In spring, weather concerns are top of mind for farmers and analysts alike as they try to guess how the growing season will pan out.

A major factor this year:  the current record-strong El Nino and when that weather pattern will transition to La Nina. That matters, because La Nina is associated with hot, dry weather that typically lowers yields.

That could boost crop prices, much to growers’ likely gratitude—but there are still plenty of unknowns, according to the team at farmdoc Daily.

First, such weather pattern shifts take time. The transition to La Nina can take months and might not occur until fall 2016, according to some forecasts. If the El Nino pattern persists through the summer, farmers—and the market—should expect higher than average yields.

Second, timing matters. Depending on when El Nino turns into La Nina, the effects on the growing season—and crop yields—will vary. “History suggests that a transition to La Niña by June, July, or August may measurably raise the risk of corn and soybean yields falling below trend,” wrote Scott Irwin and Darrel Good, both at the University of Illinois, adding that “yield risk was generally larger the earlier that La Niña conditions emerged.”

For example, a June transition to La Nina resulted in corn yields almost 10 bushels below trendline, compared to July or August transitions, which reduced yield by roughly 5 bushels, according to their analysis.

Soybeans also lost more yield based on when La Nina arrived. A June transition trimmed 2.4 bushels of soybean trendline yields, compared to approximately 1 bushel for a July or August La Nina shift.

What can farmers do? Continue paying close attention to the weather and the arrival of La Nina, so you and your advisers can respond quickly to changes in your agronomic and marketing plans.