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!