Severe thunderstorms possible this afternoon, evening

Updated 10 a.m.

After a gorgeous Fourth of July holiday weekend, we're expecting some big changes in our weather.

Dew point temperatures will rise to around 70 degrees this afternoon, as a low pressure system approaches. It will be steamy, with highs in the upper 80s to around 90 over southern and central Minnesota:

Scattered showers and an isolated thunderstorm are possible this morning, then we'll have a chance of strong to severe thunderstorms this afternoon and this evening.

The National Oceanic and Atmospheric Administration’s North American Mesoscale Forecast System shows the thunderstorms spreading from west to east this afternoon and evening:

Severe weather risk

The Storm Prediction Center of the National Weather Service shows an enhanced risk of severe weather today and tonight over much of central and southern Minnesota, including the Twin Cities:

Our best chance of severe weather in the Twin Cities appears to be late this afternoon and this evening.

Here are the risk categories that are used by the National Weather Service:

10 a.m. update

The Twin Cities office of the National Weather Service plans an extra balloon launch early this afternoon to get a fresh sounding of the atmosphere.

Weather balloons are normally launched around 7 a.m. and 7 p.m. CDT (6 a.m. and 6 p.m. CST) as part of a worldwide gathering of weather information. When there is the potential for severe weather, and the atmosphere is expected to destabilize rapidly,  an intermediate balloon launch is sometimes performed.

The balloon carries an instrument package that measures temperature and humidity as it rises several miles into the atmosphere. The weather data is transmitted to a receiver at the launch site.  The exact position of the balloon is also tracked as it rises, allowing meteorologists to determine winds throughout the atmosphere.

The Chanhassen office of the National Weather Service is one of two sites in Minnesota which launch weather balloons twice daily for atmospheric soundings. The other site is International Falls.

Here is a map of the locations in North America which conduct weather balloon launches twice daily:

Simultaneous weather balloons launches occur twice daily from hundreds of other sites around the world.

Here is how the plotted Chanhassen sounding looked this morning:

The red line shows the temperature profile as the instrument package rose above the Twin Cities this morning. Wind barbs are on the right.

Meteorologists apply formulas to soundings to determine the instability in the atmosphere. Computer models use regional and national upper air data, combined with surface observations, to predict future thunderstorm development.

Cooler Wednesday

Highs in the 80s return to southern Minnesota on Wednesday:

Wednesday should be mostly dry, with a chance of showers and thunderstorms returning on Thursday.