By Mike Goldberg, Meteorologist
November 25, 2008

Each year, it seems we talk about different extreme weather we’ve had heading into the winter season.  Back in 2004, it was a record amount of rain during the summer, in 2005 it was a record hurricane season, 2006 brought a record amount of rain during meteorological fall (September, October and November) and last year it was extremely dry during the autumn months.  This year has been somewhat quiet, but awfully cold the last few weeks, and we’ve already had two rounds of snow showers!  This could be a sign of what’s to come.

There are various factors that play a role in determining the winter outlook.  The big feature is often ENSO (the El Nino Southern Oscillation).  The two main modes are El Nino and La Nina.  This year, however, there is not a signature of either one, with neutral conditions which we call La Nada.  There are some signs that a weak La Nina is trying to form, and that potentially could have an impact later in the winter.  However, a weak signature means more variability.  For now, it appears we’ll have take a closer look at other factors when determining what the winter may offer.  The big player is the North Atlantic Oscillation (NAO), which always seems to be the wild card.  The NAO involves the atmospheric pressure patterns in the North Atlantic.  When the NAO is in its "negative" phase, high pressure over Greenland forces cold air southward from Canada into the Eastern United States.  The NAO is very hard to predict beyond a week or two, and can shift back and forth many times over the winter.  Therefore it's hard to determine the effect of the NAO over an entire season.  Our greatest threat for winter weather will come when it's in its negative phase.  There are other factors listed below which I believe could have a more significant impact than normal this winter.  A more in-depth discussion of these follows the outlook.
    
WINTER OUTLOOK 2008-09

Overall, I think this will be a colder and snowier winter than the last several years, but I know that’s not saying much!  The winter outlook represents an AVERAGE of conditions over the stated period of time (December through February).  As is typical of many winters, we will see some wide swings in the prevailing weather conditions (especially temperature).  I believe we’ll start off on the cold side, with December coming in below average.  January will bring some intense swings from warm to cold, and February is the real wild card.  Precipitation will likely be close to average throughout the winter season, so we’ll get our chances for winter weather.  As always, it will depend on timing.  We need a solid cold air mass in place before the arrival of a storm to provide significant winter weather.  I have a hunch we’re going to see some early season snow (in December), with another decent storm chance in January.  A general time to watch will be mid-January (January 15-25).  Don’t rule out a few events later in the season as well.  Even though this outlook focuses on December through February (the traditional winter months), we know it’s certainly possible to receive snow in March or April. 

The following states the odds for temperature and precipitation compared to "average" conditions over the next three months.

TEMPERATURES

OVERALL:     Near average  (the various extremes balance each other)
DECEMBER:  Below average (the recent cold spells continue—early snow possible)
JANUARY:     Near average (a general thaw, but with one good cold spell mixed in)
FEBRUARY:  Near to above average (a brief period of cold, but some significant temperature swings should balance out)

PRECIPITATION

OVERALL:     Near average (we get our share of storms, but don’t go overboard)
DECEMBER: Near to above average (several storms bring an early snow threat)
JANUARY:    Near average (the timing of storms determines whether we see winter weather)
FEBRUARY: Near average (variable storm tracks make it hard to determine winter weather potential)

SNOWFALL

Annual snowfall “averages” about 12-13 inches in the Metro Richmond area (that’s the 30-year climatic normal).  It’s so important to remember that getting snow in central Virginia depends on all the ingredients coming together at the right time.  If the cold air is in place but a storm isn't here, then it's a no-go.  One big storm can make it an average year for us, so predicting a snow total for the season is just an educated guess and not an easy thing to do.  The prevailing pattern may produce a variety of storm tracks for us this season.  The northern tracks don't typically bring a lot of precipitation to our area.  However, it’s the ones that pass south and/or develop along the Carolina coast that bring the potential for significant winter weather.  We will likely have several typical winter events that bring a variety of precipitation types (snow to ice/rain) to the region.  Taking all this into account, my "best guess" for the 2008-09 winter in Richmond is 10 inches, a near or slightly below average season.  As usual, there will be higher amounts in the higher elevations north and west and somewhat lower amounts in communities closer to the coast.  Here are some estimates for potential snow throughout the region:

Richmond             10 inches
Petersburg             9 inches
Emporia                  8 inches
Charlottesville      20 inches   
Fredericksburg     18 inches
Reedville                 9 inches
Williamsburg          9 inches
Norfolk                    6 inches

MORE ABOUT THE OUTLOOK...

