Strong afternoon thunderstorms will mark an end to the 90 degree heat in the Upper Great Lakes Sunday. The storms will be triggered by the passage of an intense cold front. Some of these storms will be capable of producing damaging winds and large hail across large swaths of Wisconsin, Illinois, and Michigan. Tornadoes will also be possible over parts of eastern Wisconsin. A lesser risk for severe storms exists in northern Texas, where a weak frontal boundary may initiate isolated thunderstorms, and in northern North Dakota, where a passing impulse will prompt scattered storms, a few of which may become severe.
Cloud debris from early morning convection should clear just in time to briefly allow temperatures to rise into the upper 80s and low 90s while simultaneously destabilizing the environment before the arrival of the cold front. Very large quantities of instability will develop early in the afternoon so that when the lifting mechanisms arrive, thunderstorms will quickly develop. Shear will generally be insufficient for supercells except over northeastern Wisconsin, so a squall line with a few bowing segments will be the primary mode of thunderstorm organization across the Upper Great Lakes. However, a few supercells will be possible in central and eastern Wisconin near the Door Peninsula. Early morning storms will persist in eastern Minnesota and northwestern Wisconsin, so severe weather will be unlikely to occur there
Storms will organize in eastern Iowa, western Illinois, and southwestern Wisconsin into a broken line of storms by lunch time and spread east throughout the rest of the day. Storms will reach Chicago and the eastern shores of Lake Michigan by early evening. Storms will be most widespread in northern Illinois, central and eastern Wisconsin, and the southwestern upper Michigan peninsula.
As the parent low of the cold front tracks northeast across the Upper Michigan Peninsula into Ontario, an unseasonably potent mid-level jet streak will cross Wisconsin. Central and northeastern Wisconsin will reside within the right entrance region of this jet streak. There is enhanced ascent within the vicinity of the right entrance and left exit regions of jet streaks, so supercell thunderstorms may develop ahead of the main line of storms in these areas. A modest risk for tornadoes exists where the jet streak’s right entrance region overlaps areas where shear is sufficiently strong along and to the north of the Upper Peninsula.
Tornadoes that do form will neither be long-lasting, nor will they be particularly strong but they may bring isolated destruction if they cross populated areas. Cities at risk for tornadoes include Appleton and Green Bay. Elsewhere, scattered wind damage and isolated large hail will be likely with these storms, with the risk extending down to Saint Louis, MO.
The severe threat is much lower over Texas and northern North Dakota. The only source of lift over north-central Texas will be from a weak cold front. With subsidence from high pressure just south of this area, the frontal boundary will only be enough to spawn isolated storms. But with very high instability, storms that do pop-up may be capable of producing strong wind gusts and large hail. In northern North Dakota, moisture and instability will be lacking. However, a shortwave trough passing through southern Sasketchewan and Manitoba will provide ascent for scattered thunderstorm development. Storms that develop in isolated pockets of modest instability will be capable of producing strong wind gusts.
Monday, a shortwave trough may trigger severe thunderstorm development over parts of the Plains and Upper Midwest. Meanwhile, parts of the Northeast should be on the lookout for afternoon storms to quell the heat. The ridge of high pressure responsible for this weekend’s excessive heat will be flattened by a shortwave trough over southern Ontario and Quebec. This shortwave may spark thunderstorms along and to the west of the mountain of Pennsylvania, New York and northern New England.