Spring and Summer Outlooks for Iowa – 2017

Introduction

Now that we’re finally exiting the winter months and well on our way to spring, you’re probably wondering what might be in store for the remainder of spring and summer.  While seasonal forecasting is far from perfect, we can often times get a feel for what may occur based on teleconnections.  What is a teleconnection you ask?  Good question.  In atmospheric sciences, teleconnections are anomalies that have shown a relation to each other.  A great example of a teleconnection that many have heard about is El Nino, or more properly, El Nino-Southern Oscillation (ENSO).  For those that have seen previous seasonal outlooks from our blog, you are probably already familiar with what ENSO is.  For those that are not, take a peek below.

ENSO refers to a phenomenon that exists along the equatorial Pacific Ocean.  It is categorized as El Nino or La Nina depending on the type of anomaly or Neutral if there is no anomaly or a very weak one.  More specifically, El Nino is characterized by anomalously warm seas surface temperatures that propagate eastward towards the South American coast.  Conversely, La Nina is characterized by anomalously cool sea surface temperatures off of the coast of South America that propagate into the central Pacific Ocean.  The oscillation between the two is ENSO.  Do determine whether or not El Nino or La Nina conditions exist, the Oceanic Nino Index (ONI) is used.  The ONI is a measure of above and below normal sea surface temperatures within specific regions of the equatorial Pacific.  For an event to be declared, 5 consecutive over-lapping 3 month periods must be anomalous one way or another by 0.5 deg C or more.  El Nino would be for positive anomalies and La Nina for negative ones.  Anything that does not meet the 5 consecutive period criteria or is within +/- 0.5 deg C is declared Neutral.  Note the difference between conditions and event.  Conditions exist during any 3 month averaging period, while an event takes the 5 consecutive over-lapping 3 month periods mentioned previously. 

At this point, you may be wondering what Pacific Ocean sea surface temperatures have to do with weather in the in the United States, let alone Iowa.  The atmosphere is constantly in flux, responding to various inputs such as ENSO conditions in the Equatorial Pacific in an attempt to reach equilibrium.  Phenomena that last long periods of time and occur regularly can affect atmospheric conditions in ways that lead to tendencies in other areas, teleconnections.  A number of phenomena across the globe have been researched and found to correlate to weather patterns in other locations across the globe, and ENSO is the most researched and well known teleconnection for the United States.  We’ll proceed to take a look at the tendencies for forecasted neutral to El Nino conditions in Iowa for the upcoming Spring and Summer seasons.

For more detailed information about El Nino and La Nina, atmospheric conditions, thresholds, U.S. impacts, global impacts, and more, check out: https://www.climate.gov/enso

For more detailed information about the Oceanic Nino Index, check out: http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml

Spring

Currently, Neutral conditions exist within the Nino 3.4 region, having just come out of relatively weak La Nina conditions through winter.  Official ENSO probabilistic forecasts indicate a likely transition towards El Nino conditions within the Nino 3.4 region as spring and summer progresses.  Though focusing on Spring alone, forecasts heavily favor Neutral conditions, which makes sense in the short term considering the region just came out of La Nina conditions.

(Image 1: Official probabilistic ENSO Forecast from the Climate Prediction Center and International Research Institute for Climate and Society)

Noting the heavy lean to neutral conditions within the forecast, we will focus strictly on historical Neutral events for Iowa and surrounding areas during Spring.  It should be noted that with conditions freshly coming off of a La Nina event, atmospheric response, especially as it pertains to the United States, may be in transition throughout Spring as neutral conditions set in.  Historically, temperatures during Neutral conditions have leaned on the cooler side of average for the Upper Midwest and Upper Mississippi Valley’s with Iowa on the southern edge and with only minimal cool departures (Image 2: Temperature Anomalies (F) for previous Neutral events during the Spring).  The analysis for precipitation points towards above average precipitation throughout much of the Mississippi Valley and Upper Midwest, including the entire state of Iowa.  Once again, though, the state finds itself on the lower end of the departures (Image 3: Precipitation anomalies (in) for previous Neutral events during the Spring). For both temperature and precipitation, the largest departures are well away from Iowa, predominantly centered on the south and west. 

Comparatively we’ll take a quick look at the official Climate Prediction Center (CPC) outlooks for Spring as well.  As it pertains to temperatures, the CPC suggests a slightly better opportunity for above normal temperatures versus normal or below normal through Iowa (Image 4: Spring temperature outlook from the Climate Prediction Center).  Looking at precipitation, the CPC indicates a slightly better opportunity for above normal precipitation over northern Iowa, while central and southern Iowa has equal chances for above, below, or normal precipitation (Image 5: Spring precipitation outlook from the Climate Prediction Center).  While the CPC precipitation forecast roughly resembles a nod towards the Neutral analysis, temperatures do not.  Which possibly indicates lingering La Nina influence as the atmosphere continues to respond to incoming Neutral conditions or the influence of other phenomena and atmospheric patterns being taken into consideration.

Summer

Now with summer a couple of months away yet, a bit more uncertainty comes into play for the ENSO forecasts.  Recall the forecast at the beginning showed increasing likelihood of El Nino conditions setting in, but hovered around 40-50%, with Neutral conditions taking up much of the remaining likelihood.  With that in mind, we’ll take a quick look at what both Neutral and El Nino conditions have resulted in historically.  

Examining Neutral conditions first, there is no strong tendency for above or below average precipitation within the state, let alone the surrounding states (Image 6: Precipitation anomalies (in) for previous Neutral events during the Summer). Conversely temperatures do show a tendency to be above average throughout most of Iowa and much of the United States for that matter (Image 7: Temperature anomalies (F) for previous Neutral events during the Summer).   The best tendencies for precipitation reside through the Appalachian region and for temperature the southwest US.  

On the El Nino side of the equation, precipitation tendencies show up within the state and surrounding areas, however they are a bit of a mixed bag of both above and below average (Image 8: Precipitation anomalies (in) for previous El Nino events during the Summer).  On the temperature side, history favors below average temperatures throughout Iowa and surrounding areas, which opposite that of historical Neutral conditions (Image 9: Temperature anomalies (F) for previous El Nino events during the Summer).  In the El Nino case, it is hard to pinpoint the best tendency for precipitation within the United States given the mixed responses, whereas the northern Great Lakes region shows the best tendency for temperatures.   

Once again to give some perspective, we’ll take a quick look at the Climate Prediction Center forecast for the summer months.  At first glance, both the precipitation and temperature summer outlooks from the Climate Prediction Center loosely resemble the Neutral conditions depicted above versus the El Nino ones (Image 10: Spring precipitation outlook from the Climate Prediction Center) (Image 11: Spring temperature outlook from the Climate Prediction Center).  Recall once again that the atmosphere takes time to respond to changes such as ENSO.  Should El Nino make an appearance it will have to traverse through Neutral conditions first, remember we just came out of La Nina, on its way to El Nino.  So void of other influences, it would make a degree of sense that the atmosphere may still be responding to Neutral conditions even if El Nino creeps in.  Of course, ENSO is not the only teleconnection that influences our weather conditions.  Others that have been and are continuing to be researched include the North Atlantic Oscillation, the Madden-Julien Oscillation, the Pacific Decadal Oscillation, and more.  In future editions we may explore how some of these other teleconnections can affect Iowa and the United States.  Until then, we will see how spring and summer play out and whether or not ENSO tendencies appear.

Blog post by: Allan Curtis, Meteorologist Intern