In the fall of 2010, there was a transition from an El Niño event to a La Niña event, with a moderate to strong La Niña event evolving in the winter of 2010-2011. As we approached the winter months of 2011-2012... La Niña conditions returned to the equatorial Pacific for the second winter in a row. The Climate Prediction Center (CPC) has published theOfficial Winter Forecast for the United States. So what does this mean for our areas of Northern Indiana, Southwestern Lower Michigan, and Northwestern Ohio? Read below to learn more...
La Niña refers to the periodic cooling of sea surface temperatures in the central equatorial and central-eastern Pacific. La Niña events typically occur every 3 to 5 years, but occasionally they can occur for several years. La Niña represents the cool phase of the El Niño/Southern Oscillation (ENSO) cycle. La Niña weakened last spring through early summer, but showed signs of strengthening again in late summer as negative temperature anomalies returned to the equatorial Pacific. Several computer models predict that this cooling will continue through the winter and therefore thelatest official forecastfrom the Climate Prediction Center (CPC) this urges La Niña to strengthen and persist until early spring 2012.
During a La Niña event, changes in Pacific Ocean temperatures affect tropical precipitation patterns from Indonesia to the west coast of South America. These changes in tropical precipitation patterns are affecting weather patterns around the world. These effects are usually strongest in the winter months, when the jet stream is strongest over the United States. As shown in the image below, La Niña episodes during the winter months feature an undulating jet stream flow across the United States and Canada, causing colder and stormier than average conditions in the north. and warmer, less stormy conditions in the south. Historically, fall in this part of the Midwest has tended to be warmer and drier than normal, while winters have tended to be wetter than normal. However, there are many other complicating factors in the atmosphere and oceans that can also influence our weather patterns.
The following images show temperature, precipitation, and snowfall anomalies over the continental United States (CONUS) based on previous La Niña events dating back to 1950. Each image consists of an anomaly map on the left and an abundance map on the right. . The anomaly map shows the calculated anomaly based on all past events. The frequency map to the right shows how often these anomalies occurred. A high frequency leads to greater confidence that the abnormality can recur. The maps are divided into composites at the top, trends in the middle, and a combination of composites and trends at the bottom. For a more detailed explanation of these letters, seehere. The images on the left correspond to the December-February period (meteorological winter) and the images on the far right correspond to the January-March period (late winter to early spring). For our area, you can see that the temperature anomalies are close to zero, but the negative anomalies are only in our northwest. This reflects that some La Niña years have been warmer than normal and others cooler than normal. Precipitation and snowfall anomalies in our area become wetter as we progress from winter to early spring, with reasonable frequency of occurrence. Please note that these maps are based solely on past La Niña events and do not take into account other phenomena that work for or against a La Niña pattern and affect our climate. Some of the better known phenomena are described in the following sections.
Scientific research in recent years has found correlations between conditions in the Arctic and North Atlantic areas and the climate throughout the CONUS. These conditions have been called the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO).
The AO is an indication of changes in atmospheric pressure patterns at high latitudes in the Arctic region. With relatively high pressure over the northern polar region and relatively low pressure in the mid-latitudes, the AO is in a negative phase. When this pattern reverses, it is known to be in a positive phase. During the negative phase, cold air typically rushes south into the central and eastern United States as the jet stream twists and high pressure over the polar region helps push very cold polar air toward the South. In the positive phase, the winds associated with the polar jet stream are generally zonal and confined to northern latitudes. As a result, the very cold air in the polar regions is practically “bottled”. The phases of AO can be seen in the following diagram. For more information on the AO, seehttp://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml.
