By Mike Goldberg, Meteorologist
November 21, 2011

Well isn’t this funny.  For the past two years, at the beginning of the outlook, I’ve said  “it seems each year I start out discussing the winter outlook, we’ve just experienced some sort of extreme weather the few months prior.  Last year, it was the extremely hot summer and the Chicago “chiclone.”  This year, it was an even HOTTER summer and then the surprise October snowstorm from the Virginia mountains into the Northeast.

There are always numerous factors that play a role in determining the winter outlook.  The role of ENSO (the El Nino-Southern Oscillation) seems to be the most famous.  La Nina and El Nino are the two main modes of the ENSO (the El Nino-Southern Oscillation).  Last year, we experienced a moderate La Nina and while it disappeared earlier this year, it is now making a return, albeit a bit weaker.  Regardless, there is another factor which we deem the “wild card” that has taken control the last two years, leading to massive cold outbreaks and more snow than “normal” in many places along the East Coast.  That is the NAO, or North Atlantic Oscillation.  The NAO involves changes in 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 United States.  This leads to colder and snowier weather along the East Coast.  The NAO is very hard to predict beyond a week or two, and often fluctuates back and forth many times over the winter.  However, it has had a tendency toward the negative phase over the longer term, and this has offset some of our other factors the last two seasons.

There is a lot of uncertainty in this year’s outlook, but I do expect extremes to continue in our overall pattern.  While I don’t expect the NAO to have quite as much impact as the last two years, frequent spells of the negative phase will offer harsh cold spells, along with the potential for snowy periods.  However, many of these cold outbreaks can be dry ones, so for snow lovers we will need to take advantage during these blocking periods.

Let’s take a look at what I expect during the coming winter…

*** WINTER OUTLOOK 2011-2012 ***

DECEMBER (Average highs go from 55°F to 48°F, average lows from 35°F to 29°F)

Temperatures:  Slightly below average (-1°F to -2°F) (a few extreme swings in temperatures giving way to more significant cold spells over the holidays)

Precipitation:  Near to slightly above average (frequent dry spells, but a few events could produce significant precipitation)

Highlights:  The recent progressive pattern will give way to a few prolonged cold outbreaks, especially during the second half of the month…the holidays will feel like winter!

JANUARY (Average highs in the mid to upper 40s, average lows in the upper 20s)

Temperatures:  Slightly below average (0°F to -1°F) (the cold continues, although a few swings in temperature bring a period or two of warmer weather)

Precipitation:  Slightly above average (a shifting storm track could cause significant snow to be confined to the mountains)

Highlights:  Probably the coldest month of the winter, with some snow and ice likely…probably the best shot at prolonged winter weather (a week or two)

FEBRUARY (Average highs go from 49°F to 55°F, average lows from 29°F to 33°F)

Temperatures:  Above average (1°F to 2°F) (above average overall, but that doesn’t mean there won’t be any cold weather)

Precipitation:  Near average (being on the temperature battleground, mixed precipitation events will be a threat)

Highlights:  Winter tries to hang on, but milder spells become more frequent

OVERALL (December/January/February combined)

Temperatures:  Near to slightly below average (Wild extremes average out a little colder than average, particularly in December and January)

Precipitation:  Slightly above average (the active Ohio Valley/Northeast storm track keeps things busy, with a wide variety of precipitation across the region)

SNOWFALL

The National Weather Service recently issued new 30-year climatic averages, incorporating the years of 1981-2010.  The new snowfall “average” for Richmond is 10.3” inches, down a bit due to recent lean years.  Predicting the amount of snow over the course of the entire winter is not easy.  It’s an educated guess, based on the overall weather patterns that we expect.  It’s 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.  On the other hand, one big storm can give us our entire annual average in one day!

So here it is:  My "best guess" for the 2011-2012 winter in Richmond is 11 inches.  This could happen in just a couple of events, or even one.  I expect the overall pattern to favor a very cold and snowy winter for the Great Lakes into the Northeast and New England.  The storm track will likely shift north and west of our area, but redevelopment along the coast is always the key.  With the shifting pattern, redevelopment may occur too far north to provide much of our area with significant snow.  That being said, I do think it will be an above average season for the higher elevations to our west.  Here are some estimates for potential snow throughout the region.

