At the overwintering sites in Mexico, "We know temperatures below -8 °C (17.6° F) will kill about 50% of the monarchs," says Dr. Lincoln Brower.

This year's combination of early migration and late-season cold has the potential to set the monarch population back.

"These jet stream conditions are similar to those in 1997. Monarchs got off to a great start that spring but the population subsequently crashed apparently due to freezing temperatures in early April that reached well into Texas," observed Dr. Chip Taylor of Monarch Watch.

This week's weather separated winners from losers, punishing those who advanced early and favoring those who lagged behind.

"Losses like this leave far greater imprints on the world than we suspect, or remember for very long," said nature writer Elizabeth Hunter from her home in North Carolina. She gave an example of a plant—butterfly—bird association that was also hurt by the cold.

"I wonder what the result will be of the loss of the spicebush. The spicebush swallowtails had begun to emerge. Now they have no leaves on which to lay their eggs. And what about the birds next fall that normally eat the high lipid spicebush fruits on their southward journey?"

From Unseasonably Warm to Cold
April 3-9, 2007

Watch how tempeartures change across the eastern two-thirds of the U.S. beginning April 3rd.

Temperatures had averaged 9+ degrees above normal between March 25-31st. After these record high temperatures, unseasonably cold air and snow moved in on April 3rd.

• Put your mouse over the map to see the dramatic change in average temperatures over the next 7 day period.