Strong high pressure has allowed for above normal temperatures in much of the central and eastern United States. That high pressure system will slowly move from the Central Plains on Saturday to the Southeastern United States by the early part of next week. This will take the above normal temperatures and above 100 degree heat index, which takes into consideration humidity for a truer feels-like temperature, into the Southeastern United States. Temperatures have been and will continue to be over 90 degrees for much of the central and eastern United States. Heat waves will most likely be reached across many areas including Lawrence, Massachusetts which reached 91 on Thursday, 99 on Friday and already reached 90 before midday on Saturday. Minneapolis, Minnesota reached 90+ degrees as high temperatures from Wednesday to Friday and could reach 90+ on Saturday too. In addition, Ann Arbor,, Michigan reached 90+ degrees for high temperatures on Thursday and Friday and could reach 90+ on Saturday for a third day in a row, the criteria for a heat wave.
All this heat is usually followed by thunderstorms during or by the end of the day. As such, by sunset Saturday, a low pressure system will be moving northeast out of the Rockies into Southern Canada and will drag its cold front through the Northern Plains by Sunday morning. This will turn above normal temperatures into more normal temperatures for this time of year. However, this temperature change will mean strong to severe thunderstorms will be forced along by this cold front. A stronger than normal low level jet stream is in the Central and Northern Plains on Saturday and then it will be in the Great Lakes on Sunday before being in the Northeast on Monday. This low level jet stream will help to keep thunderstorms sustained as they pop during the day. During this time, the low pressure system in Southern Canada will be deepening allow the better forcing for showers and thunderstorms to be closer to the deepening upper level low pressure system in Southern Canada. Convective energy will also support thunderstorms along the northern periphery of the United States as the cold front moves east with 3000+ J/kg of CAPE on Saturday and Sunday. On Monday, after the front crosses the Great Lakes and with a little more marine influence stabilizing the airmass, the convective energy will be a little less, but will still reach into the 1000s of J/kg.
On Tuesday, another low pressure system is forecast to eject off the Rockies and allow for more severe weather in the Northern Plains with more warm air around the region. Warm air will once again traverse east. While there won’t be a strong high pressure system around the region indicating warm air, 90 degree temperatures are still possible with above normal warm air in the low levels of the atmosphere in the northern US. The Northeastern US will have a chance to be cooled off Wednesday into late week with temperatures below 90 degrees should onshore winds occur.