Originally by Matt Ross, November 1, 2006

We'll start with a caveat: Long range seasonal forecasting is still in its infancy. Although great strides have been made, there is still considerable uncertainty and the potential for significant errors in these outlooks. We believe we can provide more value and accuracy than a mere coin flip, but the ideas presented should be viewed as general guidance and themes. The more specific we get, the more chance there is to err. This is especially true for snow amounts, as 1 or 2 storms or lack thereof can make or break snowfall totals. As always, we will grade the outlook both during and after the winter.

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Executive Summary

The following factors were considered in preparation of this outlook:

• Weak to moderate El Nino event


• Temperature/Precipitation Pattern


• Negative North Atlantic Oscillation (NAO) possibly becoming neutral


• Average to below-average hurricane season
• Other Indices Considered: Quasi-Biennial Oscillation (QBO), Pacific Decadal Oscillation (PDO), North Pacific Index (NP)


• Analog years


• Miscellaneous Factors

The Forecast

This winter will be colder and stormier than normal with above average snowfall. We will see both extended cold outbreaks and thaws, but as has been the case this fall, the cold will overwhelm the warmth and thus will be more commonplace and long lasting. The core of winter will be colder with respect to normal than the beginning and end, and I expect the January/February timeframe to also be the stormiest. The higher level of storminess also means we will see more mixed events then normal. I expect a number of events will not be all snow, but rather snow to rain, rain to snow, sleet, freezing rain, or a combination of all. Overall, I expect the coldest winter since 03-04 and possibly colder, and the snowiest since 2002-03, although not as snowy.

The temperatures predicted below will be measured against the official readings at National Airport (DCA), and snowfall measured against the airports as indicated.

Overall Temperatures for December through March:

-1.25°F below normal
December: -1° to +1°, leaning very slightly above normal
January: -2° to -4°
February: Normal to -2°
March: Normal to -2°

Total Snowfall:

20-30% above normal
National Airport (DCA): 20" (versus average of 16")
BWI: 24" (versus average of 20")
Dulles (IAD): 27" (versus average of 22")
Arlington/Alexandria/DC:
19-25"
Fairfax Co.: 22-28"
Loudoun Co.: 26-33"
Mo Co: 23-29"
PG: 17-23"

What can go wrong

• El Nino becomes much stronger, or alternatively weakens much more quickly, than predicted. In both cases the result could be warmer conditions with less snow. Chance of either scenario: 10-15%


• The NAO is very positive for much of the winter. This would likely lead to warmer conditions and would also lead to many events being rain or snow to freezing rain/rain, thus less snow. Chance: 25%


• We never see a real pattern change/positive NAO emerge as we head through November, and December ends up much colder than forecast. Chance: 25%

Outlook Factors

The factors below were colletively considered. No single factor overwhelmed the others, although some were weighted more heavily.

El Nino and the North Pacific:

This was probably the most important factor in making the forecast. Currently we are in the midst of a weak to moderate El Nino event. Sea surface temperatures in the Pacific Ocean along the equator are running 1-2 degrees Celsius above normal. Currently this event is basin wide with the warmest water anomaly off the South American coast in Nino region 1+2. This El Nino event is expected to remain weak to moderate and persist throughout the winter. I believe that this event will peak sometime during the early part of winter and start to slowly moderate as we head through January/February/March. Model guidance suggests a December/January/February tri-monthly peak in Region 3.4 of 0.8C to 1.3C above normal. I think the higher end of this range is most likely. Typically weak to moderate El Nino events support a positive Pacific Decadal Oscillation (PDO) and thus a ridge in the west and a trough in the east with cooler than average temperatures(especially in January/February) and more precipitation/snow than normal with an active subtropical jet. In addition to the Equatorial Pacific, the North Pacific has an effect on our weather. Currently there is an expanding cool pool of water in the Gulf of Alaska. The water profile can change very quickly, but if this cool pool should grow stronger, this suggests that when the North Atlantic Oscillation (NAO, see discussion below) is positive that warmer air would be able to make it into our region more easily as the amplitude of the ridge/trough axis would be flatter. I expect that warm and cold will battle in this region throughout the winter.

I looked for years with similar ENSO profiles both in the benchmark region 3.4 and in the other Nino regions. I also looked at the progression of the Nino event over the past year and favored events where Nino was strengthening out of a La Nina or neutral state rather than weakening.

Temperature/Precipitation Trends

This summer was extremely hot across the continental US, especially in the western part of the country, particularly the northern Plains. Even here in our backyard, after a normal start to summer, July and August were both well above average including an extended heat wave. Additionally, we had much above average rain this summer in the Mid Atlantic, mostly due to the deluge in late June when we received record flooding rains.

September and October featured back to back below normal temperatures with above average precipitation. In fact, much of the country was below normal during this stretch. November has started off very cold, but may modify a bit this week. We are running well ahead of pace for average yearly precipitation despite long stretches of dry weather in between very rainy periods.

I looked for past years that had similar temperature anomalies and progression in the summer and fall, both nationally and locally. Additionally, I looked for years that had above average precipitation and/or a pattern of feast or famine locally, with anomalies over the US similar to 2006.

