO the AMO usually sticks in one phase for a very long time (hence the word decadal in the name). Each phase can last anywhere between 30-50 years or so. From about 1970 or so to the early to mid 1990's the AMO was dominated by the negative phase. However, since the mid 1990's the AMO has once again become dominated by the positive phase. What impacts does the AMO phase have on our weather and the weather pattern?

Well, wince the AMO is involved with SST's and SSTA's across the North Atlantic it can certainly have an impact on our weather, especially during the summer months. When the AMO is in it's positive phase this means water temperatures across the Atlantic are much warmer than usual, during the summer this can really help to shape up the upper-air pattern across the Atlantic so that we see a good deal of ridging, if strong enough this ridging can extend back towards the United States, allowing the potential for much warmer than normal weather. Research shows that the majority of summers with a positive AMO tend to lead to much warmer than normal summers across most of the United States, while this is not always the case the majority of the time this holds true.

Last summer we saw an AMO which was very positive, in fact one of the strongest positive phases on record, this really helped to intensify much of the heat across much of the United States which lead to the very warm to downright hot summer for many. However, as the summer faded the strongly positive AMO began to weaken some and this continued through the fall and into the winter, in fact the AMO continues to weaken. Given the current state of the SSTA's across the Northern Atlantic it doesn't seem as if the AMO is going to rebound any time soon, or at least if it does the process will be on the slow side. Remember it can take a good deal of time for waters of such size to really warm or cool. With this said I expect the AMO to remain positive through the summer with a likely increasing trend, however it will be rather slow to take place and it might not have much of an affect on the pattern, by this it won't really due enough to really ensure another above-normal summer for much of the east.

PDO

The PDO (Pacific Decadal Oscillation) is very similar to the AMO except it's regarding SST's and SSTA's across the Northern Pacific Ocean. Just like the AMO this oscillation also alternates between positive and negative phases, but usually around every 20-40 years. From about the late 1970's to the early 2000's the PDO was dominated by the positive phase, however, since about the early 2000's the PDO has been dominated by a more negative phase, although there was a minor period from about 2002-2004 where the positive phase took shape but since then it's been mainly negative. Like the AMO the PDO can have an influence on our weather across the United States, especially across the western portion of the country. The PDO is also linked to the state of ENSO. Since La Nina episodes are accompanied by cooler than normal waters in the Pacific the PDO tends to either be more negative or closer to negative during Nina years. The opposite holds true for EL Nino years.

The PDO did briefly go positive during the late summer of 2009 and early spring of 2010 when we were in a strong EL Nino phase, however, once the EL Nino weakened and quickly transitioned to a La Nina the PDO quickly went back into the negative phase. With some recent warming across the Northern Pacific the past few months though the negative PDO has slightly weakened but it appears the PDO will remain slightly negative through the summer. With this said due to the PDO not being overly positive or negative and expecting this to last through the summer I don't think the PDO will have a major say in the overall weather pattern, however, it might be just negative enough to where it could have some sort of impact across the Pacific northwest.

NAO/AO

The North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) are said to have some of the biggest impacts on the overall weather pattern, especially across eastern North American, the northern Atlantic and most of Europe, especially during the winter and early spring months. While neither the NAO or AO have a significant impact on the overall summer pattern it can have an influence, especially if the phase (positive or negative) is strong. Like the AMO and PDO the NAO/AO also go through periods where one phase is dominant, however, these periods tend to only last several years instead of decades.

Over the past few years we have seen the NAO/AO take on a predominately negative phase, and even very negative at times, reaching historical levels a few instances. The past three winters and summers all featured negative NAO/AO's. As we have moved into April though the NAO has gone from quite negative to fairly positive. There really isn't a solid correlation between the April state of the NAO and the NAO during the summer months and forecasting the NAO is very difficult as it can fluctuate very rapidly but given what we have seen the past few years with regards to the NAO I would think we see a NAO/AO state that is slightly negative, meaning we probably won't see the NAO or AO have much of an impact on the summer pattern.

