Spring into summer: Unseasonable heat helps pave way for violent weather

“February showers bring March flowers,” National Weather Service forecasters wrote in an early morning discussion of upcoming weather patterns, posted on the Chicago Forecast office's website. Forecasters say they expect rain and cooler temperatures Friday, bringing an end to the “dog days of March.” Until then, however, “our historic and unprecedented stretch of record-breaking temperatures generally looks to continue.”

As it collides with this unusually warm, soggy air, the storm working its way east is expected to dump between 7 and 11 inches of rain over parts of Texas, Louisiana, Arkansas, Oklahoma, and Missouri.

Broader atmospheric conditions contributing to these records begin with La Nina, part of a warm-cool cycle that sloshes back and forth along the tropical Pacific. During a La Nina event, the ocean surface in the eastern Pacific is colder than normal, while water in the western tropical Pacific is warmer than normal.

The effects of a shift between La Nina and its mirror opposite, El Nino, are felt most strongly in the tropics. But the siblings can affect atmospheric circulation patterns much farther north and south. Alaska, for instance, has been experiencing record snowfalls – increased winter precipitation in keeping with previous La Ninas in kind, if not magnitude. The mountain West also has seen heavy snow this winter.

La Nina on average tends to drive storm tracks over North America farther north than usual, sending powerful storms off the North Pacific across the northern tier of the US.

For most of this winter, storms have scooted across the northern US and into Canada, allowing warmer-than-usual temperatures to hit the usually frigid northern US.

This pattern received a boost from another in the North Atlantic – the North Atlantic Oscillation. The NAO shows up as a cycle in which the difference in air pressure between a zone of low pressure around Iceland and high pressure near the Azores swings between strong and weak. The NAO set up this winter in ways that helped keep the storm track from dropping down into the eastern US.

Finally, another feature of the tropics, dubbed the Madden-Julian oscillation, has had La Nina's back. The feature consists of a region of thunderstorms that travels along the equator, making one circuit of the planet every 30 to 60 days. It's behaved in ways this winter that have contributed to warmer temperatures in the eastern US.

Despite the arrival of this week's storm system and the rainfall it's expected to bring, much of the southern tier continues to register moderate to exceptional drought, with the most-severe conditions centered in Texas and Georgia.

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