Recap of the 2019 Growing Season
By Eric Snodgrass, Principal Atmospheric Scientist for Nutrien Ag Solutions
The state of Missouri has been at the epicenter of precipitation extremes during the last two growing seasons. Drought from Texas, Oklahoma, and Kansas spread east during summer 2018 before torrential rains returned during harvest. Winter 2018-19 followed suit with an active storm track that at times brought flooding rain, ice, and snow with each new low-pressure system that tracked across the US, especially in the latter half of winter. In late winter, a split in the Polar Vortex combined with a moderate strength El Niño to create an anomalously fast Pacific jet stream that led to repeated, large low-pressure systems that tracked from the Southwestern US, through Missouri, toward the Northeast, setting up Spring 2019 to be the 9th wettest April-June time period on record for Missouri.
Now drought free, 200 mph jet stream winds plus destabilizing atmospheric conditions and an abundance of moisture set up Missouri farmers to endure daily rounds of severe storms in May and June. In total, Missouri had 89 tornadoes, 192 reports of hail larger than 1 inch, and over 500 reports of wind damage. May 22 was a particularly nasty day tornadic supercells in southwest Missouri produced several tornadoes, one of which had 160 mph winds and did damage consistent with an EF-3 rating. The hook echo on the back of this supercell, seen on radar in Figure 2, followed a path eerily close the May 22, 2011, Joplin EF-5 tornado, which is one of the deadliest tornadoes in US history. Nearly fifty tornadoes were produced by these storms across the Midwest on May 22, 2019, making it one of the most active severe weather outbreaks of the year.
Spring rains continually plagued US corn and soybean farmers by repeatedly flooding lower ground near our river systems. At one point, over 600 river gauges monitored by the USGS (United States Geological Survey) were at or near flood stage – especially on the Missouri and Mississippi Rivers and their tributaries. Planting progress was the slowest for US corn in over 40 years. Figure 3 shows year-to-date precipitation departure from normal for the US. Some farms in western Missouri are off the charts having their wettest year on record with a surplus of 20 inches of rain!
So what caused 2019 to be such a wet year with above average severe weather? To answer that question we always look at the weather events that control the jet stream position and speed. A fast, unblocked jet stream in the North Pacific Ocean often enters the west coast of North America and then dips south over the Rocky Mountains before racing toward the east. This configuration leads to near perfect upper atmosphere support for the development of low-pressure systems, which are the parent storm systems for all of the severe weather experienced in the central US this year. A weak to moderate El Niño plus above average global wind speeds helped keep the Pacific jet stream on its breakneck pace through the end of June. This pattern started to break down in July as pressure centers changed over the central Pacific Ocean allowing for slower winds, an end to El Niño, and a more stagnant pattern in the jet stream. The result was a large ridge that formed over the southwestern US which changed the trajectory of storm systems over Missouri which now began to come from the northwest instead of the southwest. The problem is that northwest flow in the jet stream in summer across Missouri brings a lot of storms along with it. July-August 2019 again soaked the western side of Missouri and frequent thunderstorms dropped 2-4 inches of rain at a time.
Harvest Outlook
Fall is notoriously one of the most difficult times of year to predict weather patterns. As the jet stream slowly begins its southward retreat, low pressure systems become more frequent leading to larger daily changes in temperature. The forecast is further complicated by tropical cyclones that peak in frequency in mid-September and often track northward out of the Gulf of Mexico into the lower- and mid-Mississippi River Valley. The front half of the hurricane season has largely avoided the Gulf of Mexico, but ocean temperatures are well above normal and very supportive of tropical cyclone development. Notice in Figure 4 that water temperatures are 1-3°C warmer than average.
September 2019 started off with near average precipitation in northern Missouri but was very dry south. Mid-month rainfall was quite heavy in places due to a tropical disturbance and moved north through Texas and met a slow moving cold front over Missouri during Sept. 19-24.
Looking longer term, Figure 4 offers a few clues to predicting Fall temperature and precipitation patterns. Extending west off the coast of South America is a tongue of cooler water that emerged mid-summer. At first glance, we might be tempted to think a La Niña is forming for winter, but the odds of that happening are low due to the very large body of above average water temperatures in the North Pacific. Warmer than average water along the west coast of North America is part of a larger circulation called the Pacific Decadal Oscillation (PDO). It is rare to develop a strong La Niña while the PDO is positive.
Putting these pieces together for Fall to produce an accurate forecast is very difficult as the Pacific Ocean wind and temperature patterns are not in complimentary phases. In 2018, a developing El Niño added to a blocked jet stream that produced a large ridge over the Southeast. Storms fired along the periphery of that ridge for over three months while hurricanes Gordon, Florence, and Michael slid in underneath the ridge to produce widespread flooding in the east. Fall 2019 is not forecast to be as wet, but there are indicators that above average rainfall could continue at times across Missouri this Fall. Figure 5 illustrates this point and shows the forecast for precipitation anomalies for October, November, and December as predicted by National Multi-Model Ensemble. Long range forecast models predict near average to slightly above average temperatures for both October and November, but given the lack of a dominant feature in the global wind patterns, expect Fall temperature patterns to be closer to average.
Winter 2019-2020 Outlook
There are two primary features driving the long range forecast for winter. As discussed above, the warm north Pacific Ocean and cool central Pacific Ocean are at odds with one another, so let’s discuss the potential impacts of both. El Niño/La Niña have been studied extensively and looking back over the last 70 years we can identify winters with ENSO neutral (neither El Niño or La Niña) conditions and use those winters as analog years in our forecast. Figure 6 shows the precipitation patterns (top) and temperature patterns (bottom) during ENSO neutral winters (Nov. – Jan). Early winter temperatures tend to be slightly drier and colder due to more frequent troughs digging into the eastern half of the US.
The second part of our winter forecast discussion focuses on the warm water in the north Pacific (+PDO) and the forecast by the best global model – the ECMWF. November and December temperature and precipitation patterns are focused on a jet stream flow trajectory that will likely favor a western US trough and ridge over the southeast. This will take the jet stream right over Missouri for the start of winter and lead to a very active storm track and potentially above average precipitation. Figure 7 illustrates the forecast trend from the ECMWF model over the last two months and shows the very active central US storm track that could set up in November and December (unlike in 2018). Troughing in the jet stream over the western US often leads to storm track that follows the “Panhandle Hooker” track, which emerges from the panhandles of Oklahoma and Texas before tracking across Missouri. That particular track causes weather conditions to change rapidly for Missouri farmers and in mid-winter, produces a lot of ice and snow storms.
Seasonal forecasting is speculative at best. We are attempting to gather information on the primary drivers of our winter weather patterns, but in reality, sub-seasonal fluctuations in the jet stream dominate winter weather behavior. These features, like the Arctic Oscillation (AO), Madden Julian Oscillation (MJO), and North Atlantic Oscillation NAO) are not easily forecast beyond 2 weeks. Yet, they are the factors that ultimately determine the weather patterns for winter. Cold air outbreaks are often associated with the negative phases of the AO and NAO when the Polar Vortex is disrupted or weakened. Furthermore, the strength and position of the jet stream is often determined by the phase of the MJO. The MJO is also the dominant factor that controls the Brazilian monsoon which in turn controls rainfall amounts in Brazil’s prime growing regions. Speaking of Brazil, widespread drought during July-Sept. 2019 will likely delay the planting of the first crops beans which needs to be watched closely as harvest commences in the US.
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