By Mike Goldberg, Meteorologist
November 19, 2012
The overall trend for more extreme weather events to occur over the past couple of years has been very clear. Global climate change certainly has something to do with this, but to say future storms will just be like Sandy would be foolish. The hybrid hurricane-blizzard was one for the record books, a rather unique phenomenon in the meteorological world.
Every year, we seem to get caught up in discussing the role of ENSO (the El Nino Southern Oscillation, which includes El Nino, La Nina and La Nada) for determining the winter outlook. However, the last couple of years have made it very clear that other factors are at work here. Specifically, the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) have played a huge role in formulating the weather that occurred during the winter seasons along the East Coast of the U.S. While many forecasters were quick to jump on board with the development of an El Nino this year, it has clearly not occurred and we are more likely to spend most if not all of the winter in “La Nada” conditions. Therefore, I have looked very closely at the data from the NAO and AO, as well as various other factors (listed in the “science” portion of the outlook below) to come up with a bunch of analog (similar) years to analyze.
In reviewing all of the data in these analogs, one consistency I found is the trend for cooler than average winter months. What isn’t so clear is the precipitation side of things. While a trend in the NAO to go negative will create blocking patterns, it doesn’t guarantee that Virginia will experience a lot of precipitation from any developing storms. Some of the analogs show much above average precipitation, including the winter of 2002-03, which I took a close look at. However, the dryness in the nation’s mid-section may play a role in the location of any eastern trough. My feeling is to temper the outlook and side with a more typical or below average winter in terms of the precipitation. Regarding snow, it all depends on if the cold and precipitation occur together. You can certainly have a winter with below average precipitation and above “normal” snowfall, as long as the storms come when the cold air is in place.
I can say with fairly high confidence that this winter will be colder and a bit snowier than last winter! Overall, I think it will tend colder and somewhat drier than “normal.”
So let’s take a look at what I expect during the coming winter…
*** WINTER OUTLOOK 2012-2013 ***
DECEMBER (Average highs go from 55°F to 48°F, average lows from 35°F to 29°F)
Average Temp: 41.0°F
Temperatures: Slightly below average (0°F to -1°F) (a few warmer spells, but continued cold air invasions)
Precipitation: Slightly below average (several near misses with coastal events)
Average Snow: 2.1”
Highlights: The current pattern continues with substantial cool weather outweighing milder spells. While coastal development may continue, chances are there will be a near miss or two.
JANUARY (Average highs in the mid to upper 40s, average lows in the upper 20s)
Average Temp: 37.9°F
Temperatures: Below average (-1°F to -2°F) (the cold continues, and will likely intensify a bit)
Precipitation: Below average (not without a winter storm threat)
Average Snow: 3.9”
Highlights: May end up being the coldest of the winter, with at least some snow likely)
FEBRUARY (Average highs go from 49°F to 55°F, average lows from 29°F to 33°F)
Average Temp: 40.9°F
Temperatures: Below average (0°F to -2°F) (Chances are the colder winter continues, but there is a chance the cold will let up a bit, particularly if a weak El Nino can develop later in the season)
Precipitation: Slightly above average (could be the stormiest of the three winter months, with the potential for coastal events, depending on blocking)
Average Snow: 3.4”
Highlights: The cold possibly hanging on and some stormy weather create a potent mix for potential winter precipitation)
OVERALL (December/January/February combined)
Temperatures: Below average (Extreme swings are possible with the cold outdoing the milder periods)
Precipitation: Slightly below average (Placement of coastal events depending on the prevailing pattern will determine how much winter precipitation we get. Overall, precipitation numbers should be a bit below “normal”)
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 2012-2013 winter in Richmond is 12 inches. This could happen in just a couple of events, or even one. I definitely think this winter will be colder than last winter, with more snow. The higher elevations to our west could be looking at a very snowy season. Here are some estimates for potential snow throughout the region.
“Best Guess” Forecast Range
Richmond 12” 10”-15”
Petersburg 10” 9”-13”
Emporia 10” 8”-12”
Charlottesville 23” 20”-26”
Fredericksburg 18” 16”-22”
Reedville 10” 8”-12”
Williamsburg 10” 8”-12”
Norfolk 8” 6”-10”
Lynchburg 17” 15”-20”
MORE ABOUT THE OUTLOOK...
Predicting a season's weather in advance is never an easy task, and we sometimes have a hard time dealing with storms that are just a few days away. Scientific advances continue to give us more data and insight in order to provide a general outlook several months in advance. This helps provide valuable 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"...
