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:
...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:
In Eau Claire, Wisconsin:
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.
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.