The talk all winter and spring was El Niño and how it would affect large portions of the United States. As spring ended and summer took over, El Niño conversations were quickly replaced with La Niña thoughts as conditions began to transition. Taking a step back, for those that are unfamiliar with the terms El Niño or La Niña, they are phenomena that exist along the equatorial Pacific Ocean. An El Niño event is characterized by anomalously warm sea surface temperatures that propagate eastward towards the South American coast. Conversely, a La Niña event is characterized by anomalously cool sea surface temperatures off the coast of South America that propagate into the central Pacific Ocean. The oscillation between the two is called the El Niño-Southern Oscillation (ENSO). For an event to be categorized as an El Niño or La Niña, the Oceanic Niño Index (ONI) is used. The ONI is a measure of above and below normal sea surface temperatures within a specific region of the equatorial Pacific. Anomalous values of 0.5 C or greater for 5 consecutive over-lapping seasons would be an El Niño. Conversely, values of -0.5 C or less for 5 consecutive over-lapping seasons would be a La Niña. In between 0.5 C and -0.5 C or for periods that do not meet the 5 consecutive over-lapping seasons, would be categorized as neutral.
At this point, you may be wondering what Pacific Ocean sea surface temperatures have to do with weather in the United States, let alone Iowa. The atmosphere is constantly in flux, responding to various inputs such as El Niño or La Niña 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, known as 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 El Niño and La Niña are probably the most researched and well known teleconnections. We’ll proceed to take a look at the tendencies in Iowa during neutral and La Niña conditions and how they compare to the current seasonal outlooks from the Climate Prediction Center.
For more detailed information about El Niño and La Niña, atmospheric conditions, thresholds, U.S. impacts, global impacts, and more, check out: https://www.climate.gov/enso
For more detailed information about the Oceanic Niño Index, check out: http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml
As hinted at above, El Niño conditions existed through the winter and spring months, but weakened into summer and are forecast to continue into at least neutral conditions and possibly transition into La Niña conditions through the fall months (Image 1).
First, focusing on neutral conditions, when looking at previous historical neutral events that occurred during the fall months, temperatures have a tendency to be cooler throughout the state (Image 2). On the other hand, precipitation does not have a strong tendency state wide, and instead is a mixed bag, indicating a weaker correlation (Image 3).
Should La Niña conditions set in, once again the strongest tendency is seen in temperatures, however this time it is for warmer temperatures throughout the state (Image 4). Then similarly to neutral, precipitation does not show a strong tendency, but does lean toward slightly drier conditions (Image 5).
While all of that is well and good, how does all of it compare with the Climate Prediction Center’s (CPC) official fall outlook? For their most recent fall outlook, temperatures nationwide show an increased chance for above normal temperatures (Image 6). The precipitation outlook shows no tendency for above or below normal precipitation for much of the United States, including Iowa (Image 7). When comparing to the neutral and La Niña tendencies above, the outlooks do not closely resemble either, possibly indicating low confidence or another teleconnection playing a significant role.
Shifting towards winter, forecasts show a likely probability of La Niña conditions setting in and that’s what we will focus on here. La Niña has a pretty strong signal across the United States, however Iowa is caught in the transition zone of above normal temperatures to the south and below normal temperatures to the north (Image 8). Precipitation, also has a decent signal across the United States, but once again Iowa does not fall within an area of anomalies one way or another (Image 9). Instead, the best signals are found through the Ohio River valley, Pacific Northwest, the Southeast, and California.
When compared to the CPC outlook for winter, striking resemblances are immediately seen. The strongest resemblance is seen within temperatures, where the CPC highlights the Southeast and South for above normal temperatures and the North for below normal temperatures (Image 10). Strong resemblances are also seen in the CPC outlook for precipitation, where the Ohio River Valley and Pacific Northwest are highlighted for above normal precipitation, and the Southeast, South, and California for below normal precipitation (Image 11). Unfortunately for Iowa, with the seeming strong tendency for the CPC outlooks to lean towards La Niña conditions this winter, the CPC does not depict strong feelings one way or the other for precipitation or temperatures.
Blog post by Allan Curtis, Meteorologist Intern, NWS Des Moines
The statewide average temperature for July 2016 was 73.0°F which was 0.6°F below normal (See Figure 1). July 2016 ranks as the 50th coolest July on record out of 144 years of statewide climatic records. The monthly average temperature at Des Moines was 76.8°F or 0.5°F above normal with the highest temperature coming in at 97°F on the 21st and lowest temperature at 58°F on the 1st. Waterloo’s monthly average temperature was 72.8°F or 0.8°F below normal. The highest temperature in Waterloo for the month of July was 93°F on the 21st and 52°F on the 3rd.
