La Nina and the Winter Outlook
With the approaching Winter Season coming up, the big questions that everyone wants to know are how much snow are we going to get and how warm or cold is it going to be. While long term forecasting can be highly variable and sometimes unpredictable, you can always look back on previous seasons to see what has happened in the past. With La Nina conditions developing in the eastern equatorial Pacific, this can help guide us in knowing what the average conditions will be over the winter season. Here’s a look into the past and what you can possibly expect over the next winter.
First, we’ll take a look at the developing La Nina and how long we can expect it to persist.
Your first question is, what the heck is La Nina and how does the Pacific Ocean affect our weather? Well, La Nina is the periodic cooling of the ocean water’s surface temperature in the central and east-central equatorial region in the Pacific Ocean and occurs about every 3 to 5 years. La Nina is somewhat the opposite of an El Nino year as the La Nina phase is the cold phase and El Nino represents the warm phase of the El Nino/Southern Oscillation cycle in the eastern Pacific. Here is an image showing current sea surface temperature anomalies in the Equator region of the eastern Pacific. Notice the large area of blue representing the cooling taking place:
Figure 1: Observed SSTs in the Equatorial Pacific
For it to be classified as a La Nina onset, you need to have a three month average of temperature anomalies less then -0.5ºC in the Nino 3.4 region. A La Nina episode is not official until there are at least 5 consecutive three month periods with sea surface temperature anomalies less than -0.5ºC. The onset conditions were expected to be reached with the July-August-September average. The images show the different regions of climate variability in the equatorial Pacific as well as the sea surface temperature anomalies in these regions, showing the distinct cooling that has taken place there.
Figure 2: Climate Variability regions in the Pacific. Nino 3.4 is used for monitoring El Nino/La Nina conditions
Figure 3: Sea Surface Temperature anomalies in the Nino regions.
These La Nina conditions are expected to last through at least through the early portion of Spring 2008, meaning that they will persist through our upcoming winter. These conditions are forecast to peak over this fall and gradually warm toward next spring. Here’s a look at what the forecast models are spitting out for the conditions.
Figure 4: Model forecasts of SST anomalies in the Nino 3.4 region
Now that you know more about La Nina than you probably ever expected to know, you’re probably asking yourself, what does this have to do with our weather in the Chicago area. Well, La Nina episodes feature a very wave-like jet stream flow over the United States and Canada, with colder and stormier than average conditions across the northern US, and warmer and less stormy conditions across the southern US. For us in northern Illinois and northwest Indiana, it typically means a lot more Alberta Clipper type of quick moving storm systems moving down from southern Canada with a high amplitude ridge off to the west and a deep upper level trough in the east. These type of storm systems typically give us a lot of minor accumulating (generally 2-5 inches) snowstorms, particularly even more so further north.
From looking at past La Nina years, the data shows that winter snowfall can be quite variable, with no real clear trend present. However, some of the largest winter snowfall seasons have occurred while La Nina conditions were present. In the 1973-1974 Winter season, Chicago experienced the 5th snowiest winter season (54.2”), while Rockford had its 4th snowiest season (47.3”). The ’73-’74 La Nina was one of the strongest episodes on record, with anomalies at one point reaching -2.0ºC. It actually was one of the longest La Nina events on record as well (data pertaining to the El Nino/La Nina cycle dates back to 1950), lasting from April-May-June of 1973 through April-May-June of 1976, with only one three month average briefly dropping below the -0.5ºC standard. In the 1961-62 winter season, Chicago had its 6th heaviest snowfall season (50.0”) and Rockford had its 10th snowiest winter (39.9”), albeit with a fairly weak La Nina. The largest calendar day snowfall in Chicago’s history fell on January 2nd, 1999 when 18.6” of snow fell while moderate La Nina conditions were present in the Pacific.
Here are comparisons of Snowfall, Temperatures and Precipitation during the past La Nina years at Chicago and Rockford. La Nina records date back to 1950. Data colored red represents data that is higher than the 1971-2000 NCDC normals, while data colored blue represents data that is lower than the normals.
