VISIONS OF FALL SUGARPLUMS

Temperatures were down some Tuesday, but as expected, a reduction in humidity lagged well behind. That issue will be corrected in time as drier air slips into the region later Wednesday and Thursday. Along with that welcome news, will be the fact temperatures will cool even more, going from the range of 80 to 84 Wednesday to 78-82 Thursday. That's just a hint of what's to come late weekend and next week. More on that big news in a minute.

First, I came across an interesting graphic from the Iowa Mesonet that uniquely encapsulates our weather pattern the last 6–8 weeks. The graphic below represent a time series of 14 day standardized departures of average temperature and precipitation for the Quad Cities. They are plotted every 7 days beginning April 28th. There are 4 quadrants and the upper right one with the red arrows represents (above average precipitation and temperature departures). What's shown is that from June 25th to August 11th, the Quad Cities were squarely in the upper right quadrant, indicating a prolonged period of heat and wet weather. We're all are aware of it, but It's cool to see it quantified in such a way.

The data the plots were generated from is broken out on the right. Average 7 day temperature and precipitation is expressed in sigma units, along with the dates. From June 9th to August 11th, 10 consecutive weeks were above normal in the Quad Cities with this approach. Additionally, 6 straight weeks saw above normal precipitation.

Going forward, plots will be shifting to the lower left quadrant as below normal precipitation and temperatures look likely much of the next 2 weeks. This occurs as a pattern change emerges, highlighted by NW flow and the development of an east coast trough. You can watch it unfold in this animation of the 500mb jet stream flow. You can also see hurricane ERIN moving off the east coast, clearing the path for the upcoming trough.

With the initial energy diving into the region Friday, a quick emergence of moisture occurs ahead of the cold front that brings the cool change. That may allow some scattered showers and storms to develop ahead of it as it sweeps south Friday night. However, the daylight hours look dry and seasonal, with highs upper 70s to low 80s.

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FALL IS KNOCKING ON THE DOOR

Saturday is a transition day and with it will come with some passing clouds. Temperature will begin to turn cooler and humidity will steadily fall. Highs should remain in the mid to upper 70s near and north of I-80, with low 80s elsewhere.

Sunday, cold air advection is in full swing, with highs of 65 north to 70 south. Cold air aloft should generate some passing clouds and perhaps some light showers, especially in the north. Brisk winds will make it feel pretty fresh, with dew points barely above 50.

Monday morning, make sure the windows are closed, particularly if the GFS is right. It sends lows into the 40s areawide, with some down to 45. That's getting very close to records. Dew points will also dip into the 40s, a place they haven't been for quite some time. I've got visions of fall sugarplums dancing in my head.

The cool temperatures are shown on the GFS for much of the next 2 weeks. Here's the GFS meteogram. It indicates highs in the 60s and 70s, 11 of the next 13 days, nothing higher than 80! That's impressive considering normal highs are still around 80 much of that period.

The National Blend of all models shows good agreement, so this is a high confidence trend.

The Climate Prediction Center has significantly increased its chances of below normal temperatures along with below normal precipitation.

The EURO ensembles the next 2 weeks show precipitation well below the mean. We can afford that trend with all the rain the past 6 weeks.

The bottom line is that we are in for a notable change as the shorter days and a lower sun angle are starting to do their thing. That said, summer is by no means finished and it will bounce back. Hopefully with diminished force next time around. Roll weather...TS