Through the 4th of July weekend, cooler than normal temperatures prevailed across the state (See Figure 2). In fact, a few locations recorded low temperatures in the 40s on the morning of the 3rd (See Figure 3). Estherville, Elkader, and Cresco all had low temperatures at 47°F on July 3rd. Then the hot and humid conditions developed and dominated throughout much of the remainder of the month. There were a few days peppered into the last 3 weeks of the month when cloud cover and thunderstorms kept temperatures below normal. Regardless, the hottest stretch occurred from the 17th to the 27th. The hottest temperature of the month throughout the entire state was 97°F on the 20th at Sioux City and at Des Moines and Lamoni on the 21st. Much of the state had heat index readings well over 100°F between the 20th and 23rd (See Figures 4 & 5) due to the high humidity and dew points.
The monthly statewide precipitation total for Iowa during the month of July 2016 was 6.13 inches (See Figure 6). This was 1.63 inches above normal, resulting in the 16th wettest July on record among 144 years of statewide climatic records. Des Moines totaled 6.98 inches for the month, with the bulk of that total occurring on the 19th when 3.53 inches of rain fell. This daily total nearly doubled its monthly total up to that point. In fact, only 0.02 inches of additional rainfall accumulated the remainder of the month, but Des Moines was still 2.51 inches above normal for the entire month. Waterloo had a different outcome with 4.01 inches or 0.90 inches below normal for July.
After a dry first five days of the month, an active weather pattern developed and brought a multitude of precipitation through the 20th. The statewide average from July 6th to the 20th was 4.86 inches or 2.63 inches above normal during that stretch (See Figure 7 & 8). The heaviest rainfall occurred from the southwest to north central to northeast portions of the state. Enough rain fell over central and southern Iowa to relieve some of the drought conditions except for a small area in far south central Iowa (See Figure 9).
The statewide average temperature in May 2016 was 59.5°F which was 0.6°F below normal (See Figure 1). This ranks as the 65th coolest May on record out of 144 years of statewide climate statistics. Des Moines’s average temperature was 63.0°F or 0.7°F above normal while Waterloo’s monthly average temperature 59.9°F or 0.6°F below normal.
A cold snap occurred during the first five days of May as a few spots reported light frost on the mornings of the 2nd, 3rd, and 5th (See Figure 2). However, by the second week of May, temperatures were above normal with a few locations topping 90°F for the first time in 2016 in Iowa. Marion, Guttenberg, and Hawarden reached 90°F on the 6th. More cool temperatures filter into the state from the 12th to the 21st. In fact, freezing temperatures were recorded on the 14th, 15th, and 18th with the coldest day and most widespread frost occurring on the 15th (See Figure 3). Cresco, Decorah, and Elkader all recorded 27°F low temperatures on the morning of the 15th. The remainder of the month, temperatures finished above normal with a string of 10 consecutive warmer than normal days. The hottest temperature was 89°F in Little Sioux and Lamoni on the 30th.
The statewide average precipitation for May 2016 was 4.64 inches or 0.08 inches above normal. This ranks as the 52nd wettest May among 144 years of statewide climate records. Des Moines was 0.62 inches below normal for the location’s monthly precipitation as it totaled 4.12 inches. Waterloo totaled 3.24 inches of rainfall for the month of May, which was 1.29 inches below normal.
The first day of May was cool and wet before generally dry weather prevailed from the 2nd to 8th. From the 9th to the 13th, a wet period settled into the state with several weak systems that brought light to moderate rainfall. The highest precipitation accumulated over southwest Iowa (See Figure 4). A couple of brief tornadoes were spotted in Davis and Guthrie Counties County on the 9th, mainly damaging crops and trees. Another relatively dry period dominated the middle of the month from the 14th to the 22nd before a fairly active weather pattern developed to finish off the month. Severe weather plagued the state every day during the final week of the month with the most active night occurring on the 25th. Widespread damaging winds along with some large hail were reported in 19 counties across the northern two-thirds of Iowa. The dries weather was found over central to southeast Iowa, while the bulk of the monthly precipitation accumulated over northwest to southwest Iowa (See Figure 5). The highest monthly precipitation was 10.75 inches at Atlantic while the lowest monthly total was 2.19 inches in Albia. In fact, Atlantic’s total was its 2nd highest total out of 130 years of records. The highest total is 12.37 inches set in 1903.
Blog post by Kenny Podrazik, Meteorologist, NWS Des Moines
The statewide monthly average temperature was 73.2°F or 3.5°F above normal (See Figure 1). June 2016 ranks as the 14th warmest June among 144 years of statewide climate records. A warmer June was last recorded in 1991. The average temperature at Des Moines and Waterloo was 77.3°F and 71.9°F respectively. Des Moines was 5.5°F above normal while Waterloo was only 1.9°F above its monthly average. On the 15th, the hottest temperature of 96°F was recorded for the month at Des Moines while the coldest temperature was 56°F on the 2nd. The hottest temperature at Waterloo was 95°F on the 10th while just two days earlier the low temperature dropped to 46°F.
