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Winter 2009-10 Outlook
Updated November 4, 2009 - Jeff Boyne

On October 15, 2009, NOAA's Climate Prediction Center (CPC) released their first outlook for the 2009-10 meteorological winter (December 1, 2009 to February 28, 2010).  El Niño in the central and eastern equatorial Pacific Ocean is expected to be a dominant climate factor that will influence the December through February winter weather in the United States. They are forecasting above normal temperatures across southeast Minnesota, northeast Iowa, and western Wisconsin.  Meanwhile their precipitation forecast is for equal chances of above normal, below normal, and near normal precipitation. See information below for additional historical climate information relevant to this upcoming winter.

Climate Prediction Center (CPC) Outlook

CPC Temperature Outlook:

For the upcoming 2009-10 winter, NOAA's Climate Prediction Center (CPC) temperature forecasts strongly reflect typical El Niño temperature anomalies through the United States.  This includes higher probabilities of warmer-than-normal conditions across the northern United States (including southeast Minnesota, western Wisconsin, and northeast Iowa) and Alaska, and cooler-than-normal conditions across the mid Atlantic, Gulf Coast, and Hawaii.  Though temperatures may average warmer-than-normal from Montana to Wisconsin, periodic outbreaks of cold air are still possible.

CPC's Winter 2009-10 U. S. Temperature Outlook

CPC Precipitation Outlook:

For the upcoming 2009-10 winter, NOAA's Climate Prediction Center (CPC) precipitation forecasts strongly reflect typical El Niño precipitation anomalies through the United States.  This includes higher probabilities of wetter-than-normal conditions across the southern United States and drier-than-normal conditions in the Pacific Northwest and in the Tennessee and Ohio Valleys.  Meanwhile the remainder of the United States (including southeast Minnesota, western Wisconsin, and northeast Iowa) will see equal probabilities for drier, wetter, and near normal.

CPC's Winter 2009-10

Review of Past Winters with Similar Global Characteristics

What is an El Niño?

The term El Niño was first coined more than 100 years ago to describe the unusually warm waters that would occasionally form along the coast of Ecuador and Peru.  This phenomenon typically occurred late in the calendar year near Christmas, hence the name El Niño (Spanish for "the boy child", referring to the Christ child).  Today the term El Niño is used to refer to a much broader scale phenomenon associated with unusually warm water that occasionally forms across much of the tropical eastern and central Pacific (between longitudes 120W and 170W and latitudes from 5N to 5S).  The time between successive El Niño events is irregular, but they typically tend to recur every 3 to 7 years. 

Although El Niño episodes are characterized by warmer than average sea surface temperatures in the tropical Pacific, they are also associated with global changes in wind, pressure, and rainfall patterns.  In the tropics where El Niños form, rainfall tends to occur over areas having the warmest sea surface temperature.  Normally, the warmest water is typically found in the western Pacific, as is the greatest rainfall.  Surface winds typically travel from east to west across the Pacific (these winds are called easterlies).  During an El Niño, the easterlies weaken and this allows the warmer than average sea surface temperatures to shift eastward into the central and eastern tropical Pacific.  This in turn shifts the region of heaviest rainfall eastward as well. 

The two figures below (courtesy of NOAA's Climate Prediction Center) are schematic views of sea surface temperature and tropical rainfall in the equatorial Pacific Ocean during Normal and El Niño conditions.  The sea-surface temperature is shaded: blue-cold and orange-warm.  The dark arrows indicate the direction of air movement in the atmosphere: upward arrows are associated with clouds and rainfall and downward-pointing arrows are associated with a general lack of rainfall.
 

Normal Conditions	El Niño Conditions

While the tropical ocean affects the atmosphere above it, so too does the atmosphere influence the ocean below it. In fact, the interaction of the atmosphere and ocean is an essential part of El Niño.  During an El Niño, sea level pressure tends to be lower in the eastern equatorial Pacific (near Tahiti) and higher in the western equatorial Pacific (near Darwin, Australia). The opposite occurs during a La Niña (colder-than-normal waters in the central and eastern equatorial Pacific).  This see-saw in atmospheric pressure between the eastern and western tropical Pacific is called the Southern Oscillation, often abbreviated as simply the SO.  A standard measure of the Southern Oscillation is the difference in sea level pressure between Tahiti and Darwin, Australia.  Since El Niño and the Southern Oscillation are related, the two terms are often combined into a single phrase, the El Niño-Southern Oscillation, or ENSO for short.  Often the term ENSO Warm Phase is used to describe El Niño and ENSO Cold Phase to describe La Niña.

