Record highs set Wednesday. Open circles indicate records were tied, circles with an x indicate records were broken. (National Climatic Data Center)

Since Sunday, an amazing 943 new record highs have been broken or tied across the U.S. compared to just 9 record lows

On Wednesday alone, an incredible 400 new record highs were were broken (307) or tied (93). Record heat spanned from Florida to Montana.

Here’s a selection of new record highs set Wednesday:

Tampa, FL: 87
St. Louis, MO: 86
Topeka, KS: 84
Louisville, KY: 82
Evansville, IN: 82
Chicago, IL: 81
Des Moines, IA: 81
Traverse City, MI: 81
Myrtle Beach, SC: 79
Madison, WI: 78
Atlantic City, NJ: 77
Minneapolis, MN: 73
Green Bay, WI: 73
New York, NY (Kennedy): 72
Duluth, MN: 64

Some of the most impressive heat relative to normal has occurred in the Midwest.

In Chicago, the high of 81 - four degrees above the old daily record of 77 - was 35 degrees above normal. The only 81 degree reading to occur earlier than Wednesday since 1871 occurred on March 12, 1990. The Windy City’s average temperature in March is 11 degrees above normal thus far. (Source: Capital Climate)

Minneapolis broke its record high Wednesday by 9 degrees, hitting 73. Meteorologist Paul Douglas of the Star Tribune predicts “5 more record-breaking, 70-degree-plus days are likely between [today] and next Tuesday.” Minneapolis’ average temperature in March is running 8 degree above normal to date.

Traverse City’s record high of 81 was a jaw-dropping 42 degrees above average, and four degrees warmer than any previous day so early in the year since 1897 (when records began, source: Jeff Masters)

What’s the cause of this remarkable heat?

NOAA’s Climate Prediction Center forecasts very high chances of above normal temperature in the eastern two-third of the U.S. continuing through March 20-24 in its 6-10 day outlook. (NOAA)

This veritable heat wave is part of a large, continental-scale, transfer of warmth from the subtropical Pacific to the Lower 48. High altitude winds have recently begun to blow maritime air over the States in a way that has created a bubble of warm air near the surface capable of sending temperatures as much as 30°F above climatology at its core.

Wunderground’s Jeff Masters added the following Wednesday, noting the unusual nature of this warm pattern - going nowhere fast (bold indicates emphasis added):

The weather system responsible is a large upper-level ridge of high pressure that is “stuck” in place--a phenomenon known as a “blocking pattern.” The jet stream is bending far to the south over the Western U.S., then bending far to the north over the Rockies and into Canada, and lies far to the north of the eastern U.S. Since the jet stream acts as the boundary between cold air to the north and warm air to the south, the current looping pattern is bringing colder than normal temperatures and snow to the mountains of the West, and summer-like warmth to the Eastern U.S. It is common for the jet stream to get stuck in a blocking pattern for a period of a week or more in summer, but not in March. If the current model forecasts prove correct, a high pressure ridge over the U.S. bringing heat this intense and long-lasting in March will be unprecedented in the historical record, going back to 1872.

Is the heat related to global warming?

At Climate Central, Andrew Freedman put this current stretch of extraordinary warm weather into a broader context:

In a long-term trend that has been linked to global climate change, daily record-high temperatures have recently been outpacing daily record-lows by an average of 2-to-1, and this imbalance is expected to grow as the climate continues to warm. According to a 2009 study, if the climate were not warming, this ratio would be expected to be even.

Consider - too - the baseball and steroid analogy I used to describe how global warming stacks the deck for more record warm weather:

Just as homerun numbers got an artificial boost in the 1990s, warm weather statistics are inflated today. In the U.S., over the last 12 months [spanning February 2011 to January 2012], daily warm weather records outpaced cold weather records by a factor of more than two* (60,024 warm weather records compared to 22,474 cold weather records). In Virginia and Maryland, the last 12 months (spanning February 2011-January 2012) were the warmest on record. Washington, D.C.’s last two Julys were the hottest two on record.

The backdrop for these warm weather records is an atmosphere that’s bulking up. Levels of carbon dioxide and methane (two key greenhouse gases) are higher than they’ve been in at least 800,000 years, and global temperatures over the last decade are unsurpassed in the modern climate record. All 11 years of the 21st century rank among the 13 warmest globally since 1880 according to NOAA.