Above normal temperatures prevailed for the majority of month of June across the state. A long hot spell occurred from the 9th to the 20th when the hottest temperature of 100°F was recorded at Little Sioux on the 11th (See Figure 2). This 100°F ended up being the hottest temperature for the entire state during June 2016 and was Iowa’s first triple-digit temperature since September 10, 2013. The state’s second longest streak without a 100°F reading ended on the 11th with 1,004 consecutive days. The longest streak is 1,438 consecutive days from August 3, 1991 to July 9, 1995. The maximum temperature at Des Moines from the 9th to the 27th was 85°F or higher for 19 consecutive days in June. This became the longest streak of its kind during the month of June since 1878 at Des Moines. The previous longest stretch was 17 consecutive days set in June 1956.
The statewide average precipitation was 3.69 inches which was 1.33 inches below the state’s normal for June (See Figures 3 & 4). This ranks as the 45th driest June out of 144 years of statewide climate records. The monthly totals at Des Moines and Waterloo were 1.47” or 3.47” below normal and 8.97” or 4.98” above normal. Des Moines recorded its 10th driest June since 1878 and Waterloo recorded its 4th wettest June since 1895. This was common throughout the state as the precipitation was highly variable (See Figure 5). For example, Cedar Falls had a whopping 11.22” while Salem only recorded 0.25” for the entire month. By the end of the month, much of south central to southeast Iowa were placed into Moderate Drought by the U.S. Drought Monitor with much of the southwest to western part of the state classified as Abnormally Dry (See Figure 6). Severe weather was limited throughout the month of June, even though June is Iowa’s peak severe weather month. In fact, the first half of 2016 was fairly “quiet” with respect to severe weather (See Figures 7a/7b). The most active day came on June 14th when several central to northern Iowa counties reported wind damage. Heavy rain and damaging winds plagued a few counties during the evening of the 25th into the early morning on 26th (See Figure 8). Flooding was also limited during the month with only 1 river reaching flood stage and just a handful of Flash Flood Warnings issued by National Weather Service Office in Des Moines.
Blog post by Kenny Podrazik, Meteorologist, NWS Des Moines
It has been a slow fire weather season. The spring was cool and damp and warm season grasses were a little slower to green up. As we transitioned into June we did warm up and were dry but grass fires, or at least grass fires that were known by the NWS, were minimal. As we went into July, we continued to have hot spells though any period of heat through mid July was both preceded and followed by a cooler spell which again helped keep grass fires in check.
As we roll into fall we will be preparing for the fall fire weather season which will begin on September 1st. Beginning September 1st we will be issuing the Fire Weather Planning Forecast twice a day…at 6 am and by 4 pm daily. We will also be alert to the potential for red flag warnings as we head into harvest season then late fall. In addition, we will use crop curing as a guide to fire danger during the months of September and October and transition to prairie grass curing in later October through mid November. We have found that using these two different fuels in the fall better assesses the fire threat and will alert people to the proper threats. The threats will be displayed in the form of a state map on the fire weather web page of our website. Please check out http://www.weather.gov/dmx/fire for the latest fire weather forecasts.
Assessment of prairie grass curing is provided by local County Conservation Board employees. They provide the National Weather Service with curing values of prairie grass on a weekly basis through the entire dry down of the grass and I would like to take this time to recognize their efforts in helping us to provide the most accurate forecast and warning service possible.
As we head into the fall season I would like to remind everyone to heed forecasts and warnings of dry conditions. Harvesting in extreme dry and windy conditions is the number one cause of crop fires in the fall and crop fires by far exceed any other cause of fall fires.
Blog Post by Frank Boksa, Meteorologist, NWS Des Moines
Temperatures have remained warmer than normal for much of the spring into the first half of the summer. The warm temperatures have been interrupted from time to time by cooler periods, especially in May when monthly temperatures averaged just below normal, but for the most part readings have remained warmer than normal. The warmest readings have been across western Iowa with a couple periods of intense heat during June and July with heat index values going above 110 degrees during the third week of July.
Since the beginning of March, precipitation has been most robust and reliable along the Minnesota border into east central Iowa as well as the far southwest corner of the state. Much of the remainder of Iowa has seen at or below normal rainfall, especially in central into south central and southeast Iowa. The month of June was particularly dry in these locations with some areas remaining below one inch of precipitation or only about a quarter of normal. Recent storms across much of Iowa in July have helped to alleviate the incipient drought conditions in the state, although much of the rain was quickly used by maturing crops. The map shows the percentage of normal precipitation from the March through July time frame.