Why would a city, such as New Orleans, at the mouth of the Mississippi River, be especially concerned with precipitation in other parts of the country? What could happen to New Orleans if areas to the north of it experienced heavy precipitation?
Assuming you are asking if heavy rains or other runoff would raise the level of the Mississippi River, that happens every year during Spring. It’s why the river is lined with high levees – to prevent the overflowing river from flooding the areas around it.
The "Great Flood of 1927" was the event that caused the construction of the modern levee system, though levees were built along the river since Europeans settled in Louisiana.
Note that the levees protecting New Orleans in 1927 were not breached by the high waters.
Also note that the city is not at the mouth of the Mississippi. New Orleans is 90 miles from the mouth of the river.