QBO

The Quasi-Biennial Oscillation (QBO) is defined as a quasi-periodic oscillation of the equatorial zonal winds between easterlies and westerlies in the tropical stratosphere. The positive phase is associated with westerly winds and the negative phase associated with easterly winds. The importance of the QBO in the summer is mainly associated with tropical activity in the Atlantic. Usually the positive (westerly winds) phase of the QBO is associated with increased tropical activity throughout the Atlantic. The opposite is said for negative (easterly winds) phase of the QBO. The negative phase of the QBO is also associated with stratospheric warming, stratospheric warming can tend to lead to increased blocking across the Arctic region. This can lead to the NAO and especially the AO going more towards the negative phase. Strongly negative QBO's in the summer can lead to increased blocking across the Arctic which can also lead to slightly above normal conditions across the northern tier of the United States and cooler than normal temperatures across the southern tier of the United States. The opposite actually holds true in the winter.

Over the past decade each phase of the QBO has lasted an average of 13.6 months; each positive phase has lasted an average of 13.2 months and each negative phase has lasted an average of 14 months. Currently the QBO is in the positive phase and has been so since August of 2010. Since December the QBO has been slowly trending downward through the positive phase. Going by the average over the past decade the QBO should remain positive through the rest of the spring and most of the summer; likely going into it's negative (easterly) phase by late summer. Given this the QBO should have little impact on the atmospheric pattern over North American, however, it could have some effect on the upcoming tropical season.

Continued on post below this.

[weatherwiz]

Given everything stated above it's now time to move on and go forward with the outlook. Going all the way back to the beginning where ENSO was discussed we were in a moderate/strong La Nina this winter and we have seen the La Nina weaken all the way down to a weak/moderate state. Indications are that we will continue to see the Nina weaken through the spring and into the summer.

From here I went back to look at all other winters featuring a La Nina and of those years looked to see which ones had the Nina weakening through the spring and into the summer, preferable to a weak Nina/neutral ENSO state. Off the years looked at three years popped out. These three years also tend to match very well with how this current Nina has progressed. The three years are as follows; 1976, 1989, and 2008. All three of these years saw a La Nina which peaked in the strong category and all saw the Nina really weaken by late winter and spring. All three years also saw an ENSO state of weak Nina/neutral by the summer. Considering these three years have shown a similar trend to what we have seen now I have went about and used these as my top analogs for the upcoming summer. Let's take a look at the following years month-by-month.

One thing to consider here with this outlook is I have included the month of May. While meteorological summer does not begin until the first of June I always like to include the month preceding the start of the actual season as in May it can be fairly hot and you can get severe weather; just like in November it can be quite cold and you can get some decent snow events. With this said some of the indices such as the NAO/AO hold much more merit on the weather pattern in May then the rest of the summer months. This is important to consider when looking at the months of May for each of the listed years.

As you can see on the images above of the 3 years pretty much all of the months from each of the years are strikingly similar as far as temperature anomalies go; the only exception is in May, where 1989 is completely different from the May's of the other two years. Here you had a few differences within some of the global indices that weren't listed above which lead to the differences in the patterns each of the years. The reasons these indices weren't listed above is b/c they have little impacts on the summer pattern.

When putting all of that together this is what is generated when you combine the May-August temperature anomalies for the three years:

When doing this you can see most of the country is experiencing temperatures that are somewhat below-average, with most of the real warmth occurring across portions of the western US. Finally before composing my thoughts on how the temperature anomalies will look across the nation for the summer of 2011 I took into account soil moisture. There seems to be a decent correlation between how dry soil moisture content is over an area and temperature anomalies. Below is a map of the precipitation anomalies across the country for the past 90-days.

With all the above information stated and taking into account current precipitation departures here is what I believe the summer of 2011 will look like as far as temperature anomalies go:

Here is the break down month-by-month:

Now that we have established the temperature anomaly outlooks it's time to go ahead and look at precipitation outlooks. With somewhat of a ridging type scenario towards the inter-mountain west and Plains region this should yield to some sort of trough over the eastern portion of the United States, not only will this help keep things a tad cooler than normal but it should also yield to precipitation be slightly above normal as well. Unlike for temperatures the precipitation anomaly map will only compose of the entire May-August summer season, rather than also broken down into months. With what was just stated above here is what I think we will see as far as precipitation anomalies go:

The final step in my 2011 summer outlook will be looking ahead towards the very late spring and summer severe weather season across the United States; which region or areas are likely to experience above-average activity and which areas will see a quiet season. I understand the severe weather aspect of thing sis not normally done in outlooks as these occurrences are on a much smaller-scale than most other phenomena but this is something that interests me and I think it's possible to pin down some areas that may experience increased activity. By areas I don't mean any one single location; more like a broad area. This will be done within the next week.

Comments and criticism gladly welcomed as are any suggestions, tips, etc.