EL NINO/LA NADA (ENSO)
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.
Coming off a second straight La Nina last winter, there were some signs that an El Nino could develop this fall, but that has yet to happen. Sea surface temperatures in the tropical Pacific over critical areas are slightly above average. In Virginia, the effects we feel depend on the relative strength of the El Nino or La Nina, and the placement of the warmer/cooler waters in the Pacific. With no clear signal, other factors will likely play a much more important role in what takes shape this winter.
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 SITUATION NOW ** The NAO has tended negative the past couple of years, and its stronger signal led to colder and snowier winters for many along the East Coast during the 2009-10 and 2010-11 winters. A switch to more positive readings last year brought us a milder and drier winter. The NAO has been leaning negative for six months now and is currently in major negative territory. This tendency over the long term leads to more blocking patterns, which ultimately can mean colder and stormier days along the Eastern seaboard. 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 SITUATION NOW ** The AO has been very consistent with the NAO, predominantly negative during 2009-10 and 2010-11, but positive last season. During October, it took on a strong negative signal, and this is combination with the strong NAO led to the Sandy situation. Remember, a signal during the fall months won’t necessarily hold throughout the winter, but this one has a pretty good chance.
POSITIVE PHASE NEGATIVE PHASE
PACIFIC DECADAL OSCILLATION (negative—since middle of 2010/cool phase)
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 SITUATION NOW ** The North Pacific right now is generally in the negative or “cool” phase, which tends to support cooler and stormier weather for the West Coast. However, the PDO may be shifting direction, as it has tended away from a strong negative reading it showed in September. Also, we had a negative PDO during the 2010-11 season, yet still had a colder than average winter in the East.
COOL PHASE WARM 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 SITUATION NOW ** 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. This is even more true over Eurasia, where the snowpack is looking a bit more significant. The question is if and when this Arctic can translate south into the U.S.. There is a pretty good chance we’ll see a few major arctic outbreaks during the coming winter, that could reach into the Deep South.
Soil moisture often plays a role in storm tracks and can be very helpful in seasonal outlooks.
** THE SITUATION NOW ** Maps are posted below for this year and the three previous years. You’ll notice that the dry conditions has expanded north over the central U.S. I have to wonder if this will have an impact on northern stream system’s this winter, possibly robbing some of the limited moisture they have. For those thinking that because of the strong NAO and AO could lead us to another winter like 2009-10, I caution that on the 2009 map (which I was unable to upload here), there was an abundance of moisture over the Mississippi and Tennessee Valleys all the way to the East Coast. We had some very rough weather and cold across the Deep South, but this year’s soil moisture anomaly map is nothing like the one from 2009. So while I think this winter could trend colder and stormier, this supports the thinking that it won’t be as harsh as the 2009-10 winter.
long-term drought indicator map below shows extremely dry conditions
over the central U.S., potentially supporting ridging to our west and a
tendency for an East Coast trough. Notice the difference
between this map and last year’s map below it, where long-term drought
was more confined to the Deep South.
THE HURRICANE SEASON (busy season…most in central ATL, a few hits)
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 maintaining a negative NAO, so it is something to be considered. This season was active in the Atlantic, as seen below. Notice that the 2011 season was somewhat similar, and we ended up having a rather “quiet” winter, while the relatively busy 2010 season produced a colder winter. The harsh winter of 2009-10 was preceded by a very quiet hurricane season. I also included a graphic of the 2002 hurricane season, since I looked closely at 2002-03 as an analog winter. There were a fair number of tropical systems, but not nearly as many in the central Atlantic as this year. Again, there’s not necessarily a correlation between the hurricane season and winter along the East Coast, but the pooling of very warm water in the north Atlantic could be a sign for more blocking to occur.
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 up to two years 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.
** THE SITUATION NOW ** Right now, the QBO has been strongly negative (favoring blocking patterns), but is going up slightly. Will this trend continue?
MADDEN-JULIAN OSCILLATION (MJO) (similar to last year…dry?)
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. I have specifically gone back and followed each factor’s history, taking into account individual analog years and their resulting weather conditions. Each factor presented different analog years, but there was a good consistency of below average temperatures. Precipitation is not as clear. The winter of 2002-03 was a benchmark for me to use, but I’m not sure we’ll be able to match that season’s much above average precipitation.
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.
In addition to daily computer model runs, we have models that help with seasonal forecasts. The
Climate Forecast System model (or CFS) has not been all that consistent
over the last few months, but has trended in a “cooler” direction for
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!