Predicting a season's weather in advance is not an easy task, as we sometimes have a hard time dealing with storms that are just a few days away.  However, scientific advances have enabled us to give a general outlook several months in advance that provides valuable and accurate information that can be used by the public and businesses to plan appropriately for the weather that may have an effect on their daily lives.  Long-range forecasting of trends and weather patterns is known as climatology, or weather over an extended period of time.  It is much different than the day-to-day weather we analyze and forecast on a daily basis.  Now for more of the "science"...

THE FACTORS

EL NINO/LA NINA

El Nino is an abnormal warming of the surface water in the central and eastern Pacific Ocean.  Its opposite is La Nina, and both affect pressure patterns over the Pacific, which in turn can bring changes in the weather for the United States and around the globe.  The shifting pattern in the Pacific affects the placement of the jet stream, a band of strong winds in the upper atmosphere that directs the path of storms at the surface. 

El Nino and La Nina are two modes of the El Nino Southern Oscillation (known as ENSO for short), which deals with the pressure and temperature patterns in the ocean and atmosphere in the tropical Pacific.  When neither is apparent, the conditions are neutral, which is usually referred to as La Nada.  The Southern Oscillation Index (SOI) measures the pressure differences across the tropical Pacific from Tahit to Darwin, Australia.  ENSO is a key force in determining winter weather patterns over the United States. 

Since we are currently experiencing La Nada conditions, other factors may play a more prominent role in this winter’s outlook.  However, I should mention that there are signs that a weak La Nina is trying to return.  Sea surface temperatures in the tropical Pacific are inching back below average.  This could potentially have an impact on the winter, especially the second half.  Winter patterns associated with La Nina often (but not always) produce cold and wet winters for the northern states (particularly the Northwest U.S.), while warm and dry weather dominates the south.  The storm track shifts north, and sometimes snowy winters dominate from the Upper Midwest through the Northeast.  In Virginia, the effects we feel depend on the relative strength of the La Nina and the placement of the warmer/cooler waters in the Pacific. 
           
PACIFIC DECADAL OSCILLATION

The Pacific Decadal Oscillation (PDO) is a large scale oscillation in the Pacific Basin that appears to relate to large scale thermohaline (both salinity and ocean temperature are factors) circulations that periodically speed up and slow down and control positions of the large warm and cool water pools in the ocean basin.  This usually determines the mode of ENSO.  The North Pacific right now remains in the negative or “cool” phase.

                        COOL PHASE                                                                          WARM PHASE


QUASI-BIENNIAL OSCILLATION

The Quasi-biennial Oscillation (QBO) is an oscillation in the wind at 10-12 miles above the equator in the Pacific.  This is an oscillation between easterly and westerly winds in a cycle that averages about 28-29 months in length.  Dr. Bill Gray, the famous hurricane forecaster, uses the state of the QBO to help determine the frequency of hurricanes in the yearly forecast.  Currently, the winds are blowing westerly in the QBO.  This means that cold air masses settling south might potentially be broken up by a rush of milder, Pacific air.  If the current pattern breaks down (and there are some signs of that), we could have more favorable conditions for winter storms later in the season (with more “blocking” taking place)…but it’s a big “if.”

