Weather Outlook: Chances of rainfall increasing? | Michigan Farm News
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Weather Outlook: Chances of rainfall increasing?

2018-09-06 181 Jeff Andresen | MSU Extension

Occasional rounds of scattered showers and thunderstorms brought welcome moisture to sections of Michigan during the first half of August, but most areas received only limited relief from prolonged dryness.

Drier-than-normal weather since June has resulted in increasing levels of moisture stress for many crops, including some passing through water-sensitive stages of development.

As of mid-August, D1 Moderate Drought conditions as classified by the National Drought Mitigation Center have continued or expanded to three sections of the Lower Peninsula: the northern Tip of the Mitt area, south central through east central sections and much of the Thumb, and more recently, to west central sections.

Abnormally Dry (D0) conditions were noted across much of the remainder of Lower Michigan as well as the eastern Upper Peninsula. Observed precipitation totals for the 30-day mid-July through mid-August reporting period ranged from less than 1.5 inches across northern sections of the Lower Peninsula to more than 5 inches across portions of southwestern Lower Michigan.

Climatologically, the 30-day totals ranged from 50 percent-75 percent of normal across most sections of the state. One continuing characteristic of rainfall during this season’s drought has been unusually high spatial variability, with heavy rainfall totals observed in some spots while many other nearby surrounding areasremained dry.

Mean temperatures during the same period ranged from slightly below normal across far northwestern sections of the state to slightly above normal across western sections of Lower Michigan.

The moderate temperatures helped reduce potential stress levels given the existing moisture deficits. Seasonal base 50 degree F growing degree day totals through mid-August remained several calendar days to more than 1.5 weeks ahead of normal.

As noted in the last column, there are continued signs of at least a temporary jet stream pattern change during late August. Latest medium- range guidance suggests that the recent weak troughing pattern across the region will gradually give way to ridging to our east across the eastern U.S. by early September.

This should result in a more active weather pattern for Michigan, with gradually warming temperatures (back to above normal levels) and for more frequent chances for rainfall and at least in some portions of the state to above-normal precipitation totals.

Further ahead, the new NOAA Climate Prediction Center outlook for September now calls for the near-equal odds (ie no forecast direction) scenario for both mean temperatures and precipitation totals (earlier versions had called for warmer than normal mean temperatures).

The new 3-month outlook for the September through November period calls for warmer than normal mean temperatures and with no forecast direction on precipitation (equal odds of below, near, and above normal precipitation totals).  

Latest guidance also suggests a 70 percent chance for El Nino conditions in the tropical Pacific region later this fall and winter (more on that in the next column).

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.

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