National Weather Service Weather Forecast Office

The heat wave which gripped the upper midwest last week has broke records for temperature and dew point, and even heat indices across the region. Click here for a quick explanation of what dew point and humidity really mean.

In the Twin Cities: 

• A record high minimum temperature was set on July 18th, when a low temperature of 80 degrees was recorded at Minneapolis - St. Paul International Airport.  The previous record was 78 degrees which was set in 1986. 
• A record high minimum temperature was also set on July 20th, when a low temperature of 80 degrees was recorded. The previous record was 76 degrees which was set in 1901, 1935 and 1940.
• The record high minimum temperature was tied on July 17th, with a low temperature of 79 degrees.  The record was previously set in 1936 and 1942.
• A heat index value of 119 degrees was calculated from a temperature of 95 and dew point of 82 at the airport at 4PM on July 19th.  
• A new hourly dew point record was set on July 19th: 

...UNOFFICIAL DEW POINT RECORD SET IN THE TWIN CITIES ON JULY 19TH...

THE DEW POINT TEMPERATURE AT THE MINNEAPOLIS ST PAUL INTERNATIONAL
AIRPORT WAS 82 DEGREES ON THE 3 PM AND 4 PM OBSERVATION. THIS WAS THE
HIGHEST DEW POINT TEMPERATURE REPORTED ON AN HOURLY OBSERVATION AT
THE MINNEAPOLIS ST PAUL INTERNATIONAL AIRPORT SINCE HOURLY DEW POINT
RECORDS BEGAN IN 1945.

THE PREVIOUS HOURLY DEW POINT RECORD WAS 81 DEGREES...WHICH WAS SET
ON JULY 30TH 1999. A DEW POINT OF 81 DEGREES WAS ALSO REPORTED AT
THE AIRPORT ON SUNDAY...MONDAY...AND EARLIER TODAY /JULY 17-19/.

THE MINNESOTA STATE CLIMATOLOGY OFFICE IS THE OFFICIAL SOURCE OF
DEW POINT RECORDS ACROSS THE STATE. THIS NEW RECORD WILL BE
CONSIDERED UNOFFICIAL UNTIL IT IS CONFIRMED BY THE STAFF AT THE
STATE CLIMATE OFFICE.

In St. Cloud, Minnesota:

• The second longest consecutive stretch of low temperatures at or above 70 degrees was set this week.  Five days in a row, temperatures bottomed out at or above 70 degrees, ending July 20th.  The longest streak of low temperatures at or above 70 degrees in St. Cloud is 10 consecutive days, set between July 16th and July 25th 1981.  

In Eau Claire, Wisconsin:

• The record high minimum temperature was tied on July 18th, with a low temperature of 76 degrees.  The record was previously set in 1942.
• The second longest consecutive stretch of low temperatures at or above 70 degrees was tied this week.  Four days in a row, temperatures bottomed out at or above 70 degrees, ending July 20th.  A four day stretch of lows at or above 70 degrees was also set between July 17th and 20th 2006. The longest streak of low temperatures at or above 70 degrees in Eau Claire is 5 consecutive days, set between July 16th and July 20th 1981.  

 Regionwide...

The record amount of water in the air, in combination with temperatures in the 90s pushed heat index values to the extreme.  Click the link below to see a list of maximum heat index values from automated sensors across central and southern Minnesota, and west central Wisconsin during the heat wave.  

Maximum Heat Index Values During the Heat Wave

 Precipitable Waters...

    Precipitable water (PWAT) is a term used commonly by meteorologists to describe the moisture within the vertical, that is from the ground up to near the top of the troposphere (the part of the atmosphere where weather occurs).  If all the water vapor were to be rung out in that thin atmospheric column, it would equal the precipitable water.  Hence the term can be helpful for predicting thunderstorms and heavy rainfall potential.   PWAT also can indicate air mass type, with low values under one half inch being commonly from Canadian or Arctic air masses, and those values of 1.75 inches or higher often of tropical or subtropical origin.  PWAT is thus used in the forecast process, and is regularly mentioned in our Area Forecast Discussion.  PWAT is measured directly by radiosondes on weather balloons, which are released twice daily by many NWS offices.  Our office in Chanhassen is one of those offices, and the weather balloon data from us can be found here.

    During the warm and humid stretch of the past week, it appears that NWS Chanhassen had a record high PWAT measurement for the period of record (1948-present) during the 6 am balloon launch on July 18th.  The PWAT measured on that morning was 2.44 inches unofficially.  The record had been 2.37 inches measured on the evening balloon launch at Chanhassen on August 12th, 1995 (00Z August 13th).  Note the period of record is a combination of measurements taken at St. Cloud prior to 1995 and then from Chanhassen since 1995.  It appears that this also may have been the highest recorded PWAT on record for NWS balloon launches in the state of Minnesota.

    To see a plot of the July 18th balloon launch plot, see here.