                        “Best Guess”        Forecast Range

Richmond                11”                         9”-13”
Petersburg               9”                           7”-11”
Emporia                    5”                            4”-8”
Charlottesville        21”                        18”-24”
Fredericksburg      15”                        13”-18”
Reedville                   8”                           6”-10”                          
Williamsburg            7”                           6”-10”
Norfolk                       5”                            4”-8”


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

LA NINA

La Nina is an abnormal cooling of the surface water in the central and eastern Pacific Ocean.  Its opposite is El Nino, 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. 

Coming off a moderate La Nina last winter, we are now in the midst of another developing La Nina, classified as a “weak” event.  Sea surface temperatures in the tropical Pacific over critical areas are below average.  This will likely have an impact on the winter over North America.  The La Nina is expected to strengthen slightly, and may eventually become moderate, although not as “strong” as last winter.  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.  Below is a graphic from NOAA of the typical impacts from La Nina across North America.

Graphic courtesy of NOAA

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 is generally in the negative or “cool” phase, which is the tendency during La Nina years.  This supports cooler and stormy weather for the West Coast, which often leads to milder conditions for the central and eastern U.S.

                       COOL PHASE                                                                        WARM PHASE

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



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.  Warmer sea surface temperatures over the North Atlantic help to encourage such a blocking pattern.  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 has tended negative the past couple of years, and its stronger signal has led to colder and snowier winters for many along the East Coast.  This tendency over the long term leads to more blocking, which can ultimately offset the negative PDO (which tends milder along the U.S. East Coast).  It can sometimes be hard to maintain the negative NAO throughout the winter, and for snow lovers, we need storms at just the right time to “cash in.” 

                      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.  The AO has been predominantly negative the last couple of winters, but it is currently tending in the positive direction.  However, I caution that a signal during the fall months may not hold throughout the winter.

                POSITIVE PHASE                                NEGATIVE PHASE

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.  The current snow cover across North America is similar to this time last year.  However, there is a bit less over the United States and eastern Canada. Regardless, the coverage over central and western Canada indicates there’s a good breeding area for Arctic air masses.  The question is if and when this Arctic can translate south into the U.S..  I think we’ll definitely see some impressive Arctic outbreaks, but the prolonged brutal cold (and snow) may be confined to the north.  The Great Lakes into the Northeast and New England could be in for a very rough winter.

SOIL MOISTURE

Soil moisture often plays a role in storm tracks and can be very helpful in seasonal outlooks.  This year’s main feature is the extended drought over the Deep South.  The lack of moisture over this region coupled with the tendency for storm tracks to shift north during La Nina years could be a good indicator of what to expect.  I have posted this year’s soil moisture map below, along with the two previous years.  On the 2009 map, note the abundance of moisture over the Mississippi and Tennessee Valleys all the way to the East Coast.  This is the winter we had some very rough weather and cold across the Deep South.  Last year, we still had some impressive cold spells, but snowfall was lower across the Southeast and Mid-Atlantic.  The corresponding soil moisture anomaly (departure from average) shows some lingering abundance along the East Coast, but a lot less than 2009.  Now in 2011, that abundance of soil moisture is now across the northern Mid-Atlantic into the Northeast.  This further supports the idea that we’ll see some very cold weather this year, but the worst winter conditions will be felt across the Northeast back into the Great Lakes. 

The long-term drought indicator map below shows extremely dry conditions over the Deep South and further supports a stormy pattern across the Upper Plains, Midwest and into New England.



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.  The relatively busy 2011 season tracks are shown below.  It’s not all that much different from the 2010 map, but 2009 was much quieter.





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.  A westerly phase (or positive QBO) makes it less favorable for blocking patterns to set up.  An easterly phase (or negative QBO) favors blocking patterns.  Right now, the QBO is tending toward the easterly phase (negative).

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.

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 usually look to find analog years where these conditions were similar and see what the resulting winter weather was like.  The past couple of years, this method seemed to be virtually useless.  However, it’s still important to take into account.  That being said, considering the variety of factors we are using in this year’s outlook, it’s difficult to find similar years in the past.  They all seem to lead us to different winters.  For example, there are some resemblances to the 2009-10 year in terms of the La Nina we are currently experiencing, but the common ground doesn’t go much further than that.  Another recent winter of possible comparison would be 2000-01.  There are also a few similar hurricane seasons I have considered, but again they don’t match up with all the other factors.  So it basically comes down to the unique circumstances we face this year, and whether the NAO can once again set up negative to please winter and snow lovers.

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!