Image (click to enlarge): U.S. daily high temperatures for May 14, 2013, from Unisys
While Mid Atlantic residents were complaining about the unseasonable cold temperatures on Tuesday, heat records were being crushed in the Northern Plains and upper Midwest. As shown in the table below, triple-digit readings smashed long-standing May 14 daily records, with a margin as much as 7° at Norfolk, Nebraska. At Sioux City, Iowa, the 106° high also broke the all-time May maximum temperature record. Sioux City climate records date back to 1889. At both Omaha and Lincoln, Nebraska, it was the earliest 100° temperature on record, surpassing the previous records of 5/29/1934 and 5/24/1967, respectively.
The map below, from NOAA/ESRL, shows the temperature departures from average (°C) for the U.S. It shows that nearly all of the country was at least 2°C above or below average, but the positive departures significantly outweighed the negative, both in area and magnitude.
The temperature extremes were a reflection of a strongly anomalous circulation pattern shown in the map below. A very strong ridge of high pressure extended from the eastern Pacific to the Great Lakes at the 500 mb level (near the middle of the atmosphere), while a deep low pressure area was located off the Mid Atlantic coast.
The National Weather Service report for the record at Sioux City:
AT 135 PM THE TEMPERATURE REACHED 106 DEGREES AT THE SIOUX CITY GATEWAY AIRPORT ESTABLISHING A NEW RECORD DAILY MAXIMUM TEMPERATURE FOR THIS DATE AND A NEW RECORD FOR THE HIGHEST TEMPERATURE FOR THE MONTH OF MAY. THE PREVIOUS RECORD DAILY MAXIMUM TEMPERATURE WAS 97 DEGREES SET IN 2001 AND THE PREVIOUS RECORD MONTHLY MAXIMUM TEMPERATURE FOR MAY WAS 105 DEGREES SET ON MAY 30, 1934. THE TOP TEN HIGHEST TEMPERATURES FOR MONTH OF MAY FROM 1889 TO 2013 DEGREES DATE 1. 105 5/30/1934 2. 103 5/29/1934 3. 102 5/25/1967 4. 102 5/31/1934 5. 101 5/15/2001 6. 100 5/06/1934 7. 99 5/19/1934 8. 99 5/18/1934 9. 98 5/28/2006 10. 98 5/24/1939