| Well above normal rainfall occurred over southern and central Wisconsin during the month of
May, 2004. At some official observation sites in southern Wisconsin, new all-time May precipitation records
were set, and in some cases, new all-time monthly records were set for any month of the year. In general,
official monthly rainfall totals ranged from around 6 inches in the extreme southwest corner to around 13.75
inches in the Union Grove to Racine area (13.85" in Union Grove and 13.55" in Racine). In addition, the Wisconsin
Dells area of northwest Columbia County received 13.31 inches.
Images of river flooding across south central and southeast Wisconsin.
The official monthly departure from normal ranged from +50% over the extreme southwest corner to an
extraordinary +350-425% in parts of Racine, Dodge, and northwest Columbia Counties. Most of southern
Wisconsin had official monthly precipitation departures of +200-300% Another way of looking at these
numbers is to compare them to the normal annual precipitation. Some locations in southern Wisconsin
received 33 to 42% of their annual precipitation in only 30 days!
Much of May 2004's precipitation came during two periods of time - May 7-14 and May 20-23. Rainfall that
fell during the period of May 7-14 left the ground very wet or simply saturated, setting the stage for
widespread river and farm field flooding in the last 10 days of the month (the worst since 2003). Roughly
the southeast two-thirds of the state picked up 2 to almost 6 inches of rain between May 7th and May 14th,
with Burlington (5.67"), the Fond du Lac airport (5.20"), Sheboygan (5.02"), 2N Cedarburg (4.99"),
Jefferson (4.94"), and the La Crosse NWS office (4.18") having some of the greatest totals.
Rainfall totals during the period of May 20-23 were generally in the 2.5 to 5 inch range across southern
Wisconsin, with scattered spots picking up more. Madison gathered 6.18 inches during this period, with 3.66
inches coming down on the 21st alone! For the period of 7 am May 21st through 7 am May 22nd, Lynxville
Dam 9 in Crawford County picked up 5.89 inches of unneeded rain, while other totals included Madison's
airport with 4.37", Juneau (Dodge Co.) with 4.20", and Jefferson with 3.49 inches. These totals all exceed
the normal monthly May rainfall - in only 24 hours!
Meteorologically, the reason for the record-setting monthly rainfall totals in May 2004 can be blamed on an
unusually stagnant weather pattern that persisted over southern Wisconsin. Several weak cold fronts moved
south across Wisconsin due to an active polar jet stream over the Great Lake region. However, most of these
cold fronts stalled over central or northern Illinois after passing over Wisconsin, rather than moving further
southeast. These fronts would then move back across southern Wisconsin the next day or so, not giving
southern Wisconsin the usual 2 to 3 day break between thunderstorm rains. Warm, muggy, unstable air south
of the fronts supplied the fuel for repeated rounds of thunderstorms with heavy rains.
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