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Winter forecast 2009-10 Part 1
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Winter 2009-10 Forecast/Discussion
- "The Winter will be rather mild for the upper Midwest and northern plains"
- “The southern US could experience their coldest winter since 2002-03”
- “The theme of the winter will be warm early with a flip to significant cold sometime between Christmas and MLK day”
My Usual Seasonal Forecasting Disclaimer:
I will start with a standard disclaimer that I will put on any and all seasonal forecasts, I ever attempt. Seasonal forecasting is a difficult area of weather forecasting. As most of you know, the further out in time you go from an initial state, the more error is associated with a weather forecast. The interactions of the atmosphere with the ocean, land masses, and also with solar influences is extremely non-linear and therefore very difficult to predict with any degree of accuracy beyond several days. From my reading of seasonal forecasts from different meteorologists and hobbyists, there seems to be 3 methods of forecasting seasonal weather.
1. Persistence and extrapolation.
2. Seasonal Indices method.
3. Past foretells the future.
Numbers 1 and 3 are somewhat similar. Persistence and Extrapolation means, identifying what the weather pattern has been for the past 30 days, 60 days, 90 days, etc., and making assumptions about how this persistent weather pattern will evolve over the course of several months, taking into account normal seasonal changes and other factors of what would either cause this pattern to persist or change. The Seasonal Indices method is determining what mode/phase the prominent ocean, pressure, solar, etc. indices are in at the current time and then predicting what they will be for the next several months. Then drawing conclusions on the physical impacts these indices will have on the weather pattern and/or picking certain past years that have exhibited similar values or patterns to those indices (analogs) and formulating a seasonal forecast. The third method involves identifying what the weather has done over the past several months or some other arbitrary length of time, and then identifying years that have behaved similarly and seeing if similar signals existed in those winters, and then applying those findings to a winter forecast that exhibited similar antecedent conditions. This method is usually a more regional or local seasonal forecast.
All of these methods have flaws, and no one approach is always successful. But at the moment, it seems to be the state of the seasonal weather forecasting business. My own opinion is that a combination of all three is the way to go. I readily admit that I haven't figured out how to give a successful seasonal forecast year in and year out, for if I did or anyone did, they would be rich men/women because of it. However, in the interest of learning and fun, I will continue to take a stab at it and hope I/we learn from the attempts and that this field of research will improve in the future. So without further adieu, my forecast discussion for the winter of 2009-10:
Overall Theme of the winter:
The evolving nature of our current El Nino event from a weak to what looks to be a moderate event, has decreased the chances in my opinion for a widespread frigid winter for a good part of the US. Although there is some debate as to how to exactly classify weak, moderate, and strong El Nino events, using my own subjective classification scheme, no moderate El Nino has produced the type of severe cold on the scale of weaker El Nino events such as 1976-77 or 1969-70. The one possible exception is the winter of 1977-78, which by my classification scale which factors in the MEI, SST’s, and the SOI, could be classified as a borderline weak/moderate event. It was a frigid winter for 2/3 of the US.
I think the major populated areas of the Midwest and northeast will have a rather mild early to mid winter period, but a cold pattern could set up for the 2nd half of the winter. The southern US in particular the southeast, stands the best chance at seeing the winter average below normal. I think we will see a rather warm December for most of the US evolve into a colder pattern as we head through the winter.
As of now I do not think the western US will experience a warm winter such as many El Nino examples in the past suggest, but more of near normal type of winter. I think this because I think the PDO will not go raging positive but average near to slightly above normal thus we may see semi-frequent troughs move into the Pacific Northwest and western US, especially early in the winter. Late winter may turn warm in the western US though.
I think an active subtropical jet stream associated with the moderate El Nino will allow for some significant storminess to occur across the country, specifically the southern US, but the potential will exist for some significant winter storms that could affect many populated areas. When the polar jet does assert itself at times, particularly in mid to late winter the active southern branch will allow for opportunities for phasing of the two branches of the jet and therefore the potential for significant low pressure systems to form. There will also be times where systems will stay suppressed and only bring the threat for wintery precipitation to the southeast or lower mid-Atlantic.
In general, I think this winter will be remembered as a cold and snowy winter for areas of the country that have not experienced significant wintery weather on a whole since the winters of 2002-03 and 2003-04, specifically the southern US. In the northern US it may not be remembered as much of a winter, except for the potential for some significant storms in the mid to late winter time period. At least this is how I see it. Below is an outline of my reasoning along with some graphics.
Outline of Reasoning:
ENSO:
**This El Nino event will peak as a moderate event in November or December and only slowly weaken or maintain itself through the winter**
I always start with ENSO state when it comes to making a seasonal forecast. The phase of ENSO many times can have a dramatic impact on the drivers of the weather pattern across the globe. This happens as the warmer or cooler than normal sea surface temperatures affect the distribution of convection and consequently the effects of the trade wind circulations in the equatorial Pacific Ocean. This often times will have significant impact on the downstream jet stream pattern and hence the corresponding weather conditions across the mid-latitudes.
