Figure 1: Anomalous Surface Temperatures (C) for June 17th - 26th, 2009 ... note the warmer colors situated across the southeastern portions of Kansas into northeastern Oklahoma and the spread through the Mississippi Valley Region of values of +3-4 C (5-7 F).
Figure 2: Mean Surface Relative Humidity (%) for June 17th - 26th, 2009 ... note the 90 to 95% mean relative humidities that existed across much of the Mid-Mississippi Valley Region.
Figure 3: Mean Surface Winds (m/s) for June 17th - 26th, 2009 ... note the mean southerly flow over western portions of Missouri with southwesterly flow over Illinois.
Figure 4: Mean Surface Pressure (mb) for June 17th - 26th, 2009 ... note the cyan color across the southern Gulf Coastal Regions indicating higher pressure.
Figure 5: Mean 500 mb Heights (m) for June 17th - 26th, 2009 ... note the dark red shaded region which indicates the location of the mean 500 upper level high pressure center. Keep in mind that high pressure rotates clockwise and winds are nearly parallel to the contrast curves between the different colors.
Figure 6: Anomalous 500 mb Heights (m) for June 17th - 26th, 2009 ... note the warmer colors across the Mid-Mississippi Valley Region which illustrate higher heights over the area indicating higher pressure.

NWS
St. Louis, Missouri

June 17th - 27th, 2009 Heat Wave

Overview:

A strong upper level ridge of high pressure built across southern portions of the United States during mid-June allowed for the transport of very hot air from the Desert Southwest via southwesterly flow aloft.  Meanwhile a series of weather systems over north-central portions of the United States in concert with surface high pressure over the Southeast allowed for southerly flow to lift very moist and humid air from the Gulf Coastal regions across much of the central United States. 

The combination of heat and humidity resulted in a period of 11 days of above 90 degree temperatures, with heat index values at 100 degrees or above, for locations in the mid-Mississippi River Valley and to the south and west.

Climatological Review for the period from June 17th - June 27th:

Date

Saint Louis
Lambert Field

1874 -

Saint Louis
Science Center
1987 -
Columbia
Regional Airport
1890 -
Columbia
Univ. of Missouri
1997 -
Quincy
Regional Airport
1948 -
Average High
94.8 (9th all-time) 95.2 (max record)
91.4
92.2 (max record)
91.1 (4th all-time)
Average Low
76.2 (max record) 75.5 (max record) 71.1 (3rd all-time)
72.9 (max record)
70.3 (max record)
Average Temp
85.5 (max record) 85.4 (max record) 81.2 (9th all-time) 82.6 (max record)
80.7 (2nd all-time)
Max Heat Index 107 on 27th   106 on 27th

110 on 27th
Average Max Heat Index
104   100
99
 

Additional Notes:

Numerous volunteer observing stations across both Missouri and Illinois reported tieing or breaking their all-time maximum low temperatures on various dates through the entire period of the heat wave.  Below are several all-time maximum low temperature records set at official long-term observing stations: 

June 18th:

St. Louis Lambert Field: 78 (previously 77 set back in 1984)

June 22nd:

St. Louis Lambert Field: 80 (tied previous record set back in 1914)
Quincy Regional Airport: 74 (previously 72 set back in 1988)

June 23rd:

St. Louis Lambert Field: 79 (previously 78 set back in 1988)
Quincy Regional Airport: 73 (tied previous record set back in 1950)


Analysis:

Surface Temperature Anomaly for June 17th - 27th
Surface Mean Relative Humidity for June 17th - 27th
Surface Mean Wind for June 17th - 27th

Mean Surface Pressure for June 17th - 27th

 Mean 500 mb Heights for June 17th - 27th

 Anomalous 500 mb Heights for June 17th - 27th

Images courtesy of the NOAA Earth System Research Laboratory - Physical Sciences Division

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