Jeff Andresen | August 15, 2018
Scattered rains helped ease dryness across portions of Michigan during late July and early August, but totals and a real coverage were limited. Elsewhere, the effects of prolonged dryness led to increasing levels of moisture stress for most crops, including much of the state’s corn crop, which moved through the vulnerable silking/pollination stage.
As of the end of July, the United States Drought Monitor classified large sections of Lower and eastern Upper Michigan (69 percent of the state in terms of area) as either ‘Moderate Drought’ (category D1) or ‘Abnormally Dry’ (category D0), with driest areas of the state located across eastern and south central sections of the Lower Peninsula.
As a reminder of how quickly dryness has become an issue this season, the same index indicated none of the state in these categories as recently as mid-June.
For July, observed precipitation totals ranged from less than 1 inch (less than 50 percent of normal) across sections of southern Lower Michigan to more than 4 inches across portions of northeastern Lower and western Upper Michigan.
Mean temperatures during July ranged from near-normal across far southern sections of the state to more than 3 degrees F above normal across northeastern sections.
The first half of the month was much warmer than normal statewide, but cooler, more seasonable temperatures moderated monthly means following the development of an upper-air troughing pattern across the region during the third and fourth weeks of the month.
Seasonal base 50 degree growing degree day totals through the end of July generally remained from 50-200 units above normal levels, with greatest departures from normal across southern sections of the state.
One consistent meteorological theme during this season’s dry spell has been the relative low number of potential rainfall-producing weather systems passing through the state, or even when they did, by a lack of moisture ahead of the system or by a weakness of the system itself. These features were associated with a persistent broad upper-air ridging pattern across the Midwest earlier in the season and more recently by a weak troughing pattern.
While recent medium-range forecast guidance still suggests more warmer and drier-than-normal in the short term (through at least the middle of August), longer lead guidance suggests changes later in the month which would lead to more a more transient jet stream flow across North America, and to more frequent chances for precipitation.
There is some reflection of this in the updated monthly outlook for August, which now calls for normal to above-normal precipitation totals over most sections of the state.
The three-month outlook for the August through October period calls for a continuation of warmer-than-normal temperatures with no forecast direction on precipitation (equal odds of below, near, and above-normal precipitation totals).
Late-season precipitation can have major impacts on crops, as was observed during 2012 when above-normal precipitation late in the season helped boost expected soybean and some forage yields in some areas of the state following earlier prolonged dryness and heat.
Andresen is a professor of Meteorology/Climatology with Michigan State University’s Dept. of Geography, Environment, and Spatial Sciences; MSUE specialist and the state climatologist for Michigan.