Ag Weather Forum

By now, you have likely seen the extended forecast for next week shows a burst of cooler air working into the United States east of the Rockies. The drop in temperatures will bring sweet relief after the heat of this week. But how long will the mild weather stick around?

A much more complicated weather scenario is working out to be the case. What looked like a long and hot dry stretch for August has turned quite a bit. The background relating to the August forecast can be found here: https://www.dtnpf.com/…. But since that forecast was made last week, we saw subtle but very important changes in the upper-level pattern that will provide a burst of cooler air to work through the U.S.

A trough developing over Hudson Bay will be stronger than originally forecast last week and will challenge an upper-level ridge across the Rockies. An initial cold front will move into the Dakotas and northern Minnesota Aug. 4. It will stall there briefly but then get pushed southward by another system moving through the Northern Plains Aug. 5 and then across the Great Lakes for Aug. 6.

The cold air pushing in behind this system should bring temperatures down a solid 10-15 degrees from the highs noted this week. In some areas of the Plains, the drop should be more dramatic, going from 100s down into the 80s. The real difference will be with the lower humidity and overnight low temperatures, which have been excessive in much of the Corn Belt and South this week. Overnight lows may go below normal, allowing for crops and livestock to get some stress relief.

Showers and thunderstorms will also follow the front southward, probably being more widespread than the clusters of storms we have seen so far this week. But with the front moving, areas that will see moderate or heavy rain are likely to be limited as the front drops south.

The American GFS model first started picking up on this cool burst late last week. The European model noticed a few days later. That is a common feature in the models. The GFS is first to see the change in the pattern, and the European develops it later. However, the GFS often gives some false positives that never turn out.

But as we look beyond Aug. 10, what do we notice in these models? Well, for one, the trough that settles into Hudson Bay is likely to give way next weekend, though it may be slow to work its way toward Greenland. It will be well-entrenched across much of Eastern Canada and the Eastern U.S. next week and may be slow to move out of there as well. Some leftover troughing features could hang out across the middle of the U.S. beyond Aug. 14. But both models are suggesting that ridging returns eastward into the Central U.S., turning the heat back up for the third week of the month. They both suggest the same sort of setup that is forecast for next week could return by the end of the month, creating a more up-and-down forecast across the U.S. than originally thought.

Also, the back-and-forth nature in the upper levels could lead to increased precipitation across the Corn Belt this month as well. As the ridge starts to redevelop in week three (Aug. 12-16), it could encounter another stalled front that may bring heavy rainfall and severe weather. And a return to a cooler pattern late in the month could still bring about frequent precipitation, though more focused in the east than the west.

But of course, models can and probably will change. Neither of these models showed the potential for a strong trough in Hudson Bay last week, and it has been hard to trust these models beyond the next 10 days. So, while the forecast for a hotter second half of the month is in the forecast, as well as the potential for another cool shot, the confidence in that is still a bit low.

As I mentioned in last week's blog linked above, the MJO was forecast to be in a steady state throughout most of August. That is still in the forecast and leads to a hotter look for the majority of the U.S. But that is not the only driver of the weather, as we will see next week. The other sorts of weather drivers have been difficult to trust this summer and will probably continue to be that way.

Even with these disruptions, though, the August temperature forecast continues to be above normal in most areas, though not as severe as it was last week. And precipitation should be mostly regular but potentially patchy. In other words, the longer-range forecasts you may see may not show the wide variability that may occur in August, even if the overall temperature and precipitation trends may be close to correct.

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John Baranick can be reached at john.baranick@dtn.com

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