Changes Coming to River Flood Warnings

Beginning on November 1st, NWS Central Illinois will implement polygon-based Flood Warnings and Flood Statements for river forecast points in our area of responsibility. In the past, these county-based warnings highlighted entire counties on the Watch/Warning/Advisory (WWA) map on the front page of our website. Soon, only the area along the river in proximity to the forecast point will be highlighted. Other than outlining a more defined area, this change will also allow additional hazards to be displayed on the WWA map in areas impacted by other weather hazards.
Important Note: These polygons are not meant to signify areas of inundation. They are simply a means to graphically represent select rivers where we provide flood forecast and warning services.

The format of the text products will remain unchanged with the exception of a series of latitude/longitude pairs at the end. These are the anchor points for the polygons. Each river forecast location has a set of static points that will be included in all river Flood Warnings and follow-up Flood Statements. Below is an example...

LAT...LON 3878 8846 3871 8829 3960 8820 3860 8832
      3867 8838 3870 8846
This change to river Flood Warning Polygons has been coordinated and implemented across all NWS offices that service portions of Illinois. This includes:
NWS Central Illinois:




Old county-based river flood warnings vs. New polygon-based warnings.


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