Chicago, IL

Weather Forecast Office

July 4-7, 2012 Heat Wave

Fast Facts

Chicago

The highest temperature recorded during the heat wave at Chicago O'Hare International Airport (the official climate site) was 103° on July 5th and 6th. These were the highest temperatures since July 13, 1995 when 104° was observed.
Chicago reached 100° for three consecutive days from July 4th-6th, which ties the records of July 3-5, 1911 and Aug 4-6, 1947. The average temperature in this 3-day period was 91.4°, and ranks as the second warmest 3-day average on record for Chicago behind July 28-30, 1916.
The three 100° days in July ties the most on record for the month with 1911.
There have been four 100° days so far in the summer of 2012. That ties for second most behind 1988 (7).
Chicago has had 65 days with a temperature of 100° or greater in 140 years of records, which averages to about once every two years. (although numerous years have had several 100° days)
Chicago tied a daily record high on July 4th and broke the records on July 5th and 6th.

Rockford

The warmest reading in the heat wave was 105° on July 7th, which was the warmest reading in Rockford since 112° on July 14, 1936. (76 years) This also was the warmest reading at the Rockford International Airport in its period of record since 1951.
Rockford reached 100° for four consecutive days from July 4th-7th, which is the longest stretch since the Dust Bowl Era, last occurring July 28-31, 1936.
Rockford has had 110 days with a temperature of 100° or greater in 106 years of records, which averages to about once a year. (again numerous years have had several 100° days)
Record highs were broken on July 4th-6th and tied on July 7th.

Area-wide

The highest recorded temperatures in the area from multiple NWS sites during the heat wave were 105° at the Rockford ASOS station on July 7th, the NWS Cooperative site at 3SW Midway Airport on July 6th, and at the Northerly Island site in downtown Chicago and the NWS Cooperative site at the Chicago Botanic Gardens on July 5th.
The NWS's longtime observer in Polo, IL observed 100° on July 6th and 102° on July 7th. These were the first 100°+ readings at Polo in almost 24 years, since 104° on Aug. 17, 1988.
The NWS Cooperative site at the Chicago Botanic Gardens tied an all-time record high of 105° in their 30 year period of record (since 1982). That reading was reached on July 5th, tying the record set on July 14, 1995.
The NWS Cooperative site at Northern Illinois University in DeKalb reached 100° on July 6th and 101° on July 7th, which was the first triple digit observation since the July 1995 heat wave.

Nationwide

Below is a table showing the amount of daily, July, and all-time records broken in the first week of July 2012. These are for NWS ASOS and Cooperative Observer sites.

Temperatures

High Temperatures
July 4th	July 5th	July 6th	July 7th

...THE HIGHEST AREA TEMPERATURES DURING JULY 4-7 2012...

LOCATION TEMPERATURE DATE OBSERVATION TYPE

------------------------------------------------------------------

CHICAGO BOTANIC GARDENS 105 7TH COOP

CHICAGO MIDWAY 3SW 105 6TH COOP

CHICAGO NORTHERLY ISLAND 105 5TH HO83

ROCKFORD AIRPORT 105 7TH ASOS

OTTAWA 5 SW 104 7TH COOP

WAUKEGAN AIRPORT 104 4TH/5TH ASOS

WHEELING/CHI EXEC 104 6TH ASOS

AURORA 103 7TH COOP

CHICAGO OHARE 103 5TH/6TH ASOS

MORRIS 1 NW 103 7TH COOP

MUNDELEIN 4 WSW 103 5TH/6TH COOP

AURORA/SUGAR GROVE 102 7TH ASOS

ELGIN 102 6TH/7TH COOP

JOLIET 102 7TH AWOS

JOLIET BRANDON ROAD DAM 102 7TH COOP

MARSEILLES LOCK 102 7TH COOP

PONTIAC 102 7TH AWOS

DE KALB 101 7TH COOP

DRESDEN LOCK&DAM 101 5TH/7TH COOP

LANSING 101 6TH AWOS

MC HENRY-WG STRATTON L&D 101 5TH COOP

PERU 101 7TH AWOS

RENSSELAER IN 101 7TH COOP

WFO ROMEOVILLE 101 6TH/7TH NWS

DEKALB 100 7TH AWOS

DUPAGE AIRPORT 100 6TH/7TH ASOS

INDIANA DUNES 100 4TH/5TH COOP

MARENGO 100 5TH COOP

PARK FOREST 100 6TH COOP

PONTIAC 100 7TH COOP

VALPARAISO IN 100 5TH/6TH/7TH ASOS

BARRINGTON 3 SW 99 5TH/6TH COOP

DWIGHT 99 7TH COOP

KANKAKEE 99 7TH AWOS

ROCHELLE 99 7TH COOP

VALPARAISO 5 NNE IN 99 6TH COOP

MORRIS 98 7TH AWOS

ROCHELLE 98 7TH AWOS

RENSSELAER IN 97 6TH/7TH AWOS

Click for the Highest Temperature List and Day by Day Breakdown (pdf)

Heat Indices

July 4th	July 5th

July 6th	July 7th

Records

Chicago Record Highs Broken & Tied

A RECORD HIGH TEMPERATURE OF 102 DEGREES WAS TIED AT CHICAGO-OHARE
IL ON JULY 4TH. THIS TIED THE OLD RECORD OF 102 SET IN 1911.

