August 2007 and Summer 2007 Saint Cloud Weather Summary

Only Treading Water During This Growing Season's Lack of Water

Saint Cloud Weather Summary for August 2007 and Summer 2007

Issued: Tuesday, September 4, 2007 7:59 AM

For the first time in five months, the Saint Cloud area picked up greater than normal rainfall during August 2007. The rainfall at the Saint Cloud Regional Airport totalled 4.14 inches, 0.21 inch above the normal August total of 3.93 inches. The bulk of the rain was produced in three mid-August storms. On August 10-11, 1.46 inches of rain fell at the Saint Cloud Airport, the bulk of it during the early morning hours of the 11th (see summary of August 10-11 severe weather in central Minnesota and the rest of the severe weather reports from August 11 from the NWS Storm Prediction Center). The second round of heavy thunderstorm rain hit St. Cloud on the afternoon of August 13th, when 1.08 inch fell after an earlier line of storms produced straight-line damaging winds and hail hit a swath of central Minnesota just to the north and east of St. Cloud (see summary of August 13-14 Severe Weather from NWS Chanhassen). The third wave of thunderstorms produced 1.48 inches of rain over four days on August 18-21. This period of rain was the same one responsible for the August 18-20 Southeast Minnesota Flash Flood (report from Minnesota State Climatology Office) that produced a new 24-hour Minnesota record rainfall of 15.10 inches, recorded near Hokah in Houston County.

The heavy rainfall in Saint Cloud snapped a severe growing season rainfall deficit. Just before the rainfall, the growing season (April 1 on) deficit reached more than 6 1/2 inches on August 9, just before the first rain event happened. In fact, between June 4 and August 9, there were only 2.08 inches of rain, 6.11 inches below normal. The rainfall deficit was associated with very dry conditions for both agricultural and hydrologic interests. Nearly all of central Minnesota was classified as being in an extreme drought by the US Drought Monitor (see 12-Week Loop of US Drought Monitor from the National Drought Mitigation Center). This led to very low river flows in the lowest 10 percentile from northeastern Minnesota into the Twin Cities (see Lowest Streamflow from Minnesota DNR Waters). This included a 3.72 foot streamflow on the Mississippi River in St. Cloud on August 9 which ranks as the third lowest recorded streamflow as seen on the Record Mississippi River Low Flows at St. Cloud from the National Weather Service. Along the Sauk River, the flow reached 0.39 feet on August 9, which is the 4th lowest level seen on the Record Low Flows on the Sauk River from the National Weather Service.

In contrast, the St. Cloud Airport picked up 2.54 inches during August 11-14 (SCSU 3.14 inches) More amazingly, 1.53 inches of the St. Cloud Airport rainfall happened in a total of 35 minutes (we had .85 inch in 17 minutes between 2:32 and 2:49 AM August 11 and .78 inch in 18 minutes between 9:53 and 10:11 PM August 14): 3/4 of the 2+ month rainfall in a little more than half an hour. The heavy rainfall was caused by a change in the weather pattern for a two-week period in mid-August. For that period, most of central and southern Minnesota was located just along the cool side of the boundary between very warm and sticky air to the south and Canadian air to the north. Given that there were a series of storms during the week that pumped the sticky air over the cooler air and that the front tended to stall, persistent, slow-moving rain bands developed over a line from the Black Hills through South Dakota and Nebraska into Minnesota and Iowa, then into Wisconsin.

However, the heavy rainfall didn't make the dent that was expected in the rainfall deficit because St. Cloud returned to the very dry weather pattern. The very humid and warm air got pushed southward, so most fronts ended up passing through dry. During the last 10 days of August, only .11 inch fell, keeping the August rainfall surplus to less than one-quarter inch. This left central Minnesota in the severe drought category, as seen on the August 28 US Drought Monitor from National Drought Mitigation Center. There are still two areas of extreme drought in central Minnesota, one from Little Falls and Brainerd to eastern Benton County to Lake Mille Lacs and Milaca to Mora. The second extreme drought area lies between Willmar and Alexandria. As can be seen on the latest 2007 Drought Situation Report from the Minnesota State Climatology Office, rainfall is still 30-50% below normal in most of central Minnesota. The growing season rainfall deficit over central Minnesota is 5.33 inches through the end of August. River flow is still in the lowest 10 percentile from St. Cloud eastward and northward, as seen on the Streamflow from Minnesota DNR Waters.

For the summer season (June 1 - August 31), St. Cloud only had 8.69 inches of rain, 3.09 inches below normal, but .07 inch more than last summer's total.

Along with the return of rainfall, cooler temperatures did move in during August 2007. The average temperature of 68.2°F was 1.0°F above normal. There were 5 days with highs of at least 90°F during the month including the only heat wave (at least 3 consecutive days with a high at least 90°F) of the summer on August 9-11, including 96°F, our highest temperature of the summer.

The dry periods of the summers of 2006 and 2007 were intense and stressed water resources, but they were not nearly as long as the 1986-1989 drought or the 10 years of rainfall deficit that made up the "dust bowl" years of the 1930s. In order to compare the rain shortfalls, Kathleen Rogers, a 10th grade participant in SCSU's Scientific Discovery Program (coordinated by SCSU Director of Minority Studies Robert C. Johnson and Biology Professor Oladele Gazal), studied periods of consecutive growing season months with below normal rainfall in Saint Cloud history. Ms. Rogers found that the four straight months of below normal rainfall this year (April-July) tied for the 8th longest dry period in St. Cloud history. The longest such streak was nine months, set during the 1960-1961 growing seasons. The biggest surprise is that the 1930's drought nor the 1986-1989 ever produced more than two months in a row with a rainfall deficit. In contrast, the four-month dry streak also produced a total rainfall deficit of 5.95 inches, the second largest deficit among the four-month streaks and larger that the deficits associated with the five-month streaks. In this way, the comparison to this year's severe dry period is more comparable to the rainfall shortage in 1976, the last single-year rain shortfall that matched the intensity of these year's shortfall.

Why didn't the longest droughts show up in Kathleen's work? We suspect that these long droughts had more of the severe heat component (forcing great loss of moisture due to evaporation), rather than the severe rainfall deficit, but only further research will show this.

    August 2007 Statistics

Summer (June 1-August 31) 2007 Statistics