Heat and Humidity Beginning to Return
Cool, high pressure that originated from the Northern Plains and brought lower humidity and comfortable temperatures the last several days is moving off to the east. Increasing south winds on its backside will begin to usher warmer and more humid conditions into Wisconsin today, but will be more noticeable on Sunday. Highs today will be a few degrees warmer than Friday, reaching the middle 80s most locations.
The surge in warmth and humidity will also set off isolated showers and thunderstorms today.
The south winds will continue to pump very warm and increasingly humid air into Wisconsin for Sunday. These conditions are then expected to persist for much of the coming work week, at least through Wednesday or Thursday.
High temperatures will reach the middle 80s to lower 90s each day. These temperatures combined with the humid air will cause heat indices to peak in the 90s each day.
A growing upper level high pressure ridge (approximately 15,000 ft above the ground) over the central portions of the United States, will allow the heat to build into the Upper Mississippi Valley and western Great Lakes. This is the same heat that has plagued much of the south and east over the last few weeks. The image below shows how this upper level ridge builds by Monday and Tuesday:
The next image shows how the heat will surge in from the west, southwest:
Surface low pressure will move through the Northern and Central Plains, pushing a warm front into Wisconsin on Sunday. This front will mark the leading edge of some very warm and humid air. See image below:
The warm front will make it into southern Wisconsin as early as Sunday, then linger in the area through at least the first half of next week. It will vary in both location and strength through the week, acting as a focus for showers and thunderstorms from time to time.
Dewpoints, which are a measurement of how much moisture is in the air, will rise back into the lower and mid 70s. This will combine with the very warm air to produce heat index values in the upper 80s to mid 90s. At this point, it is just below our criteria for issuing advisory/watch/warning headlines for excessive heat.
For your information, the NWS offices that serve Wisconsin issue the following Watches, Advisories or Warnings...
Heat Advisory - Maximum heat index values of 100 to 104 for one day, or maximum heat index values of 95 to 99 for four (4) consecutive days or more.
Excessive Heat Warning - Maximum heat index values of 105 or more during the day and a minimum heat index value of 75 or more for at least a 48-hour period. Also, a maximum heat index value of 100 to 104 during the day for four (4) consecutive days or more will trigger a warning.
Excessive Heat Watch - Issued when the following conditions are expected: 1) Maximum heat index values of 105 or more during the day and a minimum heat index value of 75 or more for at least a 48-hour period. 2) A maximum heat index value during the day of 100 to 104 for four (4) consecutive days or more.
In any case, don’t leave children or pets in a car while you run an errand – even if it’s just going to be for a few minutes. Temperatures can reach dangerous levels in a matter of minutes. Beat the Heat - Check the Backseat! Temperatures inside a closed vehicle can rise to 35 to 45 degrees above the outside air temperature. Over 450 children have died inside a closed vehicle due to the effects of heat during the past 12 years.