Terry’s Take: The flood of 2013-(the ingredients)
Last week was a classic setup for heavy rain across not only the immediate area but for much of eastern Iowa, northern Missouri, Illinois, and Michigan. When predicting and analyzing heavy rain, research and forecasting experience have shown the following elements are critical when determining where flooding rains may fall:
*REPLENISHMENT OF MOISTURE
*DURATION OF THE ABOVE FACTORS
All of these factors were established over the region from late afternoon on the 17th through the morning of the 18th. This also was coming on the heels of inherently wet grounds from rounds of rain earlier in the month (see bottom of page).
In regards to moisture content/concentration, here is a look at deep layer moisture (precipitable water) and 700 mb dew points. The atmosphere like a wash cloth can literally be “rung out”. In fact, precipitable water basically means if the atmospheric column above that point was ”rung out”, that is how much rain would fall. So why did we receive 4-8 inches of rain when precipitable waters were around an inch and a half? That’s because the moisture which condensed through lift and fell through precipitation was continuously replenished. This was accomplished through a low-level jet that stretched all the way from the Gulf of Mexico, which often is the case for heavy rain events in this area. One variable that can quantify the degree of moisture movement is moisture transport, which basically is the magnitude of moisture times the wind speeds. This is what it looked like at 850mb which is roughly (5,000 ft) up.
Note in the above image how the moisture transport quantities (shown in contours or vectors) is rapidly diminishing in a small amount of area across northern Illinois. This indicates strong moisture convergence which acts as a focus for moisture to pool. This focus was along a stationary front aloft, or a “battle zone” between the warmer air to the south and cooler air to the north. Such a focus not only helps to develop precipitation by causing convergence and lift, but also causes it to be persistent if the focus itself does not shift. Such was the case during the night of April 17th and into the morning of the 18th.
Critically important to push a heavy rain event into a major flash flood is rainfall rate. A key factor in this situation was the development of thunderstorms. Storms were plentiful, especially on the night of the 17th when over 10,000 cloud-to-ground lightning strikes were measured across the state of Illinois! Under these were heavier precipitation rates of an inch or more per hour.
The final ingredient which sent this event over the edge was the fact the ground was already saturated due to a wet pattern the first half of April, particularly during the 8th-11th. Below is a look at some of the precipitation totals during the first half of April.
With all these ingredients in place at such extreme levels, it’s no wonder record flooding was found on major rivers such as the Rock and Illinois.