we had this symbol come up on a G1000 and could not find a reference for what it indicates. Could anyone tell me what it means?
It's a county flood warning:
County data (Figure 6-38) provides specific public awareness and protection weather warnings from the National Weather Service (NWS). This can include information on tornadoes, severe thunderstorms, and flood conditions.
Displaying County Warning information:
1) Select the Weather Data Link (XM) Page.
2) Press the MORE WX Softkey.
3) Press the COUNTY Softkey.
To display the County Warnings legend (Figure 6-39), press the LEGEND Softkey when County Warnings are selected to be displayed.
G1000 Pilot's Guide for Cessna Nav III p. 304 (the symbology is the same in all G1000 variants with XM weather and the legend can probably also be found in the Pilot's Guide for your variant if not a Cessna Nav III).