Once developed, El Niño are known to shift the seasonal temperature and precipitation patterns in many different regions of the world, even areas that are distant from the equatorial Pacific Ocean. These shifts are known as teleconnections.

Teleconnections occur because rainfall, which is associated with sea surface temperatures in the tropics, affects wind patterns in the atmosphere. In the tropics, air that rises to form clouds and precipitation at a certain location must subside somewhere else (what goes up must come down). This is how one tropical region that is persistently wet, for example, can lead to another region being persistently dry. Shifts in tropical rainfall and winds can also affect regions outside of the tropics by altering prevailing wind patterns that circulate around the globe.

In several parts of the tropics, and some areas outside of the tropics, these seasonal shifts are fairly consistent from one El Niño to the next. It is important to remember, however, that no two El Niños are identical and that the seasonal shifts in temperature and precipitation patterns associated with them can vary from one event to the next; thus, an El Niño only means that there is enhanced probability of an anomaly.  The image below from the Climate Prediction Center shows where these enhanced probabilities are located.

Enhanced probabilities associated with El Niños

Since the winter of 1949-50, there have been 18 El Niño episodes.  These episodes are monitored by the Oceanic Niño Index (ONI) which is a 3-month running mean of sea surface temperature anomalies in the Niño 3.4 region (5^oN-5^oS, 120^o-170^oW in the equatorial eastern and central Pacific basin).  An El Niño episode is defined as 5 consecutive (3-month) seasons which are 0.5^oC or greater.  El Niños are classified as weak (0.5 to 0.9 ONI), moderate (1.0 to 1.4 ONI), and strong (1.5 ONI or greater).  The table below provides more details on these past El Niño episodes.

As of early November 2009, this El Niño is classified as weak to moderate.  The Climate Prediction Center is forecasting that this event will likely remain moderate through the winter, and will likely weaken during the spring of 2010.  The looping image below shows how the current sea surface temperature anomalies have steadily increased across the Pacific Ocean in the Niño 3.4 region (5^oN-5^oS, 120^o-170^oW) since mid August 2009.

Pacific Ocean Sea Surface Temperatures Anomalies

While sea surface temperatures are important when evaluating an El Niño, the sub surface temperatures are even more important, because they allow climatologists a way to monitor the true heat content of the oceanic basin.  The image below shows a loop of how these temperatures are been evolving during the past couple of months.  When looking at this loop, it is important to concentrate on how deep the anomalous warm water has grown in the Niño 3.4 region (5^oN-5^oS, 120^o-170^oW) since mid August 2009.

Temperatures:

In La Crosse, WI, composites show that El Niño's are typically warmer-than-normal.  This was the case in eleven of the seventeen (64.7 percent) El Niño events since the 1949-50 winter.
From the late 1950s through late 1970s, five of eight (62.5 percent) El Niño events were colder-than-normal.  Since 1980, eight of nine (88.9 percent) El Niño events have averaged warmer-than-normal.  This may be related to the change of the phase of the Pacific Decadal Oscillation (PDO) which went from negative to positive around 1980.  Just within the past couple of years, we have shifted back toward the negative phase of the PDO. Research has not clearly identified a link between the temperature impact of El Niño events and PDO phase, but some scientists speculate there may be a relationship between the two.  The local data supports that possibility.  It will be of interest to see whether the El Niño event this winter is more similar to the recent (positive PDO) El Niños or those during the previous negative PDO regime.
	Variability of Temperatures during El Niño Winters in La Crosse, WI
The two coldest winters since 1949-50 were both El Niño winters (1977-78 with an average temperature of 11.4 degrees and 1976-77 with an average temperature of 12.1 degrees).  The warmest El Niño winter occurred back in 1997-98 when the average temperature was 28.4 degrees.  There is a 17 degree difference between the coldest and warmest El Niño winters in La Crosse since 1950. In Rochester, MN, composites show that El Niño's are typically warmer-than-normal.  This was the case in ten of the seventeen (58.8 percent) El Niño events since the 1949-50 winter.
From the late 1950s through late 1970s, six of eight (75 percent) El Niño events were colder than normal.  Since then, eight of nine (88.9 percent) El Niño events have ended up averaging above normal.  This may be related to the change of the phase of the Pacific Decadal Oscillation (PDO) which went from negative to positive around 1980.  Just within the past couple of years, we have shifted back toward the negative phase of the PDO. Research has not clearly identified a link between the temperature impact of El Niño events and PDO phase, but some scientists speculate there may be a relationship between the two.  The local data supports that possibility.  It will be of interest to see whether the El Niño event this winter is more similar to the recent (positive PDO) El Niños or those during the previous negative PDO regime.
	Variability of Temperatures during El Niño Winters in Rochester, MN
The coldest of these winters occurred in 1977-78 when the average temperature was just 8.6 degrees (9.0 degrees below normal).  Meanwhile the warmest of these winters occurred in 1997-98 when the average temperature was 24.3 degrees (8.6 degrees above normal).  There is a 15.7 degree difference between the coldest and warmest El Niño winter in Rochester since 1950.

