The tropical Pacific has has been experiencing bursts of convection that are showing up nicely in the daily contributions of the SOI (Southern Oscillation Index). Note how the values on the daily's are all over the board going from negative to positive on a regular basis the past 30 days.
You can see in Cedar Rapids how that's caused significant spreads in temperatures relative to normal. Despite the fluctuations readings are running only about 1/2 degree below normal for the month.
From what I can see on the MJO (Madden Julien Oscillation) which is tied to the convective energy measured by the SOI, the trend towards cooler than average will continue across the central Midwest. Here's what the MJO forecast looks like the rest of August on the EURO.
If you follow the dotted green lines in the box on the left, you can see how the MJO is forecast to pass through phases 2 and 3 between now and September 2nd. To the right you can see the temperature anomalies associated with those phases by following the red arrows. My area is well into the cooler regions, especially during phase 3.
If the operational EURO is in sync with its own MJO, it should also show the trend to cooler weather. Sure enough, you can see below the major trough the EURO is carving out over the Midwest August 29th.
This is the temperature departure for August 29th. A big surge of pre-fall chill.
Some temperatures are shown as much as 25 degrees below normal August 28th in Nebraska and South Dakota.
Originating in the Arctic this air is really dry with dew points down around 40 in spots.
That results in lows that are comparable reaching the low to mid 40s the morning of the 29th.
I have a strong suspicion that much of September may end up on the cool side too. However, that's a tougher call due to the long range aspect of the forecast. I'm much more confident in saying the rest of August will get the short end of the stick. We'll know soon enough! Ride low....TS