I proceeded to send an email to the National Weather Service asking them what they thought of the USGS readings.

A member of the National Weather Service responded.

Yes...the high water upstream of St. Louis will be causing some flooding south of St. Louis. 
We have been monitoring the water situation very closely, and we do have
flood warnings for the Mississippi River south to Chester.

The basic differences in the flood threat north and south of St. Louis comes
from the Missouri River.  Fortunately, the Missouri River has not been exceptionally high over the past few weeks
(unlike 1993...when both rivers were running at record crests).   The capacity of the Mississippi River to
hold water increases quite a bit south of the confluence of the rivers...so if
the main contributor to the flooding is the Mississippi, the flood threat
lessens as you head south of St. Louis. 

Currently, we think that the Missouri water levels will not become
exceptionally high, so the flood threat is expected to abate somewhat south of
St. Louis.  Or perhaps a better way to say it is that the flooding south of St. Louis is not expected to be as bad as
the flooding to the north. 
Now, if the Missouri River does come up considerably, then the flooding threat
will be worse. 

Could evacuations become necessary?  Yes, depending on how close to the river you live.

If you follow the AHPS/River Info link on our home page, you can get hydrographs
for all of the river points in our area, and below each graph is a list of
river stages for that point, and the impact that occurs at each stage (which
roads are closed, etc...etc...)