North Atlantic Oscillation(NAO)

The NAO is measured several different ways, but is generally indicated by high or low pressure over Greenland. Typically, though with notable exceptions, when the NAO is negative(high pressure or blocking over Greenland) we see cooler than normal readings, and when it is positive(low pressure over Greenland), warmer than normal temps. Some of our bigger winter storms occur when the NAO is negative or transitioning from negative to positive. Currently we have had 3 straight months where the NAO averaged very negative. Climatology/statistics suggest a possible flip toward a positive average over the winter months, while the current temperature profile of the northern Atlantic Ocean suggests otherwise with very warm anomalies east of Newfoundland. Overall, I expect the NAO to average in the neutral range during this winter. I looked at past years with similar negative fall averages.

Hurricane Season

We had an average to below average Hurricane Season with very few US landfalls and lots of recurvature well out in the Atlantic. I looked at other years that had similar quantities and overall pattern as 2006. This was a minor factor.

Other Indices That Were Less Important Factors

Pacific Decadal Oscillation(PDO): The PDO runs in both long term and short term cycles. There is some indication that the long term cycle of the PDO switched to negative in the late 90s, although the last few years would suggest otherwise. Typically, El Nino winters are positive PDO winters. A positive PDO supports the placement of a trough and thus colder air on the East Coast. While the PDO has flipped to negative this fall, this is likely just a short term oscillation. As with the QBO, this factor was given just passing attention, although I do expect it will average positive this winter.

North Pacific Index(NP): This is a measure of the mean surface pressure over an area in the North Pacific Ocean. There is some indication that higher pressures in the autumn correlate to a more positive NAO in the winter. Unofficially, the October reading was somewhere in the middle, and this factor was given limited attention.

Quasi-Biennial Oscillation(QBO): The QBO is the measure of an area of wind in the straosphere over the Equatorial Pacific Ocean. This index cycles from positive to negative on a 2 year cycle. Currently we are in a positive or westerly QBO that is weakening toward neutral. The effect of the QBO is less well understood, so this was given just passing attention.

Other Miscellaneous Factors

• High Latitude Blocking: Throughout the fall we have seen an abundance of blocking(high pressure) in Canada, Greenland and points north. Should this pattern persist, it would lead to colder and stormier weather in the mid-latitudes.


• Cold Weather: Not only has the autumn been cold, but the periods of cold have usually been colder and more prolonged than forecast. I am skeptical of mid range models that try to warm up the East Coast for too long.


• Sea Surface Temperatures in the North Pacific and Atlantic: As explained in more detail above, while the battle of cold and warm water in the North Pacific is giving us mixed signals, the very warm water in the North Atlantic is robust. This pool of warm water is conducive to blocking over Greenland(-NAO), and likely trumps any statistic or correlation that suggests otherwise.


• Snowcover: Snowcover is pervasive over much of the Northern Hemisphere including Canada. While snowcover is fickle this early in the season, expansive snowcover is conducive to deeper and colder troughs over the east coast, as air has little opportunity to modify as it heads into the US.

The Outlook

This winter will be colder and stormier than normal with above average snowfall. We will see both extended cold outbreaks and thaws, but as has been the case this fall, the cold will overwhelm the warmth and thus will be more commonplace and long lasting. The core of winter will be colder with respect to normal than the beginning and end, and I expect the January/February timeframe to also be the stormiest. Historic storms are by definition rare, and I do not expect one this winter though the chances are elevated a bit. However, I do think we can expect up to several 6"+ snowfalls with some of the more favored areas receiving 10"+. With colder air, snow when it does fall, will stick more easily and stay around for longer. The higher level of storminess also means we will see more mixed events than normal. I expect a number of events will not be all snow, but rather snow to rain, rain to snow, sleet, freezing rain, or a combination of all. Overall, I expect the coldest winter since 03-04 and possibly colder, and the snowiest since 2002-03, though not as snowy.

The temperatures below will be measured against the official readings at DCA, and snowfall measured against the airports as indicated.

Analog Analysis

We evaluated the current state of a number of climatic factors (such as temperature anomalies,large scale patterns and hurricane activity described above), and compared them to seasons past to identify the seasons sharing similar characteristics to this year. Those seasons from the past with similar overall characteristics to this season were selected as the "analogs" that we applied to project this winter's conditions (based on the winters from those analog years).

Caveats -- What Can Go Wrong With Our Forecast?:

1. The NAO is very positive for much of the winter. This doesn't automatically mean a warm winter. But with a cooler pool of water currently in the Gulf of Alaska among other factors, the chances of a warm east coast would be significantly higher. This would also lead to many events being rain or snow to freezing rain/rain, thus less snow. Chances: 25%


2. We never see a real pattern change/positive NAO emerge as we head through November, and December ends up much colder than forecast. Chances: 25%


3. The El Nino becomes stronger than predicted. If this happens the chances greatly increase for the warmer air that is typically found in the Plains and northwest during Enso events to penetrate further east. Our winter would likely be much warmer and less snowy than forecast. Chances: 15%


4. The El Nino weakens much more quickly than forecast. If this happens, the effects are less clear. But chances of a warmer and less stormy winter would increase. This could also signal an earlier end to winter. Chances: 10%

WeatherTalkRadio has created a page summarizing other winter outlooks for the area. You can view these other outlooks here.[No Longer Active]