Independence Day Climatology for Chicago

A RECORD HIGH TEMPERATURE OF 103 DEGREES WAS SET AT CHICAGO-OHARE
IL ON JULY 5TH. THIS BREAKS THE OLD RECORD OF 102 SET IN 1911.

A RECORD HIGH TEMPERATURE OF 103 DEGREES WAS SET AT CHICAGO-OHARE
IL ON JULY 6TH. THIS BREAKS THE OLD RECORD OF 99 SET IN 1988.

Rockford Record Highs Broken & Tied

A RECORD HIGH TEMPERATURE OF 102 DEGREES WAS SET AT ROCKFORD IL
ON JULY 4TH. THIS BREAKS THE OLD RECORD OF 101 SET IN 1911.

Independence Day Climatology for Rockford

A RECORD HIGH TEMPERATURE OF 102 DEGREES WAS SET AT ROCKFORD IL
ON JULY 5TH. THIS BREAKS THE OLD RECORD OF 100 SET IN 1911.

A RECORD HIGH TEMPERATURE OF 104 DEGREES WAS SET AT ROCKFORD IL
ON JULY 6TH. THIS BREAKS THE OLD RECORD OF 102 SET IN 1936.

A RECORD HIGH TEMPERATURE OF 105 DEGREES WAS TIED AT ROCKFORD IL
ON JULY 7TH. THIS TIES THE OLD RECORD OF 105 SET IN 1936.

Meteorology Behind the Heat

The heat wave of early July 2012 had several meteorological ingredients come together on large (synoptic) and local (meso) scales. For much of the spring into the early part of the summer, the jet stream has remained well to the north of the area. The first week of July was similar, but even more extreme with the upper level westerlies amplified and a deep ridge over the central U.S. under very light upper level winds.

To the right is an image of the 500 mb height (~18,000 ft) anomalies. Note the well above normal heights associated with the central North American ridge. Height anomalies are not uncommon, but are usually most pronounced in the transition seasons of spring and autumn. To have such well above normal height anomalies in summer and over such a large area is quite rare. Above normal heights are almost always associated with warmer than normal temperatures in the atmosphere. In order for a heat wave to persist and be extreme at the surface, much of the lower atmosphere needs to be warmer than normal. NWS weather balloons observed such well above normal temperatures aloft, as the below upper air maps color contoured with temperatures show. These were from the evening balloon launch on July 6th. The -3°C observed at 500mb on the KDVN balloon launch that evening tied for the second warmest 500 mb reading with multiple other days from the period 1995 to present.

Early July 500 mb Height Anomalies

500 mb	850 mb	925 mb

The below composite maps of temperatures at the same three pressure levels as above are averages from the only three other days where Chicago reached 103° since 1980. Note the similarities to the above maps, implying a similar very anomalous pattern.

500 mb	850 mb	925 mb

May 1st-July 4th % of Normal Precip

In many respects, the heat wave had begun before the month had started. June saw well above normal temperatures, including triple digits on the 28th. Prolonged warmth and dryness across the middle Mississippi Valley and lower Great Lakes regions leading up to this period had allowed for the already hot conditions to be further enhanced. A map of the dryness can be seen to the left. A dry and warm ground can be more readily heated by the sun than can a moist surface. This is one reason why Rockford and Waukegan were some of the warmer sites during the heat wave, as they had seen below half of their normal precipitation since May 1st.

The prolonged warmth from the winter into the spring also allowed Lake Michigan to be well above normal (10-15°F), and minimized the influence of the lake breeze. For more on the warm Lake Michigan water, see this article.

Many of the aforementioned parameters were taken into account in the forecast process by meteorologists. These include noting local effects, bias-correcting too cool model guidance, and simple recognition of a record pattern. The NWS had issued Excessive Heat Warnings and Heat Advisories across numerous states in the middle part of the country. The below national map from the evening of July 4th depicts the headlines in effect for the subsequent days, with the magenta indicating Excessive Heat Warnings and the orange depicting Heat Advisories.

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