Official forecast gage on the Kaskaskia River at Carlyle, IL is moving

Service Change Notice
National Weather Service Saint Louis MO
825 AM CDT Wed Mar 30 2022

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
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TO:      Family of Services /FOS/ subscribers...
         NOAA Weather Wire Service /NWWS/ subscribers...
         Emergency Managers...Weather Information Network
         /EMWIN/ subscribers...NOAAPort subscribers...
         other National Weather Service /NWS/ users and
         partners...and NWS employees.

FROM:    Mark Fuchs
         Senior Service Hydrologist

SUBJECT: Moving forecast services on the Kaskaskia River at Carlyle
         from the tailwater gage downstream to the city gage.

This morning the National Weather Service (NWS) office in Saint
Louis, Missouri is moving its flood-only river forecast services
from the tailwater gage below Carlyle Dam to the United States
Geological Survey (USGS) gage at the U.S. Highway 50 bridge in
Carlyle, Illinois, about a half mile below the tailwater gage.

The primary reason for this change is to take advantage of the
current rating curve maintained by the USGS.  A rating curve is the
dynamic relationship between the water surface elevation and the
flow.  Since NWS modeling computes flow to output a forecast, it is
imperative that the rating curve is as accurate as possible.  For
several decades, the NWS used the upstream tailwater gage and a
rating curve that is not routinely updated. The USGS periodically
measures flow at the downstream gage, measurements that are
routinely used to update the rating curve for this site.  This
change should result in a more precise forecast.

Flood-only services mean services will only be provided when
forecast or observed stages are above action stage.  At this site,
flood stage will be 16.5 feet. This is more than 3.5 feet below the
previous flood stage at the tailwater gage, and will result in more
frequent warning.  This lower flood stage is intended to account for
agricultural flooding that begins near this level.  Moderate
flooding begins at 23 feet, while major flooding begins at 27 feet.
Since the river flow is regulated by the U.S. Army Corps of
Engineers in St. Louis, flood impacts below the dam are almost
exclusively minor. In fact, the last time moderate flood stage was
exceeded at this site was on May 27, 2002, when the river crested at
23.72 feet.  The flood of record of 33.69 feet occurred on May 21,
1943, decades before the construction of Carlyle Dam.

Flood stage is the river level at which minimal human impact from
floodwaters begins, and the level the National Weather Service uses
as a threhold for issuing river flood warnings.  Moderate and major
flood stages are levels at which human impact increases noticably.
At moderate levels, numerous secondary roads are often inundated and
some outbuildings may be flooded.  At major levels, primary roads
and highways can become flooded along with residences and businesses.

In addition to forecasts and observations, impact information, data
on past flood events, and additional information can be found at the
St. Louis Advanced Hydrologic Prediction Service (AHPS) website at
water.weather.gov/ahps2/index.php?wfo=lsx.

The National Weather Service welcomes public feedback.  If you have
any questions or comments regarding these service improvements,
please contact:

Mark Fuchs
Senior Service Hydrologist
National Weather Service Forecast Office - St. Louis, Missouri
12 Missouri Research Park Drive
St. Charles, MO 63304
Phone:  (636) 447-1876 extension 493