The worst windstorm to bear down on the upper Midwest in decades is beginning to fall into place. The extremely powerful low pressure system is expected to reach a minimum pressure of 957mb (or 28.26 inches of mercury)! Typical air pressure values are about 1013mb (29.92 inHg), while values as low as those expected from this storm are typically found only in a major, Category 3 hurricane! The surface map below from 5AM this morning shows minimum pressure values of 970.9mb, so the storm is still strengthening! Also note the red lines on the map. These are isobars – lines where the pressure is equal along them (approximately, of course). The important factor with these isobars is that the closer they are packed together, the stronger the wind field will be. These isobars are packed very tightly together, and this trend is only expected to increase. This low pressure system has a very powerful cold front attached to it which is poised to spawn a severe weather outbreak across the upper Midwest (namely Illinois) into the Ohio Valley, and potentially even farther east, possibly throwing some severe wind gusts in the Pittsburgh area by late evening Tuesday. In...