In La Crosse, WI, composites show that El Niño's are typically drier-than-normal.  This was the case in ten of seventeen (58.8 percent) El Niño events since the 1949-50 winter.  However this percentage is not enough to be considered to be a strong statistical signal. The driest of these winters occurred in 1963-64 when the total precipitation was a mere 0.92 inches (2.49 inches below normal).  Meanwhile the wettest of these winters occurred in 1982-83 when the total precipitation was 5.19 inches (1.78 inches above normal). This is a spread of 4.27 inches.
	Variability of Precipitation during El Niño Winters in La Crosse, WI
Meanwhile in Rochester, MN, composites show that there is a nearly even split between drier (8) and wetter-than-normal (9) El Niño events since the 1949-50 winter. The driest of these winters occurred in 1957-58 when the total precipitation was a mere 0.71 inches (2.00 inches below normal).  Meanwhile the wettest of these winters occurred in 1982-83 when the total precipitation was 4.92 inches (2.21 inches above normal). This is a spread of 4.21 inches.
	Variability of Precipitation during El Niño Winters in Rochester, MN

Snowfall:

Composites showed that El Niño winters had highly variable seasonal snowfalls (July 1st through June 30th).

In La Crosse, WI, eleven of seventeen (64.7 percent) El Niño events since the 1949-50 winter were below normal. The least amount of snow in a season occurred back 1965-66 when a mere 20 inches (24.3 inches below normal) fell.  Meanwhile the greatest amount of snow in a season occurred in 1991-92 when 64.1 inches of snow (19.8 inches above normal) fell. This is a spread of 44.1 inches.
	Variability of Seasonal Snowfall (July 1 to June30) during El Niño Winters in La Crosse, WI

In Rochester, MN, eleven of seventeen (64.7 percent) El Niño events since the 1949-50 winter had below normal snowfall. The least amount of snow in a season occurred back in 1957-58 when a mere 21.6 inches (31.1 inches below normal) fell.  Meanwhile, the greatest amount of snow in a season occurred in 1982-83 when 62.6 inches of snow (9.9 inches above normal) fell. This is a spread of 41.0 inches.

Variability of Seasonal Snowfall (July 1 to June30) during El Niño Winters in Rochester, MN

Potential Wild Cards for this upcoming Winter

The following wild cards may affect the Upper Mississippi River Valley temperatures and precipitation during this upcoming winter:

