September 2011 rainfall varied from much below normal across the far northwest end of the HSA, to near and above normal over the central and southeast two-thirds of the HSA. A slow moving low and cold front brought light to locally heavy rains to mainly the south and east parts of the HSA on the 2nd into the 3rd. The 4th to the 17th was a dry period for the HSA due to rather persistent high pressure over the Upper Midwest. Slow moving low pressure brought mainly light rain to the west and north parts of the HSA on the 18th and light to moderate rain to mainly the southwest to east parts of the HSA on the 19th. Scattered light rain showers fell across mainly the northern two-thirds of the HSA from the 20th to the 23rd. A deep low that stalled low near Chicago brought light to locally heavy rains to the southeast three-quarters of the HSA from the 26th to the 28th. As this low finally departed it brought light to moderate rains to the northeast two-thirds of the HSA on the 29th.
Normal rainfall for September ranges from around 3.25 inches across the west and southwest ends of the HSA, to around 3.75 inches in the northeast end. Rain totals for this month ranged from 25 to 35 percent of normal across the far northwest end of the HSA, to 125 to 150 percent of normal in localized areas of the eastern two-thirds of the HSA. Most totals for the month were 60 to 90 percent of normal. Extreme totals recorded for the month ranged from 0.93 inch at Dodge Center in Dodge Co. MN and 0.94 inch at Lake City in Wabasha Co. MN, to 5.56 inches at Blair in Trempealeau Co. WI and 5.64 inches at Neillsville in Clark Co. WI.
Temperatures for September 2011 averaged 57 to 61 degrees. This was 1 to 4 degrees below the normals. Temperatures were much above normal on the 1st, above normal on the 2nd then near to below normal 3rd to the 8th. Readings were then above normal from the 9th to the 12th, below and much below normal on the 13th to the 18th, near normal on the 19th and 20th, and near to below normal on the 21st to the 27th. The month ended with near to above normal temperatures on the 28th and 29th and below normal temperatures on the 30th. Extremes for the month ranged from lows in upper 20s to upper 30s on the mornings of the 15th, to highs in the mid 80s to lower 90s on the 1st.
Flows in the tributary rivers and streams of the HSA were mainly near normal for September. Flows in the Mississippi River started out above normal but were near normal by the end of the month. Most crops continued toward maturity during the month but were slowed by the cooler than normal temperatures, and were a few days behind normal at the end of the month. Frost and freezing temperatures on the morning of the 15th brought the growing season to an early end over much of the western half of the HSA, where many crops still needed a couple weeks to reach full maturity. The below normal rainfall for parts of the area during July, August and September continued to create drought stress on crops, especially those planted in light or sandy soils.