A  Severe Thunderstorm Watch has been issued until 7 p.m. Thursday, and includes the entire metro area.

Thunderstorms are beginning to form over southern Wisconsin with scattered development farther to the south over northern Illinois anytime after 2:30 p.m. or 3 p.m. Storm clusters will continue to develop Thursday and will move through in waves through about 4 a.m. Friday. Any thunderstorm that pops may easily become severe. Storms will be more isolated to scattered between 2:30 p.m. and 6 p.m. and a little more numerous between 6 p.m. through 4 a.m. Thursday. Some computer models indicate the heaviest storms will be across southern and central Wisconsin and near and south of I-88 with scattered activity in between.

The timeframe to be concerned is between 2:30 p.m. through 4 a.m. Friday.

Storms may produce:

-Hail

-High winds (40 to 60 mph)

-Heavy rainfall between 0.50 to 1.50 inches

-minor flooding possible

Severe Weather Index Scale: (Moderate Risk for the North Shore)

Ryan’s Severe Weather Index Scale

1.)   Slight Risk (30 percent probability)

2.)   Moderate Risk (50 to 60 percent probability)

3.)   High Risk (70 percent probability)

4.)   Significant Event (80 to 90 percent probability)

5.)   Critical weather situation (95 to 100 percent probability)