NAPERVILLE, Illinois, March 14 (Reuters) - An incredibly mild and dry finish to winter has facilitated the spreading of drought across the Midwestern United States just weeks before the start of spring crop planting.

That marks the fourth consecutive year of heightened dryness in the U.S. Corn Belt at this time of year, a pattern that has preceded various crop outcomes. Planting is quick when soils are dry in the spring and moisture delays are minimal, and acreage expansion can sometimes result.

Spring dryness increases emphasis on summer rains, which have been ample in recent years that featured a transition from El Nino to La Nina by mid-year, as is predicted for 2024. But temperatures are less of a guarantee.

DROUGHT HAS SPREAD

U.S. Drought Monitor data on Thursday showed 72% of the Midwest is abnormally dry this week, up from 54% at the end of January. That is by far the most for mid-March since 2003.

That follows the warmest and sixth-driest Midwestern February in 130 years of records, as precipitation was just 41% of normal. Warm and dry is common in El Nino Februarys, though no other years featured such an extreme combination, potentially worrisome for the months ahead.

Some of the warmest Februarys preceded drier-than-normal Julys, though the driest Februarys were associated with a mixture of both wet and dry Julys, so no solid summer predictions can be made off last month’s weather.

Good news is that the extent of U.S. corn areas in moderate or worse drought (36%) is not significantly worse than in mid-March of the previous three years, though that is up 10 percentage points in the last three weeks.

Additionally, some 56% of top corn grower Iowa is experiencing severe or worse drought, a 24-year high for the date. Soils there are remarkably warm, at least 10 degrees Fahrenheit above normal and close to fitness for planting.

Those Iowa soils are the warmest for mid-March since 2016, when U.S. farmers planted a much larger corn acreage than analysts expected, despite a mid-spring shift in the price economics that appeared more friendly to soybeans.

EL NINO TO LA NINA

The U.S. Climate Prediction Center on Thursday placed 62% odds on La Nina’s formation by June-August, up from 55% estimated last month. El Nino, characterized by unusually warm sea surface waters in the equatorial Pacific Ocean, remains in place but has 83% chances of fading between April and June.

Since 1950, there are only four instances of a 2024-magnitude February El Nino fading to La Nina by June-August: 1973, 1998, 2010 and 2016, though the latter was borderline La Nina.

U.S. corn yields were OK in 1973 and 1998 but disappointing in 2010, though the 2016 soybean yield remains the all-time high due to record July-August Midwest rainfall.

High July-August nighttime temperatures, which are limiting to corn yield, were notorious in the U.S. Corn Belt in 2010 and 2016, though both years featured abundant summer precipitation.

Last year’s record U.S. corn yield brings out the weather cynics as the result came despite the driest June in 35 years, which pummeled crop conditions. This has caused many market participants to quip that weather no longer matters and that there will never be another summer weather rally, or at least a justifiable one.

Flawed logic, of course, because there was no guarantee that the water works would turn on come July, essentially saving the crop. Although the timing was off and on, Midwestern rainfall was overall normal in both July and August 2023, and temperatures were normal to slightly cool. Karen Braun is a market analyst for Reuters. Views expressed above are her own.

(Writing by Karen Braun Editing by Matthew Lewis)