The NAO is similar to the AO, but is a measure of the difference in atmospheric pressure at sea level between the Icelandic Low (also known as the Polar Low) and the Azores High (also known as the Subtropical High). The positive phase of the NAO indicates a stronger than normal Azores maximum and a stronger than normal Iceland minimum. This results in a greater pressure differential between the two. The negative phase of the NAO indicates a weaker Azores maximum and Icelandic minimum, resulting in a weaker pressure differential between the two pressure centers. The positive phase is generally associated with above-average temperatures in the eastern CONUS, while the negative phase is associated with below-average temperatures. The phases of the NAO can be seen in the diagram below. You can find more information about the NAO herehere
Although AO and NAO have been shown to have significant effects on climate at CONUS, they can only be predicted up to a period of about two weeks with reasonable experience. Therefore, while La Niña is a relatively long-term, continuous pattern, AO and NAO fluctuations are on a much smaller time scale and their effects can amplify or suppress conditions typically associated with La Niña. When AO and NAO are in their negative phases, they can send cold air through the CONUS. If these phases coincide with a strong storm system generated in a La Niña pattern, a significant blizzard could result. However, if the AO and NAO are in a positive phase when a storm system develops, there could be less cold air and potentially more liquid precipitation. These complex interactions are only a small part of what makes predicting winter weather in the central and eastern United States so difficult.
The images below show temperature and precipitation anomalies typically associated with the AO positive phase (AO+), the neutral phase (AON), and the negative phase (AO-) across the CONUS. The images on the left correspond to the December-February period and the images on the right correspond to the January-March period. You can see that with positive AO, much of the central and eastern CONUS has positive temperature anomalies. When the AO is negative, many of these areas have negative temperature anomalies. The effects on precipitation are not as pervasive as temperatures, but there is a trend for less precipitation in a negative AO pattern in areas where La Niña tends to produce above-average precipitation, particularly in the Ohio and Lower Mississippi Valley regions. . These contrasting patterns add to the uncertainty in the winter precipitation forecast. However, La Niña systems are typically very strong and moisture-rich and tend to outweigh the negative precipitation anomalies associated with negative AO in our area.
The following map shows the conditions typically associated with the NAO.
The experts ofClimate Prediction Center(CPC) provide temperature and precipitation forecasts on weekly, monthly, and seasonal time scales using data sets, computer models, and other research tools. Below are the current outlook for this winter (December, January and February). Thatcurrent winter outlookshows equal chances of normal, above-normal, or below-normal temperatures and higher chances of above-normal precipitation for northern Indiana, southwestern lower Michigan, and northwestern Ohio. It is important to remember that these graphs show the probabilities of these conditions occurring and do not imply certainty. Also, the precipitation map is not an indicator of the probability or amount of snow, but simply of overall precipitation. As discussed in previous sections, a prolonged positive AO or NAO could result in a milder winter in northern CONUS. However, if they turn negative for an extended period, it could spell a very cold winter for the north. These indices have been negative for the past few winters, and some research suggests that these indices could remain negative or generally positive for several years. We must also remember that storm development and winter tracks across the Midwest are critical to knowing where and in what form the heaviest precipitation will fall. La Nina tends to shift storm tracks a little further south than normal, and the Ohio River Valley region is a prime area for winter storms. This places our region in the far north for many winter storms.
Winter temperature and precipitation forecast (December to February), issued on November 17
After examining national level research and the CPC winter outlook, a local research study was completed to examine how past La Niña events have affected the local area. There have been 14 documented La Niña events since 1950. Due to a limited data set and local climate variability, these results alone cannot accurately predict what will happen during the current La Niña event, but it is interesting to look at the trends. Monthly data for average temperatures, precipitation, and snowfall were compiled for each episode for Fort Wayne and South Bend. The charts below show the deviation from normal for all La Niña episodes combined.
In general, Fort Wayne was warmer than normal during all months except March, May, and November, which were cooler than normal during La Niña events. Regarding rainfall, the months of January, February, May, August and October were wetter than normal, while the remaining months were drier than normal. Snowfall was below normal in January, February, April, and October through December and above normal in March.
In general, South Bend was warmer than normal for all months except February, March, and May, which were cooler than normal during La Niña events. Regarding rainfall, the months of January, February, April, August and October were wetter than normal, while the remaining months were drier than normal. Snowfall was below normal in January, February, November and December and above normal in March, May and October.
Check back each month as the latest La Niña monthly averages are added to the charts!
