The combination of several rounds of widespread moderate to locally heavy rains and melting snow during the first two-thirds of the month produced strong within bank rises to minor flooding on most of the tributary rivers across the HSA. During the second half of the month, this tributary inflow produced a strong within-bank rise of 4 to 9 feet on the Mississippi River from Lake City MN to Guttenberg IA.
April 2013 precipitation ranged from above normal across the east third of the HSA to much above normal across the central and west thirds of the HSA. A passing low brought widespread light to moderate, mainly rain, to the HSA on the 5th and 6th. A strong and slow moving low brought widespread heavy rain and some snow to the HSA from the 9th to the 11th. Additional lows and cold fronts brought widespread moderate to heavy rains and some snow to the HSA on the 14th, 17th into the 18th and 21st into the 22nd. A stronger cold front brought light to moderate rain to mainly the northwest half of the HSA on the 30th.
Normal rainfall for April ranges from around 2.75 inches across the north end of the HSA, to around 3.5 inches across the south end. Totals for the month ranged from 150 to 200 percent of normal across the east third of the HSA to 200 to 250 percent of normal over most of the central and west thirds of the HSA. Most totals for April were 175 to 225 percent of normal. Extreme totals recorded for the month ranged from 4.23 inches near Necedah in Juneau Co. WI and 4.56 inches at Friendship in Adams Co. WI, to 7.63 inches at Strawberry Point in Clayton Co. IA and 8.39 inches at St. Ansgar in Mitchell Co. IA. April 2013 snowfall was below normal across the southern third of the HSA to much above normal across the northern third, ranging from trace amounts over the south end to 8 plus inches over the north end. Oddly enough, the above normal snowfall did not occur with one or two large systems but accumulated with many small snowfall events during the first 3 weeks of the month. Normal April snowfall ranges from 1.5 to 3 inches from south to north across the HSA. Snowfall totals this month ranged from near zero percent of normal across the south end to 200 to 300 percent of normal over the north end.
Temperatures for April 2013 averaged 38 to 44 degrees. This was 4 to 8 degrees below the normals, making this month one of the 5 to 10 coldest Aprils of record. Temperatures were below and much below normal on the 1st to the 3rd, on the 5th and during a long stretch from the 9th to the 25th. Temperatures were near to above normal on the 4th and 6th to the 8th, above normal on the 26th to the 29th and much above normal on the 30th. Extremes for the month ranged from lows of around 10 to around 20 on the morning of the 3rd, to highs in the middle 70s to middle 80s on the 30th.
Flows in the rivers and streams of the HSA were near to much above normal during April. By the end of the month, the above normal precipitation across the HSA for December through March combined with the much above normal precipitation across most of the HSA in April were finally enough alleviate the areas of moderate to severe drought that had lingered from 2012. Ground frost and snow cover remained much later than normal across the HSA during this April. Spring field work and planting activity were generally 3 weeks behind normal at month’s end.