Prepared by: Craig Cogil, Lead Forecaster – NWS Des Moines
El Niño conditions currently exist across the equatorial Pacific Ocean and are expected to persist into the upcoming winter. These conditions are characterized by above normal sea surface temperatures near the equator in eastern and central portions of the equatorial Pacific Ocean. While these conditions exist many thousands of miles away from the United States, impacts from El Niño can be observed in the continental United States including Iowa. El Niño and La Niña conditions are determined by the Oceanic Niño Index (ONI) calculated at the Climate Prediction Center. For more information on the ONI, please click here. According to the ONI definition, water conditions meeting and exceeding the 0.5C threshold for 5 consecutive over-lapping seasons would be considered an El Niño.
Impacts to both temperature and precipitation become apparent across the United States, including Iowa when El Niño years are compared to the average conditions. This paper will look at the historical impacts in Iowa from late fall into spring during El Niño years versus the historical average from 1951-2010. During this time, eighteen El Niño events occurred of varying strength. Generally, events that range from ONI 0.5-0.9 are considered weak El Niño’s with moderate events having an ONI of 1.0-1.4. Strong events have ONI values of 1.5 or greater.
The following image shows temperature departures from the long-term average (1951-2010) of all El Niño events from 1950-2014. Temperatures anomalies across Iowa average from around 0.5 to 1.5 degrees above normal during the winter months with northern Iowa having the warmest departure. However, not every year has above normal temperatures. For both Des Moines and Waterloo, 8 out of the 19 El Niño years had below normal readings or 42% of the time. In fact, several very cold winters occurred during El Niño episodes including the winters of 1976-1977, 1977-1978 and more recently 2009-2010. In other words, El Niño does not guarantee a warm winter, but tends to shift the probabilities in that direction.
The next image shows the temperature anomalies from the strongest El Nino’s since 1950. An even stronger signal for above normal readings is apparent with Iowa temperatures ranging from about 2.0 to 3.0 degrees above normal. 5 out 7 years were above normal at both Des Moines and Waterloo or 71% of the time.
The next two graphs look at temperature departure for Des Moines and Waterloo respectively during El Niño cool seasons. The temperature averages are divided into three different categories: all El Niño events, only moderate and strong El Niños and finally just strong events. These temperature averages are then compared to the 1951-2010 normal average temperatures for the given three month period to come up with the departures for each category. These graphs appear to indicate that as the strength of El Niño increases, there is a greater threat for a warm departure during the winter months with a greater departure in northern Iowa than southern Iowa. The historical frequency of warm winters is also greater for strong El Niños at 71% versus 58% for all events at both sites.
The next image indicates that precipitation across the county also has a distinct pattern during El Niño. The southern United States sees above average rainfall along with portions of the High Plains and the East Coast. The Pacific Northwest, the Lower Ohio and Tennessee River Valleys generally see below normal precipitation. Iowa generally sees near normal precipitation in the west with slightly below normal precipitation in the east when all El Niño episodes are considered.
The next image below is for strong El Niño cases (ONI > 1.4) only. This indicates a stronger historical correlation for higher precipitation across the southern half of the Midwest. California, the Deep South and the East Coast all continue to have strong indications of above normal precipitation as well. The drier than normal areas remain in the Pacific Northwest and the Ohio River Valley. Iowa certainly trends wetter during the strong episodes with the highest departure across the south.
The following graphs are similar to the previous graphs but are for precipitation departures in Des Moines and Waterloo. Both graphs show the precipitation departures for all El Niño events, moderate and strong events combined, and finally only strong events at both sites. A wet departure is seen at both Des Moines and Waterloo in fall and into early winter. However, the departures disappear later in Winter with a negative departure developing. This is especially true in Des Moines the following spring.
The historical frequency for wet conditions in the fall of all El Niño events for Des Moines is 53% and 68% in Waterloo. This jumps to 86% for both locations when considering only strong El Niños. The historical frequency of wetter than normal winters is 47% in Des Moines and 53% in Waterloo for all events. However, when looking at only strong El Niños, the frequency increases to 71% for both sites.
Quick Facts about El Niño in Iowa:
- Moderate to strong El Niño events pose a greater chance of affecting Iowa weather compared to weak events.
- The meaningful impacts in Iowa are most common from fall through spring.
- Moderate to strong El Niño’s increase the probability for warmer conditions during the winter with the best threat in northern Iowa.
- Moderate to strong El Niño’s increase the probability for wetter conditions in the fall and to a lessor extent into the winter.
- Moderate to strong El Niño’s have a greater chance of seeing dry conditions during the spring.
The upcoming seasonal outlooks for the nation can be found at the Climate Prediction Center website.
For the specific 3 – month outlooks across the nation, click here.
For local temperature outlooks for central Iowa, click here.
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