Mike's 9th Annual Winter Outlook
By Mike Goldberg, Meteorologist
November 24, 2009

It seems each year I start out discussing the winter outlook, we’ve just experienced some sort of extreme weather the few months prior.  Mother Nature just keeps throwing more surprises at us.  This month, one of the more historical November nor’easters left boatloads of rain and brought high winds and flooding to coastal Virginia and North Carolina.  Let’s review the highlights from the past several years:

2004:  record rainfall during the summer
2005:  a record Atlantic hurricane season
2006:  record rainfall during the fall
2007:  extremely dry weather during the fall

Last year, we had some awfully cold weather during the fall which led us into some major cold spells during the winter.  We had already seen two shots of snow showers by this time last year and we finally got a significant snow during the winter, albeit late in the season (March 1). 

This autumn season has certainly become known for its numerous significant rain events.  We’ve already recorded the wettest November on record in Richmond!  Interestingly enough, despite the chilly spells we’ve had, average temperatures are running above “normal” and we haven’t seen any readings at or below the freezing mark at RIC International Airport.  We often look at the fall patterns to help determine what the upcoming winter may bring and this year bears some resemblance to 2006.  I find this particularly interesting because the El Nino data for the 6-12 months is almost identical to that leading into the winter of 2006-07.  While I have considered several analog years, this one is the most striking, as others did not “fit” as well with the ongoing weather patterns.  Of course, no two seasons are ever identical, but analog years can be very helpful in composing a seasonal outlook.

There are various other factors that play a role in determining the winter outlook.  El Nino is certainly the most prominent this year.  El Nino and La Nina are the two main modes of the ENSO (the El Nino-Southern Oscillation).  The current El Nino is well-developed and has been strengthening over the last few weeks.  Our modeling shows that it will likely stick around through the winter months, but may have reached its peak.  There are signs it could begin to weaken a bit, particularly during the second half of the winter.  This is a key to our outlook, as a weakening El Nino will lead to greater chances of colder weather across the Mid-Atlantic.

Another big player is the North Atlantic Oscillation (NAO), which usually is a 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 sets up a blocking pattern which forces cold air southward from Canada into the Eastern United States.  Warmer sea surface temperatures over the North Atlantic help to encourage such a blocking pattern.  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. A more in-depth discussion of the various factors considered follows the outlook.
    
WINTER OUTLOOK 2009-2010

Overall, due to the presence of El Nino, I think when we average out conditions over the three winter months (December, January and February), we’ll come out above average (milder) in the temperature department.  However, I caution that this does NOT mean we won’t see any major cold outbreaks or snow.  Remember this represents an AVERAGE of conditions over the entire season.  As is typical of most winters, we’ll see some wide swings in the prevailing weather conditions (especially temperature).  While we’ve seen some cold spells during the fall and may see one or two outbreaks over the next few weeks, I believe when all is said and done December will come in above average.  January will probably still trend to the milder side, but a couple of cold outbreaks are likely.  The best shot at a prolonged cold spell will likely come in February when the El Nino starts to weaken and the overall large-scale pattern shifts.  Due to El Nino, the subtropical jet will stay busy and keep frequent storms racing across the south.  If the timing is right, this could bring us some winter weather and I think we have an above average chance at seeing a decent snowfall this season, particularly during the second half of the winter.  We’ll certainly get our share of precipitation (near to slightly above average overall), but as always it will depend on timing for snow lovers.  There will be numerous winter weather threats, and ice could be an issue as well, depending on the depth and strength of cold air masses over the Eastern U.S.  

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

OVERALL (December/January/February combined)

Temperatures:  Above average (several significant cold spells, we start milder and end colder)
Precipitation:  Near to slightly above average (the stormy pattern eases a bit, but still plenty of moisture across the south with a busy subtropical jet)

DECEMBER

Temperatures:  Above average (a milder than average month ahead—but one or two cold spells)
Precipitation:  Near to slightly above average (still an active pattern, can’t rule out a few early snowflakes)
Highlights:  Cold spells occur early on, relatively mild for the holidays

JANUARY
Temperatures:  Near to above average (a good cold outbreak or two mixed in, especially later in the month)
Precipitation:  Near average (the timing of storms determines whether we see winter weather)
Highlights:  A mild start to 2010, colder later in the month with a few winter weather threats

FEBRUARY
Temperatures:  Below average (more significant cold, although several temperature swings)
Precipitation:  Near average (an above average chance of winter weather)
Highlights:  A prolonged cold spell possible, particularly during the first two weeks…probably our best shot at a decent snowfall

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.  While we’ve already experienced a few coastal events this fall, I think the storm track may shift inland during December.  The subtropical jet will keep moisture and storminess across the south through much of the winter.  I expect to see numerous “chances” for winter weather, with several mixed precipitation events (snow/ice/rain all tossed in).  Since I expect a more prolonged cold spell later in the season, I’d say the time to watch for a decent snow will be later in the season, particularly late January into mid-February. 

So here it is:  My "best guess" for the 2009-2010 winter in Richmond is 9 inches, a slightly below average season.  Keep in mind, one big storm can sometimes bring it all and skew the season total.  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             9 inches
Petersburg             7 inches
Emporia                  6 inches
Charlottesville      18 inches   
Fredericksburg     15 inches
Reedville                 6 inches
Williamsburg          5 inches
Norfolk                    4 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 has 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

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 and are usually referred to as La Nada.  The Southern Oscillation Index (SOI) measures the pressure differences across the tropical Pacific from Tahiti to Darwin, Australia.  ENSO is a key force in determining winter weather patterns over the United States. 

We are now in the midst of a full-blown El Nino, now classified as a “moderate” event.  Sea surface temperatures in the tropical Pacific over critical areas are significantly above average.  This will certainly have an impact on the winter over North America.  The El Nino may have reached its peak and is expected to eventually start weakening.  Winter patterns associated with El Nino often (but not always) produce cool and wet winters for the Southeast U.S., while mild and somewhat dry weather dominates the Northern Plains through the Great Lakes.  A very active subtropical jets keeps plenty of moisture zipping across the Deep South.  In Virginia, the effects we feel depend on the relative strength of the El Nino and the placement of the warmer/cooler waters in the Pacific.  The moderate El Nino will likely lead to milder than average conditions during the first half of the winter, but if the expected weakening takes place, look for a colder finish to the season.  As I mentioned earlier, the El Nino data over the last 6-12 months virtually mirrors that leading into the 2006-2007 winter, so that might be a good analog year.


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

The following is a graph displaying values of the PDO index since 1900:

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 easterly in the QBO.  This is favorable for blocking conditions to set up in the north Atlantic, which could eventually lead to more cold/stormy conditions along the East Coast.

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.  The NAO had a very strong negative signal in October, but looking back at historical data, I found that most years that had such a strong signal in October did not hold that through the following winter. 

                       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 negative in October, like the NAO it historically has not held that signal throughout the winter. 

                  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 been below average this fall.  This could potentially impact the sustainability of cold air masses moving south into our area.  However, the snow cover over Eurasia is running above average.  The question is if and when this Arctic can translate to the Western Hemisphere.  It could happen during the second half of the winter.

SOIL MOISTURE

Soil moisture often plays a role in storm tracks and can be very helpful in seasonal outlooks.  As you can see from the map below, soil moisture content is very high over the Mississippi Valley and supports my idea of the general storm track shifting a bit inland during December.
 

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 particularly El Nino keep bringing me to the 2006-2007 winter, so I that is the one year I have weighted heavily in my consideration.  There were several other seasons considered (the most recent being 2002-03 and 2004-05), but none really fit the pattern that seems to be evolving.  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!