Mother Nature will provide her own spooky weather for Halloween. A powerful storm system will be moving across the country over the next few days. There is a 160 mph splitting jet stream over the southern Rockies that will be moving east. A strong flow of mild Gulf air has moved north ahead of the system. The low level moisture will provide the fuel for not only heavy rains, but also an outbreak of severe storms with highs winds and a few tornadoes.
It is not unusual to see a fall severe storm episode. The only ingredients needed for severe storms are adequate moisture, instability, and shear (change of wind speed and/or direction with height). The video explains wind shear. During fall all these ingredients usually come together at least for one day, and usually over the south. What makes this event unusual is the severe potential is all the way north into southern Michigan and will stretch over a two day period beginning today. The full storm outlook from SPC can be found here.
For the Chicago metro area, a heavy overcast is over the area today, Wednesday. The overcast is the result of a strong push of warm and moist air that is occurring aloft. The graphic from the NWS shows the expected average rainfall over the country ending Saturday morning. As can be seen, the metro area is in a zone where 2+ inches are expected. There will be locally higher amounts where any thunderstorms train over an area. Precipitable water values are forecast around 1.5 inches which is way above normal for the end of October.
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