State Climatologist Justin Glisan says preliminary data shows a statewide average of two inches of precipitation fell in Iowa last month. While that’s exactly what’s average for November, Glisan says precipitation amounts were far below average in northwest Iowa.
“We saw precipitation departures of anywhere from one to two inches,” Glisan says.
That means some areas of northwest Iowa, where there are drought conditions, recorded little, if any precipitation last month. On the flip side, there’s south central and south east Iowa.
“We kind of had the storm track locked on that part of the state,” he says. “That’s where we saw the largest positive departures, anywhere from 1.5 to 2 inches.”
That means some pockets of southern Iowa had twice as much rain as average. Glisan says due to a lack of precipitation over the past three to six months, a few counties in northwest Iowa are in extreme drought.
“You can kind of cut Iowa in half, typically right down I-35. Anywhere east of that we’re above average precipitation wise and anywhere west of that we’re below average and that’s where we see…abnormally dry conditions all the way to the extreme drought conditions up in that northwest corner.”
Based on astronomy, winter starts on December 21, but based on climatology and meteorology, December 1 is the first day of winter. Glison says that means the drought conditions are likely to persist.
“Drier soils will freeze faster and they’ll freeze deeper,” Glisan says. “If we get into a cold period in which we dip below freezing for a good amount of time and the soils freeze, any precipitation in the form of rain or snowfall getting into a melt period will not infiltrate into that soil very deep, if at all.”
The other factor is subsoil moisture is depleted, especially in western Iowa. Temperatures in Iowa averaged five degrees above normal for the month of November.
“Typically when we do see warmer temperatures along with these windy days that we’ve had, especially in November, that produces atmospheric demand for water vapor,” Glisan says
That exacerbated evaporation of what moisture there had been in the soil. Glisan says the short term outlook is for warmer and drier than normal conditions in December for the western three quarters of the United States.
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