The month of August 2013 featured well below normal temperatures and precipitation over a large portion of central, east central and southeast Illinois. Moderate drought conditions developed over parts of west central Illinois by late in the month, with most other areas, except southeast Illinois, experiencing abnormally dry conditions.
Figure 1 below is an overview of the upper air pattern at 500 millibars, or approximately 20,000 feet above the surface of the earth as the month of August began. A rather strong upper level low for early August was positioned near Hudson Bay. This brought an unusually strong northwest flow pattern across our area for the first week of August with periodic thunderstorm complexes developing along the periphery of the cool and warm air masses well to our southwest over parts of Kansas and southern Missouri. This is the area that received several episodes of heavy rainfall associated with large thunderstorm complexes as depicted in the 1 day observed precipitation graphic below (Figure 2) ending on the 5th of August. Much of the rainfall that occurred the first week of the month track over the same areas producing flash flooding.
During the second week of August, the upper flow pattern began to deamplify, or flatten out gradually as the unusually strong upper low that was near Hudson Bay began to shift off to the east (Figure 3 below). Our steering level winds were still out of the northwest but not as strong as what occurred during the first week of the month. Temperatures continued on the cool side over most of the area with the main complex of storms and what would be for us, beneficial rainfall, remaining well to our southwest (Figure 4). A cold front pushed across the area on the 11th and 12th producing scattered rainfall over parts of central, east central and southeast Illinois.
During the third week of August, the upper flow pattern continued to weaken over our area with the stronger flow shifting back to the north (seen in Figure 5 below). This resulted in some slight moderation in temperatures across the region. Further south over the Ohio Valley, a weak upper level disturbance was responsible for producing some scattered rainfall over parts of the area on the 22nd and early into the 23rd (Figure 6), but other than some isolated heavier rainfall totals, most areas did not see much in the way of any beneficial rainfall.
The upper flow pattern saw some significant changes develop towards the end of the third week and during the last week of August as a strong upper level high built north across the Midwest (Figure 7 below) bringing above normal temperatures to a large portion of the region. With such an expansive high in the upper levels of the atmosphere, the stronger wind fields, supportive of organized rainfall, shifted well north of the Midwest.
Summary for the month...
The weather across central Illinois featured below normal temperatures and precipitation (figures below curtousy of the Midwest Regional Climate Center). During the early portion of the month, northwest flow aloft kept the significant rains to our southwest with the pattern gradually changing by the middle of the month where the stronger flow shifted more to the north. Finally, by the end of the month, a strong upper level ridge brought above normal temperatures to a large part of the Midwest, but not enough to make up for the rather cool start to the month. Rainfall was hard to come by in most areas in August with only a few weather systems able to produce scattered showers and thunderstorms, but widespread heavy rainfall stayed mostly to the south and north of central Illinois. Peoria had their 2nd driest August on record (0.33"), and Springfield saw their 4th driest August ever (0.34").
August temperature departure from mean
Here are the monthly climate summaries for area cities. Only the summaries for Peoria, Springfield and Lincoln are considered "official", meaning they are the station of record for their respective locations. The other summaries are "supplemental", meaning another location in the area is the official climate station for that city.
Climate data for other area cities is available at http://www.nws.noaa.gov/climate/xmacis.php?wfo=ilx .