Today's IEM Daily Feature chart likely needs a bit more discussion. Here's the featured chart:
So not all rain events are created equal. Take for example this analysis of precipitation on 11 July 2017 using NOAA MRMS data:
The analysis places a good chunk of southern Iowa picking up a quarter to an inch and a half of rain. Did all of this area need to receive this rainfall? A mechanism to assess this is to produce a plot of recent precipitation departures, lets say over the past 31 days.
So we have a plot of one day's rainfall and another plot of precipitation departures over the previous 31 days. The highlighted chart in this news item above simply looks at the area that received at least 0.20 inches and partitions it by coincident departure area and expresses this overlap in terms of statewide areal coverage. Yeah, so that was a mouthful!
Using real numbers in the chart as illustration, prior to 11 July 2017 about 25% of Iowa was between 1 and 2 inches below average for precipitation over the previous 31 days. About 36% (value not depicted on chart) of this area (9.4% of Iowa / 25.7% of Iowa) received at least 0.20 inches on 11 July 2017. So you can then compare this to what happened on 10 July 2017. Here's the precipitation that day:
And here is the chart of that day's precipitation coverage along with the one from the 11th.
10 July 2017
11 July 2017
So which day's rainfall was more important? Of course, the answer is not simple, but comparing the two charts would put more value on the rainfall for the 11th as more rain fell over drier locations than the event on the 10th. The peak coverage for the 11th event happened over the 1-2 inch deficit whereas the 10th event was over the 0-1 inch deficit. On the other hand, the overall coverage on the 10th was larger and slightly larger over areas with deficits.
There is no easy answer here to the question and I am hoping to get some feedback on the usefulness of this type of chart for future refinement. You can generate this chart on demand via IEM Autoplot #182 for a Midwestern state, date since 2011, threshold, and trailing period of days of your choice.