Rainfall Not Spread Evenly, More on the Way
by Cody Matz
June 21, 2012 8:33 PM
So far this Spring and first couple of days of Summer have been pretty unusual with record heat across not only our area, but much of the country, and heavy but sporadic rainfall that has been dotting the landscape. Now, once we get into summer, these isolated very heavy rain events become more common as moisture levels continue to increase and the atmosphere is able to dump more and more water when thunderstorms develop. But receiving such lopsided totals this early in the year is a bit unusual and creating chaos for farmers with some screaming for more rain and others screaming for sunshine.
Below are a few maps that show rainfall totals over a period of time. The first one is a map indicating actual rainfall in inches over the last 30 days. The map underneath that one is rainfall totals from average over the last 30 days.
May and June are the two wettest months on average for our area, so don’t be alarmed when you see that many locations have topped 4 inches over the last month or so. But also notice the areas that have not by looking at that second map one more time. Very easy to see who needs more rain and who doesn’t. The majority of the area is actually sitting in pretty good shape with normal to slightly above normal precipitation. But the I-90 corridor southward from Sioux Falls to Mitchell is in rough shape with many areas 1-2 inches below average and parts of Gregory and Charles Mix counties some 3 inches below average. The other spot hurting for rain appears to be the Aberdeen area. But watch what happens when you look at the next 2 maps as they show rainfall totals over the last 90 days and departure from average rainfall over the last 90 days.
It becomes VERY clear who really needs the rainfall. Much of the area has see 10-15 inches of rain in just the last 90 days with the Highway 14 corridor, western Minnesota, and northwest Iowa all in great shape; 2 to 8 inches above average. But other areas are struggling with obvious holes in the rain buckets for the Aberdeen area, the city of Sioux Falls, and many areas south of I-90 in South Dakota where we are sitting close to average in some spots to as much as 6 inches below average in Gregory and Charles Mix county as well as Boyd county in Nebraska.
The good news for those who have been dry, rain is in the forecast, and could be locally heavy. The picture below is a 6-hour precipitation forecast for the morning hours of Saturday.
Notice that the area is fairly small but rainfall totals are quite intense with upwards of an inch of rain. This is usually caused by something we call convective feedback. The forecasting model can only view data a few miles apart, so when it sees a couple of thunderstorms, which can output tremendous rainfall amounts this time of year, it can often over estimate rainfall totals over a broad area. However, this also tells us that the forecasting model is picking up thunderstorm activity versus just standard light rain. The big difference is that with thunderstorms, local spots can pick up much more then the forecast model shows which would indicate some spots could get more then an inch of rain. Hopefully, that heavy rain fall in areas that really need it.