Early Harvest Weather Recap
The transition into late summer and early fall weather has been a wild ride with one common theme for the state of Illinois – it has been dry.
Most of the state has seen anywhere between 10-60 percent of normal precipitation in the last 30 days leading into the first week of October, making this one of the top 10 driest ends to the growing season on record. Thankfully, the drier conditions for the last 45 days did not bring untimely heat stress, which prevented losses to corn and soybean crops.
- Nearly all navigable rivers in Illinois are very low creating issues with barge traffic and weight. For example, the Illinois River at the La Grange Lock in Brown County was reporting 6.1 feet; 17 feet below flood stage.
- August 2017 was the tenth coolest and nineteenth driest on the 123-year record.
- From Aug. 30-Oct. 1, much of Illinois had just two recorded precipitation events resulting in a quarter of an inch of rain or more.
- After a cool end to August, the first 11 days of September followed suit until a large ridge formed over Illinois from Sept. 12-26. Near-record heat with temperatures in the 90s was recorded most days.
- Early harvest progress was very fast as fields dried down at the end of September but brought the risk of field fires.
Harvest Weather 2017
Figure 1 shows Central U.S. has been very wet for the last 30 days thanks – in large part – to a broad trough that occupied the Western U.S. in late September. The end of the first week of October marks the beginning of a new active jet stream pattern that will bring more rain to the state of Illinois than we have seen over the last 30 days.
This will delay harvest efforts at times, but every day this month is not expected to be really wet. Expect a rapidly transitioning trough-ridge pattern, which will result in highly variable weather – unlike last October.
- Overall, the month will carry a slight warm bias for Illinois, but temperatures will be highly variable. A cool down is forecast for the second week of October. Expect weekly changes in temperature patterns rather than a persistent warm or cool pattern.
- Average first frost dates range from the second to the fourth week of October for Illinois. Any deeper trough in the jet stream will bring the potential for the first frost, but the latest forecast guidance does not support an early frost.
- We are watching for a stronger cold front to pass through Illinois around Oct. 11, which will take the state from above-average temperatures back down to more typical October weather.
- Oct. 4-8 will bring the first rounds of rain for the state in several weeks. Heaviest rainfall amounts will be concentrated in the northern two thirds of the state.
- The more active jet stream will bring weekly chances of rain, but remember that October is one of Illinois’ driest months.
- Overall, harvest weather is expected to remain favorable for farming operations.
- Over the last 100 years, Illinois’ fall temperature trend has been positive. This will lead to later first frost events and overall warmer weather October-December.
The last two winters had a very late start and temperatures were mild compared to average – especially last year when Illinois had its sixth warmest December-February on record.
A big player in Illinois winter weather is the phase of El Niño and La Niña. The development of a weak La Niña was shown this fall. As these slightly cooler-than-average sea surface temperatures occupy the equatorial Pacific Ocean during winter, Illinois historically has responded by having a cooler and snowier winter. La Niña is just one of the many factors we will be watching this winter.
- Expect an active freeze-thaw cycle.
- There is a very high probability that this winter will be colder and snowier than last winter.
- Watch for strong northwest flow at times, which will cause short-lived outbreaks of well-below average temperatures.
- During the winter, pay close attention to the phase of the Arctic Oscillation (aka Polar Vortex) and the phase of the North Atlantic Oscillation. These two features will control the timing and movement of the jet stream, which will determine Illinois’ weekly weather patterns throughout winter.
- Expect an active Ohio Valley storm track, which often puts Illinois into the ice, sleet, and snow regime of the large winter storms.
- La Niña tends to favor drier growing conditions in Brazil and planting delays have already occurred due to the dry start to their growing season. However, despite yield loss due to adverse weather, Brazil’s exponential growth in new farmland development will still lead to large production output.