Last winter featured a weak La Nina that turned neutral this spring and actually flipped to a weak El Nino for this past summer. This El Nino has essentially been in place by all indicators since June. This El Nino has just crossed the line to a moderate phase after being weak for much of the summer and early fall. As of now I think that this event will likely peak in the coming weeks as a moderate event with weekly anomaly values in region 3.4 possibly reaching as high as +1.5 for several weeks before slowly weakening but remaining in the low moderate range through most of the winter, perhaps weakening closer to the weak/moderate border by February or March.
Week Ending | Region 3.4 departure from normal |
8/5/2009 | 0.8 |
8/12/2009 | 0.7 |
8/19/2009 | 0.7 |
8/26/2009 | 0.9 |
9/2/2009 | 0.9 |
9/9/2009 | 0.9 |
9/16/2009 | 0.9 |
9/23/2009 | 0.8 |
9/30/2009 | 0.7 |
10/7/2009 | 0.7 |
10/14/2009 | 0.9 |
10/21/2009 | 1.1 |
Let’s look at why I think this will happen. The current SST anomaly map shows a well developed El Nino event across the equatorial Pacific Ocean with positive anomalies showing up across most of the entire basin.
When compared with the configuration two months ago you can see this has only strengthened in recent weeks.
This is the result of westerly wind burst in October which has weakened the trade winds and even resulted in westerly anomalies far to the east of the dateline. This is indicative of El Nino conditions and has led to the waters warming in this region.
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Another indicator of ENSO phase and strength is the SOI which has been essentially in a weak negative stage for most of the summer but has turned sharply negative in recent weeks, another indicator of a strengthening El Nino.
Av.SOI Av.SOI
----------------------------------------------------------------1-Oct-2009 1012.76 1010.45 -3.74 3.60 0.25
2-Oct-2009 1013.00 1010.65 -3.48 3.46 0.19
3-Oct-2009 1014.24 1010.85 3.22 3.38 0.14
4-Oct-2009 1015.04 1011.10 6.77 3.08 0.14
5-Oct-2009 1014.51 1011.85 -1.48 2.76 0.06
6-Oct-2009 1014.68 1011.50 1.87 2.98 0.06
7-Oct-2009 1014.14 1010.75 3.22 2.99 0.14
8-Oct-2009 1012.65 1010.45 -4.45 2.46 0.08
9-Oct-2009 1012.31 1011.40 -12.77 1.67 -0.17
10-Oct-2009 1012.74 1012.40 -16.44 1.19 -0.56
11-Oct-2009 1013.49 1013.50 -18.70 0.75 -0.98
12-Oct-2009 1013.71 1013.40 -16.63 0.29 -1.33
13-Oct-2009 1013.56 1012.00 -8.58 -0.19 -1.46
14-Oct-2009 1014.11 1011.40 -1.16 -0.08 -1.41
15-Oct-2009 1013.49 1010.85 -1.61 -0.27 -1.37
16-Oct-2009 1012.50 1011.50 -12.19 -1.25 -1.60
17-Oct-2009 1009.51 1011.70 -32.75 -2.96 -2.06
18-Oct-2009 1008.19 1011.70 -41.26 -4.79 -2.55
19-Oct-2009 1008.76 1012.10 -40.16 -6.21 -2.93
20-Oct-2009 1010.43 1012.55 -32.30 -7.32 -3.28
21-Oct-2009 1011.07 1012.70 -29.14 -8.48 -3.61
22-Oct-2009 1011.96 1012.80 -24.05 -9.71 -3.84
23-Oct-2009 1011.28 1012.35 -25.53 -11.07 -4.13
24-Oct-2009 1010.99 1011.10 -19.34 -11.92 -4.32
25-Oct-2009 1010.63 1011.30 -22.95 -12.70 -4.57
26-Oct-2009 1010.09 1010.25 -19.66 -13.19 -4.72
27-Oct-2009 1009.49 1009.80 -20.63 -13.36 -4.78
28-Oct-2009 1009.00 1010.35 -27.33 -13.93 -4.87
29-Oct-2009 1010.24 1010.50 -20.31 -14.61 -5.03
30-Oct-2009 1011.84 1010.15 -7.74 -14.98 -5.06
The subsurface temperatures show abundant warmth below the sea surface which has only strengthened as well in recent weeks.
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Finally the computer models, primarily the ECMWF and IRI plume models show the event peaking in the moderate range in the coming month or two before slowly weakening as we head through the mid to late winter.
So my key assumption is that this El Nino event will peak in November or December in the moderate range and then only slowly weaken through the winter or hold rather steady. This means I do not expect this El Nino to fall apart this winter but only to weaken slightly.