The 2010-2011 winter season was cooler and drier than normal, but snowfall was above normal in both Fort Wayne and South Bend due to the occurrence of a moderate to severe La Niña. The winter of 2010-2011 demonstrates why precipitation forecasts are not indicative of the potential amount of snowfall.
fort wayne---All three months, December through February, were below normal temperatures in Fort Wayne. The average temperature was 5.3 degrees Fahrenheit below normal in December, 3.1 degrees below normal in January, and 1.8 degrees below normal in February. This means that the average winter season temperature is 3.4 degrees below normal. Rainfall was below normal in both December and January (1.69 inches less in December and 0.25 inches less in January). February saw above-average rainfall (1.27 inches above normal). During the winter season, the precipitation was 0.67 inches below normal. While precipitation was below normal, snowfall was above normal every 3 months. Snowfall was 2.6 inches above normal in December and 6.2 inches above normal in January. February ended at 18.3 inches (10.7 inches above normal) as the second snowiest February on record! Seasonal snowfall totaling 45.3 inches was 19.5 inches above normal and was the third snowiest winter season on record.
South Bend--- All three months, December-February, were below normal temperatures in South Bend. The average temperature was 4 degrees below normal in December, 2.2 degrees below normal in January, and 1.7 degrees below normal in February. The winter season average was 2.7 degrees below normal. Rainfall was below normal in both December (1.25 inches below normal) and January (0.11 inches below normal). February was above normal precipitation (1.53 inches above normal). Total precipitation for the winter season was 0.17 inches below normal. While precipitation was below normal, snowfall was above normal for all 3 months. December snowfall was 4.4 inches above normal. Snowfall for January totaled 53.8 inches, which is 30.6 inches above normal! This makes it the second snowiest January on record, after January 1978. February snowfall was 10.0 inches above normal. The seasonal snowfall total of 102.9 inches was 45.0 inches above normal, making this winter the second snowiest winter season on record. .
Through February 25, the 2011-2012 winter season was warmer and wetter than normal, but saw below-average snowfall in both Fort Wayne and South Bend due to the occurrence of a weak to moderate La Niña. . Like the previous winter, the winter of 2011-2012 demonstrates why precipitation forecasts are not indicative of the potential amount of snowfall.
Fort Wayne (through February 25)--- Every three months, from December to February, temperatures in Fort Wayne were above normal. The average temperature was 6.0 degrees Fahrenheit above normal in December, 4.7 degrees above normal in January, and 4.6 degrees above normal in February. This means that the average winter season temperature is 4.0 degrees above normal. Rainfall was above normal in both December and January (1.00 inches over December and 0.83 inches over January). So far, the precipitation in February has been below normal (0.35 inches below normal). For the winter season, the precipitation was 1.48 inches above normal. Snowfall was below normal in December (1.9 inches below normal) and February (0.7 inches below normal) and above normal in January (4.5 inches above normal). the normal). The seasonal total snowfall on February 25 was 27.5 inches, which is 1.2 inches above normal.
South Bend(until February 25) --- Every three months, from December to February, temperatures in South Bend were above normal. The average temperature so far has been 6.3 degrees above normal in December, 5.0 degrees above normal in January, and 4.0 degrees above normal in February. The winter season average was 5.0 degrees above normal. Precipitation was above normal in both December (0.01 inches above normal) and January (0.68 inches above normal). So far, February has fallen below normal precipitation (0.24 inches below normal). Total precipitation for the winter season was 0.45 inches above normal. While precipitation was above normal, snowfall was below normal in December and February and above normal in January. Snowfall has been 11.6 inches below normal in December, 14.4 inches above normal in January, and 4.7 inches below normal in February thus far. Seasonal snowfall totaling 49.7 inches was 3.2 inches below normal.
So if each of the last two winters has had La Niña conditions, why is this winter so drastically different from last winter? Although the causes of the differences are difficult to pinpoint precisely, the difference may lie in theThe Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) discussed above.When AO and NAO are in their negative phases, they can send cold air through the CONUS. If these phases coincide with a strong storm system generated in a La Niña pattern, a significant blizzard could result. However, if the AO and NAO are in a positive phase when a storm system develops, there could be less cold air and potentially more liquid precipitation. This was the cause of the current winter with bottled cold air in Northern Canada. Last winter, the AO and NAO were in negative phase, allowing cold air to enter the region and causing more snowfall instead of liquid precipitation.
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Updated on 02/26/2012