|
La Nina Winters
Chicago
|
Snowfall
(28.3”-Normal)
Winter Season
|
Ave. Temps
(25.5º-Normal)
Winter Season
|
Precipitation
(5.81”-Normal)
Winter Season
|
|
1949-1950
|
24.9”
|
29.2º
|
13.79”
|
|
1950-1951
|
40.1”
|
23.5º
|
7.13”
|
|
1954-1955
|
21.2”
|
28.5º
|
5.11”
|
|
1955-1956
|
16.5”
|
27.9º
|
3.16”
|
|
1956-1957
|
24.3”
|
28.3º
|
5.09”
|
|
1961-1962
|
50.0”
|
23.3º
|
6.23”
|
|
1964-1965
|
36.4”
|
26.4º
|
7.72”
|
|
1967-1968
|
20.3”
|
27.2º
|
6.44”
|
|
1970-1971
|
22.0”
|
26.3º
|
5.21”
|
|
1971-1972
|
26.7”
|
28.2º
|
7.96”
|
|
1973-1974
|
54.2”
|
26.5º
|
9.67”
|
|
1974-1975
|
32.6”
|
28.7º
|
8.32”
|
|
1975-1976
|
30.3”
|
29.1º
|
6.05”
|
|
1983-1984
|
35.6”
|
21.8º
|
5.53”
|
|
1984-1985
|
38.8”
|
21.9º
|
7.86”
|
|
1988-1989
|
20.5”
|
26.6º
|
3.99”
|
|
1995-1996
|
16.1”
|
25.2º
|
2.88”
|
|
1998-1999
|
32.5”
|
30.4º
|
7.31”
|
|
1999-2000
|
28.7”
|
29.8º
|
6.09”
|
|
2000-2001
|
34.6”
|
22.2º
|
5.80”
|
|
Seasons Higher Than Norm
|
11/20
|
14/20
|
12/20
|
|
La Nina Winters
Rockford
|
Snowfall
(29.0”-Normal)
Winter Season
|
Ave. Temps
(24.4º-Normal)
Winter Season
|
Precipitation
(4.81”-Normal)
Winter Season
|
|
1949-1950
|
15.5”
|
26.0º
|
5.61”
|
|
1950-1951
|
30.3”
|
19.7º
|
5.59”
|
|
1954-1955
|
18.2”
|
23.8º
|
4.90”
|
|
1955-1956
|
11.1”
|
23.9º
|
1.97”
|
|
1956-1957
|
20.0”
|
24.4º
|
3.98”
|
|
1961-1962
|
39.9”
|
19.6º
|
4.82”
|
|
1964-1965
|
25.6”
|
21.4º
|
6.14”
|
|
1967-1968
|
7.2”
|
23.4º
|
2.50”
|
|
1970-1971
|
17.4”
|
22.1º
|
5.15”
|
|
1971-1972
|
13.6”
|
22.9º
|
6.30”
|
|
1973-1974
|
47.3”
|
23.1º
|
8.52”
|
|
1974-1975
|
37.0”
|
24.4º
|
6.04”
|
|
1975-1976
|
16.3”
|
24.7º
|
3.95”
|
|
1983-1984
|
29.3”
|
20.2º
|
4.33”
|
|
1984-1985
|
35.9”
|
20.5º
|
6.17”
|
|
1988-1989
|
26.4”
|
24.1º
|
3.17”
|
|
1995-1996
|
15.5”
|
21.7º
|
2.38”
|
|
1998-1999
|
15.5”
|
28.0º
|
5.17”
|
|
1999-2000
|
31.2”
|
26.5º
|
5.40”
|
|
2000-2001
|
35.7”
|
18.9º
|
7.26”
|
|
Seasons Higher Than Norm
|
8/20
|
4/20
|
13/20
|
As you can see, snowfall is fairly variable with about half of the seasons less than normal and the other half more than normal at both Chicago and Rockford. The most interesting fact that came out of this research is in the temperature data. At Chicago, La Nina years seem to be on the warm side with 14 out of 20 cases being on the warm side…while 90 miles to the northwest in Rockford, it is overwhelmingly colder with only 4 out of the 20 seasons being warmer than normal. One possible explanation for this may be due to station location. Prior to 1980, the official observation location was at Midway airport, which is closer to the lake and further into the urban heat island that is Chicago. Now, the official observation is taken at O’Hare, which is further away from the lake (which normally has a warming effect) and is likely cooler than Midway. The normal that is computed for Chicago is based on data out of O’Hare, so perhaps the trend should be that Chicago’s temperatures are normally colder, but this is masked by the station swapping. Both stations typically show an increase in precipitation during La Nina episodes, particularly at Chicago where the 13.79” of precipitation in the 1949-1950 winter is the highest winter total in its history.
The Climate Prediction Center is mirroring this precipitation assessment and is forecasting higher probabilities that the winter season will be above normal.
Figure 5: Winter Precipitation Forecast from the CPC
The forecast then deviates a bit for temperatures. With the high amplitude ridge in the west for a good portion of the winter, one would expect our region to be in a colder, northwesterly flow for a good portion of the winter. But with recent trends in temperatures, the CPC is going with a higher chance for above normal temperatures.
Figure 6: Winter Temperature Forecast from the CPC
One word of caution: Even with an above normal temperature forecast from the CPC, that doesn’t mean that we are not going to be cold or have extreme cold spells. It just means that the overall trend for the entire three month period may be above normal. So, perhaps low 30s for highs instead of mid 20s. It will be interesting to see how this season unfolds both snow-wise and temperature-wise.
Tim Halbach
Climate Focal Point
National Weather Service Chicago
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