Analyzing The Large Scale Pattern: April & May 2011

Introduction
The synoptic, or large scale, weather pattern during April and May was supportive of the record-breaking rainfall over a broad section of the lower Midwest and most of Ohio. This large-scale pattern during each month was remarkably persistent and promoted a steady stream of storm systems across the region. The imagery and discussion that follows will present data collected during April and May 2011 at the surface, 850 mb, 500 mb and 300mb and compare it to the long-term mean observations for all Aprils and Mays (from 1968 to 1996) collected at the same levels. This analysis will show that in addition to being perfect setups for significant rainfall across a broad swath of the country's mid-section, the synoptic patterns during the two months represented a relatively significant deviation from the long-term height averages.

Before we turn to the analysis, a brief word about mean charts is in order. A mean chart is simply the average of the particular parameter being presented over the specified time period. As was shown on the series of 500 mb height charts on the background page, meteorological parameters vary over a single day and significantly over the course of a month or more. A chart that displays the mean over the course of a month or more tends to mask the daily extremes in order to present the overall mean value for the period. As a result, comparisons of mean charts over longer periods have a tendency to show subtle, but important, differences instead of the dramatic differences that might be expected from a comparison of two different days. The charts presented on the following pages show mean data and do not represent the conditions on any specific day in April or May 2011.