One possible effect that we have already seen from this El Nino event is a more active than normal subtropical jet stream this fall. I have pasted below a map of the 250mb zonal wind anomalies from 9/1 to 10/29. You can see the warmer colors from just south of Hawaii into the southern US indicating stronger than normal winds at that level.
The result has been above average precipitation across most of the lower Mississippi River Valley and interior southeast which is typical of a building El Nino in September and October. I think this area of heavier than normal precipitation will shift east as we head into the winter.
When looking for ENSO analogs I looked primarily for years that featured a similar progression to this year’s ramp up, and also an event where the winter was not embedded in a multi-year El Nino event.
Past years that fit this description and the nature of this evolving El Nino to an extent were in no specific order:
2006-07 (close match but this event may weaken at a slower pace and be a stronger event in general)
2002-03 (close match)
1986-87 (this event strengthened in late winter, so while this could be a good match for early winter, I am not sure about it’s viability for late winter)
1965-66 (this event actually peaked during the summer but remained at low end moderate levels through the winter weakening to weak in March and also followed a La Nina event. So while this may not have been a good summer/fall analog it could become a better and better one as we head into late fall and winter)
1957-58 (this event peaked in Jan/Feb as a moderate event and is probably stronger than what this event will be, but it followed a multi-year La Nina and is not a bad match)
Earlier I was considering weak El Nino years such as 1951-52, 1963-64, 1976-77, and 1979-80 but it appears this event will be stronger than those events. The year 1994-95 also got consideration but it occurred on the tail end of a 4 year El Nino event, so while it will be a reference year during this winter it is not a primary tool for putting together this forecast.
Now caution is urged as each one of these years featured differences. So while I will refer to them to see how the winter progressed these years, they are only used as guidance and not as the sole tool.
If one were to look at the monthly progression of these winters all 5 of these winters featured very warm conditions in the northern plains and upper Midwest in December with the northeast also warm except in 2002, while the southeast was normal to below normal except in 2006 when it was very warm. January was a transition month with a good bit of variability while February was cold in the east and in 2 years very cold in the Ohio valley (2007 and 1958). The west was more variable. So we will keep that in mind as we move on.
PDO (Pacific Decadal Oscillation)
“The PDO will average in a neutral to perhaps slightly positive range this winter”
The PDO is a decadal index meaning it has long-term positive and negative phases that usually span a few decades. While there has been some argument over when we began the latest negative decadal phase, some say the late 90s other say in just the past few years, there seems to be little doubt that we are in a negative long-term phase at the moment. The last decadal negative phase was from the mid 1940s to the mid 1970s with the last positive decadal phase being from the late 1970s to either the late 1990s or sometime this decade. One consequence of this is that El Nino events in negative decadal PDO phases can often times be weaker in shorter lived than those in positive decadal PDO phases. This index can also have short term phases that are the opposite of the long term phase so it is a useful tool to track monthly and even weekly. Often times the sign of the PDO value will be directly proportional to ENSO phase and strength, with El Nino’s typically being associated with +PDO phases.
We have two main sources for the PDO value, they are located here http://jisao.washington.edu/pdo/PDO.latest ftp://ftp.ncdc.noaa.gov/pub/data/ersst-v2/pdo.1854.latest.ts
A positive phase of the PDO usually corresponds to ridging in western North America and troughing in eastern North America and the opposite is true of a negative phase of the PDO in the winter.
According to the first source we had been in a negative PDO period from September of 2007 through July but recently have turned weakly positive for August and September. The 2nd link shows that we have essentially been in a negative phase since February 2006. This source still has us with a negative value as of September. Usually when these two sources have different phases I consider the PDO in a neutral phase.
The latest SST anomaly maps show a relatively neutral to slightly positive PDO look although it has been looking more and more positive in recent days.
Often times the PDO ends up in a positive phase during an El Nino winter, but this was not the case in 2006-07 or in 2004-05, although one could argue based on which PDO source you use, at the very least you could say these two winters had a neutral PDO phase. Could it be that a positive PDO is harder to sustain during its decadal cold phase even in an El Nino? This makes me hesitant to predict anything more than a neutral to weakly positive PDO phase this winter. Therefore if we say that we are in a longer term negative phase of the PDO, than one might want to pay attention more to El Nino’s that occurred during those time periods. Looking at our 5 ENSO analogs, this could mean less weight should be given to 1986-87. However here are the average DJF PDO values for the above 5 years:
- 1957-58 0.11
- 1965-66 -0.26
- 1986-87 1.8
- 2002-03 1.98
- 2006-07 0.06
As you can see only two had a very strong positive PDO for the winter 1986-87 and 2002-03, the other 4 years average between -0.26 and 0.11 or essentially neutral.
I think the PDO for the DJF period will average neutral or in this general range of +/- 0.25 therefore narrowing down our analog years a bit to 1957-58, 1965-66, and 2006-07.
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