A low-pressure system will slide into the Midwest by Monday, and this boundary will help to provide some of the lift needed to produce thunderstorms. This area of low pressure will then push eastward Tuesday. Farther to the west, a separate low-pressure system will sweep from Montana into the northern Plains by midweek, bringing a risk of severe weather to those areas, as well.
On Tuesday, the severe weather moves into Pennsylvania. The risk of severe thunderstorms will be found from southeast Iowa and northeast Missouri eastward into portions of Pennsylvania, New Jersey, New York and into much of New England. Portions of Minnesota, eastern North Dakota and northwest Wisconsin could also see severe storms ahead of the second low-pressure system, with a cluster of storms likely overnight in these areas. Damaging wind gusts and large hail will be the main threats, but an isolated tornado is also possible.
What can you expect? Heavy rainfall will also be a concern. Rounds of showers and thunderstorms will fall on already saturated ground in many areas and could lead to flash flooding. A widespread area of 1 to 3 inches of rain is expected from southern Wisconsin and northern Illinois into western Pennsylvania and parts of New York state through Wednesday, with locally higher amounts in some thunderstorms. There is also the potential for thunderstorms clusters, or mesoscale convective systems (MCS), to develop and persist overnight. These complexes of thunderstorms can contain very heavy rain and damaging winds.