North Atlantic Oscillation (NAO): The North Atlantic Oscillation (NAO) was discovered in the 1920s by Sir Gilbert Walker. Similar to the El Niño phenomenon in the Pacific Ocean, the NAO is one of the most important drivers of climate fluctuations in the North Atlantic and surrounding areas. The NAO phase can greatly affect the temperatures across the eastern half of the United States.  This includes the Upper Mississippi River Valley.  When the NAO is in its positive phase, low pressure is located across Iceland and high pressure is located across the Azores.  In this phase, temperatures across the Upper Mississippi River Valley average above normal.  Meanwhile when the NAO is in its negative phase, the high and low pressure systems are reversed and the Upper Mississippi River Valley experiences below normal temperatures. The images below from NOAA's Earth System Research Laboratory show what the typical surface pressure and temperature anomaly pattern looks like in each phase.
Sea Level Pressure Anomalies during the NAO's Positive Phase			Sea Level Pressure Anomalies during the NAO's Negative Phase
Temperature Anomalies during the NAO's Positive Phase			Temperature Anomalies during the NAO's Negative Phase
While meteorologists know that the NAO can greatly affect the temperatures across the Upper Mississippi River Valley, its predictability beyond a week to two weeks is quite low. As a result, meteorologists can't base a seasonal forecast, such as winter, on it. In addition, the phases can oscillate back and forth during the winter which can make temperature predictability very low.
Madden Julian Oscillation (MJO): The Madden-Julian Oscillation (MJO) is a tropical disturbance that propagates eastward around the global tropics.  It typically originates over the Indian Ocean and then propagates around the world anywhere from 30 to 60 days.  As this occurs, it can affect temperatures and precipitation from the tropics into the mid latitudes.  For example when the MJO is over Indonesia, the temperatures in the Upper Mississippi River Valley typically range from near to below normal. Meanwhile when the oscillation is located over the equatorial western Pacific basin, the temperatures in the Upper Mississippi River Valley typically range from near to above normal. The temperature then returns to near to below normal as the oscillation moves into the central equatorial Pacific. With its time scale, it mainly affects the weather on a sub seasonal time scale and it is not useful in determining the average temperature for a 3-month season such as winter.  However, it can have a higher impact on periodic shifts in regional temperatures and circulations patterns during the entire season.

Winter Climatology

La Crosse, WI

Temperatures (in degrees F):

La Crosse, WI averages 20.0 degrees during a typical meteorological winter (December through February).  This is based upon the 1971 through 2000 (30-year) climate normals.  The coldest winter occurred during the winter of 1872-73 when the average temperature was 8.2 degrees.  Meanwhile the warmest winter occurred during the winter of 1877-78 when the average temperature was 34.2 degrees.  The image below provides average temperatures from the winter of 1872-73 through the winter of 2008-09.

Precipitation:

On average, La Crosse, WI receives 3.41 inches of precipitation during a typical meteorological winter (December through February).  This is based upon the 1971 through 2000 (30-year) climate normals.  The driest winter occurred during the winter of 1963-64 when just 0.92 inches of precipitation fell.  Meanwhile the wettest winter occurred during the winter of 1875-76 when 7.47 inches of precipitation fell.  The image below provides precipitation totals from the winter of 1872-73 through the winter of 2008-09.

Snowfall:

On average, La Crosse, WI receives 44.3 inches of snow during a typical snow season (July 1 through June 30).  This is based upon the 1971 through 2000 (30-year) climate normals.  The least amount of snow in a season occurred during the 1967-68 snow season when just 7.7 inches of snow fell.  Meanwhile the snowiest season occurred during the 1961-62 snow season when 78.7 inches of snow fell.  The image below provides snow totals from the 1884-85 snow season through the 2008-09 snow season.

Rochester MN

Temperatures (in degrees F):

Rochester, MN averages 15.7 degrees during a typical meteorological winter (December through February).  This is based upon the 1971 through 2000 (30-year) climate normals.  The coldest winter occurred during the winter of 1978-79 when the average temperature was 5.6 degrees.  Meanwhile the warmest winter occurred during the winter of 1930-31 when the average temperature was 26.1 degrees.  The image below provides average temperatures from the winter of 1929-30 through the winter of 2008-09.

Precipitation:

On average, Rochester, MN receives 2.71 inches of precipitation during a typical meteorological winter (December through February).  This is based upon the 1971 through 2000 (30-year) climate normals.  The driest winter occurred during the winter of 1957-58 when just 0.71 inches of precipitation fell.  Meanwhile the wettest winter occurred during the winter of 1887-88 when 5.47 inches of precipitation fell.  The image below provides precipitation totals from the winter of 1929-30 through the winter of 2008-09.

Snowfall:

On average, Rochester, MN receives 52.7 inches of snow during a typical snow season (July 1 through June 30).  This is based upon the 1971 through 2000 (30-year) climate normals.  The least amount of snow in a season occurred during the 1967-68 snow season when just 9.1 inches of snow fell.  Meanwhile the snowiest season occurred during the 1996-97 snow season when 84.7 inches of snow fell.  The image below provides snow totals from the 1949-50 snow season through the 2008-09 snow season.