NORTH ATLANTIC OSCILLATION

Probably the most significant factor in determining winter weather on the East Coast is the North Atlantic Oscillation (NAO).  This has to do with weather patterns in the North Atlantic and involves a flip-flop in the relative strength of pressure systems north to south over the open Atlantic.  Normally, low pressure is located near Iceland (known as the Icelandic low) and high pressure sits just off of Portugal or the Azores (known as the Azores high).  When these systems strengthen in these positions, a fast jet stream flow tends to drain cold air off of North America.  This is the "positive" phase of the NAO and allows temperatures to moderate frequently over the Eastern U.S. and produces milder winters.  If the NAO flip-flops, high pressure pushes north toward Greenland and low pressure develops farther south replacing the Azores high.  The resulting "negative" phase of the NAO tends to produce harsh winter weather over Eastern North America.  The high pressure over Greenland retards the passage of cold air, which then expands south over the Eastern U.S.  This pattern is known as the "Greenland block," as the cold air is literally blocked from exiting the continent.  The storm track is then suppressed south, and more snow often falls in the major metropolitan areas up and down I-95.  These "blocks" are transitory by nature, but can repeat frequently and when this happens, we experience colder and snowier winters here in Virginia.  In negative NAO years, the water tends to be warmer than normal in the tropical Atlantic and far North Atlantic, which it is right now.  I will say the last few years, we haven’t seen the negative NAO sustain itself for very long periods, but that may have to do with the effects of El Nino and La Nina…so will this winter will be different?

                       NEGATIVE NAO                                                  POSITIVE NAO
    

ARCTIC OSCILLATION (AO)

The Arctic Oscillation refers to opposing atmospheric pressure patterns in the northern middle and high latitudes.  The oscillation is in its “negative” phase when relatively high pressure is over the polar regions and low pressure exists at the mid-latitudes (about 45 degrees north).  The “positive” phase is when the pattern is reversed, and high pressure at the mid-latitudes drives storms farther north, while frigid winter air does not extend as far south into the middle of North America.  This keeps much of the U.S. east of the Rockies warmer than average.  While the AO has been quite variable lately, it has taken a positive turn during October. 

                             POSITIVE PHASE                                 NEGATIVE PHASE
            
MADDEN-JULIAN OSCILLATION (MJO)

The Madden-Julian Oscillation is a pattern of tropical rainfall, the movement of large regions of enhanced and suppressed tropical rainfall over the Indian and Pacific Oceans.  A region of above normal rainfall usually first appears over the western Indian Ocean and then shifts east over the western and central tropical Pacific.  The wet phase of the MJO is often followed by a dry phase.  The abundant tropical moisture from the MJO can sometimes be incorporated into storms entering the U.S. from the Pacific.

NORTH ATLANTIC SNOW COVER

Looking at snow cover across North America (mainly Canada) during the fall months can be a tell-tale sign of what's to come.  When there is significant snow cover, arctic air masses have a breeding ground to expand and intensify, thereby making cold air outbreaks over the United States more significant and sustained.  On the contrary, when snow cover is below average, these cold air masses have a tendency to modify before moving southward.  Snow cover across North America has increased rapidly over the past few months and may now be a bit above average.  This could potentially impact the sustainability of cold air masses moving south into our area.

THE HURRICANE SEASON

There is not necessarily a correlation between the Atlantic hurricane season and the following winter's weather.  However, a busy hurricane season usually is due to the pooling of very warm water throughout the Atlantic, often the North Atlantic.  This can potentially have an effect on developing a negative NAO, so it is something to be considered.

ANALOG YEARS/CLIMATE MODELS

The above are the major factors that are taken into account when making a long-range seasonal prediction.  After examining what state these factors are in and will likely be in this winter, we then look to find analog years where these conditions were similar and see what the resulting winter weather was like.  This year, the various factors and the associated data do not point to any clear analogs.  Another major component in the outlook is analyzing our climate "models" of the atmosphere.  You may occasionally hear us refer to "computer" forecasts on our daily weather reports.  We have many computer models (or simulations) of the atmosphere that are made up of thousands and thousands of mathematical equations.  Current data from all over the world is plugged into these equations and a supercomputer quickly comes up with solutions that help predict the future state of the atmosphere and the resulting weather.  There can be a wide variety of solutions and this is why you often hear different forecasts from varying sources.  We all look at the same data, we just interpret it differently.  Computer modeling of the atmosphere has come a long way over the last few years and we now have some very reliable climate models that can help predict seasonal trends and averages.

You can see that the process of coming up with this winter outlook has been a very involved and complicated task.  There are many factors that could easily change the impacts on our day-to-day weather and whether the outlook comes to fruition.  The important thing is to be prepared and stay safe!