Unprecedented early-year heat smashes all-time records in Midwest

The first day of spring doesn't arrive until Tuesday, but Mother Nature has fast-forwarded past spring and gone straight to summer over the Midwest. Yesterday, large portions of Michigan, Illinois, Wisconsin, Minnesota, and Indiana recorded their all-time warmest temperatures for so early in the year, including the cities of Chicago, Minneapolis, Indianapolis, Milwaukee, and Lansing. Temperature records for these cities go back as far as 1863. Perhaps the most extraordinary record was set in Traverse City, Michigan, which hit 81°F--a temperature 42°F above the average high for the date, and four degrees warmer than any previous day so early in the year. Records go back to 1897 in the city.


Figure 1. Spring has arrived early in Sherrard, Illinois, and much of the Midwest, thanks to a record-breaking March heat wave. Image credit: wunderphotographer JourneysEnd.

Major airports that set all-time heat records yesterday

Chicago, Illinois hit 81°F; the warmest it's ever been this early in the year, going back to 1872, and 35°F above average. This ties the record set on March 12, 1990.

Springfield, Illinois hit 83°F, the warmest it's ever been this early in the year, going back to 1879, and 32°F above average. This ties the record of 83°F on March 13, 1918.

Rockford, Illinois hit 79°F, the warmest it's ever been this early in the year, going back to 1893, and 33°F above average. Previous record: 78°F on March 7, 2000.

Peoria, Illinois hit 81°F, the warmest it's ever been this early in the year, going back to 1883, and 31°F above average. Previous record: 80°F on March 12, 1990.

Minneapolis, Minnesota hit 73°F, the warmest it's ever been this early in the year, going back to 1872, and 33°F above average. This ties the 73°F reading of March 7, 2000.

Grand Rapids, Michigan hit 80°F, the warmest it's ever been this early in the year, going back to 1892, and 37 degrees above average. Previous record: 78°F on March 8, 2000.

Muskegon, Michigan hit 77°F, the warmest it's ever been this early in the year, going back to 1896, and 35°F above average. Previous record: 73°F on March 8, 2000.

Traverse City, Michigan hit 81°F, the warmest it's ever been this early in the year, going back to 1897, and 42°F above average. Previous record: 77°F on March 7, 2000.

Lansing, Michigan hit 79°F, the warmest it's ever been this early in the year, going back to 1863, and 36°F above average. This ties the record of 79°F on March 8, 2000.

Indianapolis, Indiana hit 81°F, the warmest it's ever been this early in the year, going back to 1871, and 30°F above average. Previous record: 80°F on March 8, 1974 and March 13, 2007.

South Bend, Indiana hit 81°F, the warmest it's ever been this early in the year, going back to 1893, and 35°F above average. Previous record: 80°F on March 8, 2000.

Evansville, Indiana hit 82°F, the warmest it's ever been this early in the year, going back to 1871, and 25°F above average. This ties the record of 82°F set on March 10, 1990.

Milwaukee, Wisconsin hit 78°F, the warmest it's ever been this early in the year, going back to 1869, and 36°F above average. Previous record: 77°F on March 7 and 8, 2000.

Madison, Wisconsin hit 78°F, the warmest it's ever been this early in the year, going back to 1869, and was 36°F above average. Previous record: 77°F on March 8, 2000.

Green Bay, Wisconsin hit 75°F, the warmest it's ever been this early in the year, going back to 1886, and 37°F above average. Previous record: 74°F on March 8, 2000.

Dubuque, Iowa hit 75°F, the warmest it's ever been this early in the year, going back to 1874, and 37°F above average. This ties the 75°F reading of March 12, 1990.

Data for the previous records was taken from NOAA's National Climatic Data Center (NCDC).

Why the record early-season warmth?
The unusual warmth is due to a loop in the jet stream that has created a large upper-level ridge of high pressure that is stuck in place over the Eastern U.S.--a phenomenon known as a "blocking pattern." Since the jet stream acts as the boundary between cold air to the north and warm air to the south, and the large loop in the jet places its axis far to the north of the eastern U.S., summer-like warmth has developed over the eastern half of the U.S. Conversely, colder than average temperatures have developed over the western third of the U.S. behind the southwards-dipping loop of the jet stream. There are at least three large-scale patterns working together right now to create an unusually strong high-pressure ridge over the eastern half of the U.S.:

1) La Niña. The on-going La Niña event in the Eastern Pacific has weakened considerably over the past month, but ocean temperatures there are still cool enough to affect the jet stream pattern, favoring high pressure and warm temperatures over the Eastern U.S., and low pressure and cold temperatures over the Western U.S.

2) The Madden-Julian Oscillation( MJO). The MJO is a 2-month cycle of thunderstorm activity that travels west to east along the Equator. The MJO is currently in phase with La Niña, and is helping create warmer temperatures over the Eastern U.S.

3) The North Atlantic Oscillation (NAO.) The NAO is in its positive phase, which means the difference in pressure between the Icelandic Low and the Azores High is stronger than usual. This tends to increase the jet stream winds and keeps the jet from sagging southwards over the Eastern U.S.

While the blocking pattern responsible for the heat wave is natural, it is improbable that the intensity of the heat would have been so great unless we were in a warming climate. Climate scientist Dr. James Hansen has posted an interesting (as yet unpublished) paper discussing how the odds of such extreme heat events have shifted in recent years. I plan to discuss these concepts in more detail next week.


Figure 2. Predicted high temperatures for Thursday, March 15, 2012 over much of the Midwest are more typical of June than March.

The forecast
The summer-like heat continues today over the Midwest, with the greatest departures from average (30°+) expected over Wisconsin, Iowa, Illinois, Indiana, and Michigan. This area of 30°+ above average temperatures will expand to stretch from South Dakota to New York on Sunday and Monday, then gradually shift eastwards, extending from Wisconsin to Maine by Wednesday. The jet stream loop is predicted to grow more pronounced this weekend, and reach a truly extreme configuration on Monday and Tuesday. On those days, the predicted strength of the upper-level ridge of ridge pressure along the northern tier of states, from Wisconsin to Maine, will be typical of what we see at the height of summer, during July and August. The GFS and ECMWF models are predicting the height at where a pressure of 500 mb is found will range between 5760 - 5800 meters on those days--7% higher than the typical mid-March 500 mb heights of 5460 meters. Record-breaking summer-like temperatures will last until at least Friday, March 23, for much of the Midwest and Northeast U.S.

Extreme jet stream patterns like this often lead to tornado outbreaks in the Midwest, at the boundary of where warm, moist air flowing north from the Gulf of Mexico collides with cold, dry air flowing south. Cold dry air aloft, combined with warm, moist air at the surface, makes the atmosphere unstable, since air rising in thunderstorm updrafts will be less dense than the surroundings, allowing the air to accelerate upwards and increase the intensity of the thunderstorm. This will be the case on Sunday, when NOAA's Storm Prediction Center has highlighted a potential risk area for severe weather from Texas northwards through Nebraska. Fortunately, the winds of the jet stream are expected to blow almost due south to north over the region by Monday, which may limit the amount of cold air aloft that can overrun the warm air at the surface, resulting in a more stable atmosphere and a reduced chance of severe weather compared to Sunday.

Jeff Masters