The last week of September 2019 is looking very similar to the last week of September 2018 -- and that is not good news for the central U.S. Upper-atmosphere high pressure (a ridge) is indicated to hover above the southeastern U.S. just like we saw in late September a year ago. On one hand, the ridge presence is offering continued above-normal temperatures for the Eastern Corn Belt, which obviously are helpful in bringing the delayed corn and soybeans farther along toward maturity. Even crops that were planted deep into June, while not likely to send the yield monitor into Vegas pinball mode, will still have a chance to attain some semblance of decent production.
However (You knew this was coming, right?) the air flow around the upper-level high, going clockwise in the Northern Hemisphere, also carries in a healthy amount of moisture from the Gulf of Mexico, streaming into the Kansas City-Chicago corridor. If that's all that was going to happen, we'd have some muggy days, some scattered thunderstorms, and that would be the end of it.
Of course, that's not all that's happening. In late September, a cold upper-air trough is forming in the western U.S. and western Canada. That cold trough heads eastward toward the Plains and Midwest. But, the southeast U.S. high makes the trough stall out -- and a stationary cold-warm frontal boundary forms in the aforementioned Kansas City-Chicago corridor. This sharp temperature contrast helps drive thunderstorm and rain formation.
All this activity would have led to moderate to locally heavy rain. That's understandable. But, there's one more wrinkle to add in to the equation -- the tropical influence. Remnants of former Tropical Storm Imelda continue to float around the southeastern Plains, enhancing the Gulf of Mexico moisture inflow into the central U.S. (Imelda, of course, brought incredible heavy rain to the Texas coastal bend during the Sept. 17-19 time frame.)
And, there's even more tropical complexity. In the eastern Pacific, Tropical Storm Mario and Hurricane Lorena, around Baja California, are offering some input into this situation also. High-altitude eastern Pacific Ocean moisture is indicated to drift northeastward across Mexico and western Texas, and find its way into that stationary frontal boundary setting up over the central and north-central U.S.
The result is the last full week of September may bring very heavy, flash flooding rain, to the western and central Midwest. We saw this feature a year ago, and it was the beginning of a harvest season that, for a while, seemed like it would never end. What is worthy of much caution is that the timeline for the southeastern ridge to modify and lose its influence is not anything that we can count on. We could see this pattern for a while. Our DTN forecast maps indicate possibly more than six inches of rain over north-central Missouri in the seven-day time frame ending Friday, Sept. 27. And, while that may seem out of bounds, our maps also called for southeastern Texas to take in more than 20 inches of rain about a week ago -- and that forecast validated; if it was in error it was on the low side of the precip scale.
What a way to go into the end of the season.
Bryce Anderson can be reached at Bryce.firstname.lastname@example.org
Follow him on Twitter @BAndersonDTN
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